Antarctipecten Beu & Taviani, 2014
BEU, A. G. & M. TAVIANI. 2014. Early Miocene Mollusca from McMurdo Sound, Antarctica (ANDRILL 2A drill core), with a review of Antarctic Oligocene and Neogene Pectinidae (Bivalvia). Palaeontology, 56 (6): 1-44, figs. 1-9. [p. 322]
«Genus ANTARCTIPECTEN gen. nov
LSID. urn:lsid:zoobank.org:act:81603C8D-0C76-429F-972E-5EB7 709C64B8
Type species. Adamussium alanbeui Jonkers, 2003.
Occurrence of type species. Oligocene – Early Miocene, Antarctica and South Shetland Islands.
Derivation of name. From Antarctica, the area of occurrence of the genus, combined with the suffix pecten (Latin, a comb), used in the names of many scallops. Gender masculine.
Diagnosis. A genus of Adamussiini with small, weakly inflated, acline, slightly dorso-ventrally elongate, thin shell (height up to 50 mm, most specimens smaller). Umbonal angle low, c. 110–115 degrees. Auricles only slightly asymmetrical, anterior slightly longer than posterior; RV anterior auricle with semicircular anterior margin, shallowly embayed below by small byssal notch, not functional in adults. Without obvious macrosculpture, other than elevation of the widely spaced commarginal ridgelets into high, thin lamellae on unusually well-preserved specimens, particularly on LV anterior auricle; some unusually well-preserved specimens bear 6–8 faint to low radial folds on LV, particularly on anterior half of valve; one specimen observed with two obvious radial ridges and a third weak one in centre of RV anterior auricle, forming very low nodules where crossed by commarginal ridgelets. Microsculpture of narrow, raised, relatively widely spaced commarginal ridgelets (3–5 per 1 mm on central and distal areas of disc) and very fine, closely spaced antimarginal ridgelets. Without internal radial ridges; one very short, thin resilial tooth present on each side of resilifer in LV; other internal characters not seen.
Included species. Only the type species is included in the new genus.
Remarks. Until now, the type species of this new genus has been placed in Adamussium, following the suggestion by Beu and Dell (1989, p. 136) – a suggestion based more on ignorance of its characters, and of scallop classification in general in the ‘dark ages’ before Waller (1991) began to establish scallop phylogeny, than on any real insights into scallop classification. It is now clear that there are numerous (five recorded so far) Oligocene–living Adamussium species around Antarctica, all agreeing with the extant type species, A. colbecki, in being wide, equidimensional shells with a wide umbonal angle (135–140 degrees), with sculpture of several low, wide, relatively obvious radial folds in the shell, crossed by low but obvious commarginal ridgelets in most specimens and by very fine antimarginal ridgelets in all. They also all have slightly longer auricles (anteroposteriorly) than Adamussium alanbeui. Therefore, it now appears that Adamussium alanbeui belongs in a genus distinct from Adamussium, having little or no obvious radial macrosculpture, smaller size, lesser inflation, slightly taller shape concomitant with a narrower umbonal angle (110–115 degrees), a much shallower byssal notch and shorter auricles than the radially folded species referred to Adamussium. Although the material seen originally by Beu and Dell (1989) lacked all sign of radial macrosculpture, one of the specimens referred here tentatively by Jonkers (2003, pl. 16f) has six or seven obvious, very low, widely spaced radial folds, and the well-preserved specimen described below from ANDRILL 2A, 376.80–376.85 mbsf, bears very faint radial ridges on the anterior half of the LV. Very weak radial macrosculpture seems to be a definite but variably present and variably preserved character of A. alanbeui, but is much weaker than in species referred more certainly to Adamussium, listed under that genus above. We propose the new genus Antarctipecten for Adamussium alanbeui alone.
Waller (2006, pp. 14–15, fig. 1.2) demonstrated that Eburneopecten Conrad, 1865 is not applicable to this or any other extra-North American pectinid. The type species, E. scintillatus (Conrad, 1865), evolved from a coarsely ribbed Dhondtichlamys species (Waller 2001) in the Early to Middle Eocene of eastern North America and is not related phylogenetically to any of the other taxa previously referred to Eburneopecten. It is still possible, as suggested by Beu and Dell (1989) and Waller (2006, p. 14), that the superficially smooth little ‘saucer scallop’ Antarctipecten gen. nov. alanbeui was ancestral to Adamussium, but that now seems unlikely, in view of the variety of smooth Adamussiini recognized in Australia and New Zealand. Beu and Darragh (2001, p. 122), when proposing the genus Victoripecten, pointed out at least three groups of superficially similar, but independently evolved, smooth ‘saucer scallops’ in Eocene–Miocene rocks of New Zealand, as well as Victoripecten in southern Australia. The three obvious New Zealand groups are Duplipecten Marwick, 1928, Lentipecten Marwick, 1928 (= Janupecten Marwick, 1928, a lineage that gradually evolved the smooth, final, Oligocene species L. hochstetteri (Zittel, 1864); tribe Serripectinini; Beu et al. 2012, p. 31) and the unnamed Miocene genus that usually has been known incorrectly as Lentipecten. Other New Zealand genera of these poorly understood, very similar taxa possibly remain to be recognized. Tribe Serripectinini evolved two other similar, almost smooth species, Serripecten semilaevis (McCoy, 1876; Middle Miocene, southern Australia; Beu and Darragh 2001, p. 89, figs 25E, 26A–D, 27A–B) and an unnamed Australian Oligocene Serripecten species (Beu and Darragh 2001, p. 81, fig. 22C). Loss of most or all macrosculpture to produce a smooth, thin, light, weakly inflated shell (the amusioid form; Waller 1991, p. 10) that could swim away from rapidly moving predators evidently was a common response to severe predation pressure during the warm period of the early to mid-Cenozoic, even in Antarctica. Relationships with several of the genera recognized in New Zealand are possible for Antarctipecten gen. nov. alanbeui, but in view of the small size of the type species (smaller than any Australian or New Zealand taxa of this group) and the fact that several, superficially similar genera evolved in neighbouring seas, a close phylogenetic relationship will always remain difficult to demonstrate. Also, the prominent commarginal microsculpture now known to be present on A. alanbeui is not present on any other species of Adamussiini we have examined, other than the type species, Adamussium colbecki. The end result in Antarctipecten alanbeui is a shell resembling a diminutive version of the adult form of Adamussium colbecki, with quite similar auricles other than a much less strongly sigmoidal LV anterior auricular margin, a narrower umbonal angle and much weaker commarginal sculpture, but with more closely spaced commarginal lamellae and much weaker radial macrosculpture on the disc.» ALAN GLENN BEU & MARCO TAVIANI, 2014
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Antarctipecten gen. nov. alanbeui (Jonkers); A. G. Beu & M. Taviani, 2014, Early Miocene Mollusca from McMurdo Sound, Antarctica (ANDRILL 2A drill core), with a review of Antarctic Oligocene and Neogene Pectinidae (Bivalvia), figures 4B-4E.
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