Austrochlamys forticosta Beu & Taviani, 2013
BEU, A. & 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, 57 (2): 299-342, figs. 1-9. [p. 311, figs. 4A-4G]
2013 Austrochlamys forticosta Beu & Taviani, 2014
A. G. Beu & M. Taviani, 2014, figure 4.
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«LSID. urn:lsid:zoobank.org:act:42F5424E-73E0-485E-8082-D38A DC312DDF
Derivation of name. From Latin, fortis (strong, powerful, robust) and costa (a rib), referring to the few, prominent radial costae of this species.
Type material. Holotype USNM 545830, umbonal areas of two formerly articulated valves cut by core splitter at an oblique angle, in ANDRILL 2A, from the pectinid concentration at 430.54–430.68 mbsf (Fig. 4B–C); three illustrated paratypes: USNM 545831, illustrated RV anterior auricle (Fig. 4A) and two other paratypes cut by both drill and core splitter (USNM 545833, Fig. 4D; USNM 545832, Fig. 4E); six paratype fragments from the same horizon not illustrated (USNM 545834); two paratypes from ANDRILL 2A, 466.5–469.0 mbsf, one a very incomplete RV cut by both drill and core splitter, with base of anterior auricle remaining (USNM 545841; Fig. 4F), the other incomplete paratype (USNM 545842) from the same horizon, a central area of disc cut by both drill and core splitter; one incomplete paratype, USNM 545839, from ANDRILL 2A, 459.0 mbsf, coarsely radially sculptured central area of disc cut by both drill and core splitter (Fig. 4G), with small Hiatella valve attached to disc.
Type locality and horizon. Collected only from the 38.5-m-thick interval between 430.54 and 469.0 mbsf in the ANDRILL 2A core (Burdigalian).
Dimensions. ANDRILL 2A, 430.54–430.68 mbsf: holotype RV fragment: H 35, L 50 mm, LV fragment: H 33, L 52 mm; H estimated from large size of LV posterior auricle to have been originally c. 130, L 120 mm; USNM 545839, paratype, 459.0 mbsf: L > 59 mm, estimated originally 70+ mm, H highly incomplete; USNM 545841, paratype, 466.5–469.0 mbsf: H > 48 mm, L > 59 mm; second specimen highly incomplete; both estimated originally 70+ mm. Austrochlamys forticosta sp. nov. may well have reached a similar size to A. anderssoni (137 mm; Jonkers 2003, p. 65).
Diagnosis. A large (H > 100 mm), coarsely and sparsely radially costate species of Austrochlamys with c. 10–12 wide primary costae; byssal fasciole swollen above rest of RV anterior auricle.
Description. Shell large (available fragments to L 59 mm, estimated
originally >100 mm), thick; umbonal angle very low (95 degrees) on one measurable umbonal fragment, presumably larger (c. 110–115 degrees?) on complete adult shells. Macrosculpture of 9–10 wide, moderately elevated primary radial plicae with almost flat to gently convex crests and gently convex edges, all without obvious subdividing grooves in material examined; each gently concave radial interspace as wide as or a little wider than one plica, bearing none to two moderately wide secondary costae or up to three narrower tertiary costellae, each arising either by intercalation or by subdivision from edges of primary costae. Obvious microsculpture of low, closely spaced, flat-topped to narrower, raised commarginal ridges, 4–5 per 1 mm over outer area of disc of large specimens; commarginal ridges anastomose frequently where flat-topped; faint antimarginal ridgelets visible between commarginal ridges on LV anterior auricle of one paratype, otherwise not discernible (abraded?). Separate RV anterior auricle paratype cut by drill around disc margin and slightly trimmed along dorsal margin, umbonal area and lower part of byssal fasciole missing; otherwise shaped as in Austrochlamys anderssoni, with moderately long, flat, upper, radial area and deeply arcuate byssal notch, byssal fasciole slightly arched above flat, radially oriented area; radially oriented area bearing four or five vaguely defined, wide, closely spaced radial ridges; entire auricle crossed by five deep, narrow grooves (growth steps), biarcuate in conformity with growth lines. LV posterior auricle large, tall, with weakly concave posterior margin sloping slightly forwards towards umbo, bearing seven narrow, raised, moderately widely separated radial costae. Resilifer normal; one low, narrow resilial tooth on each side, confluent with lower border of valve-margining ridge in LV; RV hinge and other characters not preserved. Remarks. Austrochlamys forticosta sp. nov. is much the most common pectinid in ANDRILL 2A. However, no specimen is complete. The pectinid concentration, a zone of weakly concentrated scallop shells at 430.54–430.68 mbsf was illustrated by Fielding et al. (2010, fig. 6) and Taviani et al. (2010, fig. 12a), revealing at least eight thick-shelled, coarsely costate valves of A. forticosta sp. nov. in the split core face. The sample available consists of 10 incomplete specimens cut through by both the drill and the core splitter, along with numerous other fragments, including an almost complete RV anterior auricle. Although this material includes specimens from both working and archival halves of the core, none can be reassembled to produce an even moderately complete specimen, and it is most unfortunate that the core was split before the shells were extracted. Nevertheless, as no more material might ever be collected, and it is obvious that this material represents a single species with much coarser sculpture than any previously named species of Austrochlamys, we think it worth naming here.
We interpret this new species as an extremely robust, coarsely and sparsely ribbed species of Austrochlamys, with many fewer radial costae of much higher profile and wider amplitude than in the well-known younger species A. natans and A. anderssoni, and with the byssal fasciole swollen above the flat, upper, radially oriented area of the RV anterior auricle, rather than depressed below it as in other Austrochlamys species. The many fewer radial plicae (wide costae) clearly differentiate the new taxon from all other species referred to Austrochlamys and from all other Antarctic scallops. Specimens examined have the following rib counts: (1) holotype, 430.54–430.68 mbsf: 10 primary costae on LV umbonal fragment, nine on RV umbonal fragment, secondary sculpture of one narrow costa in centre of each primary interspace on LV, two or three still narrower ones in each interspace on RV; (2) USNM 545839, paratype, 459.0 mbsf: five primary costae on fragment with L > 59 mm, and from none to two low, widely spaced, secondary costae in each radial interspace; (3) USNM 545841, paratype, 466.5–466.9 mbsf: nine primary costae on specimen with L > 59 mm, and from none to two narrow, widely spaced secondary costae in each wide radial interspace, and three low, wide costae on the disc flank. In comparison, Jonkers (2003, p. 64) stated that A. anderssoni has c. 30 narrow primary radial costae, and intercalated and bifurcated secondary costae make up a total of c. 70 costae near the ventral margin of large specimens. Jonkers (2003, p. 61) also stated that A. natans has c. 20 primary costae and c. 10–15 intercalated and bifurcated secondary costae, although his illustrations (Jonkers 2003, pl. 14; pl. 15, figs a–b) demonstrate that this species has c. 30–39 primary radial costae. The separate RV anterior auricle (USNM 545831; Fig. 4A) has had its umbonal end cut off by the drill and its dorsal margin trimmed slightly by the core splitter, but what remains is very similar to that illustrated for A. anderssoni by Jonkers (2003, pl. 13, figs a–b; Fig. 3D). It bears about five vaguely defined radial costae on the upper, radially directed area of the auricle. We assume that Austrochlamys forticosta sp. nov. was simply ancestral to the later species of Austrochlamys and costal width and prominence decreased, while costal number increased with time from c. 10–12 to c. 30 in a simple lineage, but we only tentatively refer it to Austrochlamys.» ALAN GLENN BEU & MARCO TAVIANI, 2014
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