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J. Paleont., 75(5), 2001, pp. 947–971 Copyright q 2001, The Paleontological Society 0022-3360/01/0075-947$03.00

SYSTEMATICS OF THE ORDOVICIAN TRILOBITES ISCHYROTOMA AND DIMEROPYGIELLA, WITH SPECIES FROM THE TYPE IBEXIAN AREA, WESTERN U.S.A. JONATHAN M. ADRAIN,1 STEPHEN R. WESTROP,2 ED LANDING,3

AND

RICHARD A. FORTEY4

Department of Geoscience, University of Iowa, 121 Trowbridge Hall, Iowa City 52242, ,[email protected]. and Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom, 2 Oklahoma Museum of Natural History and School of Geology and Geophysics, University of Oklahoma, Norman 73019, ,[email protected]., 3 Center for Stratigraphy and Paleontology, New York State Museum, The State Education Department, Albany, New York 12230, ,[email protected]., and 4 Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom, ,[email protected]. 1

ABSTRACT—Lower Ordovician sections in the type Ibexian area of western Utah contain a considerably more diverse trilobite fauna than has previously been reported. Reinvestigation of these faunas, based on new field sampling, allows a reassessment of the dimeropygid genera Ischyrotoma Raymond, 1925, and Dimeropygiella Ross, 1951. These taxa have been considered synonyms, but parsimony analysis indicates each is a well supported clade, and they are best recognized as sister genera. The number of species known from Ibex has been doubled, from four to eight, and morphological information is now available for most parts of the exoskeleton. New species include Ischyrotoma juabensis (Juab Formation), I. wahwahensis (Wah Wah Formation), Dimeropygiella fillmorensis (Fillmore Formation), and D. mccormicki (Fillmore Formation). The previously named species Dimeropygiella caudanodosa, D. blanda, and D. ovata are fully revised on the basis of abundant new material. Pseudohystricurus is a paraphyletic group, with species distributed as a basal grade of the Ischyrotoma/Dimeropygiella group.

INTRODUCTION

HE IBEX region of western Utah comprises the southern parts of the Confusion and House Ranges and the interlying Tule Valley and Barn Hills (Fig. 1). It contains some of the most complete and well exposed Lower Ordovician sequences in the world and is the type area for the Ibexian Series (Hintze, 1982; Ross et al., 1993, 1997). The sections through the Pogonip Group are particularly important because they contain abundant silicified invertebrate faunas. The trilobites of these faunas (also Ross, 1951) have formed the basis for a biostratigraphic scheme widely correlative across Laurentia and elsewhere (Ross et al., 1997). With a few minor exceptions, however (see below), the systematics and paleoecology of the Ibex trilobite faunas have not been dealt with in nearly half a century, and Hintze’s original monograph (1953) remains the sole documentation of the majority of the taxa. The present study represents the first result of a comprehensive systematic revision of the Lower Ordovician faunas described originally by Hintze and Ross, based on extensive new field sampling. Although the project is in its early stages, it is apparent that trilobite species diversity in the Ibex sections has been greatly underestimated, and many new species (and genera) await description. We begin here with a full revision of the dimeropygid genera Ischyrotoma Ross, 1951, and Dimeropygiella Hintze, 1953, including the description of three previously unreported new species and the first known early growth stages.

T

HISTORY OF STUDY OF THE IBEX TRILOBITE FAUNAS

Since Hintze’s (1953) original monograph, only a few papers have been published on Ibex trilobites. Hintze (1954) published two new genus names for preoccupied taxa, and Hintze and Jaanusson (1956) named three new genera of asaphids with Ibex type species. Harrington (1957) named five new genera of pliomerids, two of which had Ibex type species, and a further two of which had Garden City Formation type species which also occur at Ibex. No new material was photographically illustrated in any of these studies. Subsequent revision of the Lower Ordovician faunas has included Demeter’s (1973) work on the pliomerids, Terrell’s (1973) study of the lower Fillmore Formation zonation, and Young’s (1973) treatment of a single rich horizon in Section H. The latter group of papers added thirteen new formally named

species and subspecies. They also contained a substantial number of unnamed yet obviously new taxa reported in open nomenclature, supplementing several such undetermined forms in Hintze’s original monograph. Fortey and Droser (1996) augmented the Ibex faunas with several new species from the earliest Whiterockian Juab Formation. Hintze’s monograph concentrated on the rich silicified faunas. Trilobites in the Juab Formation are not silicified and Hintze (1953, p. 19) regarded them as poorly preserved and largely unidentifiable. Fortey and Droser, however, obtained several good calcareous crackout collections from Section J, and proposed a new threefold subzonal division of the Juab based on the newly discovered trilobites. Finally, McCormick and Fortey (1999) illustrated some new silicified Ibex material of the telephinid Carolinites genacinaca and related forms. As a result, Hintze’s (1953) monograph remains the sole documentation of most of the Ibex trilobites, and comprises much of the basis for detailed biostratigraphy published nearly half a century later (Ross et al., 1997). Hintze’s work is exceptional, especially for the time it was published, but a desire to more fully document the taxa, including all sclerites and available ontogenetic material, coupled with the relatively high number of species described only in open nomenclature, led J.M.A. and S.R.W. to begin a comprehensive, field-based revision of the Ibex trilobite faunas. The first results are presented herein, and they are representative of our experience with many resampled horizons in the area: new collections have revealed exactly double the number of species of Dimeropygiella and Ischyrotoma as previously known. The new species were not previously reported in open nomenclature; they are newly discovered in the Ibex sections. Similar finds have been made in many other groups at many other horizons, and indicate that a significant portion of the trilobite diversity in the type Ibexian area remains to be described. This situation is likely to be at least partly a result of sampling intensity. Hintze (1953) gave qualitative estimates of relative species abundances at particular sampling horizons, but did not list total sample sizes. Terrell (1973) and Demeter (1973), however, both described their samples as ‘‘large.’’ Neither gave quantitative sampling data, but the average physical dimensions of Demeter’s (1973, p. 40) samples were described as ‘‘about 40 3 40 3 60’’, which would amount less than 1 kg of rock. Terrell’s (1973, p.

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FIGURE 1—Geographic position and line of section of sections H and J of Hintze (1953, 1973), Heckethorn Hills, Ibex area, Millard County, western Utah.

69) samples were larger, described as ‘‘5- to 10-pound,’’ or approximately 2–5 kg. This is an order of magnitude less than the level of sampling in our current field program, in which 25–40 kg of rock are typically collected from individual horizons containing silicified trilobites. Experience with silicified faunas of Ordovician (Adrain and Fortey, 1997), Silurian (Adrain, 1997, and references therein), and Devonian (Adrain and B.D.E. Chatterton, unpublished data) age has indicated that samples of these sizes are more likely to yield an accurate faunal list. Differences in sampling do not account for all of the new taxa, however, as certain new species are by no means rare. Ischyrotoma wahwahensis, for example, is common throughout the middle part of Section J, and is the most abundant species at horizon J-81, yet it has not previously been reported, and may have been conflated with I. caudanodosa. Initial sampling of section J has also yielded previously unreported species of cheirurids, odontopleurids, bathyurids, and the telephinid Opipeuter. The goal of the present study is to describe all new and revised Ibex species of the dimeropygid genera Dimeropygiella and Ischyrotoma, including the earliest known growth stages, and to consider the validity and scope of the genera, which have often been considered synonyms. LOCALITIES

Hintze (1951, 1953) originally measured and described sections through the Pogonip Group in the Ibex area of western Utah (Fig. 1). He later (1973) published revised descriptions based on remeasurements made in 1965. We remeasured and redescribed

FIGURE 2—New logs of parts of sections H and J, with collection horizons of material described herein indicated on right of columns.

Hintze’s sections H and J, relating our new measurements to Hintze’s via the painted sets of numbers left on the sections by Hintze. Horizons used in the study are identified with a number relating them to the original footage of Hintze’s sections, and their position on our new descriptions (measured in metres) is shown in Figure 2. For example, horizon J-81 is a bed positioned at 81 feet in Hintze’s (1953) scheme, and at about 24.2 m in our remeasurement of section J. The relationship between Hintze’s original numbers and the 1965 remeasurements (also in feet) are given by Hintze (1973). Complete columnar sections and new descriptions will be published elsewhere. Here, we report only those parts of sections H and J that contain the sampling horizons for taxa in this study (Fig. 2). DIMEROPYGID CLASSIFICATION

Dimeropygidae was established independently by Hupe´ (1953) and Whittington and Evitt (1954), who noted Hupe´’s work in proof. Hupe´’s concept of the family included only Dimeropyge, 1937, and Bolbocephalus Whitfield, 1890, a genus firmly established as a member of Bathyuridae by Whittington (1953). Hupe´ also erected the family Toernquistiidae to include Toernquistia Reed, 1896, and Pyraustrocranium Ross, 1951. Whittington and Evitt’s (1954) concept of Dimeropygidae included Dimeropyge,

ADRAIN ET AL.—ORDOVICIAN TRILOBITES ISCHYROTOMA AND DIMEROPYGIELLA Toernquistia, Dimeropygiella, and their new Chomatopyge and Mesotaphraspis. They excluded Pyraustrocranium (a genus requiring reinvestigation; it was placed order and family uncertain in the 1959 Treatise) and considered Toernquistiidae a synonym of Dimeropygidae. This arrangement was followed by Whittington in the Treatise (Moore, 1959), but was emended in 1963. Raymond (1925) had erected Ischyrotoma on the basis of a partial and exfoliated cephalon from the early Whiterock of western Newfoundland, which he illustrated only by a line drawing. He listed, but did not figure, a second specimen from the Mystic Conglomerate of Le´vis, Quebec. Whittington (1963), in describing the rich ‘‘crackout’’ fauna from the great alpha boulder at Lower Head, now assigned to the Shallow Bay Formation of the Cow Head Group (James and Stevens, 1986), revised I. twenhofeli on the basis of the original holotype and new material. Fully illustrated, it was evident that I. twenhofeli was very similar to the Utah species of Dimeropygiella described by Ross (1951) and Hintze (1953). Whittington placed Dimeropygiella in synonymy of Ischyrotoma, an opinion followed by all subsequent workers to date. Whittington (1963) also erected the genus Ischyrophyma as a new member of Dimeropygidae. Earlier, Jaanusson (1956) had erected the family Celmidae for the Baltic genus Celmus Angelin, 1854. Jaanusson also erected the dimeropygid subfamily Mesotaphraspidinae to accommodate Mesotaphraspis and Chomatopyge. Whittington (1963, p. 49–50) compared his Ischyrophyma tuberculata directly to Celmus, but noted also its considerable cephalic similarities with Ischyrotoma. Critically, the pygidium of Ischyrophyma tuberculata was unknown; it could not then be established that it was of the distinctive single-segment Celmus type, and Whittington assigned his new genus tentatively to the dimeropygids. He later erected a second species, I. tumida Whittington, 1965, but again the pygidium was not recovered. Bruton (1983) revised Celmus granulata on the basis of good new material and considered Ischyrophyma a probable synonym of Celmus. Recently, J.M.A. and R.A.F. (unpublished data) have collected silicified material of Ischyrophyma tumida from the Table Cove Formation in the Hare Bay Region of western Newfoundland (equivalent to Whittington’s ‘‘middle Table Head formation’’). This shows unequivocally that Ischyrophyma possesses thoracic segments and a single-segmented, flanged pygidium essentially identical to that of Celmus. This was confirmed by a new silicified species, Celmus michaelmus Adrain and Fortey, 1997, from the Tourmakeady limestone of western Ireland. Adrain and Fortey (1997) placed Ischyrophyma in synonymy of Celmus, maintaining the latter in a family Celmidae. Glaphurella Dean, 1971, described as a glaphurid, is likely related to Celmus. Apart from Glaphurella, no new genera were added to the dimeropygids for over thirty years following Whittington’s (1963) work. The only genera to receive extended and detailed treatments were Dimeropyge (e.g., Shaw, 1968; Chatterton and Ludvigsen, 1976; Chatterton, 1980, 1994; Tripp and Evitt, 1983) and Ischyrotoma (Fortey, 1979, 1980; Ingham in Ingham et al., 1986). Recently, however, Chatterton et al. (1998) resurrected Toernquistiidae, to which they assigned Toernquistia, Chomatopyge, Mesotaphraspis, and their new Lasarchopyge and Paratoernquistia. Their parsimony analysis included well known species of ‘‘toernquistiids’’ and dimeropygids, together with 11 ‘‘hystricurids,’’ and supported the monophyly of the toernquistiid and dimeropygid groups (although the celmids, represented only by ‘‘Ischyrophyma’’ tuberculata Whittington, nested deeply within the dimeropygids). At this point, then, there are potentially three families, Dimeropygidae, Toernquistiidae, and Celmidae, that may be recognized within the broad ‘‘dimeropygoidean’’ group. Whether each is monophyletic and whether together they comprise a clade is a

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question for further study. In the present work we are concerned only with a subset of the dimeropygid component, the species that have previously been assigned to Ischyrotoma and relatives, which we refer to henceforth as the ‘‘Ischyrotoma group.’’ The primitive members of the Ischyrotoma group have long been identified as the poorly known species attributed to Pseudohystricurus Ross, 1951 (e.g., Fortey and Owens, 1975). The affinities of the group beyond these species are not well established. Some workers (Ross, 1951, p. 123; Fortey and Owens, 1975, p. 228) have regarded the Ischyrotoma group as the ancestral grade from which Dimeropyge was derived. Chatterton et al. (1998, figs. 4, 5), on the other hand, resolved Dimeropyge as a paraphyletic grade from which the Ischyrotoma group was derived (although Dimeropyge was resolved as a clade sister to the Ischyrotoma group when they applied successive approximations character weighting, in agreement with the result of Chatterton, 1994, fig. 1). The nature of the relationship will probably require more new data before it can be definitively assessed. However, there are a number of reasons to suspect that Dimeropyge is a related but independent clade. First, Dimeropyge, although broadly similar in morphology and tuberculate sculpture, has many features not shared with the Ischyrotoma group, including the presence in all species for which information is available of a long thoracic axial spine, a highly characteristic ‘‘walled’’ pygidium, and a unique, tiny hypostome and highly modified rostral plate (see Chatterton, 1994, for good examples of all). It is conceivable, of course, that all of these features are modifications of the states seen in the Ischyrotoma group, but there is no positive evidence of this. With the new information presented herein, we now have a good idea of what the Ischyrotoma/Dimeropygiella pygidium looks like during the meraspid and early holaspid period; at no point does it ever develop the smooth vertical ‘‘wall’’ of Dimeropyge. Members of the Ischyrotoma group have a large, natant hypostome. Articulated individuals of I. twenhofeli are known, and lack a thoracic axial spine; no such spine has ever been associated with any other species of the group known from disarticulated material. The early cranidial morphology of all members of the Ischyrotoma group shows a distinct median preglabellar furrow, a feature conclusively absent from all known Dimeropyge ontogenies. Finally, there is some evidence that animals with the Dimeropyge pygidial morphology were already established by the very early Ordovician: Ross (1951, pl. 9, figs. 25, 29, 30) figured a pygidium from the early Ibexian (Zone B; Symphysurina Zone) which is characteristically ‘‘walled,’’ has strong fulcral spines, and aside from its quite short sagittal length is difficult to distinguish from Dimeropyge (compare, e.g., with D. clintonensis Shaw, 1968 (Chatterton, 1994, figs. 7.15, 7.16, 7.19, 7.23)). At present, Dimeropyge seems best construed as an independent clade. Hence, we assume that the Ischyrotoma group is monophyletic. With the abundant new information presented herein, it is now possible to formulate an explicit hypothesis of ingroup structure by means of cladistic parsimony analysis. STUDY TAXA

We carried out parsimony analysis on all adequately known ingroup species previously assigned to Dimeropygiella, Ischyrotoma, and Pseudohystricurus as well as one previously assigned to Ischyrophyma. Taxa used in the analysis and sources used for coding are given in Table 1. Species which are definitely part of the ingroup but not well enough known for analysis include Ischyrotoma sp. ind. of Whittington (1965) (Ischyrotoma herein), Ischyrotoma cf. caudata of Dean (1989) (Dimeropygiella herein), and Ischyrotoma sp. A of Brett and Westrop (1996) (?Dimeropygiella herein). These are discussed under the appropriate genus heading in the systematics section below.

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TABLE 1—Coding sources, sclerites known, and type of preservation of species subject to parsimony analysis. Original generic assignment is given, and species are listed in alphabetical order. Original species name Ischyrotoma anataphra Dimeropygiella blanda Dimeropygiella cf. blanda Ischyrophyma? borealis Dimeropygiella caudanodosa Dimeropygiella fillmorensis Ischyrotoma juabensis Dimeropygiella mccormicki Pseudohystricurus obesus Pseudohystricurus orbus Dimeropygiella ovata Ischyrotoma parallela Ischyrotoma stubblefieldi Ischyrotoma twenhofeli Ischyrotoma wahwahensis Pseudohystricurus sp. Ischyrotoma sp.

Coding sources, sclerites, and preservation Fortey (1979, pl. 36, figs. 1–13); cephalon, pygidium; calcareous, mostly testiferous. Hintze (1953, pl. 19, figs. 6–8), Young (1973, figs. 2, 6, 7 only [see synonymy in text]); cranidium, librigena, ?hypostome, thoracic segments, pygidium; silicified. Figure 16 herein; cranidium, pygidium; calcareous, testiferous. Fortey (1980, pl. 12, figs. 1–7); cranidium, librigena; calcareous crackout, mostly testiferous. Ross (1951, pl. 35, figs. 18, 22–28), Hintze (1953, pl. 19, figs. 5, 10), Figures 7.24–7.50, 8, 9 herein; all sclerites known; silicified. Figures 11.9, 12 herein; cranidium, librigena, ?hypostome, pygidium; silicified. Foretey and Droser (1996, fig. 15); cranidia, librigenae, pygidia; calcareous crackout, testiferous. Figure 15.12–15.33 herein; cephalon, pygidium; silicified. Ross (1951, pl. 16, figs. 25, 30, 34; pl. 19, figs. 34, 38 [see discussion on genus]); cranidium, ?pygidium; silicified. Ross (1953, pl. 63, figs. 10, 11, 15–20, 23); cranidium, librigena; silicified. Hintze (1953, pl. 19, figs. 1–4), Young (1973, pl. 2, fig. 1), Figures 10, 11.1–11.8, 11.10–11.12 herein; cranidium, librigena, ?hypostome, ?thoracic segments, pygidium; silicified. Boyce (1989, pl. 34, figs. 7, 8; pl. 35, figs. 1–10); cephala only; calcareous, mainly exfoliated internal molds. Ingham in Ingham et al. (1986, figs. 8, 9, 15h–15k); all sclerites known; silicified, but quite coarsely preserved and variably distorted. Whittington (1963, pl. 7), Fortey (1980, pl. 11, figs. 9, 10, 12–21); enrolled individual, cephalon, rostral plate, pygidium. Calcareous, crackout, mostly testiferous. Figures 5, 6, 7.1–7.23 herein; all sclerites known; silicified. Ross (1951, pl. 16, figs. 26, 27, 31); cranidium, silicified. Ross (1967, pl. 7, figs. 1–7); cranidium, librigena, pygidium; silicified but likely immature.

Dimeropygiella eos Kobayashi, 1955, from the McKay Group, British Columbia, was erected on the basis of a single pygidium, illustrated by a small photograph of the unwhitened specimen (Kobayashi, 1955, p. 456, pl. 6, fig. 10). Dean (1989) figured three similar pygidia from the Survey Peak Formation, Wilcox Pass, Jasper National Park, Alberta, Ibexian (Stairsian), Zone E (Tesselacauda Zone), as ‘‘cf. eos,’’ and discussed the species as a member of Ischyrotoma. Beyond the presence of a pair of terminal axial nodes—a feature distributed widely among Ibexian ‘‘hystricurids’’—the pygidia bear no compelling similarity with those of Dimeropygiella or Ischyrotoma, and the species seems unlikely to be relevant to their ingroup phylogeny. Full evaluation will only be possible when the remainder of the exoskeleton is discovered. Pseudohystricurus rotundus Ross, 1951, is from the early Ibexian (Skullrockian; Zone A; Symphysurina Zone) of the Garden City Formation, Hillyard’s Canyon, southern Idaho. This earliest Ordovician species was based on three crackout cranidia (Ross, 1951, pl. 16, figs. 32, 33, 35–37). Although at least superficially similar to the late Ibexian type species of Pseudohystricurus in the possession of an inflated, tuberculate glabella, rotundus has a broad, flattened anterior border, a very long palpebral lobe bounded by a long, deep palpebral furrow, and deep S1 nearly fully isolating L1. Better assessment of its relationships will require more material and knowledge of other sclerites, but its affinity with the much younger Ischyrotoma/Pseudohystricurus group is at best weakly supported. Choice of outgroup.Following from the conclusions of Chatterton (1994), the reweighted cladogram of Chatterton et al. (1998), and the discussion above, Dimeropyge must be considered as a potential outgroup, as it may represent the sister taxon of the Ischyrotoma group. The basal node of Dimeropyge is essentially unknown, however, and if the suggestion above that a Zone B pygidium has affinities to the genus is correct, then it is considerably older than previously suspected. The well known Middle and Upper Ordovician species of Dimeropyge are also highly autapomorphic, and it is difficult to specify direct homologies for states relevant to the ingroup structure of the Ischyrotoma group. It is highly likely that a variety of very poorly known earlier Ordovician ‘‘hystricurids’’ need to be considered before a robust

hypothesis of dimeropygid phylogeny can be developed. There is, however, one ‘‘hystricurid’’ species that may have bearing on the origin of the Ischyrotoma group. Parahystricurus smithiae Boyce, 1989, has an inflated, ovate glabella with strong tuberculate sculpture and weakly incised glabellar furrows (cf. Dimeropygiella fillmorensis herein), an obvious median preglabellar furrow, and a pygidium with tab-shaped fulcral flanges and terminal paired flanges. Most of its features can be related to character states seen in early members of the Ischyrotoma group, and in the present state of knowledge it appears to be the best candidate for determining the root of that clade. CHARACTERS

Cranidium 1) Shape of anterior border: 0—broad, shelf-like, nearly flat; 1—medially extended, with inverted ‘‘W’’-shaped bottom (e.g., Fig. 8.9); 2—tab-shaped, subrectangular, without pronounced median extension (Fig. 5.1); 3—rim-like, subdued. 2) Relation of anterior border furrow to preglabellar furrow: 0—separated by preglabellar field with median furrow; 1—separated by preglabellar field lacking median furrow; 2—in contact medially, preglabellar field absent, characteristic ‘‘X’’ shape in anterior view (Fig. 5.6). 3) Length of unfurrowed preglabellar field: 0—field absent or furrowed; 1—very short, nearly with ‘‘X’’ shape, but anterior border furrow and preglabellar furrow separated by narrow transverse strip of exoskeleton medially (Fig. 8.9); 2—long (Fig. 13.9). 4) Depth and incision of anterior border furrow: 0—relatively long and shallow; 1—deep, shallowing posteriorly (Fig. 10.2); 2—very short (sag.; exsag.) and sharply incised (Fig. 13.1); 3— nearly effaced. 5) Degree of effacement of sculpture on front of cranidium: 0—not effaced, anterior of glabella, fixigena, and frontal area with similar sculpture to rear of cranidium (Fig. 5.9); 1—effaced (Fig. 13.1). 6) Course of anterior sections of facial sutures in front of palpebral lobe: 0—subparallel or slightly anteriorly convergent (Fig. 5.1); 1—strongly anteriorly divergent, bowed laterally (Fig. 8.1). 7) Dorsal incision of palpebral furrow: 0—not incised, fixigena grades more or less smoothly into palpebral lobe (Fig. 13.2); 1—

ADRAIN ET AL.—ORDOVICIAN TRILOBITES ISCHYROTOMA AND DIMEROPYGIELLA TABLE 2—Characters used in parsimony analysis. Cranidium anataphra blanda cf. blanda borealis caudanodosa fillmorensis juabensis mccormicki obesus orbus ovata parallela stubblefieldi twenhofeli wahwahensis P. sp. T. sp. smithiae

31230 11221 11221 22010 11110 11110 22010 11220 01110 11110 11110 31230 22010 22010 22010 02010 22010 00000

01102 10001 1000? 0111? 12001 12001 0111? 10001 02000 01000 12001 0?102 02112 02112 01112 01000 0??1? 00000

Lib. 01 00 00 00 10 10 00 00 ?0 00 10 01 01 01 00 00 00 00

10110 11000 ????? 10111 10001 11002 10012 00001 ????? 00000 11001 10?11 10110 10111 10002 ????? 10002 00000

Pygidium 00100 00110 00110 ????? 00101 00101 11100 00110 00010 ????? 00101 ????? 10100 11100 10100 ????? 11100 00000

000 110 110 ??? 010 210 000 110 100 ??? 210 ??? 001 001 001 ??? 000 000

definite strong furrow (Fig. 5.1); 2—obvious break in slope, but no incised furrow (Fig. 8.1). 8) Shape of S0 and anterior edge of L0: 0—S0 relatively short, anterior edge of L0 sloping or grading into furrow (Fig. 13.1); 1—S0 long, with smooth, flat bottom, anterior edge of L0 sharply delineated, in some specimens actually overhanging furrow (Fig. 5.1, 5.5). 9) Width of posterior fixigena: 0—point at which posterior section of facial suture crosses posterior border furrow set abaxial to lateral edge of palpebral lobe; 1—point at which posterior section of facial suture crosses posterior border furrow aligned with or set adaxial to lateral edge of palpebral lobe. 10) Ventral rostral area: 0—only moderately bowed, with flat

FIGURE 3—Strict consensus of four most parsimonious cladograms with length 54, consistency index 0.648, retention index 0.808. See Figure 4 for alternative ingroup relationships of Ischyrotoma. Numbering of nodes corresponds to ACCTRAN and DELTAN optimizations for one of the most parsimonious cladograms given in Table 2.

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rostral plate; 1—rostral sutures fused, ventral rostral spike-like projection; 2—strongly ‘‘V’’ shaped but lacking fusion and ventral projection, narrow rostral plate retained. 11) Size of primary fixigenal tubercles in mature holaspides: 0—not obviously larger than surrounding tubercles (Figs. 5.1, 13.2); 1—large and obviously distinct from surrounding tubercles (Figs. 8.1, 10.1). 12) Width of interocular fixigena: 0—wide, more than one tubercle row developed; 1—very narrow, tubercles restricted to single row. Librigena 13) Length of holaspid genal spine: 0—long and robust (Fig. 15.19); 1—very short, thorn-like (Fig. 12.4). 14) Posterolateral margin of lateral border: 0—laterally convex, more or less evenly arcuate (Fig. 7.25); 1—with laterally concave indentation in front of genal spine (Fig. 10.10, 10.13). 15) Eye socle: 0—weak, smooth area only; 1—strongly inflated band. 16) Width of lateral border furrow: 0—narrow (Fig. 7.32); 1— very broad (Whittington, 1963, pl. 7, figs. 3, 5, 13). 17) Lateral border sculpture: 0—prominent raised lines only (Fig. 13.19); 1—tubercles and raised lines (Fig. 7.32); 2—strong tubercles, more or less completely obscuring lines (Fig. 12.4). Pygidium 18) Spacing of posterior axial tubercles: 0—separated by at least thin median strip, axis wide at rear (Fig. 7.43); 1—very closely crowded medially, axis tapering strongly at rear (Fig. 7.12). 19) Fusion of posterior axial tubercles: 0—not fused; 1—partially or fully fused medially. 20) Tab-like flanges on fulcra of pleural bands: 0—present (Ross, 1951, pl. 19, figs. 34, 38); 1—absent. 21) Shape of posterior axial tubercles: 0—rounded, nodeshaped, subconical (Fig. 7.37, 7.43); 1—tab-shaped, much wider (tr.) than long (exsag.) (Fig. 15.1). 22) Shape of posteriormost axial ring in front of posterior axial tubercles: 0—medially continuous, roughly similar length sagitally as exsagitally (Fig. 15.2); 1—subdivided into more inflated or tuberculate lateral parts and subdued median part (Fig. 12.32). 23) Tubercles on axial rings and pleural bands: 0—present and continuously distributed; 1—completely absent; 2—very subdued, clustered only on adaxial parts of pleural bands. 24) Pleural bands and furrows of second pygidial segment: 0— anterior pleural band and pleural furrow obvious (Fig. 7.21); 1— anterior band and pleural furrow lost, posterior band inflated into strong rib (Fig. 7.45). 25) Pleural furrow of first pygidial segment: 0—continuous across fulcrum; 1—interrupted near fulcrum by inflated band, separated into adaxial and abaxial parts with different course (Fig. 7.21). Characters not used. 1. Presence of a juvenile median occipital spine. In contrast to many other groups of trilobites, no members of the study group retain an adult median occipital node or spine that can be distinguished from other tubercles on the occipital ring. The median node is visible in early ontogeny, however, and its state is potentially significant. The juvenile state in Dimeropygiella is known for D. blanda, D. fillmorensis, D. ovata, and D. caudanodosa; all of these species possess a subdued tubercle in meraspid ontogeny, which becomes indistinguishable by the late meraspid or early holaspid stage. The condition in Ischyrotoma is known only from I. wahwahensis, which bears an elongate spine during the meraspid period. The spine (as, for example, in the aulacopleurid Cyphaspis, see Adrain and Chatterton, 1995) is reduced in relative

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JOURNAL OF PALEONTOLOGY, V. 75, NO. 5, 2001 TABLE 3—Synapomorphies for nodes in the preferred cladogram of Figure 4.2, optimized using accelerated transformation (ACCTRAN) and delayed transformation (DELTRAN). Equivocal apomorphies indicated by asterisk. Nodes numbered following Figures 3 and 4.2. Node

ACCTRAN Synapomorphies

Node

1 2 3 4 5 6 7

2(1)*, 4(1), 7(1)* 3(1)* 1(1), 20(1) 3(2)*, 13(1)*, 17(1)* 6(1), 7(0)*, 10(1), 24(1) 3(1)*, 7(2)*, 11(1), 22(1) 4(2), 21(1)*, 23(1)*

1 2 3 4 5 6 7

8 9 10 11 12 13 14 15

5(1), 14(1)*, 17(0)* 14(1)*, 23(2)* 1(2), 8(1), 10(2), 15(1)*, 16(1)* 1(3), 4(3), 12(1)* 2(2)*, 3(0)*, 9(1), 18(1), 25(1)* — 7(2)*, 12(1)* 15(0)*, 17(2)*

8 9 10 11 12 13 14 15

DELTRAN Synapomorphies 4(1), 7(1)* 2(1)*, 3(1)* 1(1) 13(1)*, 17(1)*, 20(1) 6(1), 10(1), 24(1) 7(2)*, 11(1), 22(1) 3(2)*, 4(2), 7(0)*, 21(1)*, 23(1)* 5(1) 14(1)*, 23(2)* 8(1), 10(2), 16(1)* 1(3), 3(2)*, 4(3), 12(1)* 1(2), 2(2)*, 3(0)*, 9(1), 18(1) 15(1)* 7(2)*, 12(1), 25(1)* 17(2)*

FIGURE 4—Topology of Ischyrotoma in each of the four most parsimonious cladograms. Node numbers for tree 2 correspond to ACCTRAN and DELTRAN character-state optimizations given in Table 2.

the case of number of rings, three appears to be the plesiomorphic number. length through ontogeny, but is retained as a distinct node much later than in Dimeropygiella. It seems possible that this is a general condition for at least some species of Ischyrotoma, and if so is likely apomorphic. The only other species of Ischyrotoma for which very small cranidia are known, however, is I. stubblefieldi, and although the retention of a node late in ontogeny is evident, none of Ingham’s (in Ingham et al., 1986) smallest specimens bear an occipital spine. The material is more coarsely preserved than that of I. wahwahensis, and the absence may be preservational. If presence of a spine is confirmed in more species, it should be incorporated as a character in subsequent analyses. 2. Hypostome. The hypostome of most hystricurids and dimeropygids is unknown, and that of the Ischyrotoma group has only recently been identified with certainty. Ingham (in Ingham et al., 1986, fig. 15h–15k) assigned specimens with question to I. stubblefieldi. Chatterton et al. (1998, fig. 11.8) then identified a similar sclerite from Ibex section H-434 as either blanda or ovata. With the multiple silicified occurrences documented herein, the hypostomal assignment is confirmed. There are certainly morphological differences among the known examples. The hypostomes assigned herein to I. wahwahensis, for example, possess a distinct ventral bulge on the posterior lobe of the middle body and have nearly parallel sided, versus arcuate, lateral margins of the anterior wings. Hypostomes will likely prove to be a source of phylogenetic information. At present, however, the small number of species for which associations are well supported precludes their use. Only I. stubblefieldi, I. wahwahensis, and Dimeropygiella caudanodosa have definitely assigned hypostomes. The first species is the only member of the group in the collections in which it occurs, whereas the latter two occur in collections from section J in which one or the other is much more common. At H-434, three species co-occur and very few large hypostomes have been recovered. All have a similar morphology. Hence, general statements about the likely condition in all three species are possible, but direct assignment of sclerites is thus far impossible. 3. Number of pygidial axial rings and pleural ribs. Easily counted meristic characters are natural candidates in a search for homology. Both of these counts are stable within ingroup species. However, no obvious pattern is discernable between species. In

ANALYSIS

The character matrix for 25 characters and 17 ingroup taxa initially coded is given in Table 2. All multistate characters were treated as nonadditive, with the exception of character 10, which was constrained by a state-tree as outlined in the character list above. Parsimony analysis was performed using PAUP* 4.0b2 (Swofford, 1999), employing the exact Branch and Bound algorithm. Networks were rooted using the designated outgroup, P. smithiae. RESULTS

Initial runs used all 17 ingroup species, and resulted in 13 equally parsimonious trees of length 55. These trees indicated support for clades encompassing Ischyrotoma and Dimeropygiella as understood herein. Ingroup relationships of Dimeropygiella were stable and fully resolved. Ischyrotoma, however, was poorly resolved, with the only ingroup structure in the strict consensus involving the sister species anataphra/parallela and twenhofeli/ stubblefieldi. These pairs formed a basal polytomy with the remaining four species. Most of this lack of resolution was caused by a single labile species, Ross’s (1967) Ischyrotoma sp. This taxon is known only from juvenile material, and its codings are therefore highly ambiguous. Intuitively, it appears very similar to Ischyrotoma juabensis n. sp. and to share several obvious apomorphies. However, other codings either contradict this relationship or suggest alternatives, and result in multiple possible positions for the species. We regard many of these contradictory codings as uncertain, because they may reflect juvenile versus adult states. Because of this uncertainty, Ischyrotoma sp. of Ross was excluded from the final analysis. Discussion of its possible relationship to I. juabensis is given under that species below. Analysis of the remaining 16 ingroup species together with the outgroup yielded four equally parsimonious cladograms of length 54 and consistency index 0.648. The strict consensus cladogram is shown in Figure 3. The cladograms differ only in the structure of part of the Ischyrotoma clade; the alternative topologies are shown in Figure 4. The ambiguity is likely influenced by the missing pygidial codings of I. borealis. On grounds of stratigraphic congruence, the topology of Figure 4.2 minimizes ghost lineage

ADRAIN ET AL.—ORDOVICIAN TRILOBITES ISCHYROTOMA AND DIMEROPYGIELLA hypotheses and is preferred. Synapomorphies for all nodes of the preferred cladogram, optimized using the assumptions of accelerated transformation (ACCTRAN) and delayed transformation (DELTRAN), are shown in Table 3. Relationships are discussed under each genus in the systematics section below. SYSTEMATIC PALEONTOLOGY

Repository.Figured material is housed in the Paleontology Repository, Department of Geoscience, University of Iowa, Iowa City, with specimen number prefix SUI, and in the Department of Palaeontology, Natural History Museum, London, with prefix It. Terminology.Terms for the trilobite exoskeleton are applied in standard fashion, as summarized by Whittington and Kelly in Whittington (1997). Number of pygidial pleural rib pairs and terminal pieces follows the scheme typically applied to encrinurids (Ramsko¨ld, 1986). For example, a species described as having 5 1 1 pairs of ribs bears 5 pairs and a single, median rib behind the axial terminal piece, whereas a species with a count of 5 1 0 bears five pairs but lacks a terminal median rib. Family DIMEROPYGIDAE Hupe´, 1953 Genus PSEUDOHYSTRICURUS Ross, 1951 Type species.Pseudohystricurus obesus Ross, 1951, Garden City Formation, Ibexian (Stairsian), Zone F (Rossaspis superciliosa Zone), crest of ridge on west side of Hillyard’s Canyon, southern Idaho. Other species.Pseudohystricurus orbus Ross, 1953, Garden City Formation, Ibexian (Tulean), Zone G1 (Hintzeia celsaora Zone), crest of ridge on west side of Hillyard’s Canyon, southern Idaho; Pseudohystricurus sp. of Ross (1951), Garden City Formation, Ibexian (Stairsian), Zone E (Tesselacauda Zone), east side of Hillyard’s Canyon, southern Idaho. Discussion.Despite noting that Pseudohystricurus obesus was ‘‘relatively abundant,’’ Ross (1951, p. 74) illustrated only a single immature cranidium when he erected this type species. Terrell (1973, p. 88, pl. 5, fig. 1) assigned a fragmentary cranidium from either Zone E (Tesselacauda Zone) or low Zone F of section EE, near the Ibex area, western Utah. Cranidial characters were coded herein on the basis of Ross’s illustration, but both occurrences will require more complete documentation. Ross illustrated a variety of unassigned pygidia from the Rossaspis superciliosa Zone (Zone F) of the Hillyard’s Canyon sections, including two types (Ross, 1951, pl. 19, figs. 33, 36, 37 and pl. 19, figs. 34, 38) which show some similarity to those of Dimeropygiella and Ischyrotoma. The wider variety, which occurs in one specimen with an articulated thorax including a thoracic axial spine, has been shown by Boyce (1989) to belong to Hillyardina. The other (Ross, 1951, pl. 19, figs. 34, 38) is extremely similar to pygidia of, for example, D. blanda, with the exception that it has prominent flanges at the fulcra of the pleural bands, in addition to the pair of pygidial axial terminal nodes. This pygidium is tentatively assigned to Pseudohystricurus obesus herein, and coded as such for the analysis, though the association will obviously require confirmation. Ross (1953) tentatively assigned to Pseudohystricurus orbus pygidia bearing a robust, elongate spine running posteriorly from the rear of the axis. It is difficult on present evidence to assess whether this is correct. The pygidia of all other species that have definitely been assigned lack such a spine, but it is possible that it is a homologue of the pair of terminal axial nodes seen in all other taxa, derived through fusion and extension. However, such a spine is relatively common among bathyurines. For coding purposes, the pygidium of P. orbus was considered unknown, pending new collections. Pseudohystricurus sp. of Ross (1951) was illustrated by a single cranidium. This species is actually more fully known than Ross’s

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named type species, P. obesus, because it is based on a mature holaspid (more than twice the size of the immature holotype of P. obesus). Terrell (1973, p. 89, pl. 2, figs. 2, 4) assigned a very similar cranidium from Zone E (or possibly Zone D) of the Fillmore Formation in western Utah (either section AA or section EE). Since the analysis indicates that Pseudohystricurus as presently conceived is a paraphyletic group, no diagnosis is given. All of the species are inadequately known and will require revision on the basis of new collections before strong hypotheses of their relationships can be developed. Only P. orbus is known from sclerites other than cranidia, and as noted above Ross’s (1953) tentative pygidial assignment remains questionable. Unlike the other species, it shares a characteristic inverted ‘‘W’’-shape of the anterior border with Dimeropygiella. In the present state of knowledge, this state is optimized under both ACCTRAN and DELTRAN at node 3 (Fig. 3), and transforms to the tab-shaped state of Ischyrotoma between nodes 4 and 10. Hence, as coded, it cannot be construed as a synapomorphy uniting P. orbus exclusively with Dimeropygiella. This coding will need to be reassessed in light of new information. It depends on the notion that the specialized inverted ‘‘W’’ shape can plausibly transform to the equally specialized, but very different, tab-shape of Ischyrotoma. Were this transformation to be disallowed via a charactertree constraint, the inverted ‘‘W’’ shaped anterior border structure would be liable to uniquely group orbus as a primitive member of Dimeropygiella. Genus ISCHYROTOMA Raymond, 1925 Type species.Ischyrotoma twenhofeli Raymond, 1925, from a large early Whiterockian boulder in Bed 14 of the Shallow Bay Formation, Cow Head Group, early Whiterockian, Lower Head, western Newfoundland (revised by Whittington, 1963, p. 45, pl. 7; Spitsbergen material described by Fortey, 1980, p. 65, pl. 11, figs. 9, 10, 12–21). Other species.Ischyrotoma anataphra Fortey, 1979, Catoche Formation, Ibexian, Port au Choix, western Newfoundland; Ischyrophyma? borealis Fortey, 1980, Profilbekken Member, Valhallfonna Formation, Whiterockian (V4 b), Profilbekken, Spitsbergen; I. juabensis n. sp., Juab Formation, Ibex Section J, Whiterockian, Zone L (Paralenorthis-Orthidiella Zone), Ibex area, western Utah, and Shingle Limestone, Shingle Pass, southern Egan Range, eastern Nevada; I. parallela Boyce, 1989, lower limestone sequence of Catoche Formation, Ibexian, Boat Harbour, western Newfoundland; I. stubblefieldi Ingham in Ingham et al., 1986, Dounans Limstone, probably early Whiterockian, Highland Border Complex, Scotland; I. wahwahensis n. sp., Wahwah Formation, Ibexian (Blackhillsian), Zone J (Pseudocybele nasuta Zone), Section J, Ibex area, western Utah; Ischyrotoma sp. ind. of Whittington (1965); Ischyrotoma sp. of Ross (1967). Diagnosis.Diagnosis is presented in the following format, derived directly from the parsimony analysis: character-state number, consistency index, whether optimized by ACCTRAN (A), DELTRAN (D), or both, and description/discussion. 1. 8(1), c.i. 5 1.0, A/D; S0 long (sag.; exsag.), front of L0 sharply defined, in some species slightly overhanging S0. 2. 10(2), c.i. 5 1.0, A/D; rostral area defining strong inverted ‘‘V’’ shape in anterior view, very narrow rostral plate retained with functional rostral sutures. 3. ? 1(2), c.i. 5 1.0, A; anterior border tab-shaped (1(2)). This state is optimized at the genus node under ACCTRAN, but this depends on the assumption that it can transform into the specialized rim-like state of the considerably older species pair anataphra/parallela. In terms of morphocline analysis (e.g., Mickevich and Weller, 1990), the rim-like state seems more plausibly considered an intermediate between the primitive flattened condition

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FIGURE 6—Ischyrotoma wahwahensis n. sp., from the Wah Wah Formation, Ibex Section J-81, Ibexian (Blackhillsian; Zone J/Pseudocybele nasuta Zone), Millard County, western Utah. All figures are scanning electron micrographs, and are 330 except where noted. 1, 5, cranidium, SUI 94254, dorsal and oblique views. 2, cranidium, SUI 94255, dorsal view. 3, 7, cranidium, SUI 94256, dorsal and oblique views. 4, cranidium, SUI 94257, dorsal view. 6, cranidium, SUI 94258, dorsal view. 8, 13, hypostome, SUI 94259, ventrolateral and ventral views, 320. 9, transitory pygidium, SUI 94260, dorsal view, 340. 10, pygidium, SUI 94261, posterodorsal view. 11, 12, hypostome, SUI 94262, ventral and posteroventral views, 325.

and the derived tab-shape, a hypothesis which matches the occurrence of the states in time. This constraint was not imposed in the analysis, but the DELTRAN optimization of state 1(2) as an apomorphy of the more restricted ‘‘twenhofeli group’’ seems preferable. 4. 15(1), c.i. 5 0.5, A; eye socle with strongly inflated band. This state is reversed with certainty in I. wahwahensis and I juabensis. Some evidence that a genuine reversal is involved is provided by the librigenae of I. juabensis, in which the smaller illustrated example (Fortey and Droser, 1996, fig. 15.3) appears to retain a subdued but distinct band (15(1)), whereas the larger specimen (Fortey and Droser, 1996, fig. 15.2) appears to have entirely lost the socle (15(0)). The specimens are from different

localities, but in all other proportions and features are nearly identical. Such a loss cannot, however, be demonstrated in the abundant silicified librigenae of I. wahwahensis. We have considerable confidence that this character is a genuine synapomorphy of the all-inclusive Ischyrotoma. It fails to optimize at the basal node under DELTRAN due to missing data—the socle is not preserved in I. parallela and DELTRAN optimizes the missing state as 0. This seems unlikely, given the strong similarity between I. parallela and I. anataphra in all available details. 5. 16(1), c.i. 5 0.5, A/D; librigenal lateral border furrow very broad in mature specimens. The character is reversed in I. wahwahensis, but otherwise seems to be a robust apomorphy. Discussion.The species borealis was tentatively assigned by

← FIGURE 5—Ischyrotoma wahwahensis n. sp., from the Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybele nasuta Zone), Millard County, western Utah. All figures are 3 10. 1, 2, 5, 6, cranidium, holotype, SUI 94246, dorsal, oblique, right lateral, and anterior views, J-81. 3, 7, cranidium, SUI 94247, dorsal and right lateral views, J-81. 4, 8, cranidium, SUI 94248, dorsal and right lateral views, J-81. 9, 10, 12, cranidium, SUI 94249, dorsal, anterior, and right lateral views, J-46. 11, 14, 22, cranidium, SUI 94250, dorsal, right lateral, and ventral views, J-46. 13, 18, cranidium, SUI 94251, dorsal and left lateral views, J-81. 15, 19, 23, cranidium, SUI 94252, dorsal, left lateral, and anterior views, J-81. 16, 17, 20, 21, cranidium, SUI 94253, dorsal, oblique, right lateral, and anterior views, J-81.

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ADRAIN ET AL.—ORDOVICIAN TRILOBITES ISCHYROTOMA AND DIMEROPYGIELLA Fortey (1980) to Ischyrophyma Whittington, 1963 (a junior synonym of Celmus Angelin, 1854, see above). With the abundant new information available on both Celmus (Adrain and Fortey, 1997 and unpublished) and Ischyrotoma (Ingham in Ingham et al., 1986, and herein), it is now evident that borealis is a member of the latter. There are four equally parsimonious resolutions of Ischyrotoma ingroup relationships (Fig. 4), of which Figure 4.2 is preferred on grounds of stratigraphic congruence and minimal ad hoc hypotheses of ghost lineages. All trees position the eastern Laurentian species pair I. anataphra and I. parallela as sister to the remainder. Ischyrotoma anataphra is well known, and occurs widely in the mid-Ibexian of Newfoundland (Fortey, 1979; Boyce, 1989), Greenland (Fortey, 1986), and Ellesmere Island, Arctic Canada (J.M.A. and S.R.W., unpublished data). Ischyrotoma parallela is difficult to assess, as most of the figured specimens are fairly coarsely preserved internal molds and the species is known only from cephala. It does, however, appear to differ from I. anataphra in its subparallel-sided glabella, as indicated by Boyce (1989). The remaining species form a well supported clade characterized particularly by narrow posterior fixigenae (character 9(1)) and very closely crowded pygidial posterior axial tubercles (18(1)). It is now clear that Ischyrotoma and Dimeropygiella represent two quite distinct groups whose monophyly is supported by parsimony analysis. Their synonymy was proposed (Whittington, 1963) at a time when much less information was available, particularly on the very different rostral structures. With the new material and explicit hypothesis of relationship presented herein, it seems best to resurrect Dimeropygiella and to consider the groups separate genera. Ischyrotoma and Dimeropygiella differ most obviously in the shape of their anterior border and rostral area. The rostral plate of Dimeropygiella bears a large, ventrally directed ‘‘spike.’’ Ross (1951, p. 123) initially misinterpreted the rostral plate as absent, but later (1953, p. 637) recognized its presence, claiming it was separated from the librigena by a ‘‘perfectly clean-cut suture.’’ Most of the librigenae in our collections are isolated from the rostral plate, but there are several examples (e.g., Figs. 9.2, 11.6, 11.8) in which the connective sutures are clearly fused and the librigenae yoked. Among isolated librigenae, many (e.g., Fig. 10.10, 10.20) show what appears to be a smooth and well-defined connective suture, as opposed to breakage from a yoked rostrum. Hence, it appears likely that the connective sutures were fused at certain times during the life of the animal, but that they could be made functional when necessary to facilitate molting (cf. Fortey, 1986). The situation is very different in all members of Ischyrotoma for which information is available. Librigenae of Ischyrotoma

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have never been found yoked and there is no evidence the connective sutures were ever fused. The rostral plate is much narrower than that of Dimeropygiella (see Whittington, 1963, pl. 7, figs. 12, 13; Ingham in Ingham et al., 1986, fig. 8i) and does not bear any form of large ventral ‘‘spike.’’ These differences are reflected in the shape of the anterior border. That of Ischyrotoma is more or less tab-shaped, not markedly longer sagittally, and has an essentially flat ventral profile (e.g., Fig. 5.1, 5.6, 5.9, 5.10). The anterior border of Dimeropygiella is always much longer medially, and the ventral margin describes an inverted ‘‘W’’ shape in anterior profile (e.g., Fig. 8.1, 8.9). Other differences between the genera include the presence in Ischyrotoma of a lower, more elongate glabella which typically nearly or completely overhangs the anterior border furrow. Dimeropygiella displays a relatively shorter (sag.), broader, and sometimes more inflated glabella, the anterior margin of which is always set back to expose the anterior border and preglabellar field (if present). The anterior sections of the facial sutures are subparallel or anteriorly convergent in Ischyrotoma, but anteriorly divergent and bowed laterally in Dimeropygiella. Ischyrotoma also generally features a much broader and deeper librigenal lateral border furrow and a more strongly inflated, band-like eye socle. Finally, separate anterior and posterior pleural bands and a pleural furrow are retained on at least the second pygidial segment in Ischyrotoma, but are lost on all but the first in all species of Dimeropygiella. ISCHYROTOMA WAHWAHENSIS new species Figures 5, 6, 7.1–7.23 Diagnosis.Interocular fixigena broad, with more than one exsagittal tubercle row; librigena lacking inflated eye socle and with narrow lateral border furrow; five pygidial axial rings; pygidial axial tubercles set close together but not fused; 5 1 0 or 6 1 0 pairs of pygidial pleural ribs; ribs abaxial to fulcrum flattened, with fine granular sculpture. Description.Cranidium with maximum width across posterior borders, 92–94 percent sagittal length; width across midpoint of palpebral lobes 94–95 percent width across posterior borders; anterior border wide and short, width 26–34 percent of sagittal length; sagittal length 44–46 percent that of L0, width 69–76 percent width of L0, anterior margin gently convex, border shorter exsagittally than sagittally, with strong dorsal convexity, nearly tube-like in sagittal profile, and dorsal sculpture of 20–30 small to medium sized tubercles; anterior border furrow incised laterally, longer and shallower medially, dorsally bowed in anterior view (Fig. 5.6, 5.10); anterior sections of facial suture bowed laterally, anteriorly convergent in dorsal view; preglabellar field

← FIGURE 7—1–23, Ischyrotoma wahwahensis n. sp., from the Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybele nasuta Zone), Millard County, western Utah. All figures are 310. 1, right librigena, SUI 94263, external view, J-81. 2, right librigena, SUI 94264, external view, J-81. 3, left librigena SUI 94265, external view, J-81. 4, left librigena SUI 94266, external view, J-81. 5, right librigena, SUI 94267, external view, J-81. 6–8, thoracic segments, SUI 94268, dorsal, oblique, and ventral views, J-81. 9, 10, left librigena, SUI 94269, external and internal views, J-81. 11–13, 18, pygidium, SUI 94270, dorsal, posterior, ventral, and anteroventral views, J-59. 14–16, thoracic segment, SUI 94271, dorsal, anterior, and ventral views, J-81. 17, 22, thoracic segment, SUI 94272, dorsal and anterior views, J-81. 19, 20, pygidium, SUI 94273, dorsal and posterior views, J-81. 21, 23, pygidium, SUI 94274, dorsal and posterior views, J-59. 24–50, Dimeropygiella caudanodosa Ross, 1951, from the Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybele nasuta Zone), Millard County, western Utah. All figures are 310. 24, left librigena, SUI 94275, external view, J-42. 25, left librigena, SUI 94276, external view, J-46. 26, right librigena, SUI 94277, external view, J-42. 27, left librigena, SUI 94278, external view, J-42. 28, right librigena, SUI 94279, external view, J-42. 29, left librigena, SUI 94280, external view, J-42. 30, 36, right librigena, SUI 94281, external and internal views, J-46. 31, left librigena, SUI 94282, external view, J-42. 32– 34, right librigena, SUI 94283, external, ventrolateral, and internal views, J-46. 35, right librigena, SUI 94284, external view, J-46. 37, 43, 47, 48, pygidium, SUI 94285, dorsal, posterior, anteroventral, and ventral views, J-81. 38, 44, pygidium, SUI 94286, dorsal and posterior views, J-81. 39, 40, thoracic segment, SUI 94287, dorsal and anterior views, J-42. 41, 42, thoracic segment, SUI 94288, dorsal and posterior views, J-42. 45, 49, pygidium, SUI 94289, dorsal and posterior views, J-81. 46, 50, pygidium, SUI 94290, dorsal and posterior views, J-59. 51–53, Dimeropygiella sp., from the Wah Wah Formation, Ibex Section J-42, upper Ibexian (Blackhillsian; Pseudocybele nasuta Zone), Millard County, western Utah, x10, pygidium, SUI 94291, left lateral, dorsal, and posterior views.

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FIGURE 9—Dimeropygiella caudanodosa Ross, 1951, from the Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybele nasuta Zone), Millard County, western Utah. All figures are scanning electron micrographs, and are 330 except where noted. 1 cranidium, SUI 94303, dorsal view, J-59. 2, yolked librigenae/rostral plate, SUI 94304, anteroventral view, J-59. 3, hypostome, SUI 94305, ventral view, J-46. 4, hypostome, SUI 94306, ventral view, J-59. 5, librigena, SUI 94307, external view, J-59. 6, hypostome, SUI 94308, ventral view, J-46. 7, transitory pygidium, SUI 94309, dorsal view, J-59. 8, transitory pygidium, SUI 94310, dorsal view, 345, J-46.

absent, anterior border furrow juxtaposed against preglabellar furrow; frontal areas with moderate dorsal inflation and strong sculpture of small, medium, and large sized tubercles; interocular fixigena slightly wider than palpebral lobes, contiguous with frontal area and with identical sulpture; eye ridge not expressed dorsally; palpebral furrow narrow and incised, completely isolating palpebral lobe; palpebral lobe narrow, length (exsag.) 150–200 percent sagittal length of L0, steeply sloped adaxially, abaxial part horizontal, sculpture varying from muted tubercles (Fig. 5.4, 5.16) to entirely smooth (Fig. 5.9); interocular fixigena grades into posterior fixigena with similar dorsal convexity and tuberculate sculpture; posterior facial sutures slightly bowed laterally and with slight posterior divergence as far as posterior border furrow, bowed strongly across posterior border; posterior border furrow long and deep, set almost exactly transversely; posterior border nearly contiguous with abaxial part of L0, axial furrow very weak, short adaxially, longer abaxially, with strong dorsal convexity and sculpture of weak, scattered tubercles; axial furrows in front of L0 very deep and strongly incised, bordered on either side by narrow strip of smooth exoskeleton on the adaxial side of the fixigena and abaxial side of the glabella; axial furrows slightly anteriorly divergent in front of L0, reaching maximum divergence opposite rear third of palpebral lobe, anteriorly convergent in front of palpebral lobe, running without break into preglabellar furrow of similar depth and incision; glabella with maximum width 80–85 percent sagittal length, maximum width achieved opposite rear of palpebral lobe; glabella with dense sculpture of interspersed moderate and large tubercules, largest equivalent to or slightly larger than those on interocular fixigena; glabellar moderately inflated, sagittal profile describing more or less even arc from L0 to dorsal aspect of anterior border; rear of

glabella sharply circumscribed by steep break in slope to S0; S0 long (sag., exsag.), with unsculptured, nearly flat bottom in sagittal profile; L0 long medially, shortened behind L1 (Fig. 5.5, 5.12), slightly longer abaxially near axial furrow; L0 with very dense sculpture of fine, medium, and large tubercles, slighty more crowded and smaller than those on glabella. Librigena with maximum width 60–68 percent exsagittal length; lateral border slightly narrower anteriorly than posteriorly, with dense sculpture of fine tubercles; lateral border tubercles slightly larger adjacent to lateral border furrow; genal spine reduced to small knob in mature holaspid (Fig. 7.4, 7.9), indistinguishable on some specimens; fine raised lines developed on ventrolateral part of lateral border, running subparallel to lateral margin; lateral border furrow arcuate and deeply incised, similar depth along entire length; librigenal field narrowest near rear of eye, wider anteriorly, with sculpture of fine and coarse tubercles. larger tubercles developed on anterolateral portion of field; eye large and slightly bulbous, separated from field by very narrow, moderately incised furrow; no distinct eye socle present; lateral border underlain by nearly flat doublure of similar width, lacking sculpture, incised in front of genal angle by prominent Panderian notch. Hypostome (Fig. 6.8, 6.11–6.13) with maximum width across anterior wings, slightly less than or equal to sagittal length; anterior margin transversely straight medially, bowing posteriorly and arcuate laterally; anterior margin gently ‘‘lipped’’ medially, set off from middle body by small transverse furrow; anterior wings long, broad, and subrectangular, occupying anterior half of hypostome; lateral border narrow anteriorly on wing, broader posteriorly, rounded in section opposite rear of middle body, grading without interruption into posterior border; lateral and posterior

← FIGURE 8—Dimeropygiella caudanodosa Ross, 1951, from the Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybele nasuta Zone), Millard County, western Utah. All figures are 310. 1, 6, 9, 16, 22, cranidium, SUI 94292, dorsal, left lateral, anterior, ventral, and oblique views, J-59. 2, 7, cranidium, SUI 94293, dorsal and left lateral views, J-46. 3, 8, 11, cranidium, SUI 94294, dorsal, right lateral, and anterior views, J-46. 4, 5, cranidium, SUI 94295, dorsal and left lateral views, J-46. 10, 13, 14, 18, cranidium, SUI 94296, anterior, dorsal, left lateral, and oblique views, J-42. 12, 15, cranidium, SUI 94297, dorsal and left lateral views, J-46. 17, 20, cranidium, SUI 94298, dorsal and left lateral views, J-42. 19, 21, cranidium, SUI 94299, dorsal and right lateral views, J-81. 23, 26, cranidium, SUI 94300, dorsal and right lateral views, J-81. 24, 27, 28, cranidium, SUI 94301, dorsal, right lateral, and anterior views, J-81. 25, 29, cranidium, SUI 94302, dorsal and right lateral views, J-81.

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FIGURE 11—1–8, 10–12, Dimeropygiella ovata Hintze, 1953, from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/ Trigonocerca typica Zone), Millard County, western Utah. All figures are scanning electron micrographs. 1, yolked librigenae and rostral plate, SUI 94323, dorsal view, 350. 2, cranidium, SUI 94324, dorsal view, 350. 3, cranidium, SUI 94325, dorsal view, 350. 4, hypostome (assignment tentative), SUI 94326, ventral view, 320. 5, cranidium, SUI 94327, dorsal view, 360. 6, yolked librigenae and rostral plate, SUI 94328, anterior view, 330. 7, right librigena, SUI 94329, external view, 330. 8, yolked librigenae and rostral plate, SUI 94330, anteroventral view, 325. 10, transitory pygidium, SUI 94331, dorsal view, 360. 11, 12, pygidium, SUI 94332, oblique and dorsal views, 330. 9, Dimeropygiella fillmorensis n. sp., from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah, scanning electron micrograph, cranidium, SUI 94333, dorsal view, 350.

borders with ventral sculpture of very fine subparallel raised lines; lateral border furrow indistinct on wing, slightly deeper posteriorly; posterior border furrow very fine and incised, deeper than posterior part of lateral border furrow; middle body sagittally inflated, keel-like along sagittal line, sloping steeply toward wings; sculpture of a few subdued tubercles on anteromedial part of middle body (Fig. 6.13), otherwise smooth; middle furrow very weakly impressed, setting off rounded, knob-like swelling on posterior part of middle body. Thorax with axial lobe occupying 36–38 percent of total width; axial ring short (sag., exsag.), with sculpture of two crowded

transverse rows of medium-sized tubercles; distinct, crescentshaped preannulus present on some segments (Fig. 7.17); articulating half-ring longer than remainder of ring, broad and smooth; axial furrow shallow; fulcrum set 60–70 distance abaxially on pleural lobe; leading edge of anterior pleural band with extremely short (exsag.) transverse articulatory ridge between axial furrow and fulcrum, separated from main part of band by very fine furrow; anterior pleural band lacking tuberculate sculpture except for a single prominent tubercle set just abaxial to fulcrum; pleural furrow shallow adaxially, not in contact with axial furrow, deeper abaxially across fulcrum, shallowing and becoming effaced on

← FIGURE 10—Dimeropygiella ovata Hintze, 1953, from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah. All figures are 310. 1, 5, 11, cranidium, SUI 94311, dorsal, right lateral, and anterior views. 2, 6, 9, 12, 17, cranidium, SUI 94312, dorsal, right lateral, anterior, oblique, and ventral views. 3, 7, cranidium, SUI 94313, dorsal and right lateral views. 4, 8, cranidium, SUI 94314, dorsal and left lateral views. 10, left librigena, SUI 94315, external view. 13, right librigena, SUI 94316, external view. 14, 20, right librigena, SUI 94317, internal and external views. 15, 18, 22, 26, pygidium, SUI 94318, dorsal, posterior, ventral, and anteroventral views. 16, left librigena, SUI 94319, external view. 19, 21, pygidium, SUI 94320, dorsal and posterior views. 23, 27, pygidium, SUI 94321, dorsal and posterior views. 24, 25, pygidium, SUI 94322, dorsal and posterior views.

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FIGURE 12—Dimeropygiella fillmorensis n. sp., from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah. All figures are 310. 1, 5, 9, 13, cranidium, holotype, SUI 94334, dorsal, right lateral, anterior, and oblique views. 2, 6, cranidium, SUI 94335, dorsal and left lateral views. 3, 7, cranidium, SUI 94336, dorsal and right lateral views. 4, right librigena, SUI 94337, external view. 8, 12, right librigena, SUI 94338, external and internal views. 10, 14, cranidium, SUI 94339, dorsal and left lateral views. 11, 15, 27, cranidium, SUI 94340, dorsal, right lateral, and ventral views. 16, right librigena, SUI 94341, external view. 17, 20, thoracic segment, SUI 94342, dorsal and anterior views. 18, 19, thoracic segment, SUI 94343, anterior and dorsal views. 21, 22, thoracic segment, SUI 94344,

ADRAIN ET AL.—ORDOVICIAN TRILOBITES ISCHYROTOMA AND DIMEROPYGIELLA articulatory facet; posterior pleural band with transverse row of fairly coarse tubercles; posterolateral tip of segment extended into very small, pointed spine behind facet; tip of segment underlain by narrow doublure; posterior edge of posterior pleural band with ventral groove to receive articulatory ridge. Pygidium with sagittal length 57–62 percent maximum width; axis 34–46 percent of width anteriorly, tapering rapidly posteriorly; articulating half-ring short (sag., exsag.), relatively smaller than those of the mid-part of the thorax, set off from axis by deep ring furrow; five axial rings, first three with sculpture of subdued tubercles along anterior edge, posterior two very small; terminal piece with two large, medially crowded but not fused tubercles; axial furrow shallow on first segment, deeper posteriorly; anterior and posterior pleural bands adaxial to fulcrum similar to those of thoracic segments; abaxial of fulcrum, pleural bands are flattened and share a subdued granular sculpture with pygidial border; pleural furrow discontinuous, separated into proximal part adaxial to fulcrum and more posteriorly set distal part abaxial to fulcrum; pleural furrows distinct on anterior three, and rarely fourth (Fig. 7.21, 7.23, right side) segments; adaxial part of posterior pleural band with narrow transverse row of tubercles on first three segments, fourth and fifth segments bearing only single tubercle at fulcrum; border broad, with fine raised lines ventrolaterally, and moderate median ‘‘notch’’; doublure narrow and tightly incurved, lacking sculpture. Etymology.After the Wah Wah Formation. Material and occurrence.Holotype, cranidium, SUI 94246 (Fig. 5.1, 5.2, 5.5, 5.6), and paratypes 94247–94274, horizons J46, J-59, J-81, Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybele nasuta Zone), Millard County, western Utah. Discussion.Ischyrotoma wahwahensis is common through the lower part of the Wahwah Formation in its type section (Section J) at Ibex, and at some horizons (e.g., J-81) is the most abundant species of trilobite. Its occurrence has never previously been reported and none of its sclerites have ever been illustrated. Ischyrotoma wahwahensis is now the best known member of the genus, and is only the second species for which any information on early ontogeny is available (Fig. 6). This shows that early transitory pygidia (Fig. 6.9) display an unreleased thoracic segment with a long pair of axial spines, a feature seen also in the early ontogeny of some species of Dimeropyge (J.M.A. and R.A.F., unpublished data). Meraspid cranidia have an essentially spinose sculpture which is transformed into robust tubercles with maturity. Juveniles of the species bear a relatively elongate median occipital spine and two or three shorter pairs of spines at the rear of L0; all of these are shortened through ontogeny and reduced to nondescript tubercles in the adult. The cranidium shows marked shape change from early to late stages, as the glabella becomes narrower and more elongate. These aspects of meraspid ontogeny differ somewhat from their counterparts in I. stubblefieldi, the only other species for which early stages are known. The material of I. stubblefieldi is more coarsely preserved, but early cranidia do not display an elongate occipital spine and are in general less spinose than those of I. wahwahensis. Although it could potentially be a function of the tectonic distortion undergone by the Dounans material, small cranidia of I. stubblefieldi are also much more consistently narrow and elongate. According to our analysis, Ischyrotoma wahwahensis is most closely related to I. juabensis, with which it is compared under

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that taxon below. Collectively, I. wahwahensis differs from the early species I. anataphra and I. parallela in the possession of an inflated, tab-shaped, tuberculate, versus low, broad, and terrace-lined (Fortey, 1979, pl. 36, figs. 6, 8) anterior border; absence versus presence of a preglabellar field; deep versus extremely shallow anterior border furrow; possession of narrower posterior fixigenae and wider interocular fixigenae; possession of weak versus strongly incised eye socle and much narrower librigenal lateral border furrow with strong versus absent tuberculate sculpture; more closely spaced posterior pygidial tubercles; and interrupted, versus continuous, first pygidial pleural furrow. Ischyrotoma wahwahensis differs from I. borealis in the presence of less incised glabellar furrows, a longer (sag.; exsag.) S0; the absence versus presence of small spines on the cranidial posterior border; larger and more robust cranidial tuberculate sculpture; absence of a strongly inflated eye socle; considerably broader band of larger librigenal field tubercles; narrower librigenal lateral border furrow; and broader, more inflated librigenal lateral border with much stronger tuberculation. In addition to the ontogenetic features mentioned above, I. wahwahensis differs from I. stubblefieldi in the possession of narrower palpebral lobes; a lower and less inflated glabella; a broader anterior border; longer S0; more widely divergent anterior sections of the facial suture (cf. Fig. 5.6, 5.10, with Ingham et al., 1986, fig. 8b, 8h); presence of additional small tubercle rows on the interocular fixigena; absence of an inflated eye socle; librigenal lateral border with strong versus essentially absent tubercles; and pygidia with five versus three or possibly four axial rings, 6 1 0 versus 5 1 0 more distally flattened pleural ribs, and a more pronounced posteroventral ‘‘notch.’’ Finally, I. wahwahensis differs from the type species, I. twenhofeli, in its less anteriorly inflated glabella (cf. Fig. 5.5, 5.12, with Whittington, 1963, pl. 7, figs. 2, 5); librigena lacking prominent banded eye socle and with more robust tubercles on the border; pygidium with terminal tubercles crowded but separate versus nearly completely fused; and distal pygidial pleural ribs that are flattened versus convex (cf. Fig. 7.11, 7.21, with Fortey, 1980, pl. 11, figs. 17, 21). ISCHYROTOMA JUABENSIS new species Not Figured Ischryotoma stubblefieldi Ingham; FORTEY AND DROSER, 1996, p. 91, fig. 15.1–15.9. ?Ischyrotoma stubblefieldi FORTEY AND DROSER, 1999, p. 191, fig. 5.1– 5.4.

Diagnosis.Interocular fixigena with several rows of tubercles; glabella relatively short (sag.) and less parallel-sided than is typical for genus; librigenal field very narrow (tr.) with relatively low number of large, coarse tubercles; eye socle very weak band in small examples, effaced in larger specimens; librigenal lateral border with dense tubercles; three and possibly minute fourth pygidial axial rings; 4 1 1 pygidial pleural ribs; pleural furrow developed only on proximal part of first pygidial segment; pygidium very short; terminal axial nodes fused. Discussion.Fortey and Droser (1996) referred early Whiterockian material from the Juab Formation at Ibex Section J and Shingle Pass, east-central Nevada, to Ischyrotoma stubblefieldi Ingham in Ingham et al., 1986, a late Ibexian species from the Highland Border Complex, Scotland. They later (1999) also referred a cranidium from the basal Whiterockian at Meiklejohn Peak in southern Nevada, and one from the basal Whiterockian

← anterior and dorsal views. 23, 25, thoracic segment, SUI 94345, anterior and dorsal views. 24, 26, thoracic segment, SUI 94346, posterior and dorsal views. 28–31 pygidium, SUI 94347, dorsal, posterior, ventral, and anteroventral views. 32, 33, pygidium, SUI 94348, dorsal and posterior views.

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at the type section in Whiterock Canyon, central Nevada. It is now evident that none of this material figured by Fortey and Droser (1996, 1999) is conspecific with I. stubblefieldi, and that in fact more than one species may be represented in basal Whiterockian strata of the Great Basin. The Ibex material figured by Fortey and Droser (1996) certainly appears to represent a single taxon. Since the cranidium, librigena, and pygidium are well known and the species is obviously distinct, it seems reasonable to formally name the taxon. Ischyrotoma juabensis differs from I. stubblefieldi in the possession of a relatively shorter glabella; interocular fixigena with several as opposed to a single tubercle row; more laterally divergent anterior sections of the facial suture; librigenal field which is considerably broader anteriorly than posteriorly opposite the eye, versus similar in length posteriorly and anteriorly (cf. Fortey and Droser, 1996, fig. 15.2, 15.3, with Ingham et al., 1986, fig. 8l, 8n, 8q); more subdued or absent band-like eye socle; librigenal lateral border with dense and robust versus essentially absent tubercles; pygidial terminal axial tubercles that are fused versus clearly separate; relatively shorter pygidium; and medial fusion or near-fusion of the fifth pygidial pleural rib pair to form the 4 1 1 condition versus definitely separate 5 1 0. Among species included in the analysis, I. juabensis is sister to I. wahwahensis, from which it differs in the possession of a shorter glabella; narrower librigenal field with fewer tubercles; four versus five pygidial axial rings; and 4 1 1 versus 6 1 0 pygidial pleural rib pairs that are convex versus flattened distally. The species share the presence of several tubercle rows on the interocular fixigena, a subdued eye socle, and dense tubercles on the librigenal lateral border. Ischyrotoma juabensis is similar to, and possibly even conspecific with, the material of similar age described from Pyramid Peak, southeastern California, by Ross (1967) as Ischyrotoma sp. Ross’s specimens are immature, hampering comparison. Nevertheless, Ross’s pygidium in particular is similar to those of I. juabensis in general proportions, rib shape, and particularly in the fully fused terminal axial tubercles. The California pygidium shows five axial rings, but this could be a function of its small size. Genus DIMEROPYGIELLA Ross, 1951 Type species.Dimeropygiella caudanodosa Ross, 1951, Ibexian (Blackhillsian), Zone J (Pseudocybele nasuta Zone), Garden City Formation, Locality 13 of Ross (1951), northeastern Utah, and Wahwah Formation, Ibex area, western Utah (see species discussion below). Other species.Dimeropygiella blanda Hintze, 1953, Fillmore Formation, section H-434, Ibexian (Blackhillsian), Zone H (Trigonocerca typica Zone), Ibex area, western Utah; D. fillmorensis n. sp., Fillmore Formation, section H-434, Ibexian (Blackhillsian), Zone H (Trigonocerca typica Zone), Ibex area, western Utah; D. mccormicki n. sp., Fillmore Formation, section H-573, Ibexian (Blackhillsian), Zone I (Presbynileus ibexensis Zone), Ibex area, western Utah; D. ovata Hintze, 1953, Fillmore Formation, section H-434, Ibexian (Blackhillsian), Zone H (Trigonocerca typica Zone), Ibex area, western Utah; Dimeropygiella cf. blanda (herein), Ninemile Formation, late Ibexian, Little Rawhide Mountain, south end of Hot Creek Range, Nevada; Ischyrotoma cf. caudanodosa of Dean (1989, pl. 28, figs. 7, 9, 10, 13, 15–17), Outram Formation, Ibexian (Blackhillsian), Zone J (Pseudocybele nasuta Zone), Wilcox Pass, Jasper National Park, Alberta, Canada; ?Ischyrotoma sp. A of Brett and Westrop (1996), Fort Cassin Formation, Ibexian, correlative with Zone H (Trigonocerca typica Zone), Sciota Road Section, Champlain Valley region, eastern New York State.

Diagnosis.Diagnosis is presented in the same format as for Ischyrotoma above. 1. 6(1), c.i. 5 1.0, A/D; anterior sections of facial sutures anteriorly divergent, and bowed laterally. 2. 10(1), c.i. 5 1.0, A/D; rostral sutures fused, bottom of rostral plate extended ventrally as median spike-like projection. 3. 24(1), c.i. 5 1.0, A/D; anterior band and pleural furrow of pygidial segments posterior to first lost, posterior band inflated into strong rib. Discussion.Dimeropygiella was compared with Ischyrotoma under discussion of that genus above. The genus comprises two clades with fairly distinct morphologies (Fig. 3). A ‘‘caudanodosa’’ group is densely tuberculate, has a strongly inverted ‘‘W’’shaped anterior border, and has a short or absent preglabellar field. Monophyly of this group is supported by characters 11(1), retention of large and obviously distinct primary fixigenal tubercles in large holaspides, and 22(1), separation of the posteriormost pygidial axial ring into separate abaxial elements with medial effacement. A ‘‘blanda’’ group shows variable effacement of at least the anterior cephalic tuberculation (on the frontal area, anterior glabella, and librigenal field), retains a fairly long preglabellar field, and has only a moderate or weak inverted ‘‘W’’-shape to the front of the anterior border. Its monophyly is supported by several character-states (Table 3), including the universal development of a very short (sag., exsag.) and deeply incised anterior border furrow. It is difficult to assess the affinity of the single fragmentary cranidium figured by Brett and Westrop (1996) as Ischyrotoma sp. A. It has a ‘‘normal’’ cranidial anterior border but lacks a preglabellar field. It is contemporaneous with, and may be related to, Ibex species such as Dimeropygiella blanda, but it also resembles older species of Pseudohystricurus. It is certain, however, that it is not a member of Ischyrotoma as understood herein. DIMEROPYGIELLA CAUDANODOSA Ross, 1951 Figures 7.24–7.50, 8, 9 Dimeropygiella caudanodosa ROSS, 1951, p. 124, pl. 35, figs. 18, 22–28; HINTZE, 1953, p. 154, pl. 19, figs. 9, 10; WHITTINGTON AND EVITT, 1954, p. 37. Ischyrotoma caudanodosa (ROSS, 1951). WHITTINGTON, 1963, p. 47; FORTEY, 1979, p. 106; DEAN, 1989, p. 36. Ischyrotoma caudinodosa [sic] (ROSS, 1951). CHATTERTON, 1994, p. 543, fig. 1, table 1.

Diagnosis.Cranidial axial furrows broad and anterior border furrow long (sag., exsag.), both bordered by area of smooth exoskeleton; anterior border strongly inverted ‘‘W’’-shaped; librigenal field densely tuberculate; librigenal lateral border furrow quite narrow; lateral border with more dense tuberculation posteriorly, raised lines ventrolaterally and anteriorly; pygidium with five axial rings, 6 1 0 or faint 7 1 1 rib pairs, and a strong median indentation in the ventral margin in posterior view. Material and occurrence.Figured specimens SUI 94275– 94290, 94292–94310, from horizons J-42, J-46, J-59, and J-81, Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybele nasuta Zone), Millard County, western Utah. Discussion.Ross (1951) established this type species on the basis of three illustrated sclerites from northern Utah (he later (1953) added one juvenile cranidium and one librigena/rostral plate) while Hintze (1953) documented its occurrence at Ibex with single dorsal views of one cranidium and one pygidium. We here extend the treatment of Section J material to include multiple sclerites, including librigenae, thoracic segments, hypostomes, and additional early growth stages. Due to the low number of illustrations of material from the Utah type locality, it is not possible to be entirely confident that the Ibex section J taxon is conspecific. Ross’s holotype cranidium is small, only about half the

ADRAIN ET AL.—ORDOVICIAN TRILOBITES ISCHYROTOMA AND DIMEROPYGIELLA size of our typical examples (Fig. 8.1–8.3). However, it closely resembles similarly sized specimens in our samples (e.g., Fig. 8.23–8.25). The single librigena illustrated by Ross appears to have a narrow field and a lateral border furrow that broadens posteriorly (Ross, 1951, pl. 35, fig. 27), but this is at least partly a function of photographic orientation. Ross’s pygidium is slightly distorted. On present evidence, it seems very likely that the section J species is Dimeropygiella caudanodosa, but this will be confirmed once the Garden City type locality is recollected. Some stratigraphic variation in cranidial morphology may occur among the samples here assigned to D. caudanodosa. Cranidia from horizon J-42 (the lowest from which samples used in this paper were obtained) appear to have a slightly more densely crowded pattern of glabellar tubercles (Fig. 8.13, 8.17), together with a more prominent dorsal expression of the eye ridge, which is topped by a larger isolated tubercle. No differences are apparent, however, in the librigenae or pygidia recovered from this horizon, and in other respects the cranidia conform closely to those higher in the section. The hypostome of D. caudanodosa was previously unknown. It shows that despite the derived rostral morphology, a large, ‘‘typical’’ natant hypostome was retained (Fig. 9.3). It differs from that of Ischyrotoma wahwahensis, which is common higher in the section, in its less parallel-sided anterior wings, lack of a ventral knob-like protrusion on the posterior lobe of the middle body, and longer posterior border. It is very similar to the older specimen recovered from H-434 (Fig. 11.4), which must belong to one of the three species of Dimeropygiella occurring there. The genal spine of the very smallest specimens is about as long as the remainder of the cheek, and remains distinct until late in meraspid ontogeny. Small librigenae (Fig. 9.2, 9.5) show that the raised subparallel lines seen on the lateral border of adults begin as rows of fine, closely spaced tubercles which eventually coalesce into ridges. Interestingly, the axial furrow and cephalic border furrows of small specimens are also lined with a row of small, fine tubercles (Fig. 9.1, 9.5). Both of these features are seen in the early ontogeny of Dimeropyge (see Chatterton, 1994), but not in what is known of Ibexian ‘‘hystricurids’’ (Lee and Chatterton, 1997). Dimeropygiella caudanodosa differs from D. ovata in the possession of broader cranidial axial furrows; a longer anterior border furrow; a relatively smaller and less inflated glabella with less obvious smooth areas marking S1 and S2; less anteriorly divergent anterior sections of the facial sutures; narrower librigenal lateral border with stronger and denser tubercles versus more prominent raised lines; five pygidial axial rings versus three with a minute fourth; 6 1 0 or 7 1 1 versus 4 1 1 or 5 1 0 pygidial pleural ribs; and a pygidium that has a relatively larger proximal pleural area, is not as tall relative to its width, and bears a prominent median indentation in the ventral margin versus a simple inverted ‘‘V’’ shape. It differs from D. fillmorensis, the sister species of D. ovata, in many of the same ways. In addition, D. caudanodosa is distinguished from D. fillmorensis in the possession of a more subtriangular anterior border; much less dense cranidial tuberculation; obscure versus incised glabellar furrows; five versus three pygidial axial rings; 6 1 0 or 7 1 1 versus a faint 5 1 1 pygidial pleural rib count; and a subtriangular pygidium with maximum width in front of pygidial sagittal mid-length. DIMEROPYGIELLA OVATA Hintze, 1953 Figures 10, 11.1–11.8, 11.10–11.12 Dimeropygiella ovata HINTZE, 1953, p. 155, pl. 19, figs. 1–4. non Ischyrotoma ovata (Hintze, 1952 [sic]). YOUNG, 1973, p. 102, pl. 2, figs. 3, 4, 6, 7 (fig. 3 and fig. 4 represent Dimeropygiella fillmorensis n. sp.; figs. 6, 7 are of a pygidium of D. blanda).

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Ischyrotoma blanda (Hintze, 1952 [sic]). YOUNG, 1973, p. 102, pl. 2, fig. 1 only.

Diagnosis.Glabella relatively large and inflated; S1 and S2 defined as smooth, unincised areas of exoskeleton; anterior sections of facial sutures strongly bowed laterally in front of palpebral lobe; librigenal lateral border broad and nearly blade-like anteriorly, with prominent, subparallel raised lines; pygidium with subdued dorsal sculpture, with three and a very faint fourth axial ring and 4 1 0 or 5 1 1 rib pairs; pygidium tall, only slightly transversely flexed in posterior view. Material and occurrence.Figured specimens SUI 94311– 94332, horizon H-434, Fillmore Formation, Ibex Section H, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah. Discussion.In Hintze’s (1953) monograph, two species of Dimeropygiella which occurred at horizon H-434, D. ovata and D. blanda, were correctly discriminated. Fieldwork for this project has revealed the presence of a third distinct species at this horizon, described as D. fillmorensis below. In Young’s (1973) study, sclerites belonging to all three species were assigned somewhat haphazardly to Hintze’s original two species. Of the five Dimeropygiella sclerites illustrated by Young, only one, a cranidium of D. blanda, was correctly assigned to species. The librigena Young assigned to D. blanda belongs in fact to D. ovata, the pygidium assigned to D. ovata is obviously D. blanda, and the cranidium and librigena assigned by Young to D. ovata represent our new D. fillmorensis. We have considerable confidence in the assignments of sclerites to particular species in the present study, based on the following criteria: 1) New species similar to D. blanda occur higher in section H (D. mccormicki, see below) and elsewhere (Dimeropygiella n. sp. aff. D. blanda)—the morphology of their librigenae and pygidia are a good guide to the correct associations for D. blanda. 2) Sculptural correspondence. Mature cranidia of D. blanda are relatively effaced, and this is reflected also in the librigenae and pygidia. Cranidia of D. fillmorensis have considerably denser tuberculation than those of D. ovata, and this is matched by the sculpture of the librigenae. 3) Frequency of occurrence. This is the main basis for assigning pygidia to either D. ovata or D. fillmorensis. Cranidia and librigenae of D. ovata are more than twice as common as those of D. fillmorensis. These relative abundances are reflected also in the two pygidial morphotypes, and hence the more common one is assigned to D. ovata. Dimeropygiella ovata was compared with D. caudanodosa above. It differs from its sister species, D. fillmorensis, in its less dense cranidial tubercles; more laterally bowed anterior sections of the facial suture; longer preglabellar area; longer anterior border furrow; impression of S1 and S2 as smooth areas versus incised furrows; wider librigenal field with less crowded tubercles; wider librigenal lateral border furrow; broader librigenal lateral border that widens anteriorly, with prominent raised lines versus dense tubercles; pygidia that are longer (sag.) relative to width, taller, and with posterior margin describing even arc in dorsal view versus inverted ‘‘W’’ shape; three strong pygidial axial rings and very weak fourth versus two strong and weak third; and 4 1 1 or 5 1 0 pygidial pleural ribs versus weak but definite 5 1 1. DIMEROPYGIELLA FILLMORENSIS new species Figures 11.9, 12 Ischyrotoma ovata (Hintze, 1952 [sic]); YOUNG, 1973, p. 102, pl. 2, figs. 3, 4 (non pl. 2, figs. 6, 7 5 D. blanda).

Diagnosis.Cranidial tubercles extremely numerous and densely crowded; glabellar furrows incised; librigenal field with

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Ischyrotoma ovata (Hintze, 1952 [sic]). YOUNG, 1973, P. 102, pl. 2, figs. 6, 7 (non pl. 2, figs. 3, 4 5 D. fillmorensis).

FIGURE 14—Dimeropygiella blanda Hintze, 1953, from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah. All figures are scanning electron microscopes and 360. 1, cranidium, SUI 94364, dorsal view. 2, cranidium, SUI 94365, dorsal view. 3, cranidium, SUI 94366, dorsal view.

dense tubercles; librigenal lateral border furrow narrow and incised; lateral border narrow and densely tuberculate; pygidium short relative to width, with three axial rings and 5 1 1 pairs of pleural ribs. Etymology.After the Fillmore Formation. Material and occurrence.Holotype, cranidium, SUI 94334 (Fig. 12.1, 12.5, 12.9, 12.13), paratypes SUI 94333, 94335– 94348, horizon H-434, Fillmore Formation, Ibex Section H, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah. Discussion.Dimeropygiella fillmorensis was compared with D. caudanodosa and D. ovata above. It is possible to assign thoracic segments to the species because of the characteristic densely crowded tubercles, reflected on the thoracic axial rings and posterior pleurae. The basis for association of sclerites was also discussed above. DIMEROPYGIELLA BLANDA Hintze, 1953 Figures 13, 14, 15.1–15.11 Dimeropygiella blanda HINTZE, 1953, p. 155, pl. 19, figs. 6–8. Ischyrotoma blanda (Hintze, 1952 [sic]); YOUNG, 1973, p. 102, pl. 2, fig. 2 (non pl. 2, fig. 1 5 D. ovata).

Diagnosis.Cranidium with strong sagittal convexity; anterior part with tubercles effaced in mature holaspides; long preglabellar field; librigenal field nearly smooth in large specimens; librigenal lateral border furrow narrow and incised; lateral border with tubercles completely effaced and raised lines on ventrolateral aspect; thoracic segments lacking dorsal sculpture; pygidia tall, with very little transverse flexure; three and rarely a minute fourth pygidial axial rings and 5 1 0 pairs of pygidial pleural ribs; ribs becoming subdued distally near contact with pygidial border; border strong and rim-like. Material and occurrence.Figured specimens SUI 94349– 94370, horizon H-434, Fillmore Formation, Ibex Section H, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah. Discussion.Dimeropygiella blanda is compared with D. mccormicki n. sp. below. Early cranidial growth stages (Fig. 14) show considerable differences with those of the ‘‘caudanodosa’’ group (Fig. 9, 11). In particular, the transverse shape of the anterior border is in place from the earliest stages known, in which it resembles a generalized, ‘‘normal’’ border (Fig. 14.3), and the preglabellar field is also present throughout ontogeny. This indicates that these features are likely to be plesiomorphic. The material does show that the axial furrows were bounded by a row of fine tubercles, as in the ‘‘caudanodosa’’ clade. Smaller cranidia show the inverted ‘‘W’’ shape to the bottom of the anterior border characteristic of the genus (Fig. 13.15), though weakly. In larger specimens (Fig. 13.9) the bottom of the border is nearly transverse. DIMEROPYGIELLA MCCORMICKI new species Figure 15.12–15.33 Diagnosis.Long, unfurrowed preglabellar field; cephalon with dense tuberculate sculpture, very slightly subdued on anterior part of glabella; long genal spine retained in mature holaspides; pygidium with three axial rings, 6 1 0 pleural ribs that are very subdued distally, and tab-shaped, well spaced terminal axial nodes. Etymology.After Tim McCormick. Material and occurrence.Holotype, cephalon, SUI 94373 (Fig. 14.16, 14.21, 14.30, 14.31), paratypes SUI 94371, 94372, 94374–94379, horizon H-573, Fillmore Formation, Ibex Section H, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah. Discussion.Dimeropygiella mccormicki shares several features with D. blanda, including the presence of a long preglabellar field, more transversely straight connective sutures, at least partial suppression of sculpture on the anterior of the glabella in mature specimens, the suppression of tuberculate sculpture on the librigenal field and dorsal surface of the pygidium, and the complete absence of tubercles on the librigenal lateral border. It differs in the generally denser cranidial tuberculate sculpture, the presence of a more pronounced inverted ‘‘W’’ shape to the bottom of the

← FIGURE 13—Dimeropygiella blanda Hintze, 1953, from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah. All figures are 310. 1, 5, 9, cranidium, SUI 94349, dorsal, right lateral, and anterior views. 2, 6, cranidium, SUI 94350, dorsal and left lateral views. 3, 7, cranidium, SUI 94351, dorsal and left lateral views. 4, 8, 12, cranidium, SUI 94352, dorsal, right lateral, and ventral views. 10, 11, 14, 15, cranidium, SUI 94353, dorsal, oblique, left lateral, and anterior views. 13, 17, cranidium, SUI 94354, dorsal and right lateral views. 16, 21, left librigena, SUI 94355, external and internal views. 18, left librigena, SUI 94356, external view. 19, 20, left librigena, SUI 94357, external and internal views. 22–24, right librigena and rostral plate, SUI 94358, external, ventrolateral, and anterior views. 25, 27, 28, thoracic segment, SUI 94359, right lateral, dorsal, and anterior views. 26, right librigena, SUI 94360, external view. 29, 30, thoracic segment, SUI 94361, dorsal and anterior views. 31, 32, thoracic segment, SUI 94362, dorsal and anterior views. 33, 34, pygidium, SUI 94363, dorsal and posterior views.

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FIGURE 15—1–11, Dimeropygiella blanda Hintze, 1953, from the Fillmore Formation, Ibex Section H-434, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah. All figures are 310. 1, 5, 9, pygidium, SUI 94367, dorsal, posterior, and right lateral views. 2, 6, 10, 11, pygidium, SUI 94368, dorsal, posterior, anteroventral, and ventral views. 3, 7, pygidium, SUI 94369, dorsal and posterior views. 4, 8, pygidium, SUI 94370, dorsal and posterior views. 12–33, Dimeropygiella mccormicki n. sp., from the Fillmore Formation, Ibex Section H, thrombolitic buildups at H-573, Ibexian (Blackhillsian; Zone H/Trigonocerca typica Zone), Millard County, western Utah. All figures are 310. 12, 14, cranidium, SUI 94371, dorsal and left lateral views. 13, 15, cranidium, SUI 94372, dorsal and anterior views. 16, 21, 30, 31, cephalon, holotype, SUI 94373, dorsal, anterior, oblique, and right lateral views. 17, 18, 22, 23, cranidium, SUI 94374, dorsal, ventral, left lateral, and anterior views. 19, 20, 24, left librigena, SUI 94375, external, ventrolateral, and internal views. 25, pygidium, SUI 94376, dorsal view. 26, 27, 32, 33, pygidium, SUI 94377, dorsal, left lateral, posterior, and ventral views. 28, right librigena, SUI 94378, external view. 29, right librigena, SUI 94379, external view.

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FIGURE 16—Dimeropygiella n. sp. aff. D. blanda, from the Ninemile Formation, Little Rawhide Mountain, south end of Hot Creek Range, Nevada. 1, 2, 4, cranidium, It. 25035, dorsal, oblique, and anterior views, 37.5. 3, 6, 7, pygidium, It. 25036, dorsal, left lateral, and posterior views, 310. 5, right librigena, It. 25037, external view, 310.

anterior border; retention of a long genal spine in large specimens, and the presence of 6 1 0 versus 5 1 0 or 5 1 1 pygidial pleural ribs. By reference to Pseudohystricurus and the outgroup, the denser sculpture and long genal spines are very likely plesiomorphic conditions relative to the more effaced and shortened states in D. blanda. Dimeropygiella mccormicki, however, occurs some 45 m above D. blanda in section H, a testament to the independence of stratigraphic versus phylogenetic data, and of the irrelevance of the former to the latter. Dimeropygiella mccormicki has never previously been reported from the Ibex area and none of its sclerites have been figured. It is the most common trilobite in a collection derived from the sponge-algal reef which forms the basal unit of Hintze’s (1973) Calathium calcisiltite member of the Fillmore Formation. DIMEROPYGIELLA new species aff. D. BLANDA Hintze, 1953 Figure 16 Material and occurrence.Figured specimens It. 25035– 25037, from section at Little Rawhide mountain, south end of Hot Creek Range, central Nevada (Siewers in Droser and Sheehan, 1995, fig. 31). The lower part of the section in the Ninemile Formation consists of dominant shales with micritic laminated limestone beds, which is succeeded by 21 metres of rubbly bedded nodular limestones with shale partings. This is succeeded by bedded limestones. The collection was made 2 m from the base of these limestones. Discussion.This taxon is obviously closely related to Dimeropygiella blanda, though nevertheless clearly distinct. It resembles D. blanda in the cranidial effacement, general dimensions, and presence of a preglabellar field. The available cranidium is large, however, yet it retained distinct tubercles on the frontal area. These are effaced even in smaller specimens of D. blanda. The pygidium is also differentiated, with a very strong 4 1 1 rib

count and smaller, more rim-like border. The small librigena found (Fig. 16.5) could well belong to another taxon, given that two or more species of the Ischyrotoma group occur together at many Ibex horizons. DIMEROPYGIELLA sp. Figure 7.51–7.53 Material and occurrence.Single pygidium, SUI 94291, from horizon J-42, Wah Wah Formation, Ibex Section J, Ibexian (Blackhillsian; Zone J/Pseudocybele nasuta Zone), Millard County, western Utah. Discussion.A single large pygidium from J-42 is clearly distinct from co-occurring Dimeropygiella caudanodosa and almost certainly represents a new, rare species. It is shorter (sag.; post.) relative to its width than pygidia of D. caudanodosa, has much stronger dorsal tubercles, and less fin-shaped pleural ribs. Its overall morphology and terminal axial tubercles indicate that it likely represents a member of the Ischyrotoma group, and its similarity in axial ring and rib counts to D. caudanodosa suggest Dimeropygiella. However, it does not closely resemble any species thus far described. ACKNOWLEDGMENTS

This work was supported by NSF grant EAR 9973065. Costs of earlier fieldwork were met by a Natural Sciences and Engineering Research Council (Canada) operating grant to S.R.W., held at Brock University, and by Natural Environment Research Council and Leverhulme Trust research grants to R.A.F. We are grateful to T. McCormick for assistance in the field and with acid processing. REFERENCES

ADRAIN, J. M. 1997. Proetid trilobites from the Silurian (Wenlock-Ludlow) of the Cape Phillips Formation, Canadian Arctic Archipelago. Palaeontographia Italica, 84:21–111.

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ADRAIN, J. M., AND B. D. E. CHATTERTON. 1996. The otarionine trilobite Cyphaspis, with new species from the Silurian of northwestern Canada. Journal of Paleontology, 70:100–110. ADRAIN, J. M., AND R. A. FORTEY. 1997. Ordovician trilobites from the Tourmakeady Limestone, western Ireland. Bulletin of the Natural History Museum, London. Geology Series, 53:79–115. ANGELIN, N. P. 1854. Palaeontologica Scandinavica. Pars II, Crustacea formationis transitionis. Academiae Regiae Scientarum Suecanae (Holmiae), 25–92. BOYCE, W. D. 1989. Early Ordovician trilobite faunas of the Boat Harbour and Catoche Formations (St. George Group) in the Boat HarbourCape Norman area, Great Northern Peninsula, western Newfoundland. Newfoundland Department of Mines and Energy, Geological Survey Branch, Report, 89- 2:1–169. BRETT, K. D., AND S. R. WESTROP. 1996. Trilobites of the Lower Ordovician (Ibexian) Fort Cassin Formation, Champlain Valley regions, New York State and Vermont. Journal of Paleontology, 70:408–427. BRUTON, D. L. 1983. The morphology of Celmus (Trilobita) and its classification, p. 213–219. In D. E. G. Briggs and P. D. Lane (eds.), Trilobites and other early arthropods. Papers in honour of Professor H. B. Whittington, F. R. S. Special Papers in Palaeontology, 30. CHATTERTON, B. D. E. 1980. Ontogenetic studies of Middle Ordovician trilobites from the Esbataottine Formation, Mackenzie Mountains, Canada. Palaeontographica Abteilung A, 171:1–74. CHATTERTON, B. D. E. 1994. Ordovician proetide trilobite Dimeropyge, with a new species from northwestern Canada. Journal of Paleontology, 68:541–556. CHATTERTON, B. D. E., AND R. LUDVIGSEN. 1976. Silicified Middle Ordovician trilobites from the South Nahanni River area, District of Mackenzie, Canada. Palaeontographica Abteilung A, 154:1–106. CHATTERTON, B. D. E., G. D. EDGECOMBE, B. G. WAISFELD, AND N. E. VACCARI. 1998. Ontogeny and systematics of Toernquistiidae (Trilobita, Proetida) from the Ordovician of the Argentine Precordillera. Journal of Paleontology, 72:273–303. DEAN, W. T. 1971. The trilobites of the Chair of Kildare Limestone (upper Ordovician) of eastern Ireland, Pt. 1, Monographs of the Palaeontographical Society, 531:1–60. DEAN, W. T. 1989. Trilobites from the Survey Peak, Outram and Skoki formations (Upper Cambrian–Lower Ordovician) at Wilcox Pass, Jasper National Park, Alberta. Geological Survey of Canada Bulletin, 389: 1–141. DEMETER, E. J. 1973. Lower Ordovician pliomerid trilobites from western Utah. Brigham Young University Geology Studies, 20:37–65. DROSER, M. L., AND P. M. SHEEHAN. 1995. Paleoecology of the Ordovician Radiation and the Late Ordovician extinction event: evidence from the Great Basin, p. 63–106. In J. D. Cooper (ed.), Ordovician of the Great Basin: Fieldtrip Guidebook and Volume for the Seventh International Symposium on the Ordovician System. Pacific Section SEPM Book 78. FORTEY, R. A. 1979. Early Ordovician trilobites from the Catoche Formation (St. George Group), western Newfoundland. Geological Survey of Canada Bulletin, 321:61–114. FORTEY, R. A. 1980. The Ordovician trilobites of Spitsbergen. III. Remaining trilobites of the Valhallfonna Formation. Norsk Polarinstitut Skrifter, 171:1–163. FORTEY, R. A. 1986. Early Ordovician trilobites from the Wandel Valley Formation, eastern North Greenland. Rapport. Grønlands Geologiske Undersøgelse, 132:15–25. FORTEY, R. A., AND M. L. DROSER. 1996. Trilobites at the base of the Middle Ordovician, western United States. Journal of Paleontology, 70: 73–99. FORTEY, R. A., AND M. L. DROSER. 1999. Trilobites from the base of the type Whiterockian (Middle Ordovician) in Nevada. Journal of Paleontology, 73:182–201. FORTEY, R. A., AND R. M. OWENS. 1975. Proetida—a new order of trilobites. Fossils and Strata, 4:227–239. HARRINGTON, H. J. 1957. Notes on new genera of Pliomeridae (Trilobita). Journal of Paleontology, 31:811–812. HINTZE, L. F. 1951. Lower Ordovician detailed stratigraphic sections for western Utah. Utah Geological and Mineralogical Survey Bulletin, 39: 1–99. HINTZE, L. F. 1953. Lower Ordovician trilobites from westerh Utah and

eastern Nevada. Utah Geological and Mineralogical Survey Bulletin, 48:1–249. (for 1952) HINTZE, L. F. 1954. Presbynileus and Protopresbynileus, new generic names proposed for Pseudonileus and Paranileus Hintze, preoccupied. Journal of Paleontology, 28:16–20. HINTZE, L. F. 1973. Lower and Middle Ordovician stratigraphic sections in the Ibex area, Millard County, Utah. Brigham Young University Geology Studies, 20:3–36. HINTZE, L. F. 1982. Ibexian Series (Lower Ordovician) type section, western Utah, U.S.A., p. 7–10. In R. J. Ross, Jr., et al. (28 authors), The Ordovician System in the United States. International Union of Geolgocial Sciences Publication No. 12. HINTZE, L. F., AND V. JAANUSSON. 1956. Three new genera of asaphid trilobites from the Lower Ordovician of Utah. Bulletin of the Geological Institution of Uppsala, 36:51–57. HUPE´, P. 1953. Classe des Trilobites, p. 44–246. In J. Piveteau (ed.), Traite´ de Pale´ontologie, 3. INGHAM, J. K., G. B. CURRY, AND A. WILLIAMS. 1986. Early Ordovician Dounans Limestone fauna, Highland Border Complex, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 76:481– 513. (for 1985) JAANUSSON, V. 1956. On the trilobite genus Celmus Angelin, 1854. Bulletin of the Geological Institution of Uppsala, 36:35–49. JAMES, N. P., AND R. K. STEVENS. 1986. Stratigraphy and correlation of the Cambro-Ordovician Cow Head Group, western Newfoundland. Geological Survey of Canada Bulletin, 366:1–143. KOBAYASHI, T. 1955. The Ordovician fossils of the McKay Group in British Columbia western Canada, with a note on the early Ordovician palaeogeography. Journal of the Faculty of Science, Tokyo University, Section 2, 9:355–493. LEE, D.-C., AND B. D. E. CHATTERTON. 1997. Hystricurid trilobite larvae from the Garden City Formation (Lower Ordovician) of Idaho and their phylogenetic implications. Journal of Paleontology, 71:862–877. MCCORMICK, T., AND R. A. FORTEY. 1999. The most widely distributed trilobite species: Ordovician Carolinites genacinana. Journal of Paleontology, 73:202–218. MICKEVICH, M. F., AND S. J. WELLER. 1990. Evolutionary character analysis: tracing character change on a cladogram. Cladistics, 6:137–170. MOORE, R. C. (ED.) 1959. Treatise on invertebrate paleontology, Pt.O, Arthopoda 1. Geological Society of America and University of Kansas, Boulder and Lawrence, 560 p. ¨ PIK, A. A. 1937. Trilobiten aus Estland. Tartu U ¨ likooli Geoloogia-IntiO tuudi Toimetused, 52:1–163. RAMSKO¨LD, L. 1986. Silurian encrinurid trilobites from Gotland and Dalarna, Sweden. Palaeontology, 29:527–575. RAYMOND, P. E. 1925. Some trilobites of the lower Middle Ordovician of eastern North America. Bulletin of the Museum of Comparative Zoology, Harvard University, 67:1–180. REED, F. R. C. 1896. The fauna of the Keisley Limestone, Pt.I. Quarterly Journal of the Geological Society of London, 52:407–437. ROSS, R. J., JR. 1951. Stratigraphy of the Garden City Formation in northeastern Utah, and its trilobite faunas. Peabody Museum of Natural History, Yale University, Bulletin, 6:1–161. ROSS, R. J., JR. 1953. Additional Garden City (Early Ordovician) trilobites. Journal of Paleontology, 27:633–646. ROSS, R. J., JR. 1967. Some Middle Ordovician brachiopods and trilobites from the Basin Ranges, western United States. United States Geological Survey Professional Paper, 523–D:1–43. ROSS, R. J., JR., L. F. HINTZE, R. L. ETHINGTON, J. F. MILLER, M. E. TAYLOR, AND J. E. REPETSKI. 1993. The Ibexian Series (Lower Ordovician), a replacement for ‘‘Canadian Series’’ in North American chronostratigraphy. United States Geological Survey Open File Report, 93–598:1–75. ROSS, R. J., JR., L. F. HINTZE, R. L. ETHINGTON, J. F. MILLER, M. E. TAYLOR, AND J. E. REPETSKI. 1997. The Ibexian, lowermost series in the North American Ordovician. United States Geological Survey Professional Paper, 1579:1–50. SHAW, F. C. 1968. Early Middle Ordovician Chazy trilobites of New York. New York State Museum and Science Service Memoir, 17:1– 163. SWOFFORD, D. L. 1999. PAUP*. Phylogenetic Analysis Using Parsimony (*and Other Methods). Version 4b2. Sinauer Associates, Sunderland, Massachusetts.

ADRAIN ET AL.—ORDOVICIAN TRILOBITES ISCHYROTOMA AND DIMEROPYGIELLA TERRELL, F. E. 1973. Silicified trilobite zonation in the Lower Fillmore Formation in western Utah. Brigham Young University Geology Studies, 20:67–90. TRIPP, R. P., AND W. R. EVITT. 1983. Silicified trilobites of the genus Dimeropyge from the Middle Ordovician of Virginia, p. 229–240. In D. E. G. Briggs and P. D. Lane (eds.), Trilobites and other early arthropods. Papers in honour of Professor H. B. Whittington, F. R. S. Special Papers in Palaeontology, 30. WHITFIELD, R. P. 1890. Observations on the fauna of the rocks at Fort Cassin, Vermont, with descriptions of a few new species. Bulletin of the American Museum of Natural History, 3:25–39. WHITTINGTON, H. B. 1953. North American Bathyuridae and Leiostegiidae (Trilobita). Journal of Paleontology, 27:647–678.

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WHITTINGTON, H. B. 1963. Middle Ordovician trilobites from Lower Head, western Newfoundland. Bulletin of the Museum of Comparative Zoology, Harvard, 129:1–118. WHITTINGTON, H. B. 1965. Trilobites of the Ordovician Table Head Formation, western Newfoundland. Bulletin of the Museum of Comparative Zoology, Harvard, 132:275–442. WHITTINGTON, H. B., AND W. R. EVITT. 1954. Silicified Middle Ordovician trilobites. Geological Society of America Memoir, 59:1–137. YOUNG, G. E. 1973. An Ordovician (Arenigian) trilobite faunule of great diversity from the Ibex area, western Utah. Brigham Young University Geology Studies, 20:91–115. ACCEPTED 10 JANUARY 2001