Pinnocaris, , Engeser and Riedel, 1996

Peel, John S., 2004, Pinnocaris and the origin of scaphopods, Acta Palaeontologica Polonica 49 (4), pp. 543-550 : 545

Protoconch of Pinnocaris

The protoconch of Pinnocaris lapworthi is a minute, isostrophic shell that is coiled through a full whorl and has open umbilici ( Figs. 1A, B , 2 ); it is not cup−shaped as assumed by Engeser and Riedel (1996). The height of the protoconch measured perpendicular to the dorsum of the adult is about 0.5 mm and its length is between 1 and 2 mm. Shell coiling expands towards the attenuated termination of the adult shell, such that the sub−apical surface in lateral perspective corresponds to the relatively short distance to the broadly rounded termination of the adult shell. The supra−apical surface is long, forming the dorsal surface towards the attenuated termination.

Two specimens with well−preserved protoconchs have been observed in a collection of more than 50 specimens preserved in the Natural History Museum, London. Both specimens form part of the Mrs Robert Gray collection and were collected from the Ardmillan Group at Balclatchie, Girvan, southern Scotland. Specimen NHM In 20372 was figured by Jones and Woodward (1895: pl. 15: 8, 9) and by Pojeta and Runnegar (1976: pl. 9: 13, 14), but accompanying descriptions make no reference to the well−preserved coiled protoconch. The external mould and the corresponding internal mould figured by Jones and Woodward (1895: pl. 15: 8, 9, respectively) represent the supposed left valve, with the protoconch thus lying close to the interpreted anterior margin. Pojeta and Runnegar (1976, fig. 13) figured the same external mould and a latex replica of this specimen (their fig. 14), which they also interpreted as a left valve. While the illustrations of Jones and Woodward (1895) indicate that the specimen is preserved in a small slab, those of Pojeta and Runnegar (1976) have been trimmed artificially free from matrix. The specimen is 29 mm long and wedge−shaped, and broadly rounded at one termination which was considered anterior by Jones and Woodward (1895) and Pojeta and Runnegar (1976); the shell narrows uniformly to the opposite, extended and attenuated, termination ( Fig. 1A 1 ). The protoconch of NHM In 20372 is visible on both the external and internal moulds although only the latter, with adherent patches of thin shell ( Fig. 2 ), is figured here. The height of the protoconch is about 0.5 mm and it has a length of between 1 and 2 mm, the precise dimension being uncertain on account of the gradual supra−apical transition towards the attenuated termination of the adult. The sub−apical transition from the protoconch to the adult shell is abrupt. The internal mould of NHM In 20372 shows a prominent radial channel representing the plate−like pegma which united the opposing lateral areas in the apical region. The pegma approaches the dorsal surface at the junction between the protoconch and the sub−apical surface and does not affect the protoconch ( Figs. 1A 2, 2 ). The groove it generates on the internal mould does not cross the dorsum.

NMH In 20400 is a partially crushed specimen with the tip of the attenuated termination and much of the margin below the apex broken away ( Fig. 1B 1 ). The protoconch is seen from the opposite side to NHM In 20372 and its external surface is apparently preserved. The supra−apical junction with the adult shell is a line of fracture with the adult shell depressed relative to the protoconch ( Fig. 1B 2 ). The protoconch is of similar size and shape to that in NHM In 20372 but less well−preserved. While the pegma is developed on the shell interior, its location on the external surface of NMH In 20400 is indicated by a dark, shallow depression in Fig. 1B 1 .

In general style of coiling, the isostrophically coiled protoconch of Pinnocaris lapworthi resembles the adult shells of a variety of helcionelloids, cyrtonellid Tergomya and bellerophontoidean gastropods, although these are variously considered to be exogastrically or endogastrically coiled (see discussion in Peel 1991a and Runnegar 1996).

Figures

Fig. 1. Pinnocaris and Eotebenna. A, B. Pinnocaris lapworthi Etheridge, 1878, Ordovician, Girvan, southern Scotland; A1, A2, lateral view of internal mould of partially crushed specimen with adherent shell patches showing the pegma and the coiled protoconch, NHM In 20372, scale bars: A1 3 mm, A2 1 mm; B1, B2, lateral view of partially crushed specimen showing the coiled protoconch, NHM In 20400; scale bars: B1 3 mm, B2 1 mm. C. Eotebenna viviannae Peel, 1991, Middle Cambrian, Andrarum Limestone, Bornholm, Denmark, right side of phosphatic internal mould showing the protoconch, MGUH 19.565, scale bar 100 µm. D. Yochelcionella greenlandica Atkins and Peel, 2004, Lower Cambrian, Aftenstjernesø Formation, Peary Land, North Greenland, lateral view of right side showing the prominent snorkel, MGUH 27.018 from GGU sample 271470, scale bar 200 µm. E. Eotebenna arctica Peel, 1989, Lower Cambrian, Henson Gletscher Formation, Freuchen Land, North Greenland, lateral view which has been horizontally mirrored to enhance comparison, MGUH 18.701 from GGU collection 315109, scale bar 1 mm.

Fig. 2. Pinnocaris lapworthi Etheridge, 1878, Ordovician, Girvan, southern Scotland, lateral view of internal mould, showing fine comarginal ornamentation on adherent patches of thin shell on the sub−apical surface, NHM In 20372, scale bar 2 mm.