Similotheca groenlandica, Malinky & Skovsted, 2004

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 : 555

Similotheca groenlandica sp. nov.

Fig. 2D .

Holotype: MGUH 27084 from GGU sample 314835.

Type horizon: Bastion Formation, Lower Cambrian.

Type locality: North−East Greenland.

Etymology: Occurring in Greenland.

Diagnosis.— Similotheca with dorsum having one narrow, keel−like ridge on each flank and coarse, widely spaced rugae.

Description.—Conch seemingly orthoconic with large apical angle such that conch expands rapidly; venter inflated into broad, rounded surface; lateral edges sharp and keel−like; dorsum high and inflated with median ridge marked by central longitudinal keel−like rib and one similar keel in the middle of each flank; surface of flank is concave next to both the central rib and the rib in the middle of each flank, whereas elsewhere the flank is convex. Dorsal apertural rim orthogonal but with very shallow median indentation, and lateral sinuses broad though shallow; ligula long and curved at anterior edge with steeply dipping sides. Cross−section has rounded triangular shape.

Dorsum covered with transverse rugae that are widely separated and irregularly spaced; each is rounded on top with gently dipping sides such that interspaces are curved on both sides; combination of rugae and keels defines cancellate pattern on dorsum. Venter with faint growth lines only.

Remarks.—This species is based on one specimen that is well−preserved in the apertural region but lack the apical termination. Inclusion under Similotheca is warranted by the longitudinal ribs that create the cancellate pattern on the dorsum and by the concavity adjacent to the dorsal keel. This species differs from S. similis (Walcott, 1890) and S. bastionensis sp. nov. in having on the dorsum only one longitudinal keel−like rib on each flank and coarser rugae, which intersect to define a cancellate pattern of ornament on the dorsum. In addition the venter is more inflated, thereby giving rise to a more rounded triangular cross−section.

Stratigraphic range and distribution.—Lower Cambrian, 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.