Adamussium necopinatum Quilty, Darragh, Gallagher & Harding, 2016
QUILTY, P. G., T. A. DARRAGH, S. J. GALLAGHER & L. A. HARDING. 2016. Pliocene Mollusca (Bivalvia, Gastropoda) from the Sørsdal Formation, Marine Plain, Vestfold Hills, East Antarctica: taxonomy and implications for Antarctic Pliocene palaeoenvironments. Alcheringa: An Australasian Journal of Palaeontology, 40 (4): 556-582. [p. 16, figs. 13A-13L]
2006 Adamussium necopinatum Quilty, Darragh, Gallagher & Harding, 2016
P. G. Quilty, T. A. Darragh, S. J. Gallagher & L. A. Harding, 2016, figure 13.
|
«Etymology. necopinatum—(L)—surprised or unexpected.
Occurrence. Locality 11.
Material. Nine specimens from Locality 11, Marine Plain, East Antarctica. Some fragmentary, but ample for a full description. Shell preserved patchily. NMV P331865, P331698–133702, 133712. Holotype: NMV P331701. Paratypes: NMV P331865, P331698–331700, P331702, P331712.
Diagnosis. Small, distinct species with umbonal angle close to 90°. A little higher than long; 7–8 costae on disc.
Description. Shell small for genus; thin shell finely laminated, decorticating as flakes; valves equilateral, right valve low convexity, left valve more inflated (Fig. 13A–L). Disc a little higher than wide; height 26–33 mm; length 25–30 mm; hingeline straight, 9–11 mm. Umbonal angle (87–90°) defines a raised area differentiated from smooth anterior and posterior region marked only by growth lines. Auricles of right valve with clear byssal notch, not obvious on left valve; surface features of left auricle unknown; auricle of right valve with row of expanding ‘tubercles’ broadening and becoming lower with distance from umbo, sitting above a well-defined, narrow auricular sulcus; teeth not obvious. AAAD (Jonkers, 2003) of left valve ca 120°. Around 7–9 costae with wide interspaces from umbo, fading before reaching margin giving way to fine concentric sculpture. Costae strongest in centre of disc. Margin smooth.
Discussion. The specimens are closely approximated and known from a single locality where they occur in very fine grained brown sandstone. They seem to constitute a monospecific fauna with a life mode similar to that of the modern A. colbecki (E.A. Smith 1902).
Until recently, Adamussium was regarded as a modern monospecific genus restricted to the Antarctic. With the studies by Stilwell et al. (2002), Jonkers (2003), Quaglio et al. (2008, 2010), Beu & Taviani (2014), it is now known to have an Oligocene–Holocene history with several species, but with a distribution still restricted to the Antarctic. Quaglio et al. (2008, 2010) and Beu & Taviani (2014) presented tables to illustrate the features used to differentiate species. Jonkers (2003) and Quaglio et al. (2008) also provided keys to the various measurements used in describing the species. The table of Beu & Taviani (2014), modified from Quaglio et al. (2010), provides a basis for comparison of this species with most known species of the genus and related genera. The outstanding differences are in size and umbonal angle, the latter the lowest at 90° of any species. Also apparently not seen or noted previously is he row of ‘tubercles’ on the right auricle. By far the most similar species, in descriptive terms, is A. jonkersi, which is the smallest, with a more convex left valve and 7–9 costae. Details of the auricles are not clear for A. jonkersi, for example the byssal notch, but they are symmetrical and, thus, different from the new species. Beu & Taviani (2014) illustrated A. cf. jonkersi from an ANDRILL core but, although incomplete, it shows clearly that it had several more (estimate 12–15) costae than in the new species. The only described species that was approximately contemporaneous with A. necopinatum is A. cockburnensis Jonkers but that is clearly differentiated by virtue of a much wider umbonal angle (>130°), more abundant costae (12–15) and much larger size (>55 mm). Stilwell et al. (2002) discussed and illustrated poorly preserved Adamussium n. sp.? from the Battye Formation in the Prince Charles Mountains, the geographically closest occurrence of a fossil species of this genus. The age was estimated as no younger than early Miocene and specimens seem to be smaller than the species listed in the tables mentioned above with height of ca 40 mm. Umbonal angle is <110° and approaches 90° in the figured specimens. It is difficult to be certain about numbers of costae but from the figures, that seems to be about 12–15. It thus does not fit into any of the described species.» PATRICK G. QUILTY, THOMAS A. DARRAGH, STEPHEN J. GALLAGHER & LUCY A. HARDING, 2016
|