The type species of the Ordovician agnostid trilobite - Springer Link

Paliiont. Z.

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Stuttgart, Dezember 1989

The type species of the Ordovician agnostid trilobite Geragnostus HOWELL,1935 PER AHLBERG, Lund* With 3 figures and 2 tables K u r z f a s s u n g: Die Typus-Art des im Ordoviz weitverbreiteten agnostiden Trilobiten Geragnostus, G. sidenbladhi (LINNARSSON,1869) aus dem Ceratopygekalk des oberen Tremadoc yon Schweden, wurde erneut untersucht und ein Lectotypus festgelegt. Geragnostus HOWELL,1935, besitzt eine grof~e A.hnlichkeit mit Arthrorhachis HAWLE~ CORDA,1847.Beide Gattungen sind offensichtlich nah verwandt, und Geragnostus wird sich eventuell als jllngeres subjektives Synonym von Arthrorhachis herausstellen. Line Trennung von Geragnostus und Arthrorhachis k6nnte aufrechterhalten werden, wenn man zu Arthrorhachis metagnostide Trilobiten rechnet, in denen der hintere Lobus (M3) der Pygidium-Spindel deutlich kiirzer ist als die L~inge(sag.) des vorderen und mittleren Lobus zusammen (M1 + M2). Abstract: The type of the widespread Ordovician agnostid trilobite Geragnostus, G. sidenbladhi (LINi'q'ARSSON,1869)from the upper Tremadocian Ceratopyge Limestone of Sweden, is redescribed and a lectotype is selected. Geragnostus HOWELL,1935 bears a strong similarity to Arthrorhachis HAWLE & CORDA,1847. Obviously the two genera are closely allied, and Geragnostus may eventually be regarded as a junior subjective synonym of Arthrorhachis. It is suggested, however, that the distinction between Geragnostus and Arthrorhachis might be upheld if the latter genus is taken to include metagnostids in which the posterior lobe (M3) of the pygidial rhachis is distinctly shorter than the length (sag.) of the anterior and middle lobes combined (M1 + M2).

Introduction Although the agnostid trilobites reached their maximum diversity during the Middle Cambrian and early Late Cambrian they are characteristic elements of many Ordovician shelly faunas, and range upwards into the pre-Hirnantian Ashgill. Ordovician agnostid trilobites have received increased attention during the past two decades and are now becoming better known (e. g. PEK 1977, CAVERAet al. 1978, FORTEY 1980, FORTEY& OWENS 1987, ZHOU 1987, AHLI3ERG1988a, 1988b). The bulk of the Ordovician species have been assigned to the genera Geragnostus and Arthrorhachis/Trinodus. FORTEY(1980: 27) restricted the genus Trinodus M'CoY, 1846 to the holotype of the type species and revived Arthrorhachis HAWLE COI~DA, 1847, a genus previously considered a junior synonym of Trinodus. This procedure is now generally accepted and is followed in this paper (cf. OWEN 1981; ZHOU ~ DEAN 1986, ZHOU 1987). The genus Geragnostus was erected by HOWELL (1935) and Agnostus sidenbladhi LINNARSSON, 1869 was designated as the type species. Since then, a great number of Lower and *Address of the author: PERArtL~ERG,Department of Historical Geology and Palaeontology, S61vegatan 13, S-223 62 Lund, Sweden.

0031-0220/89/0063-0309 $ 2.25 9 1989 E. Schweizerbart'sche Verlagsbuchhandlung, D-7000 Stuttgart 1

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Middle Ordovician species have been assigned to Geragnostus, and the genus has been recorded from many parts of the world. However, the taxonomic status and concept of Geragnostus have been discussed in the literature (e. g. DEAN 1966: 274, ZHOt5 ~ DEAN 1986: 748; ZI-tOU 1987: 657), and it is clear that the type species is in need of modern description and illustration to resolve some of the uncertainties surrounding the genus. This study includes an examination of all available museum material, and aims at revising the type species. The illustrated specimens are deposited in the type collections of the Geological Survey of Sweden, Uppsala (SGU), the Palaeontological Institute, University of Lund (LO), the Palaeontological Institute, University of Uppsala (PIU), and the Swedish Museum of Natural History, Stockholm (RM). Systematic palaeontology O r d e r Agnostida SALTER, 1864 S u b o r d e r A g n o s t i n a SALT~W, 1864 Family Metagnostidae JAEKEL, 1909 Genus Geragnostus HOWELL, 1935 Type species : AgnostussidenbladhiLINNARSSON,1869 (pp. 82-83, pl. 2, figs. 60-61); by original designatlon. R e m a r k s : The concept of Geragnostus was discussed at length by FORTEY (1980), who showed that the glabellar structure is fundamentally different from that of Micragnostus HOWELL, 1935, a genus sometimes considered a subgenus or even synonym of Geragnostus. Thus, a number of species previously assigned to Geragnostus should be referred to Micragnostus.

Geragnostus sidenbladhi (LINNARSSON, 1869) Figs. 1-3 1869 AgnostusSidenbladhi n. sp. - LINNARSSON:82-83, pl. 2, figs. 60-61. 1906 AgnostusSidenbladhi LINNARSSON MOBERG& SEGERBERG:77, pl. 4, fig. 1. 1906 AgnostusSidenbladhiLINNARSSONvar. urceolatusn. v. - MOBERG~ SEGEV,BE~G:77-78 [pars], pl. 4, figs. 2-3 only. non 1906 Agnostussidenbladhi, Linnarsson - LAKE:22-23, pl. 2, fig. 17. 1956 Geragnostussidenbladhi(LINNARSSON,1869)-T3ERNVIK:188-189, text-fig. 27A, pl. 1, figs. 5-6. ?1961 Geragnostussidenbladhi (LINNARSSON)1869 - BALASHOVA:112-113, pl. 1, figs. 3-4. non 1965 Geragnostussidenbladhi(LrNNARSSON)- BURSKIYin BONDAREVet al.: 37-39, pl. 10, figs. 9-10. 1987 Geragnostussidenbladhi (L1NNARSSON)- ZHOU, fig. 2. Lectotype: A nearly complete pygidium (SGU Type 25; Fig. 3A), illustrated by LINNARSSON(1869, pl. 2, fig. 61) and MOB~RG~ SEGERBEV,G (1906,pl. 4, fig. 1); selected herein. Although this specimen differs in some minor respects from LINNARSSONs illustration, it is almost certainly one of his two syntypes. It differs from his illustration in being slightly wider than it is long, and in having a slightly shorter (exsag.) anterior rhachial lobe (M1). In addition, the rhachis is slightly constricted at the middle lobe (M2) in the specimen to hand. An old label states that the specimen was collected by LINNAV,SSONhimself and that it is likely to be his original. I see no reason to think this statement is incorrect. A piece of the left anterolateral portion of the rhachis was lost during preparation. Type stratum and type locality: Dark grey limestone (Ceratopyge Limestone;Apatokephalus serratus Biozone of the upper Tremadoc) at Mossebo, Hunneberg, V~isterg6tland, south-central Sweden (see T31~RNWK1956:118-121 for locality description and references). Material: 14 cephala and 12 pygidia. -

D e s c rip t io n: The cephalon is gently to moderately convex, subquadrate in outline, subequal in length and width, and highest at the median glabellar node. The glabella, occupying 65-67 % of the total cephalic length, is moderately convex, nearly parallel-sided to gently tapered forward, broadly rounded anteriorly, and well defined by shallow dorsal furrows and an abrupt change in exoskeletal slope. It is 1.8 to 2.1 times as long as it is wide and faintly constricted slightly posterior to the mid-length. The glabellar median node is prominent, sagit-

Fig. 1. Geragnostus sidenbladhi (LINNARSSON,1869). Ceratopyge Limestone; Apatokephalus serratus Biozone of the upper Tremadoc. A-B, dorsal and right lateral views of a cephalon from Hunneberg (locality unknown), V~isterg6tland, south-central Sweden. Coll. G. C. y o n SCHMALENS~E1878, SGU Type 7886, x 14.2 (A), x 15.8 (B). C, dorsal view of a cephalon from Storeklev, Hunneberg, V~isterg6tland, south-central Sweden. Coll. G. HOLM 1893. SGU Type 7887, x 17.3. D-E, dorsal and left lateral views of a cephalon from Fagelsang, Scania, southern Sweden, LO 5950t, x 15.5 (D), x 15.9 (E). F-H, dorsal, right lateral, and left lateral views of a cephalon from F~igels~,ng,Scania, southern Sweden. LO 5951t, x 13.7 (F), x 16.2 (G-H). I-J, dorsal and right lateral views of a cephalon from Fagels~mg, Scania, southern Sweden. Coll. C. O. SEGEII.BERG.LO 5952t, X 15.7. K, dorsal view of a cephalon from the Oslo Region (locality unknown), Norway, RM Ar 36691, x 17.8.

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tally elongated, highest anteriorly, and situated slightly anterior to the midpoint of the glabella. In dorsal view, the rear of the glabella is nearly right-angled. The glabellar muscle insertion areas are indistinct. Immediately in front of the median node, however, there is a short, transverse furrow or depression which is continued into forward- and outward-curving shallow furrows. In some specimens there is an additional pair of furrows, directed backwards and outwards from the transverse furrow, faintly indicated. These outward-curving furrows define the anterior and posterior margins of the large anterior muscle impressions (6P) on the glabella (FoRTEY1980: 25). The basal lobes are entire, subtriangular, wider than they are long, and connected medially. The basal furrows are distinct. The acrolobe is subquadratic in outline and rounded anteriorly. The genae are smooth, subequal in width anteriorly and laterally, and slope downwards; most steeply postero-laterally where there is a gently inflated area. The border is relatively narrow, gently convex, and separated from the genae by a wide, shallow border furrow. It is widest anteriorly and anterolaterally, becoming narrower postero-laterally. Sagittally, the border and border furrow combined occupy 8-10 % of the total cephalic length. The posterior border is convex (exsag.) and defined by a deep border furrow. It slopes down beyond the fulcrum and is then curved forward to become the lateral border. The genal angles are pointed without spines. The thorax has not been recorded. The pygidium is of similar outline and convexity as the cephalon. The pygidial rhachis (excl. the articulating half-ring), occupying 50-64 % of the total pygidial length, is nearly parallel-sided, generally slightly constricted at the middle lobe (M2), broadly rounded or truncated posteriorly (generally with a faint median node), and 1.3 to 1.6 times as long as its maximum width. It is set off from the pleural fields by shallow dorsal furrows, and divided into three distinct lobes. The anterior rhachial lobe (M1) is a bit shorter (exsag.) and wider (tr.) than the middle lobe (M2). M1 and M2 are separated from each other by a furrow (F1) which is directed inward and slightly forward from the dorsal furrow, and then curved strongly forward adaxially. The posterior lobe (M3) is 1.3 to 1.6 times longer than M1 + M2, and separated from M2 by a transverse furrow (F2). The median ridge is distinct and rises on M2 to a prominent tubercle, which extends backwards above the anterior portion of M3 and forms the highest point of the pygidium. The pleural fields are smooth, subequal in width posteriorly and late-

Fig. 2. Cephala of Geragnostus sidenbladhi (LINNARSSON,1869) from the Oslo Region,Norway. Ceratopyge Limestone;Apatokephalus serratus Biozone.A, RM Ar 54288. Samsalin the Ringsaker district. Coll. G. HOLM1879; X 17.5.B, RM Ar 54298. Fure at Tyrifjordenin the Modum district. Coll. G. HOLM1879; X 17.5. C, RM Ar 54294. Slemmestadin the Oslo-Asker district. Coll. G. HOLM1877; X 17.4.

Ordovician agnostid trilobite Geragnostus HowzH,

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Fig. 3. Geragnostus sidenbladhi (LINNARSSON, 1869). Ceratopyge Limestone; Apatokephalus serratus Biozone. A, a nearly complete pygidium, original of LINNARSSON1869, pl. 2, fig. 61 and MOB~RG & SEGERB~RG 1906, pl. 4, fig. 1, and chosen here as lectotype. Mossebo, Hunneberg, V~isterg6tland, south-central Sweden. SGU Type 25, • 15.7. B, pygidium from Mossebo, Hunneberg, V~isterg6tland, south-central Sweden. Coll. T. TJERNVIK1952. PIU Vg 157, x 14.6. C-E, pygidia from F/tgels~ng, Scania, southern Sweden. Coll. C. O. SEGrRBERG. C, LO 5953t, • 15.4. D, LO 5954t, x 15.9. E, LO 5955t, • 16.7. F, pygidium from Bjerk~sholmen in the Oslo-Asker district, southern Norway. Coll. G. HOLM 1879. RM Ar 54300, x 16.4. G, pygidium from Hunneberg (locality unknown), V~isterg6tland, south-central Sweden. Coll. G. C. VON SCHMALENSEE1878, SGU Type 7888, • 15.9.

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Tab. 1. Selected cephalic dimensions of G. sidenbladhi. Measurements in mm. Sag. = sagittal. Lc = sag. length cephalon; Lbc = sag. length border (incl. border furrow); Lac = sag. length acrolobe; G = sag. length glabella; N = distance from rear of glabella to highest point of median node; Wc = max. width cephalon; Wg - max. width glabella.

LO 5951t RM Ar 54288 SGU Type 7886 LO 5952t RM Ar 54294 LO 1810t LO 5950t RM Ar 54298

Lc

Lbc

Lac

G

N

Wc

Wg

3.15

0.25

2.90

2.60 2.55 2.40 2.20 2.00

0.25 0.25

2.35 2.30 2.20 2.15 2.00 1.85

2.10 1.80 1.70 1.65 1.65 1.60 1.45 1.30

1.35 1.15 1.05 1.05 1.10 0.95 0.85 0.80

3.15? 2.30? 2.25 2.20 2.15

1.10 1.00 0.90 0.85 0.85 0.75 0.70 0.75

0.25 0.20 0.15

Tab. 2. Selected pygidial dimensions of G. sidenbladhi. Measurements in mm. Sag. = sagittal. Lpl = Sag. length pygidium (incl. articulating half-ring); Lp2 = sag. length pygidium (excl. art. half-ring); Lr = sag. length rhachis (excl. art. half-ring); Lbp ~ sag. length border (incl. border furrow); Wp = max. width pygidium; Wr = max width rhachis.

PIU Vg 157 RM Ar 54300 SGU Type 25 SGU Type 7888 LO 5953t LO 5954t PIU Vg 158

Lpl

Lp2

Lr

Lbp

Wp

Wr

2.40 2.10 -

2.95 2.55 2.50 2.40 2.15 1.90 1.80?

1.85 1.40 1.50 1.70 1.40 1.10 1.05?

0.35 0.30 0.20 0.20 0.20 0.20 0.20

3.00 2.80 2.80 2.60 2.35 2.25? 2.15

1.25 1.05 1.05 1.10 0.95 0.85 0.75

rally, and down-sloping (most steeply antero-laterally). In some specimens the region behind the rhachis is slightly inflated. The border is moderately wide, gently convex, widest posterolaterally, and separated from the pleural fields by a shallow border furrow. Postero-laterally there is a pair of strong, backwardly directed spines, commencing on a transverse line passing the posterior portion of the pleural field. The anterior border is narrow (exsag.), horizontal adaxially, down-sloping abaxially, and set off from the pleural field by a deeply incised furrow. The ring furrow is wide and deepens laterally into a pair of apodemes. Its posterior margin is curved backwards medially. The dimensions of the species are given in Tables 1 and 2. R e m a r k s : The cephalon illustrated by LINNARSSON (1869: pl. 2, fig. 60) cannot be located. However, in the collections of the Geological Survey of Sweden, Uppsala, there is a complete cephalon (numbered 08251 Trl), measuring 1.80 m m in length and collected by LINNARSSON from the type stratum and type locality. According to an old label and a label on the rock specimen, this cephalon may be his original. It differs, however, widely from LINNARSSON'S illustration and description, and from topotype material of the species, in being subcircular in outline, in having a highly convex acrolobe with strongly down-sloping genae, and in having a proportionately wider and slightly longer glabella with a distinct forward taper. In addition, the median glabellar node is in a more posterior position (at about the mid-length of the glabella), and I conclude that this specimen can hardly be the original of LINNARSSON (1869: pl. 2, fig. 60). It seems to represent Geragnostus c r a s s u s TJERNVIK,1956 or a closely comparable species.

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As revised here, G. sidenbladhi is a fairly variable species. In particular, modest variation in the shape of the pygidial rhachis can be observed. Much of this variation is due to variation in the shape and relative length of the terminal lobe of the rhachis. It is 1.3 to 1.6 times longer than the anterior and middle lobes combined, and ranges from truncated to broadly rounded posteriorly. LAKE(1906: 22-23, pl. 2, fig. 17) reported Geragnostus sidenbladhi from the Tremadoc of Wales. However, the specimen figured by him differs from Swedish material of the species in having a distinct glabellar furrow which makes a forward-pointing V in front of the median tubercle, and as suggested by FORTEY(1980: 27) it may be referable to Segrnentagnostus. Outside northern Europe, Geragnostus sidenbladhi has been reported from the Lower Ordovician of Kazakhstan (e. g. BALASHOVA1961) and the Novaya Zemlya-Pay Khoy region (BURS~IYin BONDAREVet al. 1965). The specimens from the Atkyubinsk region of Kazakhstan (BALASHOVA1961) are fairly well preserved but poorly illustrated. They appear, however, to differ from Scandinavian material of the species in having a glabella with a distinct forward taper, and a relatively wider and longer pygidial rhachis, and a subcircular cephalon. Thus, they may not be conspecific with G. sidenbladhi. The specimens from Pay Khoy (BURS~IYin BONDAR~Vet al. 1965) differ widely from G. sidenbladhi in having a considerably shorter and narrower glabella, a more elongate (sag.) cephalon, and a distinctly tapering pygidial rhachis, and I conclude that they do not represent G. sidenbladhi. O c c u r r e n c e" Ceratopyge Limestone, Apatokephalus serratus Biozone (upper Tremadoc). Sweden: Mossebo, Hunneberg, V~isterg6tland, south-central Sweden; Hlgels~ng, Scania, southern Sweden (locality H2b of MOBERG 1910: 75). Oslo Region, southern Norway (cf. FJELLDAL1966): Trefoldighetskirken (central Oslo) and Bjerk~lsholmen (Slemmestad) in the Oslo-Asker district; Fure at Tyrifjorden in the Modum district; Samsal in the Ringsaker district.

Taxonomic discussion The distinction between the genera Geragnostus and Arthrorhachis/Trinodus, as well as the proper assignment and distinctive characters of the numerous species referred to these genera, is difficult. The problems surrounding the discriminatory characteristics have been discussed extensively in the literature (e. g. WHITTINGTON1963, DEAN 1966, 1971, ROSS 1967, INGHAM 1970, OWEN~ BRUTON1980, FORTEY1980). Species with a long pygidial rhachis and a median transglabellar furrow are generally referred to Geragnostus, whereas species with a short pygidial rhachis and an effaced transglabellar furrow are referred to Arthrorhachis (sensu FORTEY 1980). However, the distinction between the two genera is arbitrary. Ross (1967) and INGHAM (1970) followed HOWELL(1935) in emphasizing the presence ofa transglabellar furrow in Geragnostus, whereas DEAN (1966, 1971) and WHITTINGTON(1968) emphasized the differences in length and shape of the pygidial rhachis. As noted by FORTEY(1980: 26), however, these characters are variable, even within some populations, and the characteristics of the two genera intergrade. The expression of the transverse furrow immediately in front of the median glabellar node is not of generic significance, because its presence or absence is affected by slight differences in the degree of effacement. Also, a transverse glabellar furrow may be effaced on the dorsal exoskeletal surface, but be prominent on the parietal surface (internal mould; cf. FORT~Y1980: 26). This is evident in a specimen referred to the type species ofArthrorhachis, A. tarda (BARRANDE, 1846), from the Ashgill of Scania, southern Sweden (AHLBERG1989: fig. 3A). The length and shape of the posterior rhachial lobe (posteroaxis) of the pygidium is a variable feature in many agnostids (cf. HUNT 1967: pl. 22, figs. 27, 29; FORTEY1980: 26). Yet the

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relative length of the pygidial rhachis is the most consistently cited feature for distinguishing between Geragnostus andArthrorhachis. FORTEY(1980) suggested that forms with the terminal lobe of the pygidial rhachis exceeding the length of the postrhachial region (sag.) inside the border should be assigned to Geragnostus. In the type species, G. sidenbladhi, however, the length of the terminal lobe may closely approach the length of the postrhachial region inside the border. This is especially evident in the lectotype (Fig. 3A). Consequently, Geragnostus may eventually be regarded as a junior subjective synonym of Arthrorhachis. However, I concur with FORTrY (1980: 26) that it is difficult to place species as different in pygidial proportions as, for instance, Arthrorhachis tarda and Geragnostus sidenbladhi or G. crassus TJERNVIK, 1956 in the same genus. Furthermore, synonymization of the much-used name Geragnostus with Arthrorhachis would cause a major change to long established procedure. Therefore, I suggest that the distinction between Geragnostus and Arthrorhachis can be maintained if the latter genus is taken to include species in which the posterior lobe (M3) of the pygidial rhachis is distinctly shorter than the length (sag.) of the anterior and middle lobes combined (M1 + M2).

Acknowledgements

Professor VALDARJAANUSSON,Stockholm, critically read the draft of the manuscript and suggested valuable improvements. Mrs NAIDAFAULKNER,Melbourne, kindly corrected the English. Mr. REzs6 LASZLOand Mr. THOMASNIHL~N,both of Lund, skillfully carried out the dark-room work. Financial support has been received from the Swedish Natural Science Research Council (NFR).

References

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HOWELL,B. F. (1935): Cambrian and Ordovician trilobites from H&ault, southern France. -J. Paleont., 9: 222-238; Menasha, Wisconsin. HUNT, A. S. (1967): Growth, variation, and instar development of an agnostid trilobite. -J. paleont., 41: 203-208; Tulsa, Oklahoma. INGHAM,J. K. (1970): The Upper Ordovician trilobites from the Cautley and Dent districts of Westmorland and Yorkshire. Part 1. - Palaeont. Soc. Mon.: 1-58; London. LAKE,P. (1906): A monograph of the British Cambrian trilobites. Part 1. - Palaeont. Soc. Mon.: 1-28 (part of volume for 1906); London. LINNARSSON,G. (1869): Om Vesterg6tlands cambriska och siluriska aflagringar. - Kongliga Svenska Vet. Akad. Handlingar, 8 (2): 1-89; Stockholm. MOBER(;,J. C. (1910): Guide for the principal Silurian districts of Scania (with notes on some localities of Mesozoic beds). - Geol. F6reningens i Stockholm F6rh., 32" 45-194; Stockholm. MOBERG,J. C. ~ SEGERBER6,C. O. (1906): Bidrag till kiinnedomen om Ceratopygeregionen med s~irskild h~insyn till dess utveckling i Fogels~ngstrakten.- Lunds Univ. A.rsskrift N. F., 2 (2): 1-116; also as Kongliga Fysiografiska Siillskapets Handlingar N. F., 17 (7): 1-116; Lund. OWEN, A. W. (1981): The Ashgill trilobites of the Oslo Region, Norway. - Palaeontographica Abt. A, 175" 1-88; Stuttgart. OWEN, A. W. ~ BI~t3TON,D. L. (1980): Late Caradoc-early Ashgill trilobites of the central Oslo Region, Norway. - Paleont. Contr. Univ. Oslo, 245: 1-42; Paleont. Mus. Oslo. PEIl, I. (1977): Agnostid trilobites of the central Bohemian Ordovician. - Sbornlk geol. ved. paleont., 19: 7-44; Prague. Ross, R. J., JR. (1967). Some Middle Ordovician brachiopods and trilobites from the Basin Ranges, western United States. - U.S. Geol. Survey Prof. Pap., 523-D: 1-43; Washington. TJERNVIK,T. E. (1956): On the Early Ordovician of Sweden - stratigraphy and fauna. - Bull. Geol. Inst. Univ. Uppsala, 36: 107-284; Uppsala. WHIttINGTON, H. B. (1963): Middle Ordovician trilobites from Lower Head, western Newfoundland. Bull. Mus. Comp. Zool., 129. 1-118; Cambridge, Mass. - (1968): The Ordovician trilobites of the Bala area, Merioneth. Part IV. - Palaeont. Soc. Mon.: 93-138; London. ZHOU ZHI-YI (1987): Notes on Chinese Ordovician agnostids. - Acta Palaeont. Sinica, 26" 639-661. [In Chinese with Engl. summary] ZHOU ZHI-YI ~ DEAN, W. T. (1986): Ordovician trilobites from Chedao, Gansu Province, north-west China. - Palaeontology, 29: 743-786; London. Eingang des revidierten Manuskripts bei der Schriftleitung am 10. 5. 1989.