Parkula bounites Bengtson, 1990a

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 : 559

Parkula bounites Bengtson, 1990a

Figs. 3G , 4A, B , 5 .

Parkula bounites Bengtson, 1990a: 223 , figs. 149–151.

Parkula bounites Bengtson, 1990a ; Demidenko 2001: 101, pl. 9: 12, 13. Parkula bounites Bengtson, 1990a ; Wrona: 197, fig. 5F.

Holotype: South Australian Museum , Adelaide SAMP30892 .

Material.— MGUH 27091–27093, MGUH 27101 and 116 additional specimens from GGU samples 314807, 314835, and 314908.

Diagnosis.—As for genus.

Description.—Operculum having broad, rounded conical shield with a relatively high profile; summit is rounded to flattened on surface; furrows are deep and well−developed although associated rooflets appear nonexistent; cardinal shield small with sides adjacent to summit also curved to be convex toward conical shield. Operculum monoclaviculate, with one pair of short cardinal processes that expand toward distal end, to terminate in blade−like projections; clavicles diverge at angle of approximately 70°. Edge of conical shield may have series of low, widely spaced radial ridges with spaces between them equal to width of ridge. Exterior may have widely spaced, generally indistinct concentric growth lines; interior smooth except for ridges at edge of conical shield.

Remarks.—Opercula from the Bastion Formation of North−East Greenland match well those of Parkula bounites Bengtson, 1990a from Australia, although there is much variability in the width of the operculum (see Bengtson 1990a: fig. 150A and E and herein). The conch of this species has not yet been identified in Greenland. Its absence supports the notion of a hydrodynamically sorted accumulation of fossils in the bed that yielded the specimens. Its absence could also be explained by differential preservation, as initially suggested by Holm (1893) for some hyolith species from Denmark, and later expanded by Malinky and Berg−Madsen (1999) to explain general disparities in the numbers of hyolith conchs versus opercula from the same levels. However in this instance, because the conch of Parkula is known from other localities, sorting provides a better explanation.

A single specimen from Greenland ( Fig. 5 ) shows a circular perforation on the right hand side of the conical shield. Its diameter is about 100 µm and the sides slope slightly inwards. Perforations of similar size and morphology in shells of Early and Middle Cambrian phosphatic brachiopods and problematic fossils were interpreted by Conway Morris and Bengtson (1994) as evidence of the activity of some as yet unidentified predator.

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

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.

Fig. 4. A, B. Parkula bounites Bengtson, 1990a. A. MGUH 27092; A1, exterior of operculum from above; A2, lateral view of operculum with conical shield to right. B. MGUH 27093, interior of operculum with cardinal processes and clavicles (top of the picture). C, D. Parkula? sp. C. MGUH ZZZ13; C1, exterior of operculum from above; C2, view from cardinal shield showing enlarged cardinal processes. D. MGUH 27095; D1, exterior of operculum from above; D2, lateral view of operculum with conical shield to right; D3, view from cardinal shield showing enlarged cardinal processes. E–I. Hyptiotheca karraculum Bengtson, 1990a. E. MGUH 27096; E1, exterior of operculum from above; E2, lateral view with conical shield to right, showing no change in angle between conical and cardinal shields; E3, view from edge of conical shield.F. MGUH 27097; F1, oblique view from above of exterior of operculum from above; F2, view from edge of conical shield; F3, exterior of operculum from above. G. MGUH 27098; G1, oblique view from above of exterior of operculum from above; G2, lateral view of operculum with conical shield to right, showing abrupt change in angle between cardinal and conical shields. H. MGUH 27099; H1, lateral view of operculum with conical shield to right, showing abrupt change in angle between cardinal and conical shield; H2, exterior of operculum from above.I. MGUH 27100, interior view showing weakly developed depressions along edges only, corresponding to rooflets on exterior. All from GGU sample 314835. Scale bars 0.2 mm.

Fig. 5. Exterior view of operculum of Parkula bounites Bengtson, 1990, MGUH 27101, showing possible borehole. From GGU sample 314807. Scale bar 0.2 mm.