Mimachlamys heterophyseta 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. 141, figs. 44A, 50A-E, 51A-G, 52B, D]
2001 Mimachlamys heterophyseta Beu & Darragh, 2001
«Description. Moderately large for genus (to c. 112 mm high), most specimens thin and brittle for genus; disc shape moderately to strongly prosocline, length equal to or slightly greater than height; umbonal angle c. 90° in juvenile specimens, c. 110-115° in adults; inflation of LV approximately twice that of RV. LV preradial dissoconch microsculpture of deep, oval, radially elongate pits, deeper than on M. asperrima. Disc sculpture of 24-30 narrow, widely spaced, narrowly round-topped, rather low primary radial costae, slightly more prominent on LV than on RV of some specimens; by disc height of 5-10 mm (earlier near anterior and posterior ends than on centre of disc) fine secondary costellae subdivide from costal edges, and strengthen down disc to almost rival primary costae by ventral margin of some specimens; at c. 30-35 mm disc height, fine tertiary costellae initiate in same manner as secondary costellae, one in centre of radial interspace and one between each primary costa and its neighbouring secondary costa; again tertiary costellae strengthen down disc to be only slightly weaker than secondary costellae by ventral margin; on some specimens, fine quaternary costellae are initiated at various heights further down disc; secondary to quaternary costellae wide and low, filling shallowly to quite deeply concave primary radial interspaces, forming complex series of fasciculate rib groups and subgroups which, in many slightly abraded specimens, show only weak distinction between primary and lesser costae. All costal crests, on primary to quaternary costae, bearing evenly but rather widely spaced, low, ventrally cupped, ventrally directed scales. All radial interspaces sculptured with fine antimarginal ridgelets, in wavy, nearly straight groups of short ridges near centre of disc, complexly herringbonepatterned near anterior and posterior ends of disc. RV anterior auricle long and narrow, with deep, narrow byssal notch, depressed, strongly ridged byssal fasciole and functional ctenolium in large adults, ctenolium with 4 stout, hooked functional teeth; main face of auricle, above byssal fasciole, sculptured with 6-7 wide costae as on disc. LV anterior auricle moderately long, with strongly sinuous anterior margin inclined strongly towards anterior, producing moderately deep byssal sinus and obvious, strongly protruding, pointed anterior extremity, sculptured with many low, closely spaced, scaly costae resembling secondary and tertiary costellae of disc. Posterior auricles small, with posterior margin inclined strongly towards anterior and weakly concave in outline, sculptured as LV anterior auricle. Interior of ventral margin weakly sinuous, but without internal rib carinae on largest thin-shelled specimens seen; a few unusually thick-shelled specimens with short, prominent carinae around interior of ventral margin. Hinge teeth weakly developed on most specimens, with low, wide resilial teeth obvious on a few specimens.
Type material. Holotype, NMV P302152, a pair of valves, PL1206, Memana Formation (Early Pleistocene), dam C on ‘Brunah’ Station, Backline Road, 2.3 km NE of junction with Wingaroo (No. 2A) Road, Flinders Island, Bass Strait, grid ref. Flinders I. 915809, coll. D. M. Shanks, H. E. Wilkinson & T. A. Darragh, 4.ii. 1969; one paratype, NMV P302153, Memana Formation, PL1216, dam A on ‘Marapana’ Station, Backline Road, 2.7 km ENE of junction with Wingaroo (No. 2A) Road, Flinders Island, grid ref. Flinders I. 926799, coll. T. A. Darragh, D.M. Shanks & H.E. Wilkinson, 5.ii.1969; 6 paratypes, NMV P302154-8, Memana Formation, PL1215, dam on SW side Wingaroo (No. 2A) Road, 1.8 km NW of junction with Melrose (No. 2) Road, Memana, Flinders Island, grid ref. Flinders I. 963815, coll. T. A. Darragh, H. E. Wilkinson & D. M. Shanks, 5.ii.1969; 10 paratypes, WAM 68.1257a-j, Memana Formation, Cook’s dragline hole, Flinders Island, coll. D. G. Smith, 1968; one paratype, LV, NMV P300273, Memana Formation, PL1217, dam B, Cole’s property, ‘Marapana’ Station, Backline Road, 2.7 km ENE of junction with Wingaroo Road, Memana, Flinders Island, gird ref. Flinders I. 927797, coll. T. A. Darragh, H. E. Wilkinson & D. M. Shanks, 5.ii.1969 (specimen studied by SEM).
Other material examined. Early Pleistocene: MEMANA FORMATION, FLINDERS ISLAND: PLI205, ‘Brunah’ B (12 specimens); PL1206, ‘Brunah’ B (18 specimens); PL1207, ‘Brunah’ D (2 specimens); PL1208, ‘Brunah’ E (9 specimens); PL1215, SW side Wingaroo Rd (35 specimens); PL12I6, ‘Marapana’ A (5 specimens); PL1217, ‘Marapana’ B (6 specimens); PL1218, Oakenfall’s (1 specimen); PLI276, ‘Nangetty’ (4 specimens); PL1280, G. Smith’s (13 specimens); WAM 95.10, Cole’s draglines 1, 2 and 3, Flinders Island (2 specimens); WAM 68.1277, Cole’s dragline No. 2, Flinders Island (2 specimens); ‘YOUNGER’ ASCOT FORMATION, PERTH BASIN: WAM 70.1607, 27.4-29.0 m, Poletti’s No. 2 bore, W side Semple Rd, Jandakot (1 specimen); WAM 72.954, 37.5 m, Jandakot Cement Works bore, Parkes Rd, Yangebup (3 specimens); WAM 74.35, 41.9-42.1 m, Paulik’s bore, Semple Rd, Jandakot (5 specimens); WAM 74.613, 40.6 m in above bore (5 specimens); WAM 75.1028, 42 m in GSWA Jandakot No. 50 Exploratory Bore, Forrestdale (3 specimens); WAM 75.1086, 32.0-34.0 m, GSWA Lake Thompson No. 200 Exploratory Bore, Orton Rd, 100 m W of Cumming Rd, Peel Estate (3 specimens); WAM 98.418, Kardinya, Melville municipal bore, spoil from 31 m in bore (1 specimen); WAM 86.927, Ascot Formation undifferentiated (Pliocene-Pleistocene?), spoil heaps at borehead, Iley’s bore No. 1, corner Forrest and Warton roads, Banjup (8 specimens); WAM 72.1302, 34.8-35.1 m, Jandakot Cement Works bore, Parkes Rd, Yangebup, WA (85 fragments). Middle Pleistocene (oxygen isotope stage 7; G.W. Kendrick, pers. comm.): WAM 89.974, sewerage excavation in grounds of Catholic school, Kelly Drive, Busselton, Geographe Bay, WA (5 specimens); WAM 94.247, spoil from dam excavation, c. 3 m from surface, E side Redgum Way, Dumbarton via Busselton, WA (3 specimens); WAM 94.743, as above, 3.1-3.7 m below surface (3 specimens); WAM 95.8, as above (2 specimens); WAM 95.9, as above (8 specimens). Recent: WAM S12615, Cockbum Sound, S of Fremantle, WA, coll. L. Marsh & A. Paterson, 1971 (1 specimen, both valves, mottled paler and darker purplish pink); WAM S12616, dredged, Cockbum Sound, WA, Harry Baker Collection, donated 1983 (2 live-collected, articulated specimens, small (c. 50 mm high), uniform pale cream, with coarse scales and a prominent secondary costa on each side of each primary costa, similar to specimens from the Ascot Formation, beneath Perth); WAM S12376, Middleton Beach, Albany, south coast of WA, coll. by E .H . Sedgwick, 1929-1982 (16 LVs and 5 RVs, beach-abraded, apparently all M. heterophyseta; purple and brick-red through to yellow, some with radial costae darker than their interspaces); WAM S I2377, Sta. 13, W of City Beach, north of Fremantle, WA, coll. by L. M. Marsh on FRV Flinders, 12.x.1974 (2 valves, pale purplish pink, with 3 small valves of M. asperrima. Occurrence and time range. Apparently restricted to Early Pleistocene to Recent; Memana Formation on Flinders Island, Bass Strait, and ‘younger’ Ascot Formation in boreholes in the Perth Basin, Western Australia; late Middle Pleistocene (oxygen isotope stage 7) deposits around Geographe Bay, southern Western Australia; living around southwestern Western Australia, at least from Fremantle to Albany. Potentially of biostratigraphy utility. Remarks. Specimens of M. asperrima are present in the same collections as M. heterophyseta sp.nov., in most cases. Mimachlamys heterophyseta sp. nov. was first distinguished from M. asperrima by the much greater inflation of the LV than of the RV and, with only RVs in some collections, by the narrower primary radial costae and the much greater number of lesser costae and costellae filling each radial interspace. Other significant differences that help distinguish the two are the markedly more strongly prosocline shape, the thinner shell of Early Pleistocene specimens with (correspondingly?) even weaker traces of internal rib carinae around the ventral margin, and the much less obvious hinge teeth of M. heterophyseta than of M. asperrima. Specimens from water bores around Perth (‘younger’ Ascot Formation) are mostly small, not over about 50 mm high, and RVs predominate over LVs, presumably reflecting sizes and shapes that survive percussion drilling; the small RVs tend to have their primary costae a little abraded, so that much of the radial sculpture is similar in prominence, or at least with the three main costae of each fasciculate group similar in height. The late Middle Pleistocene material from near Busselton, Geographe Bay, includes larger specimens, a few with thicker shells than any of the other material, and a few thick-shelled specimens have short but prominent carinae inside the ventral margin (Fig. 44G), again helping to confirm a position in Mimachlamys. A scatter diagram and a histogram (Figs 48, 49) comparing height with valve inflation in M. heterophyseta and M. asperrima clearly bring out the difference in inflation. The comparison shows that the RV in M. asperrima is slightly more inflated than the LV, whereas the RV is slightly less inflated in M. heterophyseta than in M. asperrima, and the LV is very much more inflated in M. heterophyseta than in M. asperrima. No really small specimens were available of M. heterophyseta unabraded enough for SEM examination, so a relatively large shell (H 29.5 mm; NMVP300273) was examined, and found to display the pitted preradial microsculpture well in the hollows between the umbonal area and the auricles (Fig. 51C, E-G). This confirms a position in Mimachlamys for a species whose relationships were previously obscure to us, because of the lack of internal rib carinae in almost all specimens. However, as noted above, a few of the late Middle Pleistocene specimens of M. heterophyseta sp. nov. from near Busselton, Geographe Bay, have thicker shells than the rest, and have short but prominent carinae inside the ventral margin. At a late stage in this research we realised that it was likely that M. heterophyseta is extant around southern Australia, as all other pectinids known to us in the Pleistocene fauna of southern Australia are still living. Mrs S. M. Slack-Smith (Malacology, Western Australian Museum) kindly searched the Museum’s collections of Recent specimens identified as Mimachlamys asperrima, and recognised the four lots of Recent M. heterophyseta listed above (Fig. 52B, D). It is likely that these few specimens do not represent the full modem geographic range of the species, although it is quite feasible that the formerly widespread M. heterophyseta has become limited to southern Western Australia since the Middle Pleistocene. Careful examination of more collections from southern Australia may help to clarify the present geographic range of this species. The two small, uniform cream, coarsely sculptured specimens in WAM SI2375 (dredged, Cockburn Sound, Western Australia) are livecollected examples of the form identified as M. heterophyseta from Ascot Formation in the many water bores in the Perth Basin (listed above). Their coarse sculpture and cream coloration raise the possibility that they may represent yet another, consistently small (to c. 50 mm high), previously unrecognised species of Mimachlamys, but more Recent material will be needed to verify that. Identification as M. heterophyseta is maintained at present. Etymology. The specific name (Greek, ‘heteros’, different + ‘physetos’, blown or inflated) refers to the strongly discrepant inflation that characterises this species.» ALAN GLENN BEU & THOMAS ALWYNNE DARRAGH, 2001
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