cambrian and ordovician linguliform brachiopods from the ... - BioOne

J. Paleont., 75(2), 2001, pp. 241–260 Copyright q 2001, The Paleontological Society 0022-3360/01/0075-241$03.00

CAMBRIAN AND ORDOVICIAN LINGULIFORM BRACHIOPODS FROM THE SHALLOW BAY FORMATION (COW HEAD GROUP), WESTERN NEWFOUNDLAND SEAN P. ROBSON

AND

BRIAN R. PRATT

Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada, ,[email protected]., ,[email protected]. ABSTRACT—Linguliform brachiopods were recovered from the Upper Cambrian Downes Point Member (lower Sunwaptan) and from the Middle Ordovician Factory Cove Member (Arenig) of the Shallow Bay Formation, Cow Head Group, of western Newfoundland. These rocks are a series of Middle Cambrian to Middle Ordovician conglomerates, lime mudstones, and shales that formed a sediment apron at the base of the lower Paleozoic continental slope of Laurentia. The linguliform brachiopod fauna consists of sixteen species assigned to twelve genera. Three new species are described: Picnotreta lophocracenta, Neotreta humberensis, and Siphonotretella parvaducta.

INTRODUCTION

INGULIFORM BRACHIOPODS are locally prolific members of early Paleozoic faunas, and some exhibit cosmopolitan distribution and relatively high species diversity. Prior to the application of acetic acid etching techniques to recover phosphaticshelled brachiopods (Bell, 1948), little was known about their internal morphology and many taxa were considered to be of limited use for biostratigraphy. The work of a small number of specialists, particularly in the last thirty years, has resulted in wide-ranging revision of the Linguliformea (phosphatic-shelled brachiopods). Details of valve interiors show a much wider range of morphological diversity than was evident from the exteriors, and many genera believed to be monospecific were found to contain several species. Because of these changing views, the Linguliformea are more widely recognized as important biostratigraphic indicators. Despite the recent renaissance in linguliform research, however, there is still a great deal to be learned about the taxonomy, biostratigraphy, and biogeography of the group. The sedimentology and stratigraphy of the Cow Head Group has been well studied (James and Stevens, 1986; Coniglio and James, 1990), as has the biostratigraphy of its trilobite and graptolite faunas (Williams and Stevens, 1988; Young and Ludvigsen, 1989; Ludvigsen et al., 1989). The Cow Head Group has been proposed as a global stratotype for the second series of the Ordovician System (Williams et al., 1994). This study of the linguliform brachiopods in the Middle Cambrian to Middle Ordovician Shallow Bay Formation of the Cow Head Group in western Newfoundland represents the first taxonomic survey and biostratigraphy of the Linguliformea from this area. In addition to describing new species, this study documents taxa also known from Europe, Asia, and Australia, demonstrating that many linguliform brachiopods are pandemic and therefore are potentially useful tools in both regional and global correlation. Further study of this area, particularly of the Green Point Formation, and the Tuckers Cove and Stearing Island members of the Shallow Bay Formation, may reveal a greater diversity of brachiopods than is treated here.

L

GEOLOGICAL SETTING

The Cow Head Group is a succession of Middle Cambrian to Middle Ordovician conglomerates, limestones, and shales deposited at the base of the continental slope of Laurentia. It is part of the Humber Arm allochthon, located on the northwestern coast of Newfoundland (Fig. 1). The carbonate platform that was the source of the allochthonous carbonate sediments lay to the west.

The slope consists of limestone debris-flow conglomerates proximally, and grades distally into fine-grained hemipelagic sediments, comprising interbedded argillaceous lime mudstones and shales (James and Stevens, 1986). The Cow Head Group is subdivided into two contemporaneous formations; the Shallow Bay Formation and the Green Point Formation. The Shallow Bay Formation is the more proximal of the two and consists of four members (Fig. 2) all dominated by conglomerates. The Green Point Formation is the more distal and is dominated by shale and shaley lime mudstones (James and Stevens, 1986). Well-preserved linguliform brachiopods, particularly those of the order Acrotretida, were found predominantly in the bouldersized clasts of the conglomerate. The lime mudstones and calcareous siltstones contained a fauna consisting almost exclusively of the lingulid Paterula. Large numbers of crushed and unidentifiable specimens occur on the bedding planes. The exact location of the brachiopods within each bed was not measured. Thus the stratigraphic distributions (Fig. 2) reflect only the presence of species within a bed as a whole. STRATIGRAPHY AND BIOSTRATIGRAPHY

Downes Point Member.This is the basal member of the Shallow Bay Formation, and represents beds 1 to 5 of James and Stevens (1986). The unit is approximately 100 m-thick and consists primarily of conglomerate with minor interbeds of lime mudstone and shale, and some grainstone. This member is best exposed on the northeastern shore of Cow Head Peninsula at Beachy Cove where our collections were made (Fig. 1). The strata exposed at Cow Head Peninsula range from Marjuman to Sunwaptan (Upper Cambrian) (Young and Ludvigsen, 1989; James and Stevens, 1986; Ludvigsen et al., 1989). Many of the boulder clasts in the conglomerate appear to have been reworked from older sediments. A number of Middle Cambrian taxa were recovered from clasts in beds believed to be Sunwaptan in age. Young and Ludvigsen (1989) described Middle Cambrian trilobites from strata correlative with beds 1 and 2 from Broom Point, 10 km southwest of Cow Head Peninsula. Trilobites characteristic of the Cedaria and Crepicephalus zones of the Marjuman Stage were recovered from beds 3 and 4 and trilobites belonging to the Aphelaspis, Dicanthopyge, Prehousia, and Dunderbergia zones of the Steptoean Stage were collected from bed 5 (James and Stevens, 1986). However, Ludvigsen et al. (1989) found that the base of the Tuckers Cove Member, bed 6, was high within the Sunwaptan Stage. Thus, they interpreted beds 3 to 5

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JOURNAL OF PALEONTOLOGY, V. 75, NO. 2, 2001 TABLE 2—Measurements (in mm) and ratios of dorsal valves of Picnotreta lophocracenta. Abbreviations as in Table 1.

MIN MAX MEAN

FIGURE 1—Map of western Newfoundland Cow Head Peninsula.

as early Sunwaptan in age and the Marjuman and Steptoean trilobites in these beds as due to reworking. Of the linguliform brachiopods described here from the Downes Point Member (Fig. 2), Prototreta, Pegmatreta, and Dactylotreta patriella were hitherto known only from the Middle Cambrian. Only Orbithele, Neotreta, Picnotreta, and Micromitra modesta have ranges known to extend into the Upper Cambrian. Thus, it is probable that some or all of these brachiopods were in reworked Middle Cambrian boulders. The brachiopods were sparsely distributed throughout the Downes Point Member, and are sporadic in the limestone boulders. Tuckers Cove Member.This 60 m-thick unit consists primarily of quartz-rich grainstone and conglomerate. There are also minor amounts of lime mudstone, sandstone, calcareous siltstone, and shale (James and Stevens, 1986). The member crops out poorly at Tuckers Cove, but is better exposed along the northwestern shore of Cow Head Peninsula (Fig. 1). These strata comprise bed 6, and are Late Cambrian (Sunwaptan) in age (Ludvigsen et al., 1989). No brachiopods were found in the conglomerate, although unidentifiable, crushed obolids are present on siltstone bedding planes. Stearing Island Member.This is an 80 m-thick unit of megaconglomerate with minor interbeds of shale and lime mudstone, quartz-rich grainstone, and shale, comprising beds 7 and 8 (James and Stevens, 1986). The best outcrops are found along the western shore of Cow Head Peninsula (Fig. 1), but no brachiopods were recovered from them. This member straddles the Cambro-Ordovician boundary, with bed 7 located in the uppermost Upper Cambrian, and bed 8 located in the lowermost Lower Ordovician. Factory Cove Member.This is the uppermost member of the Shallow Bay Formation, and consists of a 100 m-thick sequence of boulder conglomerate, shaley lime mudstone, and shale. The unit crops out along the entire southern shore of Cow Head Peninsula (Fig. 1) and represents beds 9 to 14 of James and Stevens (1986). The strata range in age from Early to Middle Ordovician (Williams and Stevens, 1988; Williams et al., 1994). The base of the member, bed 9, corresponds to the Tetragraptus Zone (Williams and Stevens, 1988). Two species of the obolid Paterula, P. polita, and P. subcircularis were recovered from lime mudstones of bed

A

B

B/A

C

C/A

0.59 1.20 0.96

0.79 1.56 1.26

1.19 1.44 1.32

0.23 0.63 0.47

0.39 0.57 0.49

9 (Fig. 2). Unidentified fragments of Lingulella were recovered from lime mudstones of bed 11, which correspond to the Pendeograptus fruticosus and Didymograptus bifidus Zones (Williams and Stevens, 1988). Limestone boulders of bed 13 were found to contain sparse, unidentifiable acrotretid fragments. Bed 14 corresponds to the Isograptus victoriae maximus Zone (Williams and Stevens, 1988), and contains a relatively diverse fauna of linguliform brachiopods (Fig. 2). Six species comprising acrotretids, lingulids, and siphonotretids were recovered from limestone boulders. Conotreta siljanensis and Scaphelasma are abundant and confirm a Middle Ordovician age for bed 14. Siphonotretella, however, was hitherto only known from the Lower Ordovician Bjorkasholmen Limestone of Scandinavia (Popov and Holmer, 1994). The range of Siphonotretella is extended into the Middle Ordovician, and there can be no question of reworking because it occurs in the same boulders as Conotreta and Scaphelasma. METHODS

Approximately 600 kg of boulder conglomerate, lime mudstone, and calcareous siltstone were collected from beds 2 to 14 of the Shallow Bay Formation at Cow Head Peninsula. Strata were examined in the field for evidence of fossil content, and those horizons that appeared to be fossiliferous were sampled in bulk. Samples were dissolved in 10 percent acetic acid. The resulting insoluble residue was carefully washed, sieved, dried, and examined under low-power magnification with care, for the shells are extremely fragile. Phosphatic brachiopod valves were picked from the residue with a fine brush, mounted on SEM stubs, goldcoated, and examined and photographed using scanning electron microscopy. Figured specimens are deposited in the Royal Ontario Museum (ROM), Toronto, Ontario, Canada. SYSTEMATIC PALEONTOLOGY

Class LINGULATA Gorjansky and Popov, 1985 Order ACROTRETIDA Kuhn, 1949 Family ACROTHELIDAE Walcott and Schuchert, 1908 Subfamily ACROTHELINAE Walcott and Schuchert, 1908 Genus ORBITHELE Sdzuy, 1955 Type species.Discina contraria Barrande, 1868. Occurrence.Upper Middle Cambrian–Lower Ordovician, Europe, North Africa, South Urals, Greenland, Australia, and western Newfoundland.

TABLE 1—Measurements (in mm) and ratios of ventral valves of Picnotreta lophocracenta n. sp. MIN 5 minimum recorded value. MAX 5 maximum recorded value. MEAN 5 average of all recorded values.

MIN MAX MEAN

A

B

B/A

C

D

E

F

G

H

G/H

0.91 1.45 1.18

0.95 1.58 1.40

1.08 1.29 1.19

0.30 0.58 0.43

0.25 0.41 0.32

0.13 0.22 0.16

0.13 0.21 0.18

0.040 0.090 0.057

0.020 0.045 0.030

1.31 3.60 2.04

ROBSON AND PRATT—CAMBRIAN AND ORDOVICIAN BRACHIOPODS FROM NEWFOUNDLAND

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FIGURE 2—Stratigraphic units of the Shallow Bay Formation at Cow Head Peninsula. Bed numbers in accordance with the scheme of James and Stevens (1986). Vertical bars represent undifferentiated occurrences within a bed.

TABLE 3—Measurements (in mm) and ratios of ventral valves of Picnotreta robusta Popov et al. Abbreviations as in Table 1.

MIN MAX MEAN

A

B

B/A

D

E

F

G

H

G/H

0.83 1.28 0.95

0.87 1.53 1.13

1.05 1.30 1.19

0.16 0.27 0.24

0.07 0.14 0.09

0.10 0.20 0.14

0.040 0.067 0.050

0.020 0.042 0.030

1.20 2.20 1.80

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FIGURE 3—Orbithele sp. 1, 2, Left lateral and posterior views of ventral interior, ROM 46935, 330; 3–5, ventral valve, ROM 46934; 3, plan view of ventral exterior, 370; 4, detail of ventral surface, 3145; 5, detail of larval pitting, 3460; 6, 8, dorsal valve, ROM 46937; 6, plan view of dorsal exterior, 335; 7, plan view of ventral exterior, ROM 46936, 335; 8, dorsal larval valve, 3100. All specimens from bed 3 of the Downes Point Member.

TABLE 4—Measurements (in mm) and ratios of dorsal valves of Picnotreta robusta Popov et al. Abbreviations as in Table 1.

MIN MAX MEAN

A

B

B/A

C

C/A

1.06 1.30 1.18

1.31 1.40 1.36

1.08 1.24 1.16

0.82 1.04 0.93

0.77 0.86 0.82

TABLE 5—Measurements (in mm) and ratios of ventral valves of Neotreta humberensis n. sp. Abbreviations as in Table 1.

MIN MAX MEAN

A

B

B/A

C

C/B

D

0.33 0.50 0.42

0.50 0.71 0.61

1.42 1.52 1.46

0.32 0.50 0.44

0.64 0.79 0.71

0.03 0.05 0.04

ROBSON AND PRATT—CAMBRIAN AND ORDOVICIAN BRACHIOPODS FROM NEWFOUNDLAND ORBITHELE sp. Figure 3.1–3.8 Description.Shell large, subcircular in outline; ornamentation of pustulose rugae; larval shell pitted. Ventral valve convex, forming low cone; procline pseudointerarea broad with shallowly depressed intertrough; apically located larval shell bears two anterolateral spines; large, posteriorly facing foramen, not confined within larval shell. Ventral interior with knob-shaped apical process containing pedicle tube. Dorsal valve flattened, with straight posterior margin; larval shell located at posterior margin; four spines arranged posterolaterally and anterolaterally on larval shell. Material examined.Eight ventral and four dorsal valves from beds 3 and 5 of the Downes Point Member. ROM 46934–46937. Occurrence.Upper Middle Cambrian–?Upper Cambrian, western Newfoundland. Discussion.Because available material is limited to poorly preserved fragments, these specimens are left in open nomenclature. Family ACROTRETIDAE Schuchert, 1893 Subfamily ACROTRETINAE Schuchert, 1893 Genus CONOTRETA Walcott, 1889 Type species.Conotreta rusti Walcott, 1889. Occurrence.Ordovician, South Urals, Estonia, Poland, Bohemia, Sweden, North America. CONOTRETA SILJANENSIS Holmer, 1989 Figure 4.1–4.11 Conotreta? siljanensis HOLMER, 1989, p. 84, figs. 56–60.

Diagnosis.Ventral valve procline; short external pedicle tube continues internally; small apical process. Dorsal valve with two large cardinal muscle scars; apsacline pseudointerarea with depressed median plate; high median septum with three to four anterior denticles. Description.Ventral valve low cone, height 50–65 percent of width; procline; subcircular in outline, with length-to-width ratio of 80–90 percent. External pedicle tube short with posteriorly directed foramen, continuing as short internal pedicle tube (Fig. 4.5). Ventral interior has small apical process on anterior surface, and pair of small anterolateral muscle scars. Dorsal valve convex; subcircular in outline; 75–85 percent as long as wide. Pseudointerarea broad with deeply depressed median plate; propareas steeply anacline, nearly catacline; two large posterolateral cardinal muscle scars present as raised callosities; high median septum with concave septal rod on posterior slope and three to four anterior denticles, remnants of septal rods, on anterior slope. Material examined.Seven ventral valves and six dorsal valves from bed 14 of the Factory Cove Member. ROM 46984, 46985, 46988–46990. Occurrence.Middle Ordovician, Lasnamagian–Uhakuan stages of Sweden and the Whiterock Stage of western Newfoundland. Discussion.Because the type species is poorly known, mainly from internal molds, Holmer (1989) considered assignment of species to Conotreta to be only tentative. However, as Holmer (1989) pointed out, more than 30 species have been assigned to Conotreta, and C. multisinuata Cooper, 1956 has become the standard of reference. The specimens described here are confidently assigned to Conotreta based on Cooper’s (1956) definition of the genus. They differ from those of Conotreta? siljanensis described by Holmer (1989) in the following respects: 1) the external pedicle tube is approximately 30 percent shorter in the Factory Cove material; 2) the length-to-width ratio is 5–10 percent less than the Swedish material; 3) the relative height of the pedicle valve is approximately 50 percent shorter. However, the Factory Cove

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specimens are similar to those of Dalarna in having an internal pedicle tube, minute apical process and subdued mantle canals in the ventral valve. The dorsal valves described here are identical to those illustrated by Holmer (1989, figs. 56A–E, 57A–E, and 58A–E). The differences listed above are considered to be minor intraspecific variations. Genus DACTYLOTRETA Rowell and Henderson, 1978 Type species.Dactylotreta redunca Rowell and Henderson, 1978. Occurrence.Middle Cambrian–Lower Ordovician, Antarctica, Asia, Australia, Greenland, North America. DACTYLOTRETA

Zell and Rowell, 1988 Figure 5.1–5.6

PATRIELLA

Dactylotreta patriella ZELL

AND

ROWELL, 1988, p. 131, fig. 11.1–5.

Diagnosis.Ventral valve with concave anterior slope, convex posterior slope. Ventral pseudointerarea shallow, broad; dorsal valve with low median septum. Description.Ventral valve slightly procline; posterior slope convex, anterior slope concave in lateral profile; apex rounded. External pedicle foramen facing posteroventrally, confined within larval valve. Apical process occludes apex, extending from anterior wall to posterior wall, tapering toward posterior margin, containing large, subtriangular internal pedicle. Two apical pits located posterolaterally to internal foramen; two large subcircular muscle scars on inner posterior wall, lateral to foramen; two deeply incised baculate vascula lateralia extend from apical pits, diverging anteriorly. Material examined.Eleven ventral valves from beds 4 and 5 of the Downes Point Member. ROM 46956, 46957, 46959, 46960. Occurrence.Upper Middle to lower Upper Cambrian, western Newfoundland; Upper Middle Cambrian, North Greenland. Genus PROTOTRETA Bell, 1938 Type species.Prototreta trapeza Bell, 1938. Occurrence.Middle Cambrian, North America, Europe, Asia. Discussion.Prototreta is similar to Angulotreta Palmer, 1955. The distinguishing feature is the location of the apical process, which runs along the posterior slope of Prototreta, and along the anterior slope of Angulotreta. Bell (1941) originally erected Homotreta to contain species with a ventral valve similar to Prototreta, but whose dorsal valve has a blade-like median septum. Bell and Ellinwood (1962) suggested that Homotreta is a junior synonym of Prototreta, a view that has been supported by Robison (1964) and Rowell (1965, 1986). PROTOTRETA INTERRUPTA Bell, 1941 Figure 5.7–5.12 Homotreta interrupta BELL, 1941, p. 230, pl. 30, figs. 19–26. ? Prototreta sp. ZELL AND ROWELL, 1988, p. 131, figs. 12.9–13.

Diagnosis.Ventral with poorly developed nested cone structure. Dorsal median septum blade like. Description.Ventral valve conical, procline. Well-defined intertrough on posterior slope broadens from apex to margin. Ornament consists of prominent fila; posteriorly facing circular foramen not completely contained within larval shell; pedicle tube emerges internally as large opening on apical process. Two pits located posterolateral to apical process; two lobate cardinal muscle scars lateral to pits; vascula lateralia diverge anteriorly from apex on either side of apical process. Dorsal valve subcircular in

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FIGURE 4—Conotreta siljanensis Holmer, 1989; 1, lateral view of ventral exterior, ROM 46989, 350; 2–5, 7, ventral valve ROM 46984; 2, posterior view of ventral valve, 3100; 3, detail of ventral larval valve, 3245; 4, detail of pedicle foramen, 3690; 5, ventral interior, 345; 7, anterior oblique view of apex, 3170; 6, ventral interior, ROM 46988, 365; 8, 9, plan view of dorsal interior and exterior, ROM 46985, 350; 10, lateral view of dorsal interior, ROM 46990, 355; 11, lateral oblique view of dorsal interior, ROM 46985, 350. All specimens from bed 14 of the Factory Cove Member.

outline, flat in profile. Broad pseudointerarea with shallow triangular central depression and well-defined, acutely triangular propareas. Blade-like median septum rises posteriorly from pseudointerarea to approximately two thirds of shell length, then descends to anterior margin. Material examined.Nine ventral and two dorsal valves from bed 5 of the Downes Point Member. ROM 46942, 46958, 46961, 46968. Occurrence.Middle Cambrian, Montana, western Newfoundland, and possibly North Greenland. Discussion.Prototreta interrupta is distinguished from other species of the genus by its blade-like median septum. All other species of Prototreta have a digitate septum.

Subfamily LINNARSSONIINAE Rowell, 1965 Genus PEGMATRETA Bell, 1941 Type species.Pegmatreta perplexa Bell, 1941. Occurrence.Middle Cambrian, Utah, Montana, and western Newfoundland. Discussion.Pegmatreta Bell, 1941, has a convoluted history, and its validity as a genus has been uncertain. Rowell (1966) considered Pegmatreta to be a junior synonym of Linnarssonia Walcott, 1885, but Rowell and Henderson (1978) subsequently rejected this assessment and restricted Linnarssonia to species with a high apical process and raised muscle scars. They used Pegmatreta for externally similar species that possess a low, hemispherical apical callosity and lack pronounced thickening of


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FIGURE 5—1–6: Dactylotreta patriella Zell and Rowell, 1988; 1, posterolateral view of ventral exterior, ROM 46957, 340. 2–3, lateral and plan view of ventral exterior, ROM 46959, 330. 4, 5, plan and lateral oblique view of ventral interior, ROM 46956, 320. 6, anterior oblique view of ventral interior, ROM 46960, 340. All specimens from bed 5 of the Downes Point Member. 7–12: Prototreta interrupta Bell, 1941; 7, 8, ventral valve, ROM 46968. 7, posterolateral view of ventral exterior, 355. 8, posterior view of ventral larval valve, 3175. 9, plan view of ventral interior, ROM 46961, 325. 10, lateral oblique view of ventral interior, ROM 46958, 330. 11, 12, plan and lateral oblique view of dorsal interior, ROM 46942, 340. All specimens from bed 5 of the Downes Point Member.

cardinal muscle scars. This concept of Pegmatreta is followed here. PEGMATRETA sp. Figure 6.1–6.8 Diagnosis.Ventral valve low cone, intertrough well developed. Description.Ventral valve low cone, maximum height at apex; catacline; beak slightly overhangs posterior margin; large pedicle foramen posterior to apex, not contained within larval shell, continues internally as short tube (Fig. 6.3). Ornament of fine concentric growth lines. Pseudointerarea with well-defined intertrough. Apical process low rise, posterior to foramen. Cardinal muscle scars poorly preserved; vascula lateralia indistinct.

Material examined.Four ventral valves from bed 3 of the Downes Point Member. ROM 46938, 46939. Occurrence.Upper Middle Cambrian, western Newfoundland. Genus PICNOTRETA Henderson and MacKinnon, 1981 Type species.Picnotreta debilis Henderson and MacKinnon, 1981. Emended Diagnosis.Shell biconvex, wider than long, subtriangular in outline. Ventral pseudointerarea triangular in plan view, apsacline to anacline, buttressed below by apical thickening. Internal pedicle foramen located at base of apical buttress, connected to external foramen by posteriorly tapering pedicle tube.

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FIGURE 6—Pegmatreta sp. 1–4, Ventral valve, ROM 46939; 1, plan view of ventral interior, 350; 2, lateral oblique view of ventral interior, 365; 3, detail of internal pedicle foramen, 3305; 4, anterior view of ventral interior, 340; 5–8, Ventral valve, ROM 46938; 5, plan view of ventral exterior, 355; 6, 7, posterior and lateral views of ventral exterior, 390; 8, detail of ventral posterior, 3140. All specimens from bed 3 of the Downes Point Member.

Dorsal pseudointerarea minute; blade-like median septum rises anteriorly. Occurrence.Upper Middle to Upper Cambrian, Queensland, New Zealand, North Greenland, and western Newfoundland. Discussion.The concept of Picnotreta is emended to include a wider range of features than originally described. Henderson and MacKinnon (1981) considered a strongly apsacline ventral pseudointerarea buttressed by an apical thickening as the distinctive character of the genus. Specimens from this study reveal that a single species can be apsacline to anacline, with the majority exhibiting orthocline pseudointerareas.

PICNOTRETA LOPHOCRACENTA new species Figures 7.1–7.4, 8.1–8.14; Tables 1, 2 ?Picnotreta debilis HENDERSON AND MACKINNON, 1981, fig. 9M and N, not fig. 9H–L, O, P. Picnotreta sp. ZELL AND ROWELL, 1988, p. 139, fig. 14.6.

Diagnosis.Long, thin apical process extends from triangular buttress of pseudointerarea to between one third and one half of valve length. Pseudointerarea slightly apsacline to slightly anacline.


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FIGURE 7—1–4: Picnotreta lophocracenta n. sp.; 1, reconstruction of ventral valve showing location of measurements. 2, reconstruction, ventral valve, lateral view. 3, reconstruction of dorsal valve showing location of measurements. 4, reconstruction, dorsal valve, lateral oblique view.

Description.Shell biconvex and subtriangular in outline, wider than long; external surface glossy. Ventral pseudointerarea broad and short, typically orthocline but may vary from slightly apsacline to slightly anacline (Fig. 8.5), divided by concave, deltashaped intertrough bounded by narrow triangular propareas. Apex overhangs posterior margin of valve, and bears lip that protrudes slightly over intertrough. External pedicle foramen confined within larval shell, facing in posterodorsal direction. Pseudointerarea buttressed by apical thickening through which curved, externally tapering pedicle tube passes, emerging as internal pedicle foramen on anterior face of buttress. Lobate cardinal muscle scars are located on posterolateral valve margins to either side of, and anterior to internal pedicle foramen. Strongly incised grooves, possibly vascula lateralia, run from posterior edge of cardinal muscle scars posteriorly to the apex where they terminate as a pair of pits set into apical buttress. Two additional pits located to either side of apical process anterior to internal foramen. Dorsal valve with well defined sulcus. Broad limbus narrows posteriorly. Pseudointerarea minute, narrow, apsacline, no intertrough. Median septum extends approximately 50 percent of valve length, rising from just posterior of valve center, extending anteriorly to limbus. Lobate cardinal muscle scars near posterolateral margins. Etymology.Greek, lophos, crest or ridge; Latin, cracens, slender, graceful. Type.Holotype ROM 46940, Upper Cambrian, Shallow Bay Formation, western Newfoundland, Bed 4 of the Downes Point Member. Other material examined.Eleven ventral valves and eleven dorsal valves from beds 4 and 5 of the Downes Point Member. ROM 46941, 46943, 46946, 46951, 46952, 46953. See Tables 1 and 2 for measurements. Occurrence.Upper Middle to Upper Cambrian, western

Newfoundland; possibly Middle Cambrian, Queensland and North Greenland. Discussion.Picnotreta lophocracenta n. sp. exhibits a range of variability in the orientation of the pseudointerarea. The profile is low, and most specimens have an orthocline pseudointerarea, but can range from apsacline to anacline. Specimens assigned to P. debilis Henderson and MacKinnon, 1981, appear to belong to more than one species. The ventral and dorsal valves (Henderson and MacKinnon, 1981, fig. 9M, N) appear to belong to P. lophocracenta. The ventral valve has an orthocline pseudointerarea with a concave intertrough rather than the strongly apsacline pseudointerarea with central convexity believed to be characteristic of P. debilis, and the dorsal valve is also similar to those of P. lophocracenta. Similarly, specimens assigned by Zell and Rowell (1988) to Picnotreta sp. appear to belong to two different species. The ventral valve (Zell and Rowell, 1988, fig. 14.6) is assigned to P. lophocracenta, as it possesses a distinctly orthocline pseudointerarea, concave intertrough, long, slender apical process, and well defined apical pits. PICNOTRETA ROBUSTA Holmer, Popov and Lehnert, 1999 Figures 9.1–9.4, 10.1–10.10; Tables 3, 4 Picnotreta robusta HOLMER, POPOV 9A–N.

AND

LEHNERT, 1999, p. 236, fig.

Diagnosis.Ventral valve with subdued apical process, extending anteriorly from triangular buttress of pseudointerarea to one half of valve length, surrounded by very shallow, U-shaped trough. Dorsal valve with triangular, blade-like, median septum that extends nearly entire length of valve. Description.Shell biconvex, subtriangular in outline. Pseudointerarea orthocline, divided by concave, delta-shaped intertrough bounded by narrow triangular propareas; buttressed by apical thickening, through which a tapering pedicle tube passes.

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FIGURE 8—Picnotreta lophocracenta n. sp. 1–3, Plan, lateral oblique, and anterior views of ventral interior, Holotype, ROM 46940, 335; 4, detail of pseudointerarea, Holotype, ROM 46940, 375; 5, lateral view of ventral exterior, ROM 46941, 345; 6, 7, ventral valve, ROM 46946, 6, detail of pseudointerarea, 395; 7, lateral oblique view of ventral interior, 340; 8, posterior view of ventral valve, ROM 46943, 375; 9, dorsal valve, plan view of interior, ROM 46952, 330; 10, 11, dorsal valve, ROM 46951; 10, lateral oblique view of dorsal interior, 345; 11, anterolateral oblique view of dorsal valve, 340; 12–14, dorsal valve, ROM 46953; 12, plan view of exterior, 340; 13, plan view of dorsal pseudointerarea, 3120; 14, lateral view of exterior, 380. 8.1–8.4 from bed 4, 8.5–8.14 from bed 5 of the Downes Point Member.


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FIGURE 9—1–4: Picnotreta robusta Holmer, Popov and Lehnert, 1999; 1, reconstruction of ventral valve showing location of measurements. 2, reconstruction, ventral valve, lateral view. 3, reconstruction of dorsal valve showing location of measurements. 4, reconstruction, dorsal valve, lateral oblique view.

Apex overhangs posterior margin of valve, and bears lip that protrudes slightly over intertrough. External pedicle foramen confined within larval shell, facing in posterodorsal direction. Apical process present as low rise which widens anteriorly from internal pedicle foramen, extending one half the length of the shell, and surrounded by shallow U-shaped trough (Fig. 10.1). Apical pits located anterolateral to internal pedicle foramen. Cardinal muscle scars located posterolaterally. Incised grooves run from cardinal muscle scars to apex. Dorsal valve with well-defined sulcus. Broad limbus narrows posteriorly. Pseudointerarea apsacline, with broad, concave intertrough dividing triangular propareas. Median septum extends 75–85 percent of valve length from pseudointerarea to anterior limbus. Lobate cardinal muscle scars located near posterolateral margin. Material examined.Eleven ventral valves from beds 4 and 5 of the Downes Point Member, and three dorsal valves from bed 5 of the Downes Point Member. ROM 46944, 46945, 46947, 46949, 46954, 46955. Occurrence.Upper Middle to Upper Cambrian, Argentina, and western Newfoundland. Discussion.Picnotreta robusta is very similar to Picnotreta lophocracenta. It differs in having a very low, sometimes undiscernable apical process, and a pseudointerarea that is less robustly buttressed. All specimens exhibit an orthocline pseudointerarea. Subfamily UNKNOWN Genus NEOTRETA Sobolev, 1976 Type species.Neotreta tumida Sobolev, 1976. Diagnosis.See Popov et al., 1994.

Occurrence.Middle to Upper Cambrian, Kazakhstan, Kirgizia (Central Asia), North China, eastern Siberia, Queensland, and western Newfoundland. Discussion.Neotreta has many features in common with the Linnarssoniinae, and Popov et al. (1994) mentioned its similarity to Stilpnotreta Henderson and MacKinnon, 1981. However, Neotreta is most closely similar to Rhondellina Rowell, 1986, particularly in shell shape. The only major difference between the two groups is that Rhondellina has a ventrally facing pedicle foramen, while Neotreta has a posterior facing foramen. The similarity is so great that Mei (1993) described a new species, Rhondellina sinensis, which is properly assigned to Neotreta. He sited a posterior facing foramen, no groove before the pedicle foramen (a feature present in Neotreta humberensis n. sp.), a very low apical process, apical pits, and a slender median ridge as the only features distinguishing the species from Rhondellina dorei Rowell, 1986. It is clear that the two genera are very closely related. NEOTRETA HUMBERENSIS new species Figures 11.1–11.2, 12.1–12.12; Table 5 Diagnosis.Posterior margin straight, approximately 70 percent of valve width. Apical process absent. Pedicle foramen posteriorly directed, with shallow groove extending anteriorly from foramen. Dorsal valve with vestigial median septum. Description.Shell biconvex, transversely oval in outline. External surface ornamented with fine concentric growth lines. Ventral valve deeply convex, nearly 50 percent wider than long. Larval shell pitted, extending beyond posterior margin; large posteriorly directed pedicle foramen, with shallow groove extending

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FIGURE 10—Picnotreta robusta Holmer, Popov and Lehnert, 1999. 1–3, Ventral valves; 1, anterior view of interior, ROM 46949, 370; 2, posterior view of interior, ROM 46945, 360; 3, detail of pseudointerarea in lateral profile, ROM 46947, 3170; 4, 5, dorsal exterior, ROM 46944; 4, plan view, 335; 5, detail of dorsal larval shell, 3235; 6, 7, ventral valve, ROM 46945; 6, lateral oblique view, 345; 7, plan view of interior, 335; 8– 10, dorsal interior; 8, 10, ROM 46954; 8, lateral oblique view, 340; 9, anterior oblique view, ROM 46955, 350; 10, anterior view, 365. All specimens from bed 5 of the Downes Point Member.

anteriorly from foramen on exterior of larval valve. Apical process absent. Ventral pseudointerarea reduced, catacline, present as short, broad strip. Posterolateral muscle scars poorly preserved. Dorsal larval valve extends beyond posterior margin; minute catacline pseudointerarea; posterolateral muscle scars well preserved; minute, vestigial median septum extends anteriorly to one half of valve length.

Etymology.Named for the Humber Arm allochthon of western Newfoundland. Type.Holotype, ROM 46950. Upper Cambrian, Shallow Bay Formation, western Newfoundland, Bed 3 of the Downes Point Member. Other Material examined.Three ventral valves and one dorsal valve were identified from bed 3 of the Downes Point Member. ROM 46950, 46964–46966. See Table 5 for measurements.


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Diagnosis.Outline trapezoidal; shell thin; ornament subdued; dorsal valve strongly convex, sulcate. Description.Shell 25 percent wider than long; ornament of subdued growth lines near umbo, becoming strongly lamellose toward anterior margin. Dorsal valve strongly sulcate; umbo overhanging straight posterior margin; pseudointerarea short, lacking discernable propareas; median septum a high, triangular blade with rounded apex, posteroventral edge slightly concave, anteroventral edge convex, terminates just short of anterior margin; muscle scars not apparent. Material examined.One dorsal valve from bed 14 of the Factory Cove Member. ROM 46981. Occurrence.Middle Ordovician, Nevada, western Newfoundland. SCAPHELASMA cf. MICA Popov, 1975 Figure 13.4–13.12 cf. Scaphelasma mica POPOV, 1975, p. 39, pl. 5, figs. 21–30.

FIGURE 11—1, 2: Neotreta humberensis n. sp.; 1, reconstruction showing location of measurements on ventral valves. 2, reconstruction showing location of measurements on dorsal valve.

Occurrence.Middle to Upper Cambrian, western Newfoundland. Discussion.Neotreta humberensis differs from other species in completely lacking an apical process and dorsal pseudointerarea, but possessing a catacline ventral pseudointerarea and a vestigial dorsal median septum. N. humberensis is most similar to Neotreta davidi Popov et al., 1994, the most significant difference is the presence of the dorsal septum. Subfamily SCAPHELASMATINAE Rowell, 1965 Genus SCAPHELASMA Cooper, 1956 Type species.Scaphelasma septatum Cooper, 1956. Occurrence.Middle Ordovician, Alabama, Nevada, western Newfoundland, Sweden, Kazakhstan, Russia; Lower to Middle Ordovician, Poland. SCAPHELASMA TUMIDATUM Krause and Rowell, 1975 Figure 13.1–13.3 Scaphelasma tumidatum KRAUSE 7–14.

AND

ROWELL, 1975, p. 51, pl. 7, figs.

Diagnosis.Outline transversely oval, ornament strongly lamellose in anterior two thirds of shell; short groove extending from anterior margin of foramen to apex. Dorsal pseudointerarea narrow; low, triangular median septum. Description.Shell transversely oval in outline; ornament of fine concentric growth lines in umbonal region, becoming strongly lamellose in anterior two thirds of shell. Ventral valve low cone, procline, less than half as high as wide. Large, elongate, oval foramen, posteriorly directed, not confined within larval valve; broad, deep intertrough widening from foramen to margin; interior features scant with short apical process extending from anterior margin of foramen; two small pits located anterior to foramen on either side of apical process. Dorsal valve nonsulcate, convex umbonally, remainder of valve flat; umbo overhangs straight posterior margin; minute pseudointerarea; median septum short, low, triangular blade in center of valve, posteroventral margin concave, anteroventral margin convex, apex rounded; septum as high as long, height 15 percent of valve length. Material examined.One ventral and one dorsal valve from bed 14 of the Factory Cove Member. ROM 46980, 46983. Occurrence.Middle Ordovician, western Newfoundland. Discussion.The dorsal valve differs from those of the Swedish material described by Holmer (1989) in being nonsulcate, and possessing a lower and much shorter median septum. The median septa of the Swedish material were described as extending 93 percent of valve length, while the median septum of the specimen described here extends only 16 percent of valve length. These differences combined with limited material have resulted in the specimens being left under open nomenclature. SCAPHELASMA new species A. Figure 13.13–13.15 Description.Dorsal valve subcircular in outline, convex in profile, slightly longer than wide. Ornament of fine concentric lines, becoming lamellose only at anterior margin. Umbo overhangs straight posterior margin. Pseudointerarea consists of short, shallowly depressed median plate. Median septum rectangular, keel-shaped blade extending from mid length of valve to just short of anterior margin. Material examined.One dorsal valve from bed 14 of the Factory Cove Member, ROM 46982. Occurrence.Middle Ordovician, western Newfoundland. Discussion.This specimen differs from other species in having a rectangularly shaped median septum. All other species of Scaphelasma have a triangular septum. This specimen is also distinguished by its ornament, which consists mostly of fine growth lines, with only a few lamellae near the valve margin. It is likely

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FIGURE 12—Neotreta humberensis n. sp. 1, 4, Ventral valve, ROM 46964; 1, plan view of ventral interior, 350; 4, posterior view of ventral valve, 3110; 2, 5, ventral valve, ROM 46966; 2, anterior view of ventral interior, 3100; 5, posterior view of ventral interior, 3135; 3, plan view of ventral interior, Holotype, ROM 46950, 360; 6–9, ventral valve, Holotype, ROM 46950; 6, detail of ventral larval valve, 3500; 7, lateral view of ventral exterior, 370; 8, plan view of ventral exterior, 350; 9, posterior view of ventral exterior, 385; 10–12, dorsal valve, ROM 46965; 10, plan view of dorsal exterior, 375; 11, lateral oblique view of dorsal exterior, 370; 12, plan view of dorsal interior, 365. All specimens from bed 3 of the Downes Point Member.

that this specimen represents a new species, but material too limited for detailed assessment. Order LINGULIDA Waagen, 1885 Family OBOLIDAE King, 1846 Subfamily LINGULELLINAE Schuchert, 1893 Genus LINGULELLA Salter, 1866 Type species.Lingula davisii M’Coy, 1851. LINGULELLA

Krause and Rowell, 1975 Figure 14.1–14.8

AMPHORA

Lingulella amphora KRAUSE 18.

AND

ROWELL, 1975, p. 17, pl. 1, figs. 11–

Diagnosis.Valves dorsi-biconvex, oval in outline, tapering posteriorly and anteriorly. Description.Valves oval in outline with maximum width posterior to midline. Ventral valve sharply tapered anteriorly; gently

convex in lateral profile; pseudointerarea narrow with well-defined triangular pedicle groove dividing acutely triangular propareas. Dorsal valve more deeply convex than ventral valve. Dorsal posterior less sharply tapered and more rounded than ventral valve. Dorsal pseudointerarea with broad, triangular, shallowly depressed median plate separating broad propareas; two pits located anterolateral to pseudointerarea, with mantle canals extending anteriorly from them to one half valve length. Material examined.Two ventral and twelve dorsal valves, and two conjoined shells, from bed 14 of the Factory Cove Member. ROM 46980, 46986, 46987, 46990–46993. Occurrence.Middle Ordovician, Nevada and western Newfoundland. Discussion.The specimens described here differ slightly from those described by Krause and Rowell (1975) in lacking the anterior ridge on the dorsal and ventral valve interiors. Krause and Rowell (1975) did not describe the presence of vascula lateralia in their material, but it can be seen in the ventral valve (Krause


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FIGURE 13—1–3: Scaphelasma tumidatum Krause and Rowell, 1975, ROM 46981; 1, plan view of dorsal interior, 355. 2, lateral oblique view of dorsal interior, 365. 3, posterior oblique view of dorsal exterior, 375. 4–12: Scaphelasma cf. mica Popov, 1975; 4–8, ventral valve ROM 46980. 4, plan view of ventral exterior, 350. 5, lateral view of ventral exterior, 385. 6, detail of ventral larval valve, 3220. 7, posterior view of ventral exterior, 385. 8, plan view of ventral interior, 345. 9–12, dorsal valve, ROM 46983. 9, plan view of dorsal interior, 350. 10, lateral oblique view of dorsal interior, 360. 11, posterior view of dorsal valve, 375. 12, plan view of dorsal exterior, 330. 13–15: Scaphelasma n. sp. A. ROM 46982; 13, plan view of dorsal exterior, 350. 14, lateral view of dorsal exterior, 390, 15, lateral oblique view of dorsal interior, 370. All specimens from bed 14 of the Factory Cove Member.

and Rowell, 1975, pl. 1, fig. 16), and possibly in the dorsal valve (pl. 1, fig. 14). Lingulella amphora differs from other species of Lingulella in having sharply tapered anterior and posterior margins. Family PATERULIDAE Cooper, 1956 Genus PATERULA Barrande, 1879 Type species.Paterula bohemica Barrande, 1879.

PATERULA POLITA Cooper, 1956 Figure 15.1–15.4 Paterula polita COOPER, 1956, p. 236, pl. 24A, figs. 1, 2.

Diagnosis.Small valves, subelliptical, longer than wide. Dorsal valve more strongly convex than ventral valve. Pedicle notch small.

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FIGURE 14—Lingulella amphora Krause and Rowell, 1975. 1, 4, ventral valve, ROM 46991; 1, plan view of ventral interior, 355; 4, detail of ventral pseudointerarea, 3115; 2, oblique view of ventral exterior, ROM 46993, 370; 3, plan view of dorsal interior, ROM 46984, 360; 5, plan view of dorsal exterior, ROM 46987, 345; 6, 8, dorsal valve, ROM 46992; 6, oblique view of dorsal exterior, 345; 8, lateral profile of conjoined shell, 375; 7, detail of dorsal pseudointerarea, ROM 46986, 3115. All specimens from bed 14 of the Factory Cove Member.

Description.Valves subelliptical in outline, 20 percent longer than wide. Ornament of fine concentric growth lines. Dorsal valve strongly convex. Broad limbus, with interior margin bordered by slight ridge that encircles valve. Ventral valve gently convex, posterior more narrowly rounded than dorsal valve, with narrowing of limbus, and small pedicle notch. Material examined.Two ventral and one dorsal valve from bed 9 of the Factory Cove Member. ROM 46969–46971. Occurrence.Middle Ordovician, Oklahoma, western Newfoundland. Discussion.Paterula polita is similar to P. perfecta Cooper, 1956, but differs from the latter in being smaller, more narrowly rounded posteriorly, and possessing a smaller pedicle notch. PATERULA SUBCIRCULARIS Cooper, 1956 Figure 15.5–15.8 Paterula subcircularis COOPER, 1956, p. 239, pl. 24C figs. 7–10.

Diagnosis.Biconvex shell nearly circular in outline and only

slight narrowing at anterior and posterior margins. Pedicle notch narrow and deep. Description.Nearly circular in outline, valve width equal to length. Ornament of very fine concentric growth lines. Limbus relatively narrow, widest at posterolateral margin. Valve interiors featureless; pedicle notch narrow and deep. Material examined.Two ventral and four dorsal valves, and three conjoined shells from bed 9 of the Factory Cove Member. ROM 46972–46975. Occurrence.Middle Ordovician, Oklahoma, western Newfoundland. Discussion.Paterula subcircularis differs from other species of the genus in having a nearly perfectly circular outline. Cooper (1956) cited the unusually large size of his specimens (one paratype is 4 mm long) as the other distinguishing character of the species. The specimens described in this study are much smaller, with an average length and width of only 1 mm. They otherwise accord with Cooper’s description, and so could be juveniles.


257

FIGURE 15—1–4, Paterula polita Cooper, 1956. 1, 2, ventral valve, ROM 46969; 1, plan view of ventral interior, 325; 2, anterior oblique view of ventral interior, 340; 3, plan view of ventral exterior, ROM 46970, 325; 4, oblique lateral view of ventral interior, ROM 46971, 325; 5–8: Paterula subcircularis Cooper, 1956; 5, plan view of dorsal exterior, ROM 46972, 340; 6, plan view of dorsal exterior, ROM 46973, 330; 7, plan view of ventral interior, ROM 46974, 335; 8, posterior view of conjoined shell, ROM 46975, 3150. All specimens from bed 9 of the Factory Cove Member.

Order SIPHONOTRETIDA Kuhn, 1949 Family SIPHONOTRETIDAE Kuhn, 1949 Subfamily SIPHONOTRETINAE Kutorga, 1848 Genus SIPHONOTRETELLA Popov and Holmer, 1994 Type species.Siphonotretella jani Popov and Holmer, 1994. Emended diagnosis.Shell ventribiconvex, subcircular in outline; ornament of growth lamellae, and fine, hollow spines. Ventral valve subconical; pseudointerarea procline to slightly apsacline; small apical pedicle foramen; internal pedicle tube small or absent. Dorsal valve convex; pseudointerarea reduced. Discussion.The diagnosis is emended to include a short internal pedicle tube and small dorsal pseudointerarea. Popov and Holmer (1994) described the genus as having an almost completely reduced dorsal pseudointerarea and no internal pedicle tube. Occurrence.Lower Ordovician (upper Tremadoc Series), Poland, Estonia, Scandinavia; Lower Ordovician (Koagash Stage), South Urals; Middle Ordovician, western Newfoundland.

SIPHONOTRETELLA PARVADUCTA new species Figure 16.1–16.12 Siphonotreta acrotretamorpha Gorjansky. BIERNAT, 1973, p. 105, pls. 27, 4–10; 28; 29; 30, 6–8. ?Siphonotretella sp. POPOV AND HOLMER, 1994, p. 84, fig. 75.

Diagnosis.Short internal pedicle tube and short orthocline dorsal pseudointerarea. Description.Ventral valve subconical, height approximately 25 percent of valve length; small circular foramen posterior to apex, continuing internally as small pedicle tube; pseudointerarea steeply procline, nearly catacline. Dorsal valve convex, not as high as ventral valve; shallow sulcus; pseudointerarea short orthocline plate, no discernible propareas. Several pairs of short shallow depressions radiate out from pseudointerarea on either side of median ridge (Fig. 16.12). Etymology.Latin, parvus, little, and ductus, pipe or tube, referring to short internal pedicle tube.

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FIGURE 16—Siphonotretella parvaducta n. sp. 1, 2, Ventral valve, ROM 46979; 1, plan view of ventral exterior, 320; 2, lateral view of ventral exterior, 340; 3–6, ventral valve, Holotype ROM 46977; 3, anterior view of ventral anterior, 325; 4, posterior view of ventral anterior, 325; 5, anterior oblique view of ventral interior, 325; 6, plan view of ventral interior, 320; 7–10, plan, lateral and posterior views of dorsal exterior, and plan view of dorsal interior, ROM 46978, 320; 11, 12, plan and oblique views of dorsal interior, ROM 46976, 320. All specimens from bed 14 of the Factory Cove Member.

Type.Holotype ROM 46977. Middle Ordovician, Shallow Bay Formation, western Newfoundland. Other material examined.Two ventral and two dorsal valves from bed 14 of the Factory Cove Member. ROM 46976, 46978, 46979. Occurrence.Lower Ordovician, Estonia and possibly Scandinavia; Middle Ordovician, western Newfoundland. Discussion.Specimens of Siphonotreta acrotretamorpha Gorjansky, 1969, described by Biernat (1973) are considered to belong to Siphonotretella parvaducta n. sp. Popov and Holmer (1994) concluded that these specimens were not conspecific with those described by Gorjansky (1969) but were possibly conspecific with Siphonotretella sp. of Popov and Holmer, 1994. They could not make a positive determination because they were unable to describe any internal features of their Siphonotretella sp. The specimens described by Biernat (1973) appear to be closely similar to those described here, possessing a steeply procline to apsacline conical ventral valve with a short internal pedicle tube, and short dorsal pseudointerarea. Species of Siphonotretella were previously known only from the Lower Ordovician (Tremadoc).

Siphonotretella parvaducta extends the range to the Middle Ordovician (Whiterock). Class PATERINATA Williams et al., 1996 Order PATERINIDA Rowell, 1965 Family PATERINIDAE Schuchert, 1893 Genus MICROMITRA Meek, 1873 Type species.Micromitra sculptilis Meek, 1873 by original designation. Occurrence.Lower to Upper Cambrian, North America, Europe, Asia, Australia. MICROMITRA MODESTA Lochman, 1940 Figure 17.1–17.12 Micromitra modesta LOCHMAN, 1940, p. 14, pl. 1, figs. 20, 21.

Diagnosis.High delthyrium and shorter notothyrium, homeodeltidium and homeochilidium minute. Description.Shell subcircular in outline with straight posterior margin, approximately 15 percent wider than long. Ventral


259

FIGURE 17—Micromitra modesta Lochman, 1940. 1–5, ventral valve, ROM 46962; 1–3, plan and lateral views of ventral exterior, and plan view of ventral interior, 320; 4, posterior view of ventral exterior, 335; 5, detail of ventral posterior, 355; 6–9, dorsal valve, ROM 46963; 6, plan view of dorsal exterior, 315; 7, posterior view of dorsal exterior, 325; 8–9, plan and posterior views of dorsal interior, 315; 10–12, ventral valve, ROM 46967; 10, plan view of juvenile ventral exterior, 340; 11, lateral view of juvenile ventral exterior, 355; 12, posterior view of juvenile ventral valve, 360. All specimens from bed 14 of the Factory Cove Member.

valve deeply convex, 60 percent as high as long; catacline. Interior featureless except for slightly thickened ridge along edges of delthyrium, and a pair of elongate depressions extending anteriorly from the apex, corresponding to two lateral callosities on larval valve exterior. Dorsal valve convex, shallower than ventral valve. Pronounced thickening of edges of notothyrium. Three slender processes diverge from apex extending approximately 30 percent of shell length (Fig. 17.8, 17.9). Material examined.Two ventral and one dorsal valve from bed 14 of the Factory Cove Member. ROM 46962, 46963, 46967. Occurrence.Middle to Upper Cambrian, Utah, Montana, Missouri, North Greenland, and western Newfoundland. Discussion.Micromitra modesta is distinguished from other species of the genus by a minute or absent homeodeltidium and homeochilidium. ACKNOWLEDGMENTS

We thank A. J. Rowell, whose thoughtful review and insightful comments improved this paper.

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