Mesopeplum minimum 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. 157, figs. 56A, C, D]
2001 Mesopeplum minimum Beu & Darragh, 2001
A. G, Beu & T. A. Darragh, 2001, figure 56.
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«Description. Small for genus, to c. 26 mm high; the two valves about equally inflated; weakly to moderately prosocline; umbonal angle 100-105°. LV preradial sculpture not seen. Plicae low, narrow, regular and even, indistinctly margined; 7-8 on most specimens; 1 or 2 growth ledges present on most specimens. Radial sculpture of numerous narrow, well elevated costae, each interspace narrower than one costa on plical crests and about the width of one costa in plical interspaces; plicae and radial costae closely similar on the two valves. Entire surface sculptured with narrow, far-set commarginal lamellae, raised into low scales on costal crests of most specimens. RV anterior auricle small, narrow, with deep, narrow byssal notch, narrow, depressed byssal fasciole and functional ctenolium in adults,
ctenolium with 4-5 very small, narrow teeth, situated on thin, raised ridge extending alongside byssal fasciole; anterior margin of auricle rounded, inclined towards anterior, dorsal margin weakly serrated; main face of auricle above byssal fasciole sculptured with 4-5 prominent, scaly radial costae. LV anterior auricle small, triangular, anterior margin weakly sinuous, inclined very strongly towards anterior, sculptured with 5-6 radial costae as on RV auricle. Posterior auricles very small, triangular, coarsely sculptured, with concave posterior margin inclined weakly towards anterior. Interior almost smooth, weakly plicate, with prominent, very short, very narrow carinae around outermost ventral margin only. RV hinge with long, narrow dorsal teeth and one pair of small, narrow, widely diverging resilial teeth alongside dorsal half of resilifer; LV hinge with long, narrow infradorsal teeth and narrow, obvious sockets for resilial teeth. Type material. Holotype, NMV P302101, Mannum Formation (Longfordian), PL3212, cliff on right bank Murray River, from top of cliffs to 10 m above water, ‘Cave Cliffs’ Station, 8.5 km southwest of Overland Comer, South Australia, grid ref. Pooginook 305185, coll. A. G. Beu & T. A. Darragh, 29.ix.1993; with 10 paratypes, NMV P302798-P302802, all data as for holotype.
Other material examined. Longfordian: GAMBIER LIMESTONE: Glenelg River cliff opposite mouth of Dry Creek (2 specimens); Glenelg River cliff 0.8 km from McEacheron Cave (1 specimen); GIPPSLAND LIMESTONE: Longford (1 specimen); MANNUM FORMATION: PL3188, Mannum pumping station (12 specimens); PL3190, Underwood Rd cutting (1 specimen); PL3191, cutting, Klose Rd, 6 km NE of Mannum (2 specimens); PL3192, Wongulla Rd cutting (2 specimens); PL3195, Swan Reach, SA (4 specimens); PL3196, Big Bend Reserve (4 specimens); PL3199, Murray cliffs, ‘Cooinda’ (9 specimens); PL3212, Murray cliffs, ‘Cave Cliff’ (11 specimens); lower beds, Murray cliffs at Wongulla (1 specimen); lower beds, Murray cliffs, Wongulla to Mannum (1 specimen); lower beds, River Murray cliffs, Overland Corner (7 specimens); Murray cliffs opposite centre of entrance to lagoon, Punyelroo (1 specimen); WAM 90.86, Mannum Motel gardens (2 specimens); WAM 93.486, Blanchetown, River Murray (1 specimen); PUEBLA FORMATION: cliff between Spring Creek and Bird Rock, Torquay (2 specimens); polyzoal limestone, beach N of Bird Rock, Torquay (34 specimens); mouth of Jan Juc Creek (16 specimens); Bird Rock cliffs, Torquay (4 specimens); ZEALLY LIMESTONE: Jan Juc Point, mouth of Spring Creek, Torquay (7 specimens); GIPPSLAND LIMESTONE: PL3252, Brock’s Quarry (1 specimen); PL3253, McColl’s Quarry (3 specimens). Batesfordian: BATESFORD LIMESTONE: bottom of new quarry, Batesford (2 specimens); Batesford filter quarry (1 specimen). Occurrence and time range. Longfordian to Batesfordian. Moderately common in Longfordian limestones along the Torquay coast of Victoria, and in Mannum Formation (Longfordian), Murray River. Two specimens from Gambier Limestone (Longfordian) near Mount Gambier have slightly more numerous plicae and ledges than the rest of the material referred here, but seem to be conspecific.
Remarks. Mesopeplum minimum sp. nov. differs markedly from M. incertum in a number of characters. It has finer radial costae, most specimens are less prosocline in shape, it reaches a much smaller maximum size (H = 25 mm, compared with 45 mm in M. incertum), it has the two valves about equally inflated rather than the RV being much more strongly inflated than the LV as in M. incertum, and the whole appearance of the shell is markedly different from that of M. incertum because of the lower, narrower and more numerous plicae, which have a heightened effect on the appearance because of the much smaller shell size. Despite these differences, the auricles and the ctenolium are closely similar to those of M. incertum, although the ctenolium is situated on a much lower smooth, raised costa than in M. incertum. Although M. minimum occurs consistently in older rocks than M. incertum in both South Australia and Victoria, and they do not overlap in time range, the differences between them are so marked that it seems unlikely that they have an ancestor-descendent relationship. Etymology. The specific name (Latin, ‘minimus’, smallest, least) refers to this being the smallest Australian species of the genus.» ALAN GLENN BEU & THOMAS ALWYNNE DARRAGH, 2001
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