Shetlandia multiplicata, Wrona, 2004

Wrona, Ryszard, 2004, Cambrian microfossils from glacial erratics of King George Island, Antarctica, Acta Palaeontologica Polonica 49 (1), pp. 13-56 : 34-36

Shetlandia multiplicata sp. nov.

Figs. 14, 15A–E, G–J, 16.

Holotype: Specimen shown in Fig. 14, ZPAL V. VI/36 S3 from erratic boulder Me66.

Type horizon: The inferred upper part of the Lower Cambrian.

Type locality: Erratic boulders of Antarctic origin. Me 33 in glacio−marine Cape Melville Formation (Lower Miocene), King George Island, South Shetland Islands, West Antarctica.

Derivation of name: From Latin multus, many, numerous; plicatus, folded. Referring to the surface sculpture with a number of prominent radial folds, a corrugation of the shell.

Diagnosis.—As for genus.

Material.—Four incomplete sclerites and a number of fragments from erratic boulders Me30, 32, 33, and 66. Figured specimens, ZPAL V. VI /25S2; 26S13; 32S5; 35S22; 36S3; 37S2; 39S7; 55S55; 105S7; 106S2; 108S4.

Description.—Minute phosphatic elements with left and right hand forms representing a flat sclerite. Their proximal end probably tapers to apices, whereas the wider distal part is almost flat (Figs. 14A–D, 15B–D) or convex externally (Fig. 14A, E). The phosphatic composition of the shells appears to be primary, as it is for all tommotiids (see Bengtson et al. 1990). The outer surface of the sclerite is radially multiplicated (densely folded) and covered with evenly spaced carinate growth ribs (Fig. 14I–K), which reflect the lamellar structure of the sclerite wall. The inner surface also shows transverse growth features (Fig. 14G, H, L), and the broken cross−section of the wall (Fig. 14L) shows the lamellar step−like mode of growth, i.e. the last growth layer is added to the underside of the previous one. No muscle scars on the inner side were observed. This arrangement of the lamellar structured wall is consistent with that characteristic for lapworthellids, and certainly other tommotiids too: incremental growth by basal−internal secretion ( Bengtson 1983; Landing 1984; Conway Morris and Chen 1990). In addition, the external cancellate sculpture (Fig. 14I–K) could be derived from a modified or distorted polygonal pattern probably representing epithelial imprints on the interior surface. Similar distortion of the external sculpture from regular polygons at the adapical part of the slerite up to the punctate or denticulate meshwork at the marginal part of the sclerite has been documented in other tommotiids, e.g., Dailyatia sclerites (Figs. 11A 5, A 6, 12 A 5), and Lapworthella ( Conway Morris and Chen 1990) . The polygonal pattern, possibly representing epithelial cell imprints, indicates also that S. multiplicata sclerites were secreted as in many other tommotiids by epithelial tissue partly mantling the basal margin of the sclerite exterior. These characters of the Shetlandia − type sclerites show their close relationship with Dailyatia − type sclerites, and it cannot be excluded that both sclerite types may belong to the same scleritome. The more complete sclerite assemblage or articulated sclerite arrays may allow a precise reconstruction of the relationship or synonymy of both genera.

Remarks.—Although a scleritome interpretation of disarticulated fossils is very difficult, it is not a futile enterprise, because scleritome models have been applied with some success in a number of enigmatic multisclerite−bearing animals (e.g., Bengtson and Conway Morris 1984; Evans and Rowell 1990; Bengtson 1992; Müller and Hinz−Schallreuter 1993; Conway Morris and Peel 1995; Bengtson and Hou 2001). The new

Fig. 12. A–D. Dailyatia ajax Bischoff. A. Strongly broken sclerite, form C, ZPAL V. VI /39S33, erratic Me66; A 1, anterior view; A 2, oblique view; Ą A 3, oblique posterior view; A 4, Detail of A 3, showing twinned apex with epithelial polygons; A 5, Ornamentation, detail of A 3. B. Fragment of plicate wall of the sclerite, ZPAL V. VI /47S1, erratic Me66; B 1, outer lateral view; B 2, ornamentation of plicate wall; B 3, detail of B 2, showing epithelial pattern. C. Fragment of the sclerite wall, ZPAL V. VI /47S2, erratic Me66; C 1, broken section of the wall, transverse to the sclerite longitudinal axis; C 2, detail of multilaminar structure on a slightly etched cross−section; C 3, the same fragment of the sclerite, showing longitudinal growth lines on the internal surface of wall; C 4, detail of C 3, showing longitudinal growth lines (arrowed) of the laminae building the sclerite wall. D. Asymmetric sclerite, form B, ZPAL V. VI /110S1, erratic Me66; D 1, apical view; D 2, oblique inner view; D 3, lateral view. E. Lapworthella fasciculata Conway Morris and Bengtson, ZPAL V.VI /39S4, erratic Me33, planiform sclerite in anterior view.

tommotiid type sclerites described herein represent mirror−image pairs (Fig. 16) with two kinds of such paired sclerites in the scleritome: a near planar one (Fig. 14A–D) and an externally more convex ( Fig. 15A, E). They are similar in mode of growth and morphology to machaeridian sclerites, especially to terminal (anterior and posterior) sclerites, though differing in their phosphatic composition. A small fragment of tommotiid sclerite densely covered with transverse coarse rugae on the outer surface ( Fig. 26B, C ) is superficially similar to early Palaeozoic machaeridian sclerites (e.g., Dzik 1986a: 5). The most plausible reconstruction of the tommotiid scleritome proposed earlier by Dzik could be applied also to S. multiplicata sclerites, and would be analogous to primitive plumulitid machaeridians having a bilaterally symmetric dorsal scleritome composed of two or four longitudinal series of calcitic sclerites ( Dzik 1986a, 1994). Those observations support a possible relationship between Tommotiida and Turrilepadida within the class Machaeridia, enigmatic Palaeozoic metazoans ( Bengtson 1970; Dzik 1986a).

Occurrence.—Allochthonous Early Cambrian (Botomian) boulders (Me30, 32, 33, 66), King George Island, Antarctica.

Figures

Fig. 26. A. Steinkern of initial part of Stapicyathus cera Debrenne archaeocyath cup, ZPAL Ac.I/53U7, erratic Me33.B, C. Problematic?tommotiid sclerite.B. Fragment of sclerite wall, ZPAL V.VI/49U4, erratic Me66. C. Fragment of sclerite wall, ZPAL V.VI/49U3, erratic Me66; C1, general view; C2, detail showing lamellar structure at the edge (arrowed). D, E. Aetholicopalla adnata Conway Morris. D. Specimen with external wall exfoliated, ZPAL V.VI/38S6, erratic Me32. E. Specimen ZPAL V.VI/38S1, erratic Me66; E1, oblique view, showing inner wall and tubules; E2, detail of surface of inner wall.