Similotheca similis? (Walcott, 1890)

Malinky, John M. & Skovsted, Christian B., 2004, Hyoliths and small shelly fossils from the Lower Cambrian of North-East Greenland, Acta Palaeontologica Polonica 49 (4), pp. 551-578 : 553-554

Similotheca similis? (Walcott, 1890)

Fig. 2A .

Hyolithes similis Walcott ; Walcott 1890a: 38.

Hyolithes similis Walcott ; Walcott 1890b: 622, pl. 75: 3a–d. Hyolithes similis Walcott ; Sinclair 1946: 80.

Similotheca similis (Walcott) ; Malinky 1988: 225, figs. 1.1–1.6, 1.8.

Lectotype.— USNM 18319 −A (see Malinky 1988).

Material.— MGUH 3535 ( Fig. 2A = Poulsen 1932: pl. 4: 1).

Description.—Conch with generally small apical angle creating slender appearance; dorsum seemingly high with ridge marked by blunt keel−like termination; adjacent flanks dip steeply away from the dorsal keel; flanks are flat to very slightly concave near the middle of the dorsum, and then become inflated and convex toward the lateral edges. Aperture orthogonal with shallow indentation in the middle of dorsal rim. Shell has three low and faint, widely spaced longitudinal ribs on each flank; ribs subdivide slightly more prominent transverse ribs to create a cancellate pattern on dorsum.

Remarks.—This species is represented by one specimen from the Bastion Formation which is an external mould of the dorsum; the apical and apertural ends are missing, and no traces of the venter remain. Poulsen (1932) referred it to Walcott’s species with question, owing largely to the ornament on the shell. The slightly concavo−convex dorsal flanks noted above, which is considered to be a diagnostic trait of the genus ( Malinky 1988), further support this assignment. However, the species’ identification remains in question due to the incomplete nature of the specimen. As noted by Poulsen (1932) the number of longitudinal ribs on the flanks of the dorsum of the Greenland specimen differs from the type material, in which four ribs are present. The concavity near the dorsal keel is also less developed than that seen among the types, but this may also be attributed to intraspecific variation until additional specimens from Greenland demonstrate otherwise.

The synonymy list given for a particular taxon ordinarily is taken to denote which specimens belong to that taxon. Herein the list is intended to indicate how widespread were the reports of a particular taxon, and not necessarily which specimens belong to it.

Stratigraphic range and distribution.—Lower Cambrian, southeastern Newfoundland and North−East Greenland.

Figures

Fig. 2. A. Similotheca similis? (Walcott, 1890), MGUH 3535 from the collections of C. Poulsen; A1, external mould of dorsum; A2, latex cast of dorsum. B, C. Similotheca bastionensis sp. nov. B. MGUH 27082, holotype; B1, dorsum; B2, right lateral edge; B3, venter; B4, view of dorsum from apex; B5, cross−section and view of dorsum from aperture. C. MGUH 27083; C1, cross−section and view of dorsum from aperture; C2, dorsum; C3, right lateral edge; C4, venter. D. Similotheca groenlandica sp. nov., MGUH 27084, holotype; D1, cross−section and view of dorsum from aperture; D2, left lateral edge; D3, dorsum; D4, oblique view of venter. All, except A, from GGU sample 314835. Scale bars 0.2 mm, except A1 and A2 which are 0.5 mm.