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Ceratretide brachiopods from the lower and middle Cambrian of Sweden, Kazakhstan, and Siberia a

Lars E. Holmer & Galina T. Ushatinskaya

b

a

Institute of Earth Sciences, Historical Geology & Palaeontology, Norbyvägen 22, Uppsala, S‐752 36, Sweden b

Palaeontological Institute, Akademia Nauk, Profsoyuznaya 113, Moscow, 117321, Russia Version of record first published: 09 Dec 2009.

To cite this article: Lars E. Holmer & Galina T. Ushatinskaya (1994): Ceratretide brachiopods from the lower and middle Cambrian of Sweden, Kazakhstan, and Siberia, GFF, 116:4, 203-210 To link to this article: http://dx.doi.org/10.1080/11035899409546184

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Article

volume 116 (1994), pp. 203-210.

Ceratretide brachiopods from the Lower and Middle Cambrian of Sweden, Kazakhstan, and Siberia LARS E. HOLMER and GALINA T. USHATINSKAYA

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Holmer, L.E. & Ushatinskaya, G.T., 1994: Ceratretide brachiopods from the Lower and Middle Cambrian of Sweden, Kazakhstan, and Siberia. GFF, Vol. 116 (Pt. 4, December), pp. 203-210. Stockholm. ISSN 11035897. Abstract: The poorly known Acrotreta [=Bozshakolia] eggegrundensis Wiman, 1903, is redescribed as the oldest known ceratretide from the upper Lower Cambrian of Sweden, and is compared with Bozshakolia coniformis Ushatinskaya, 1986, from the upper Middle Cambrian of Kazakhstan. The new ceratretide genus and species Erbotreta singularis is described from the lower Middle Cambrian of S iberia. The ceratretides evidently were separated already from other acrotretids during the Early Cambrian, but their relationships with other acrotretids are still poorly known. Keywords: Brachiopoda, Lingulata, Acrotretida, Ceratretidae, Early Cambrian, Middle Cambrian, Sweden, Kazakhstan, Siberia. Lars E. Holmer, Institute of Earth Sciences, Historical Geology & Palaeontology, Norbyvägen 22, S-752 36 Uppsala, Sweden. Fax: 018182749. Galina T. Ushatinskaya, Palaeontological Institute, Akademia Nauk, Profsoyuznaya 113, 117321 Moscow, Russia. Manuscript received 17 June 1994. Revised manuscript received 28 September and accepted 3 October 1994.

The Ceratretidae represents a small but quite distinctive family that is widely distributed in North America, Baltoscandia, Asia (Siberia, Kazakhstan, and Kirgizia), and Australia. The main range of the family is in the Upper Cambrian, but Ushatinskaya et al. ( 1986) and Roberts & M l (1990) extended the record to the Middle Cambrian. The main object of this paper is to redescribe Bozshakolia eggegrundensis (Wiman, 1903), which is the earliest known ceratretide from the upper Lower Cambrian of Sweden, and to draw comparisons with Bozshakolia coniformis Ushatinskaya, 1986, from the upper Middle Cambrian of Kazakhstan. Here we also describe a related new ceratretide genus and species Erbotreta singularis from the lower Middle Cambrian of Siberia. The origin and earliest evolutionary history of the ceratretides is still uncertain, and the relationship to the family Acrotretidae is also not entirely clear, but the new material indicates that the ceratretide clade with its distinctive characters originated quite early in the evolution of the Acrotretida.

of the boulder is the submarine sequence north of Gävle, but unfortunately, the lithology and fauna is not known in the nearby cores at Finngrundet and Västra Banken (see, e.g., Thorslund & Axberg 1979). The associated fauna from the boulder includes Vandalotretal sp., 'Westonia' bottnica (Wiman), Botsfordial sp. [fragments referred to by Wiman (1903, PL 2, fig. 9) as Kutorgina sp.], Torellella laevigata (Linnarsson), Hipponicharion matthewi Wiman, Oleneilida gen. et sp. indet., and Microdictyon effusum (see Bengtson et al. 1986). Few of these species seem to be of use for exact stratigraphie correlation; Walcott (1912, p. 684) tentatively considered the fauna to be of Middle Cambrian age. However, Bengtson et al. (1986) noted that Microdictyon is known almost exclusively from the Lower Cambrian, and outside Sweden, M. effusum is only known from the upper Lower Cambrian at Malyi Karatau, Kazakhstan. Thus, the age of the boulder is here tentatively considered as late Early Cambrian. Central Kazakhstan (Bozshakol; Fig. 1C). - Bozshakolia coniformis was described by Ushatinskaya (in Ushatinskaya et al. 1986) from the upper Middle Cambrian (Mayan), Kysyl-Kojandy Formation, Shiderty-Olenty rivers, 4 km south-west of the town of. Bozshakol, central Kazakhstan (Fig. 1C). The associated fauna includes the trilobites Basocephalus nominalis l\shin,Inogellaspis inexpectans Ivshin, Volonellus inflatus Ivshin, Corynexochus sp., Anomocare sp., Dorypyge sp., Solenopleura sp., and Semisphaerocephalus sp., as well as the brachiopods Prototreta mímica Beü,Hadrotretafragilis'Ushd.unskaya,Neotreta'}[=Stilpnotreta?] pusilla Ushatinskaya, and Acrothele sp. Siberia (Sladky Korenya; Fig. 1C). -Erbotreta singularis gen. et sp. nov. comes from the Middle Cambrian (Amgian), Sladky Korenya Formation (Oryctocephalus-Schistocephalus Biozone), at the Batenev ridge in the Sladky Korenya Mountains, southwestern Siberia (Fig. 1C). The associated fauna was described by Ushatinskaya (1992) and includes Chakassilingula erbiensis Ushatinskaya, Kyrshabactella rectangulata Ushatinskaya, K. tatjanae Ushatinskaya, Homotreta aksarinae Ushatinskaya, Batenevotretaformosa Ushatinskaya, Acrothele exquisita Aksarina, Dictyonina pannula sibirica (Lermontova), Nisusia minusensis Lermontova, and Wimanella sinuata Aksarina.

Systematic palaeontology Localities and stratigraphy Sweden (Eggegrundisland;Fig. IB). - The glacial erratic boulder containing Bozshakolia eggegrundensis (Wiman) was collected by Wiman (sample 'Eggegrund no. 3 ' in Wiman 1903, p. 55) from Eggegrund island, in the South Bothnian Sea outside Gävle (Fig. IB). The boulder consists of a soft, calcareous, coarse-grained, partly bituminous quartz sandstone. The only possible source rock

Measurements (in millimetres, if not stated otherwise) have been performed on valves (oriented in the way shown by Holmer & Popov 1990, fig. 3) as follows: W, L, H = width, length, height of valve; WI, LI = width, length of dorsal pseudointerarea; WG = width of median groove; WM1, LM1 = width, length of cardinal muscle scars. The studied material is housed in the Department of Palaeo-

204 Holmer & Ushatinskaya: Ceratreîide brachiopods from the Lower and Middle Cambrian

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drotreta Rowell, 1966, and Vandalotreta Mergl, 1988, in lacking a dorsal median ridge. Moreover, the larval shell of the latter genera also appears to have a pedicle notch similar to that of Bozshakolia; however, the ridge-like, ventral apical process is not developed mHadrotreta and Vandalotreta. The dorsal valve of Linnarssonella also lacks a median ridge but it possesses grooved propareas, furthermore the pedicle foramen is enclosed within the larval shell.

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Occurrence. -Upper Lower-upper Middle Cambrian; Kazakhstan, Sweden.

Bozshakolia coniformis Ushatinskaya in Ushatinskaya et al, 1986 Figs. 2, 5. Synonymy.-1986Bozshakolia coniformis gen. et sp. nov. Ushatinskaya in Ushatinskaya et al., p. 38, pi. 3:15-23,4:11,12. Holotype. - PIN 4113/28, dorsal valve, figured by Ushatinskaya et al., 1986, pi. 4:21.

Fig. 1. A. Position of maps in B and C. B. Location of Eggegrund island. C. Location of Bozshakol locality in Kazakhstan and Sladky Korenya in Russia.

Material. - Five figured valves: PIN 4113/ 31, dorsal valve; PIN 4113/13, dorsal valve; PIN 4113/14, dorsal valve; PIN 4113/2, ventral valve; PIN 4113/5, ventral valve. All from the type locality (see above for locality data and stratigraphy). Measurements. - See Table 1.

zoology, Swedish Museum of Natural History, Stockholm (RM); Palaeontological Museum, Uppsala (PMU); Palaeontological Institute, Akademia Nauk, Moscow (PIN).

Bozshakolia Ushatinskaya, 1986 Systematic position. - Class Lingulata Gorjansky & Popov, 1985; Order Acrotretida Kuhn, 1949; Superfamily Acrotretoidea Schuchert, 1893; Family Ceratretidae Rowell, 1965. Type species. - By original designation; Bozshakolia coniformis Ushatinskaya in Ushatinskaya et al., 1986, p. 38. Diagnosis. - Ventral pseudointerarea procline to catacline or apsacline with inter-

trough; pedicle foramen small, subcircular to elongate suboval; apical process ridgelike bridging the posterior and anterior slopes. Dorsal median ridge vestigial or absent. Species assigned. -Type species andAcrotreta eggegrundensis Wiman, 1903. Discussion. — Bozshakolia is similar to Ceratreta, Keyserlingia, andKleithriatreta in having a high, ridge-like apical process with a thickened muscle platform; it differs from all three genera by having a shorter pedicle foramen placed in a narrow intertrough. Morover, Bozshakolia differs from Ceratreta and Klethriatreta by its lower and shorter dorsal median septum and from Keyserlingia by the lack of a listrium-like pedicle track and a lamellose, thickened ventral margin. The genus is somewhat similar to Ha-

Diagnosis.-Shell transversely subcircular. Ventral valve about half as high as wide; ventral pseudointerarea procline to catacline with shallow intertrough; apical process with well developed, wide muscle platform. Dorsal valve with low median ridge; median buttress broadly triangular. Description.—Shell transversely subcircular. Ventral valve on average 78 per cent as long as wide and 48 per cent as high as wide (Table 1). Ventralpseudointerareaprocline to catacline, with narrow intertrough, somewhat tapering towards apex; pedicle foramen elongate oval, on average 0.18 mm long and 0.09 mm wide (Table 1; Fig. 2F). Ventral interior dominated by long ridgelike apical process, bridging anterior and posterior slopes, with strongly thickened muscle platform in central part; pedicle tube placed directly behind muscule platform; apical pits well defined, placed on posterior

Holmer & Ushatinskaya: Ceratretide brachiopods from the Lower and Middle Cambrian 205

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Fig. 2. Bozshakolia coniformis Ushatinskaya, 1986. A. Dorsal exterior, PIN 4113/31, x34. B. PIN 4113/13, dorsal interior, x35. C. Dorsal interior, PIN 4113/14, x48. D. Detail of pseudointerarea of C, x85. E. Ventral interior, PIN 4113/2, x45. F. Posterior view of ventral exterior, PIN 4113/5, x36. All from the type locality.

slope, directly lateral to apical process; ventral cardinal muscle scars well defined, thickened; ventral vascula lateralia, diverging anterolaterally between apical process and cardinal muscle scars (Fig. 2E). Dorsal valve gently convex in lateral profile. Dorsal pseudointerarea on average 20 per cent as long as wide (Table 1 ), occupying about half of valve width; with wide, deep median groove and anacline propareas; median buttress broadly triangular; dorsal median ridge low, originating at about 1 mm from mid-posterior margin; dorsal cardinal muscle scars on average 38 per cent as long as wide (Table 1 ), occupying about 3/5 of width, and extending

Table 1. Bozshakolia coniformis Ushatinskaya, 1986; average dimensions and ratios of dorsal and ventral valves. dorsal n x min. max. ventral n x min. max.

W 2.87 2.30 3.50 W 2.55 2.30 2.80

L L/W 9 9 9 2.34 81% 1.80 78% 3.20 91% L L/W 2 2 2 2.01 78% 1.75 76% 2.40 85%

WI

LI

7 1.45 1.00 1.80 WP 3 0.09 0.08 0.10

7 0.29 0.20 0.35 LP 3 0.18 0.15 0.20

LI/WIWM 7 7 20% 1.94 19% 1.50 20% 2.30 H H/W 5 2 1.22 48% 0.90 39% 1.40 50%

LM 5 0.74 0.70 0.80

LM/WM 5 38% 34%

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206 Holmer & Ushatinskaya: Ceratretide brachiopods from the Lower and Middle Cambrian

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F/g. 3. Bozshakolia eggegrundensis (Wiman, 1903). A. Dorsal exterior, PMU B593, x57. B. Oblique lateral view of A, x67. C. Detail of larval shell of A, xl65. D. Oblique lateral view of dorsal interior, PMU B594, x49. E. Oblique lateral view of dorsal psedointerarea, PMU B595, x l 80. F. Ventral exterior, PMU B596, x35. G. Oblique posterior view of F, x38. H. oblique lateral view of F, x35.1. Detail of ventral larval shell and ventral pseudointerarea, PMU B598, xl35. J. Ventral interior, PMU B597, x45. All from the type locality.

1/3 of total valve length (Fig. 2B-D). Larval shell poorly preserved on both valves, but apparently with marginal pedicle notch in the ventral larval shell. The larval pitting is not well preserved.

55, pl. 2:23-29. »1912 Acrotreta eggegrundensis Wiman Walcott, p. 684, pl. 70:2a-d. «1942 ?Acrotreta cf. eggegrundensis Wiman - Poulsen, p. 215, figs. 12-16.

Occurrence. - Type locality only.

Lectotype. - Here selected; PMU B39; ventral valve (W 2, L1.68, H 0.8); figured by Wiman (1903, pl. 2:26, 27, 29) and Walcott (1912, pl. 70:2a-c).

Bozshakolia eggegrundensis CWiman 190"^ Fies -\ s g J D > Synonymy. - - 1 9 0 3 Acrotreta eggegrundensis n. sp. Wiman, p.

Material. - Six figured valves; PMU B593, dorsal valve; PMU B594, dorsal valve; PMU B595, dorsal valve; PMU B596, ventral valve; PMU B598, ventral valve; PMU B597, ventral valve All frOmthet elocalit ^ y ( s e e a b o v e for locality data and stratigraphy).

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Holmer & Ushatinskaya: Ceratretide brachiopods from the Lower and Middle Cambrian 207

Table 2. Bozshakolia eggegrundensis (Wiman, 1903); average dimensions and ratios of dorsal and ventral valves. W 5 X 1.51 min. 0.96 max. 2.08 ventral W 2 n X 1.74 min. 1.48 max. 2.00

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dorsal n

L 4 1.37 0.84 1.92 L 2 1.50 1.32 1.68

LAV 4 89% 83% 92%

LAV 2 87% 84% 89%

WI 3 0.81 0.68 0.96 H 2 0.70 0.60 0.80

LI 3 0.16 0.12 0.20 HAV 2 40% 40% 41%

LIAVI 3 19% 18% 21%

WIAV 3 62% 48% 71%

WG 3 0.40 0.32 0.48

WGAVI WM1 1 3 0.80 49% 47% 50%

LM1 1 0.52

WM1AV 1 48%

Measurements. - See Table 2.

Erbotreta gen. nov.

Diagnosis. - Shell transversely suboval. Ventral valve about half as high as wide; ventral pseudointerarea strongly apsacline with widely triangular intertrough; apical process lacking well developed muscle platform. Dorsal median ridge vestigial; median buttress poorly defined.

Name. - Alluding to the village of Erba, situated close to the type locality; Greek tretos, perforated.

Description. - Shell transversely suboval (Fig. 3A, F). Ventral valve on average 87 per cent as long as wide and 40 per cent as high as wide (Table 2), strongly recurved in lateral view with umbo extending about 1 mm beyond posterior margin (Fig. 3H). Ventral pseudointerarea strongly apsacline with widely triangular intertrough. Pedicle foramen elongate oval, around 0.09 mm wide and 0.14 mm long (Fig. 3G, I). Ventral interior with low ridge-like apical process, bridging anterior and posterior slopes; well defined muscle platform lacking, but some possible muscle scars can be observed on anterior slope, directly anterior to apical process; apical pits poorly defined; ventral cardinal muscle scars well defined, but not strongly thickened; ventral vascula later alia, poorly defined (Fig. 3J). Dorsal valve on average 89 per cent as long as wide (Table 2), gently convex in lateral profile (Fig. 3B). Dorsal pseudointerarea on average 19 per cent as long as wide (Table 2), occupying on average 62 per cent of valve width (Table 2); median groove very wide occupying about half of width of pseudointerareas with anacline propareas (Fig. 3D, E). Median buttress poorly defined (Fig. 3E); dorsal median ridge vestigial to absent; dorsal cardinal muscle not defined in most valves, occupying about half of width, and extending 1/3 of total valve length in one valve. Larval shell comparatively well defined, nearly circular, around 0.2 mm across; ventral larval shell with extremely small and shallow pedicle notch; larval pitting not observed (Fig. 3C, I). Discussion. -B. eggegrundensis is similar only to the type species. In particular the dorsal valves of both species are similar in ornamentation, pseudointerareas and interior characters. The ventral valve of B. eggegrundensis differs mainly from B. coniformis in being apsacline and strongly recurved, and in having a smaller and lower apical process. The specimens of Acrotreta cf. eggegrundensis described by Poulsen (1942) from the Exulans Limestone on Bornhohn might possibly be conspecific, but a detailed comparison cannot be attempted in view of the poor preservation. Occurrence. - Type locality only.

Type and only species. - Erbotreta singularis sp. nov. Diagnosis. — Shell subrectangular. Ventral valve highly conical; ventral pseudointerarea, procline to catacline, divided by broad, weakly defined intertrough; pedicle foramen elongate lenticular but not placed in listrium-like groove; ventral interior with apical process occluding apex, penetrated by pedicle tube. Dorsal valve shallow, with poorly developed median ridge; dorsal pseudointerarea with wide median groove and narrow propareas.

Erbotreta singularis sp. nov. Figs. 4,5. Name. - From Latin, singularis, singular; different. Holotype.-PJN43n/5 (W2.65,L2.50,H 1.70), complete ventral valve (see above for locality data and stratigraphy). Material. - Total of 8 figured valves: PIN 4377/1, dorsal valve; PIN 4377/9, dorsal valve; RM Br 136340, dorsal valve; PIN4377/ 6, ventral valve; PIN4377/14, ventral valve; RMBr 136342, ventral valve; RM Br 136341, ventral valve. All from the type locality (see above for locality data and stratigraphy). Measurements. - See Table 3. Diagnosis. - As for the genus.

Table 3. Erbotreta singularis gen. et sp. nov.; average dimensions and ratios of dorsal and ventral valves. dorsal n x min. max. ventral n x min. max.

W 2.97 2.50 4.20 W 10 2.87 1.90 4.20

L LAV 8 5 5 2.58 87% 1.90 76% 3.30 101% L LAV 4 4 2.71 94% 1.90 71% 3.20 137%

WI 5 1.76 1.60 2.20 H 6 1.55 1.00 2.10

LI

LIAVI WM 5 5 5 0.39 22% 1.56 0.20 12% 1.45 0.45 27% 1.75 HAV 6 54% 50% 70%

LM 5 0.42 0.30 0.55

LM/WM 5 26% 20% 31%

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208 Holmer & Ushatinskaya: Ceratretide brachiopods from the Lower and Middle Cambrian

/g. 4. Erbotreta singularis gen. et sp. nov. A. Dorsal exterior, PIN 4377/1, xl6. B. Dorsal interior, PIN 4377/9, x20. C. Detail of dorsal apex with larval shell, RM Br 136340, xl20. D. Lateral view of ventral exterior, PIN 4377/6, xl8. E. Holotype, posterior view of ventral exterior, PIN 4377/ 5, x27. F. Ventral interior, PIN 4377/14, x44. G. Ventral exterior, RM Br 136342, x20. H. Detail of pedicle opening, RM Br 136341, x97.1. Detail of larval shell and pedicle opening of H, xl35. All from the type locality.

Description. - Shell subrectangular (Fig. 4B), with ornamentation of closely spaced, well developed rugellae, up to about 0.1 mm apart (Fig. 4A). Ventral valve on average 94 per cent as long as wide and 54 per cent as high as wide (Table 3). Ventral pseudointerarea well-defined, flattened, catacline to slightly procline

divided by broad, weakly defined intertrough. In lateral view the ventral anterior slope is somewhat concave proximally becoming convex distally (Fig. 4D, E). Pedicle foramen elongate lenticular, on average 0.28 mm long and 0.14 mm wide (Table 3), not placed in listrium-like groove (Fig. 4H). Ventral interior with massive

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apical process, occluding apex.penetratedbyinteriorpedicle tube; ventral cardinal muscle scars thickened placed on postero-lateral slope; apical pits closely spaced, placed on posterior slope directly lateral to apical process. Ventral vascula lateralia well defined, diverging antero-laterally from apex (Fig. 4F). Dorsal valve on average 87 per cent as long as wide (Table 3), flattened in lateral view with shallow median sulcus. Dorsal pseudointerarea narrow occupying somwhat more than half of the total width, on average 22 per cent as long as wide, with narrow, anacline propareas and wide, deep median groove. Median ridge poorly developed, short and low. Dorsal cardinal muscle scars on average 27 per cent as long as wide (Table 3), occupying around half the valve width and 1/6 of the valve length (Fig. 4B). Larval shell of both valves well defined, transversely oval, around 0.25 mm wide, and 0.2 mm long; ventral larval shell with extremely small and shallow pedicle notch (Fig. 4C, I); larval pits of one size, up to 1 \im across. Discussion. - E. singularis differs from all other ceratretides in having an extremely wide apical process occluding the apex. It is similar to species ofBozshakolia in having a weakly defined dorsal median ridge. The highly conical ventral valve is most similar to species of Ceratreta.

Remarks on ceratretide relationships and evolution The ceratretides have usually been placed previously as a subfamily within the Family Acrotretidae (Rowell 1965), and Williams & Rowell (1965) proposed that they were derived from an ancestor within the Acrotretinae during the Late Cambrian; in view of the new material, it now seems that the ceratretides were separated already from other acrotretids during the Early Cambrian. The ceratretides differs from the Acrotretidae mainly in having: ( 1 ) an exceptionally well-developed, ridge-like ventral apical process, which sometimes forms a high median septum, with a wide muscle platform, (2) exceptionally large ventral and dorsal cardinal muscle scars, which sometimes are thickened to form platform-like elevations, in addition to (3) a distinctive, lenticular pedicle foramen, continued as a long, internal pedicle tube. Moreover, unlike most members of the Acrotretidae, which have a pedicle foramen enclosed within the larval shell, the ventral larval shell of the ceratretides has a marginal, posteriorly unrestricted pedicle notch. In view of this, the subfamily was elevated to family rank by Ushatinskaya (in Ushatinskaya et al. 1986), and this proposal was also followed by Holmer& Popov (1990) andSobolev (1992). The earliest known representatives, Bozshakolia and Erbotreta, described above are somewhat similar to the Cambrian acrotretides Hadrotreta Rowell, 1966, and VandalotretaMergl, 1988,in being characterized by a poorly developed or completely lacking dorsal median septum (Fig. 5). The latter two genera might be related somehow to the ceratretides, but the relationships between different groups of Early Cambrian acrotretids are still very poorly known. The lack of an enclosed pedicle foramen in all ceratretides as well as inHadrotreta and Vandalotreta is probably aretained primitive character; it is likely that the acrotretides evolved directly from a lingulid ancestor, and an evolutionary scheme for the derivation of the acrotretide type of morphology from a lingulide ancestor was proposed by Popov (1992). The long internal pedicle tube, and ridge-like apical process are well developed in Bozshakolia and Erbotreta (Fig. 5), but the

.Q CO

Bozshakolia coniformis

Ü

Ü? •o •o

Erbotreta singularis

.5? .Q CO

Ü

Bozshakolia eggegrundensis

Fig. 5. Schematic cross-section of the discussed species, comparing development of dorsal median septum, apical process and pedicle tube.

more typical ceratretide characters including thickened cardinal muscle scars and well developed dorsal median septum first appear in the early Middle Cambrian Kleithriatreta Roberts in Roberts & Jell, 1990, as well as in the Late Cambrian Ceratreta Bell, 1941, and Keyserlingia Pander, 1861 [senior synonym of Clistotrema Rowell, 1963]; in these three genera the exterior pedicle opening also is enlarged and elongated as compared with Bozshakolia and Erbotreta. The family apparently became extinct by the earliest early Ordovician (Popov et al. 1989). The genus Acrotr eta Kutorga, 1848, is the only Ordovician acrotretoid that is somewhat similar to the ceratretides; it has a marginal pedicle notch in the ventral larval shell and the apical process in the type species, A. subconica, is ridge-like and has a wide muscle platform, which is similar to that of many ceratretides (Holmer & Popov 1994). Acknowledgements. - We are grateful to Albert J. Rowell (Kansas) for comments on the language and scientific content of the manuscript. The work has been supported by grants (to L.E.H.) from the Swedish Natural Science Research Council (NFR). G.T.U. gratefully acknowledges a grant from the Department of Palaeozoology, Swedish Museum of Natural History, Stockhohn, that enabled her to work in Stockholm and Uppsala during two weeks.

References Bell, W.C., 1941: Cambrian Brachiopoda from Montana. Journal of Paleontology 15, 193-255. Bengtson, S., Crosbie Matthews, S. & Missarzhevsky, V.V., 1986: The Cambrian netlike fossil Microdictyon. In A. Hoffman & M.H. Nitecki (eds.): Problematic Fossil Taxa, 97-115. Oxford University Press. Gorjansky, V.J. & Popov, L.E., 1985: Morphology, systematic position and origin of the inarticulate brachiopods with carbonate shells [Morfologiya, sistematicheskoe polozheenie i proiskhozhdenie bezzamkovykh brakhiopod s karbonatnoj rakovinoj]. Paleontologicheskij Zhurnal 1985, 3-14. Holmer, L.E. & Popov, L. E., 1990: The acrotretacean brachiopod Ceratreta tanneri (Metzger) from the Upper Cambrian of Baltoscandia. Geologiska Föreningens i Stockholm Förhandlingar 112, 249-263. Holmer, L.E. & Popov, L.E., 1994: Revision of the type species of Acrotreta and related lingulate brachiopods. Journal of Paleontology 68, 433-450.

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