Mesopeplum divergens 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, figs. 1-67 [p. 163, figs. 59A-I]
2001 Mesopeplum divergens Beu & Darragh, 2001
A. G. Beu & T. A. Darragh, 2001, figure 59.
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«Description. Moderate-sized (to c. 40 mm high and 43 mm long), acline to very slightly prosocline, with moderately wide umbonal angle (c. 100-105°), length significantly greater than height; RV weakly inflated for genus, LV almost flat, particularly over proximal quarter to third of disc. Juvenile specimens not seen, LV preradial dissoconch studied by SEM on slightly abraded, half-grown specimen, clearly with prominent, narrow, closely spaced antimarginal ridgelets, but commarginal ridges much less obvious and less prominent than on other species of genus. Plicae low and not greatly obvious, 5 present on almost all specimens, wide on RV with interspaces each about a quarter width of one plica, indistinct near anterior and posterior ends of disc, central interplical depression the most obvious fold on most RVs; correspondingly low and narrow on LV, central narrow plica the most obvious fold on most LVs. Radial sculpture of relatively few, coarse and irregularly subdivided, well elevated costae of rectangular section, varying from c. 3 to 7 on each plica on RV, with 1 to 3 narrower costae in each plical interspace, radial interspaces each as wide as one costa to very much narrower than one costa, with very low, narrow, secondary costellae intercalated in centre of some interspaces on some specimens; 2-3 prominent, weakly subdivided costae per plica on LV, with 3-5 costae of highly varied width and prominence in each interplical space, and with very low, narrow secondary and tertiary costellae intercalated in many intercostal
spaces on most specimens; costae near anterior and posterior ends on both valves conspicuously curved outwards parallel to concave anterodorsal and posterodorsal margins of valves. Entire surface sculptured with obvious, far-set commarginal lamellae, on most specimens free and separate over much or all of RV but united to form secondary surface over much of LV, particularly on costal and plical crests and on costae at anterior and posterior ends of valve, lamellae raised into low, wide, triangular, ventrally directed scales on costal crests on many LVs, particularly on costae on anterior and posterior ends of valves and on anterior auricle. RV anterior auricle triangular, with strongly pointed anterior extremity, deep, narrow byssal notch, narrow, depressed byssal fasciole, and ctenolium with 4 functional teeth in adults, main face above byssal fasciole sculptured with 5-6 prominent, narrow, widely spaced radial costae, commarginal lamellae particularly prominent over dorsal half of many specimens, strongly serrating dorsal margin. LV anterior auricle large, triangular, with weakly sinuous anterior margin inclined strongly towards anterior, sculptured with 6-8 prominent, narrow radial costae crossed by extremely prominent, widely spaced, commarginal lamellae raised into prominent triangular scales on each radial costa. Posterior auricles relatively large, triangular, with sinuous posterior margin; weakly sculptured on RV, with 2-3 low radial costae; more prominently sculptured on LV, with 4-6 narrow radial costae and prominent commarginal lamellae, costal scales less prominent than on anterior auricle in most specimens. Interior weakly corrugated, corrugations developing into prominent, sharp carinae around interior of ventral margin. Hinge with long, narrow dorsal teeth and one pair of short, narrow, prominent resilial teeth above dorsal half of resilifer in RV, sockets for resilial teeth prominent in LV. Type material. Holotype, NMV P302119, Tambo River Formation (Mitchellian), PL3110, cutting on Swan Reach- Metung Road, left bank Tambo River, c. 200 m south of Princes Highway, Swan Reach, Gippsland, Victoria, coll. F. A. Cudmore, 1936-1940; three paratypes, NMV P302120-302122, 2 RVs and 1 LV, all data as for holotype; one paratype, NMV P302118, Tambo River Formation (Mitchellian), PL3267, Serripecten carteri locality, Lake Tyers, coll. A. G. Beu & T. A. Darragh, 21 .xi.1993; one paratype, NMV P302123, Black Rock Sandstone (Cheltenhamian), above nodule bed, Beaumaris, near Melbourne, Victoria, coll. F. A. Cudmore, 1903.
Other material examined. Mitchellian: TAMBO RIVER FORMATION: PL3110, Swan Reach cutting (190 specimens); PL3267, Reedy Arm peninsula, Lake Tyers (39 specimens). Cheltenhamian: BLACK ROCK SANDSTONE: Beaumaris (17 specimens); above nodule bed, Beaumaris (22 specimens). Occurrence and time range. Mitchellian and Cheltenhamian. Common at both Mitchellian localities in the Gippsland Basin. Uncommon and a little abraded, with much of the microsculpture polished off, in Black Rock Sandstone at Beaumaris (Cheltenhamian). We have seen it from only these three localities. M. divergens has a narrow time range, but is absent from the numerous Cheltenhamian outcrops of normal silty sandstone (lower Jemmys Point Formation) throughout the Gippsland Basin, and so appears to be limited to lithologies with a low sedimentation rate, such as greensand.
Remarks. Mesopeplum divergens sp. nov. is a very distinctive species, because of its wide shape, its flat LV, its coarse radial sculpture, its weak plicae with one or a few narrow plicae near the centre of the valve more prominent on the LV than any of the others on either valve of most specimens, its very obvious secondary shell surface formed by the commarginal lamellae on most LVs, and the unique, low triangular scales on the major costal crests, present on the costae on the crests of all plicae on some specimens, and at least on the ends of the LV and on the anterior auricle on almost all specimens. The specimens from Beaumaris have the microsculpture abraded off, including the triangular scales, but agree with the Gippsland material in all other details, such as the shape of the RV posterior auricle, and are certainly conspecific. The shape of the auricles is very similar to that of the younger species M. meringae, and it appears likely that M. divergens was either ancestral to M. meringae or, more probably, they are sister species. M. meringae reaches a very much larger size than M. divergens (up to 100 mm high), has proportionally much weaker sculpture with much more numerous radial costae but more normal plicae of relatively high amplitude on most specimens, and lacks the unique triangular costal scales of M. divergens. Etymology. The specific name (Modem Latin, ‘divergens', diverging) refers to the unusually obviously curved, diverging radial costae near the anterior and posterior ends of the disc in this species.» ALAN GLENN BEU & THOMAS ALWYNNE DARRAGH, 2001
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