Skelidorygma sp.
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publication ID |
https://doi.org/ 10.4202/app.2010.0106 |
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persistent identifier |
https://treatment.plazi.org/id/03BB9544-FF89-A654-FF7F-FE6D9C29FB3C |
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treatment provided by |
Felipe |
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scientific name |
Skelidorygma sp. |
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Fig. 30F–I View Fig .
Material.—One incomplete shell, 4 incomplete ventral valves and 1 fragmentary dorsal valve from samples GB and GT.
Remarks.—The specimens are characterised by a brachythyrid, suboval shell outline with short hinge line and smoothly rounded postero−lateral margins. The ventral sulcus is bordered by a pair of stronger and wider costae. The sulcal ribbing pattern can be assessed on one of the studied specimen only ( Fig. 30G View Fig ). Besides the sulcal median costa there are two narrow costae on each slope of the sulcus which arise by bifurcation from the sulcus−bounding costae. The latter, however, bifurcate once also laterally. The costae on flanks are broad, rounded, and either simple (single ventral valve on Fig. 30F View Fig ), or with a tendency to bifurcate (single incomplete shell on Fig. 30G View Fig ). The dorsal fold on the latter specimen appears to be smoothly rounded without any evident costation (in contrast to the costate ventral sulcus; see Fig. 30G View Fig ). Interior of four incomplete ventral valves show no dental plates but thickened dental ridges are well visible ( Fig. 30F View Fig 2, F 3, H 2). Delthyrium covered by a convex delicate deltidial plate which is medially deeply angularly indented. A single well preserved deltidial plate reveals a presence of an elliptical, ca. 0.7 mm in greater diameter pedicle opening with a short tube−like collar extending externally (i.e., dorsally; Fig. 30I View Fig ).
Unfortunately, the condition of secondary denticulation, which is characteristic and diagnostic for some brachythyridid species cannot be ascertain on the material due to its preservational constrains. The bifurcate lateral costae which can be observed on one of the specimens are not typical for Skelidorygma and many other brachythyridids but in some species bifurcation can be observed, e.g., in Brachythyris chouteauensis (Weller, 1909) ( Carter 1967: pl. 39: 1), Meristorygma arctica Carter, 1974 ( Carter 1974: pl. 3: 21–24), Brachythyris atbasarica Nalivkin, 1937 ( Litvinovich et al. 1969: pl. 67: 2, 3), Litothyris alticostata Roberts, 1971 ( Roberts 1971: 238, fig. 77), Brachythyris cobarkensis Peou, 1980 ( Peou 1980: 13), and Brachythyris suborbicularis (Hall, 1858) ( Xu and Yao 1988: pl. 79: 7).
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