Gracilitheca sp.
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publication ID |
https://doi.org/ 10.5281/zenodo.13522197 |
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persistent identifier |
https://treatment.plazi.org/id/03F8878E-FF9C-FF92-FCDE-FB9AFB97FE2E |
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treatment provided by |
Felipe |
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scientific name |
Gracilitheca sp. |
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Fig. 9A View Fig .
Material.—MGUH 27108 and 27 additional specimens from GGU sample 314807, 314835, and 314908.
Description.—Conch with bluntly rounded central dorsal ridge that grades into adjacent flanks that are slightly inflated; the flanks grade into blunt lateral edges that protrude slightly to form a longitudinal ridge along each edge; the venter has a shallow median furrow bounded on each side by a low rounded longitudinal ridge. Cross−section has a generally triangular shape. Surface of shell on dorsum with faint longitudinal lines best seen near lateral edges, becoming fainter near middle of dorsum.
Remarks.—This taxon is known from 29 specimens, all of which only preserve a portion of the mid−region of the conch. Nonetheless, the cross−section and lateral protuberances are sufficiently distinct to indicate placement within Gracilitheca Syssoiev, 1968 , although identification to species is not possible. The Greenland specimens have lateral protuberances that are most similar to those of Gracilitheca gratuita Val’kov, 1987 from Siberia. In contrast, G. excavata (Val’kov, 1975) has a much deeper ventral longitudinal furrow but otherwise the shape of the protuberances and dorsum is similar to that of the Greenland forms. Gracilitheca bayonet ( Matthew, 1899) also resembles the Greenland species in terms of shape of the protuberances, but it lacks any longitudinal ornament. Gracilitheca ternata Syssoiev, 1968 , G. argasalaica Val’kov, 1987, and G. destombesi Marek, Malinky, and Geyer, 1997 may be distinguished from Gracilitheca sp. from Greenland by the shape of the protuberances.
Stratigraphic range and distribution.—Lower Cambrian, North−East Greenland.
Orthothecid? gen et. sp.
Fig. 9B, C View Fig .
Material.—MGUH 27109–27110 from GGU sample 314809.
Description.—Seemingly cyrtoconic conch with slightly inflated venter, grading into sharp, keel−like lateral edges, which in turn pass into flat, steeply dipping dorsal flanks. Dorsum is flat, with sharp intersection with lateral edges, and with one distinct longitudinal ridge in middle. Faint transverse lines on dorsum, remainder of shell seemingly smooth. Cross−section has quadrate shape.
Remarks.—This species is known from two fragmentary specimens, neither of which has the apex or aperture intact. Referral to order is especially difficult because these individuals possess a generally quadrate cross−section, seen thus far only among Orthothecida . They also display, however, an inflated venter which ordinarily is more characteristic of Hyolithida . Known orthothecids that have a cross−section somewhat resembling that of the Greenland specimens are Obliquatheca acostae Syssoiev, 1968 (pl. 2: 9b) and Trapezotheca aemula ( Holm, 1893; see Malinky 2002: text−fig. 3N) but in both of these taxa the venter has a shallow longitudinal furrow, creating a slightly kidney−shaped cross−section, whereas the venter of the Greenland specimens is inflated. Sokolovitheca sokolovi Syssoiev, 1972 has a variable cross−section although one individual ( Syssoiev 1972: pl. 2: 1e) resembles the Greenland species. However, the dorsal morphology of S. sokolovi does not match that of the Greenland individuals. No traces of growth lines or any other surface ornamentation remain, and the nature of the aperture, in particular whether a ligula was present, is obscure.
Stratigraphic range and distribution.—Lower Cambrian, North−East Greenland.
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