Microcornus eximius Duan, 1984

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 : 556

Microcornus eximius Duan, 1984

Fig. 3A, B .

Microcornus eximius Duan, 1984: 153 , pl. 1: 5; see Bengtson 1990a: 221 for synonymy to date.

? Microcornus eximius Duan, 1984 ; Wrona 2003: 193, fig. 7A–C.

Holotype: Tianjin Institute of Geology and Mineral Resources, People’s Republic of China SH 1001.

Material.— MGUH 27085–27086 and 17 additional specimens from GGU samples 314804, 314807, and 314835.

Description.—Conch orthoconic although with slight apical curvature toward right when viewed from dorsum; apical angle large such that conch has wide appearance; venter slightly inflated, grading into rounded lateral edges which in turn pass into nearly flat, only slightly inflated dorsal flanks; flanks meet in high, almost keel−like median ridge. Cross−section has rounded triangular shape. Aperture appears orthogonal with long ligula having steeply dipping sides toward aperture; anterior edge of ligula rounded; lateral sinuses well−developed, as is sinus in centre of dorsal apertural rim. Surface of shell covered with fine, equally spaced growth lines that are most prominent next to aperture but become fainter in direction of apex. Distinct bulbous protoconch separated from remainder of conch by shallow constriction.

Remarks.—This species is represented by two generally well−preserved and 17 fragmentary individuals. Overall proportions of the conch and ornament on the shell indicate placement of these specimens under M. eximius Duan, 1984 , from South China and Australia. It is distinguished from M. petilus Bengtson, 1990a with which it co−occurs, by a larger apical angle and generally finer ornament on the shell. Microcornus breviligulatus Qian, 1989 and M. egregius Demidenko, 2001 have tubular protoconchs with no apparent growth lines or other ornament on the shell, and on that basis are easily distinguishable from both Greenland species of Microcornus . Specimens of questionable affinity to M. eximius were recently described from Antarctica by Wrona (2003).

Stratigraphic range and distribution.—Lower Cambrian; Australia, South China, North−East Greenland, and possibly Antarctica.

Figures

Fig. 3. A, B. Microcornus eximius Duan, 1984. A. MGUH 27085; A1, dorsum; A2, right flank of dorsum; A3, venter; A4, cross−section and view of dorsum from aperture. B. MGUH 27086; B1, dorsum; B2, cross−section and view of dorsum from aperture. C, D. Microcornus petilus Bengtson, 1990a. C. MGUH 27087, venter. D. MGUH 27088; D1, dorsum; D2, venter. E, F. Paracornus poulseni gen. et sp. nov. E. MGUH 27089, holotype; E1, dorsum; E2, venter; E3, enlarged view of ornament at right lateral edge (venter to right). F. MGUH 27090, dorsum. G. Parkula bounites Bengtson, 1990a, MGUH 27091; G1, exterior of operculum from above; G2, exterior of operculum from conical shield; G3, lateral view of operculum with conical shield to right; G4, exterior of operculum with cardinal shield in foreground. A, D–G, from 314835; B, from GGU sample 314906; C, from GGU sample 314904. Scale bars 0.2 mm, except E3 which is 50 µm.