Ylistrum morganense (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. 175, figs. 64A, 65A-E]
A. G. Beu & T. A. Darragh, 2001, figures 64, 65.
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«Pecten lucens Tate, 1886: 115 (in part; specimens from ‘River Murray cliffs at Blanchetown’).
Description. Rather small for genus (to c. 78 mm high), acline to slightly prosocline, weakly inflated, slightly higher than long to slightly longer than high, thin and fragile, similar in general appearance to A. pleuronectes. Umbonal angle c. 120° in large specimens; posterodorsal disc margin slightly longer than anterodorsal disc margin. LV ‘preradial’ sculpture not seen. Sculpture of exterior consisting only of weak, fine, close commarginal striae and similar radial striae, but slightly abraded on all specimens; faintly darker radial rays at positions of internal carinae visible through shell in some specimens, as in A. pleuronectes. Interior with 30-32 pairs of very narrow, closely spaced carinae in LV, 34 pairs in the one available RV, clearly arranged in pairs, each pair on a more elevated area than the interspaces, weak and obscure over a wider area near anterodorsal margin than near posterodorsal margin; carinae end in short ridges, the ridges of each pair inclined inwards towards each other. Auricles separated from disc by very narrow, shallow grooves; without obvious radial sculpture, relatively large, wide, c. 1.25 times the width of auricles on similar-sized specimens of A. pleuronectes; RV auricles with dorsal margins diverging outwards dorsally much more markedly than in A. pleuronectes, ie., auricles markedly taller than in A. pleuronectes, with rounded anterodorsal and posterodorsal extremities; anterior margin of RV anterior auricle with narrow, very shallow byssal notch, very slightly swollen above neighbouring disc surface, margin outline above notch regularly convex; RV posterior auricle with evenly convex outline, inclined slightly towards anterior. LV auricles closely similar to those of A. pleuronectes, except that anterior margin of anterior auricle is slightly sinuous, with very shallow byssal sinus occupying lower half of margin. Hinge with one pair of narrow dorsal teeth in RV, narrow sockets in LV. Each auricle with low, rounded nodule at base, just inside valve margin where auricle margin meets disc margin; much lower and less obvious than similar nodules on A. pleuronectes. Dimensions.
Type material. Holotype, NMV P302133, Morgan Limestone (Balcombian), PL3198, cutting on western approach to Blanchetown bridge, west bank Murray River above Blanchetown, South Australia, grid ref. Blanchetown 723984, coll. A. G. Beu & T. A. Darragh, 23.ix.1993; paratype, NMV P302134, Morgan Limestone, ‘white calcareous bed, River Murray cliffs near Morgan, South Australia’, coll. F. A. Cudmore, 1919-1929; paratype, NMV P302132, Morgan Limestone, PL3203, cliff on left (east) bank Murray River, about 10 m above lagoon at South Australian Water Ski Assoc, facility, 7 km NNE of Blanchetown, South Australia, grid ref. Blanchetown 753044, coll. A. G. Beu & T. A. Darragh, 25.ix.1993; 3 paratypes, SAM T970A-C, Tate’s (1886) paralectotypes of Pecten lucens, ‘River Murray cliffs at Blanchetown’, ie., Morgan Limestone, Balcombian.
Other material examined. Balcombian: Cadell Marl: PL3084, Cadell Marl lens (1 specimen); Morgan Limestone: PL3189, cutting, Mannum pumping station (1 specimen); PL3195, Swan Reach, SA (2 specimens); PL3198, Blanchetown bridge cutting (16 specimens); PL3202, Adams St, Blanchetown (3 specimens); PL3203, South Australian Water Ski Assoc. (19 specimens); PL3205, Woods Flat Post Office (7 specimens); PL3208, Murray cliffs, Great Yarra Reach (3 specimens); white calcareous bed, River Murray cliffs, Morgan district (4 specimens); Tate’s bed 9, River Murray cliffs at Morgan (2 specimens); WAM 79.3043, E bank Murray River 6.4 km N of Blanchetown (2 specimens); Nullarbor Limestone: WAM 67.426, Loongana Quarry (1 specimen). Occurrence and time range. Balcombian; almost all material is from Morgan Limestone, Murray Basin; one specimen seen from Nullarbor Limestone in the Eucla Basin. Remarks. Amusium morganense sp. nov. is similar to A. pleuronectes (type species of Amusium) in many characters, but is easily distinguished by its larger auricles, its slightly narrower byssal notch, the much more markedly dorsally diverging dorsal margins of the RV auricles, other more subtle differences in auricle shape, and the very much more numerous, more closely spaced internal rib carinae, which are clearly arranged in closely spaced pairs. A. pleuronectes has 22-34 individual carinae, rather than pairs of carinae (ie., about half the number of carinae of A. morganense) and the space between the carinae in each pair is not elevated into costal remnants in A. pleuronectes as it is in P. morganense, nor are the tips of carinae pairs inclined inwards towards each other as in A. morganense. Indeed, the carinae in A. pleuronectes are evenly spaced and not obviously arranged in pairs at all, except for 1-2 pairs at the anterior and posterior ends of the disc. The shell size and the shape and inflation of the disc are very similar in the two species, and a few specimens of A. morganense are well enough preserved to show faint radial color bands on the exterior (Fig. 65D) corresponding to the internal carinae, as in A. pleuronectes. The new species therefore makes a plausible member of a lineage leading to the Recent A. pleuronectes. A single LV (Fig. 65E) from Nullarbor Limestone (Middle Miocene) has the large auricles and numerous, closely spaced, clearly paired internal carinae of A. morganense, and appears to be a typical specimen of the new species. It occurs in Nullarbor Limestone (WAM 67.426) with poorly preserved specimens of a much larger species, similar to A. balloti. Etymology. The specific name (‘from Morgan’) refers to the origin of this species from the Morgan Limestone.» ALAN GLENN BEU & THOMAS ALWYNNE DARRAGH, 2001
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«Distribution. Middle Miocene–Recent. The earliest species referable to Ylistrum seems to be Amusium morganense Beu & Darragh (2001: 175, figs 64A, F, 65A–E), from Morgan Limestone (Balcombian Australian local stage, middle Miocene), River Murray cliffs near Blanchetown, South Australia, and Nullarbor Limestone (also Balcombian), Nullarbor Plain, southern Western Australia. Ylistrum morganense is smaller and has larger auricles and much more closely paired internal rib carinae than Y. balloti. Tropical and subtropical Indo-West Pacific, living littorally.»
DIJKSTRA, H. H. & A. G. BEU. 2018. Living Scallops of Australia and Adjacent Waters (Mollusca: Bivalvia: Pectinoidea: Propeamussiidae, Cyclochlamydidae and Pectinidae). Records of the Australian Museum, 70 (2): 113-330, figs. 1-102. [p. 207]
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