Talochlamys laticostata Beu & Darragh, 2001
BEU, A.G. & T. A. DARRAGH. 2001. Revision of southern Australian Cenozoic fossil Pectinidae (Mollusca: Bivalvia). Proceedings of the Royal Society of Victoria, 113: 1–205 [p. 107, figs. 32d-f, 35d]
2001 Talochlamys laticostata Beu & Durragh, 2001
A. G. Beu & T. A. Darragh, 2001, figures 32, 35.
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«Description. Moderate-seized for genus (to 40 mmhigh), acline to very weakly prosocline, moderately well inflated, inflation of the two valves approximately equal; a few specimens slightly taller than long, most slightly longer than tall, with relatively wide umbonal angle (most specimens c. 100°, a few as low as 95°). LV preradial microsculpture not seen. Disc sculpture of 25-32 wide, well raised, convex-topped radial costae, simple, undivided and fixed in number on most specimens, identical on the two valves, developing weak, smooth lateral secondary costellae alongside primary ones on a few specimens; costal crests bearing wide, widely spaced scales that are low and inconspicuous over much of surface of most specimens (incomplete) but when not abraded are very large for genus, initially curved slightly ventrally but then strongly and deeply cupped towards dorsum, almost hemispherical on some specimens, little wider than costae, not continuing down costal sides and on to edges of intercostal spaces as in T. keiloriana. Entire surface bearing low, harrow antimarginal ridgelets, well preserved on costal crests on most specimens, forming short, weakly radiating groups of ridgelets that stop at ventrally convex growth lines in intercostal spaces; no commarginal sculpture observed. RV anterior auricle long and narrow, retaining deep byssal notch, depressed byssal fasciole and functional ctenolium in adult, main outer face above byssal fasciole sculptured with 6-7 coarsely scaly radial costae; LV anterior auricle long, with weakly sinuous anterior margin inclined strongly towards anterior, sculptured with c. 7-9 scaly radial costae; posterior auricles moderately short, with weakly concave posterior outline, sculptured with 8-10 very narrow, widely spaced, finely scaly radial costae. Dorsal margin of RV auricles coarsely serrate. Interior smooth apart from weakly notched ventral margin. Hinge with long, narrow dorsal teeth and short, very narrow resilial teeth occupying dorsal half of edges of resilifer.
Dimensions.
Holotype, P301523 H 35.8 L 36.3 2 valves Paratype, WAM 92.283 Cadell Marl 33.2 32.3 LV Paratype, P302143, Morgan 33.4 31.4 RV Paratype, P302144, Morgan 39.1 35.6 2 valves Type material. Holotype, NMV P301523, an articulated pair, from Morgan Limestone (Balcombian), ‘lower beds, River Murray cliffs at Morgan’, South Australia, coll. F. A. Cudmore; 2 paratypes, NMV P302)43–4, one RV (P302143) and an articulated pair, Morgan Limestone, ‘upper part of lower beds, Murray River cliffs, Morgan’, South Australia, coll. F. A. Cudmore; paratype, WAM 79.16, Cadell Marl member, Morgan Limestone (Balcombian), left bank Murray River 6.4 km downstream from Morgan, South Australia, coll. G. W. Kendrick, 3–4.iii.1978; paratype, WAM 92.283, locality as WAM 79.16, coll. W. G. Buick.
Other material examined. Balcombian: MORGAN LIMESTONE: PL3217, Murray cliffs, ‘Yarra View’ (2 specimens); River Murray cliffs 6.4 km below Morgan (8 specimens); lower beds, River Murray cliffs at Morgan (6 specimens); AMS F5893, cliff 44, Parish of Waikerie, Murray River (3 specimens). Bairnsdalian: PATA LIMESTONE PL3213, Ramco Point (2 specimens).
Occurrence and time range. Balcombian and Bairnsdalian; known only from Morgan and Pata Limestones in the Murray Basin.
Remarks. Talochlamys laticostata sp. nov. is similar to T. keiloriana in general appearance, but the many differences and the lack of knowledge of the LV preradial sculpture make a relationship to T. keiloriana uncertain. T. laticostata is markedly more inflated than T. keiloriana, and although the number of radial costae is closely similar, the costae of T. laticostata are significantly wider and their interspaces are a little wider in T. laticostata than in T. keiloriana, presumably because each costa of T. laticostata occupies the same space as one costa and its two subsidiary costellae in T. keiloriana.
The character taken to indicate their close phylogenetic relationship is the arrangement of the antimarginal ridgelets in intercostal spaces as short groups of weakly radiating ridgelets, each group ending at a ventrally convex growth line, producing a series of evenly spaced, radiating ‘bundles’ of ridgelets over most of the intercostal surface, closely similar in the two species. At first sight the unusually large and prominent, dorsally cupped costal scales, forming hemispherical knobs on the costae of well preserved specimens, seem to provide a major difference from T. keiloriana, but some unusually unabraded specimens of T. keiloriana have the outer tips of a few of their scales curved ventrally and then back up towards the dorsum, producing a scale similar to that of T. laticostata, but much narrower. Unfortunately, we have seen relatively few specimens of T. laticostata, and it seems likely that, if any small specimens have been collected, they are masquerading in collections of Mimachlamys sturtiana. Examination of LV juveniles by SEM is needed to confirm the position in Talochlamys adopted here, but the similarity to T. keiloriana in many characters leaves little real doubt of the generic position. Etymology. The specific name (Latin, ‘latus’, broad +‘costatus’, ribbed) refers to the relatively wide radial costae of this species.»
ALAN GLENN BEU & THOMAS ALWYNNE DARRAGH, 2001
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