Delectopecten crassistriatus 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. 35, fig. 7A-I]
2001 Delectopecten crassistriatus Beu & Darragh, 2001
A. G. Beu & T. A. Darragh, 2001, figure 7.
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«Description. Small for genus (H 4.8, L 5.2; largest fragmentary paratype 7.6 mm long), rather strongly inflated. Disc acline, almost circular, slightly taller than long. Prodissoconch remaining on umbo of LV (holotype) and presumably on RV, but abraded on available RVs. LV sculpture of many low, narrow, flat-topped, closely spaced, slightly wavy antimarginal ridgelets, crossed by low, very narrow, widely spaced commarginal lamellae with interspaces the width of about 3-5 antimarginal ridgelets. Commarginal lamellae more closely spaced over anterior auricle than on disc, raised into low folds where they cross antimarginal ridgelets, so producing 8 weakly defined radial costellae on anterior auricle only; commarginal lamellae particularly prominent in groove separating auricle from disc. Sculpture of examined RV paratype of much lower and less continuous antimarginal ridgelets than on LV, but apparently significantly abraded; commarginal sculpture apparently not present on LV. Anterior auricle strongly demarcated from disc by deep groove and deep, V-shaped byssal notch; no ctenolium observed, apparently abraded. Auricle bearing 7 prominent, wide, closely spaced radial costellae, crossed by about 15 prominent, wide commarginal ridges, forming low nodules where they cross costae. Internal characters not seen.
Dimensions. Holotype, P301766 H 4.0 L 3.7 LV Paratype, P302786 4.8 5.2 RV Type material. Holotype, NMV, P301766, Pebble Point Formation, Paleocene, PL3003, fallen blocks in cove between Buckley Pt and Pt Pember, 4.5 km SE of Princetown, Victoria, coll. T. A. Darragh, 13.xii.1994; figured paratype, P302786, and 3 incomplete paratypes, P302787-9, from same locality as holotype. Time range and occurrence. Type locality only (9 specimens and fragments); Late Paleocene. Remarks. Delectopecten crassistriatus sp. nov. differs from almost all other species of Delectopecten we are aware of in its small size, its strong ' inflation, and its prominent radial costellae on the anterior auricles, particularly on the RV. The only similar species we are aware of is D. musorstomi Poutiers (1981), living throughout the western Pacific from the Philippine Islands to Norfolk Island and New Caledonia, in 130-500 m (Dijkstra & Marshall 1997: 88, pi. 8, figs 1-6). D. musorstomi is similar in size to D. crassistriatus and has a similarly sculptured but longer RV anterior auricle, but is slightly prosocline in shape and has coarser and more complex disc sculpture than D. crassistriatus. Dr T. R. Waller (1999, pers. comm.) has pointed out that many other Paleogene Delectopecten species (although in many cases not as yet referred to this genus) are similar to D. crassistriatus sp. nov. in their small size and in their strongly costate RV anterior auricle, eg., Pecten (Pseudamussium) alternilineatus Clarke (1918: 132, pi. 13, figs 14, 15). This species is described as 5.5 mm high and 5.0 mm long, with the RV anterior auricle ‘covered by about five subnodose to spinose radiating lines’. However, this species, and all others we know of, clearly differ from D. crassistriatus sp. nov. in their much lower inflation (‘diameter of both valves, 0.75 mm’; Clarke 1918: 132). It seems likely that the more prominent sculpture on the disc of the LV than on the RV of D. crassistriatus is a real character of the new species, although it is possibly due to abrasion of the examined RV; slight differences in sculpture between the two valves occur in some other species of Delectopecten [eg., D. vitreus (Gmelin); Grau 1959: pi. 18]. This is the only fossil species of Delectopecten we are aware of in Australia or New Zealand; the only other species recorded from the region is the New Zealand living Delectopecten fosterianus (Powell) (Powell 1933: 370; 1979: 379, pi. 73, fig. 3), which differs from D. crassistriatus in its much larger size (to 20 mm high) and weaker sculpture. D. fosterianus is sculptured only with weak antimarginal ridgelets and sparse, ventrally cupped scales. Although Delectopecten is largely an abyssal taxon at present, some species were recorded by Grau (1959) from the inner shelf, and it need not indicate a deepwater element in the Pebble Point fauna. Indeed, the surprisingly good preservation of these tiny, fragile shells in the coarse-grained Pebble Point Formation attests to their derivation from a nearby location in shallow water, though the preservation is not complete enough to detect the presence of a ctenolium. Dr T. R. Waller has also pointed out (1999, pers. comm.) that Delectopecten is among many taxa occurring in deep water at low latitudes, but living in shallower water nearer the poles. The living eastern Pacific D. vancouverensis (Whiteaves), for example, has been collected in 18-40 m near Vancouver Island, British Columbia (Grau 1959: 41). Etymology. The specific name (Latin, ‘crassus’, coarse, +‘striatus' grooved) refers to the relatively coarse sculpture of this species.» ALAN GLENN BEU & THOMAS ALWYNNE DARRAGH, 2001
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