Amusium subcostatum 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. 174, figs. 63B, D, E, 64D, E]
2001 Amusium subcostatum Beu & Darragh, 2001
A. G. Beu & T. A. Darragh, 2001, figures 63, 64.
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«Description. Small for genus, to 55 mm high and 60 mm long, slightly longer than high, acline, unusually thick and solid for genus, with low, faint, radial corrugations on exterior. Umbonal angle of large specimens up to 120°; anterodorsal margin similar in length to posterodorsal margin, shape of most specimens symmetrical. LV preradial area apparently c. 4 mm high, sculptured only with very faint, low, commarginal corrugations. Radial sculpture of disc of 25-28 extremely low, faint, wide, poorly defined corrugations, each slightly wider than one interspace on RV and slightly narrower than one interspace on LV, more clearly defined as low, narrow, widely spaced costellae over proximal third to half of auricles on some specimens, with very numerous, exceedingly fine, widely spaced, truly radial threads over entire surface. Interior with pairs of narrow, well raised rib carinae, very short near anterior and posterior ends of disc and occupying distal third to half of disc height near centre of disc, carinae in each pair spaced slightly further apart in RV than LV in specimens available, 22 pairs in LV and 23 pairs in RV; each pair ending at narrow, sharp ridge just inside ventral margin of disc, the tips of each pair inclined in slightly towards each other. Auricles very large for genus, almost equilateral and almost the same shape in the two valves, with slightly rounded anterodorsal and posterodorsal extremities, anterior margin weakly sinuous on both valves, with wide, very shallow byssal notch on RV; no ctenolial teeth visible in auricular groove in any specimens available. Auricles separated from disc by being obviously depressed below disc surface, but without undercut groove. Hinge with one pair of obvious, wide, low dorsal teeth in RV and sockets in LV. Base of each auricle with a small, sharply rounded interior nodule a short distance in from valve margin, where auricle margin meets disc margin.
Dimensions.
Type material. Holotype, a LV, NMV P300332, Jan Juc Formation, ‘Spring Creek’ (= Bird Rock cliffs), Torquay, Victoria, coll. J. Jutson; paratype, a LV, NMV P3876, upper beds, Bird Rock cliffs, Torquay, Victoria (Jan Juc Formation, Janjukian), ex Geological Survey of Victoria; paratype, a RV, NMV P3877, labelled ‘Waurn Ponds, near
Geelong’, but we have seen no other specimens from Waurn Ponds Limestone at Waurn Ponds, and the shelly, pale brown, highly glauconitic matrix of this specimen matches that of specimens from Jan Juc Formation, Torquay, so there is little doubt that is the correct provenance; ex Geological Survey of Victoria; paratype, a LV, NMV P300329, Jan Juc Formation, ‘lowest bed (bluish grey sandstone) visible in Half Moon Bay (= zone 1 of Cudmore)’, Bird Rock Cliffs, Torquay, Victoria, coll. F. A. Cudmore. Other material examined. Janjukian: JAN JUC FORMATION: PL3024, Bird Rock (4 specimens); PL3025, Fishermans Steps (4 specimens); Spring Creek, Torquay (9 specimens); Torquay, mislabelled ‘Waurn Ponds’ (2 specimens); Torquay, mislabelled ‘Western Beach, Corio Bay’ (2 specimens). Longfordian: MANNUM FORMATION: PL3182, granite quarry, near Mannum (12 specimens); PL3184, quarry at Taworri (9 specimens); PL3I87, quarry at Younghusband (7 specimens); WAM 85.764, small pit by Murray River E bank road, 5.5 km S of Younghusband store (1 specimen). Occurrence and time range. Janjukian and Longfordian; seen only from Jan Juc Formation at Torquay (Janjukian) and a few small, poor specimens from Mannum Formation (Longfordian), Murray Basin. Remarks. Amusium subcostatum sp. nov. is very distinct from any of the other Miocene to Recent species of Amusium we know of, because of its small size, its wide shape, its thick, heavy shell for an Amusium species, its relatively large auricles, which are very much longer in relation to the length of the disc than in any younger species and, above all, its very weak radial sculpture. As far as we are aware, this sculpture is the first recognised on any species correctly referred to Amusium on the basis of Waller’s (1991, 1993) phylogenetically useful characters [Waller (1991: 38-39) pointed out that American ‘Amusium’ species are homeomorphs that have evolved from Euvola]. Although slight abrasion of these relatively early fossils makes it difficult to be sure of the LV preradial characters, the holotype appears to be little-enough abraded to be showing the true LV preradial sculpture, and bears only faint commarginal corrugations. This and all the other at least reasonably typical Amusium characters of this species indicate that it is correctly referred to Amusium, sensu stricto, and apparently represents a very early species of the genus that still retains the last vestiges of radial sculpture. The large auricles and the presence of weak external radial costae indicate that this species represents an early stage of evolution of Amusium from the ancestral genus Amussiopecten, but this transition evidently took place outside southern Australia. Etymology. The specific name (Latin, ‘sub-’, under, less than + ‘costatus’, ribbed) refers to the very weak, but definitely present, radial costae of this species.» ALAN GLENN BEU & THOMAS ALWYNNE DARRAGH, 2001
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