Scythentolium scutigerulus Hautmann et al., 2011
HAUTMANN, M., H. BUCHER, T. BRÜHWILER, N. GOUDEMAND, A. KAIM & A. NÜTZEL. 2011. An unusually diverse mollusc fauna from the earliest Triassic of South China and its implications for benthic recovery after the end-Permian biotic crisis. Geobios, 44: 71-85, text-figs. 1-6. [p. 77, text-figs. 4.10-4.19, 5.1, 5.2]
2011 Scythentolium scutigerulus Hautmann et al., 2011
M. Hautmann et al., 2011,
figures 4, 5. |
«Derivatio nominis: scutigerulus (Latin) = shield bearer.
Holotype: PIMUZ 28475, Fig. 4(13). Material: Nine left and three right valves (PIMUZ 28472-28483). Diagnosis: Shell biconvex and more or less equivalved, with straight dorsal margin and well-developed anterior auricular sinus in both valves. Right valve externally smooth, left valve with periodically raised commarginal growth lines and very faint radial ribs. Description: Valves convex, acline, nearly circular in outline, with straight hinge margins and orthogyrate umbones. Apical angle of disc ca. 85°. Posterior auricles small and truncated (Figs. 4(14) and 5(2)). Right anterior auricle large, with well-developed auricular sinus (Figs. 4(19) and 5(1, 2)). Left anterior auricle similar in outline but possibly slightly larger (Fig. 4(12–14, 17)). Shell exterior of left valve with faint, widely spaced radial ribs and growth lines that periodically form minute commarginal riblets (Fig. 4(13)). Right valve externally smooth except for growth lines (Figs. 4(19) and 5(1, 2)). Remarks: Scythentolium was erected by Allasinaz (1972) in order to accommodate Early Triassic entoliids with an anterior auricular sinus. S. scutigerulus differs from other species of the genus by:
Most hitherto described species of Scythentolium have been based on poorly preserved material, which hampers detailed comparisons with the new species. In particular, the species described by Wittenburg (1908) and subsequently assigned to Scythentolium by Allasinaz (1972: pp. 222–223, 311–314) are mostly not well defined. According to the descriptions of Wittenburg (1908), Scythentolium tirolicum (Wittenburg, 1908) differs from the new species in the less convex right valve and the higher number of radial ribs on the left valve [probably incorrectly separated as ‘‘var. predazzensis’’ by Wittenburg (1908)]. Scythentolium subtile (Wittenburg, 1908) has a much larger anterior auricle and more densely spaced radial ribs. Scythentolium longauris (Wittenburg, 1908) is very similar to S. subtile, differing essentially in the lack of radial ribs, which, however, might be due to poor preservation. Scythentolium rombergi (Wittenburg, 1908) differs from the new species in a flat right valve that was described as bearing radial ribs. Wittenburg’s (1908: fig. 4) figure additionally shows a posterior auricular sinus, but it is mentioned in the main text without further specification that this figure does not show all characters correctly (Wittenburg, 1908: p. 21). Scythentolium sojale (Wittenburg, 1908) has also a posterior auricular sinus and additionally differs from S. scutigerulus in a flat right valve and more pronounced incremental lines. Scythentolium eurasiaticum (Wittenburg, 1908) is poorly defined but appears to be unique in having a very narrow but relatively long anterior auricle. Scythentolium kokeni (Wittenburg, 1909) from the Smithian-Spathian of the Salt Range (Pakistan) differs in its larger size, the lack of a left anterior auricular sinus, and the coarser radial ribs on the left valve. No similar species from China are known so far. ‘‘Entolium discites microtis (Bittner)’’ figured by Gu et al. (1976: pl. 34, figs. 12, 13) remotely resembles S. scutigerulus in the presence of radial ribs but differs in the absence of an auricular sinus and does therefore not belong to Scythentolium.
Given its archetypical morphology and the stratigraphic appearance near the base of the Early Triassic, the new species is likely to be near the root of the genus, which moderately radiated in the course of the Early Triassic before it went extinct at the end of this epoch (Allasinaz, 1972). The presence of auricular sinuses hints at adult byssal attachment, possibly brought about by paedomorphic retention of the byssus, which is a remarkable atavism because Palaeozoic entoliids were unattached and possibly able to swim (Stanley, 1972). The return to a sessile or hemisessile mode of life ca. 125 Ma after the first appearance of entoliids (Waller, 2006) may reflect unusual ecological conditions in the earliest Triassic such as reduced primary production and high seawater temperatures with reduced ability to dissolve oxygen, which favoured passive lifestyles with low metabolic rates (Hautmann and Nützel, 2005).» MICHAEL HAUTMANN, HUGO BUCHER, THOMAS BRÜHWILER, NICOLAS GOUDEMAND, ANDRZEJ KAIM & ALEXANDER NÜTZEL, 2011
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