Ruthipecten campestris Quilty, Darragh, Gallagher & Harding, 2016
QUILTY, P. G., T. A. DARRAGH, S. J. GALLAGHER & L. A. HARDING. 2016. Pliocene Mollusca (Bivalvia, Gastropoda) from the Sørsdal Formation, Marine Plain, Vestfold Hills, East Antarctica: taxonomy and implications for Antarctic Pliocene palaeoenvironments. Alcheringa: An Australasian Journal of Palaeontology, 40 (4): 556-582. [p. 14, figs. 12A-12E]
2016 Ruthipecten campestris Quilty, Darragh, Gallagher & Harding, 2016
P. G. Quilty, T. A. Darragh, S. J. Gallagher & L. A. Harding, 2016, figure 12.
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«1985 Chlamys tuftsensis Turner; Beu, p. 7.
1986 Chlamys tuftsensis Turner; Adamson & Pickard, p. 70. 1998 ‘Chlamys’ (Zygochlamys) tuftsensis Turner; Jonkers, p. 165, fig. 3c. 2003 Austrochlamys tuftsensis (Turner); Jonkers, p. 66, pl. 12, fig. i. 2014 Ruthipecten tuftsensis (Turner); Beu & Taviani, p. 314, fig. 5B–F. Etymology. campestris—(L)—a plain, referring to the source on Marine Plain.
Occurrence. Localities 1, 2, 7.
Material. Several complete specimens, abundant fragments: P302292, P320605, P302684, P302296, P302297, P302300, P302310, P302315–302318, P302320, P302529, P320606. Most are internal moulds, and shell surface details generally are not well preserved except on fragments. Holotype: NMV P302316. Paratypes: NMV P302292, P302297, P302318, P320605.
Diagnosis. Ruthipecten with 5–6 very well-developed prominent nodes more commonly on costae towards the margin of adult shells. Concentric growth lines cross costae without causing costae to have irregular rough surfaces.
Description. Shell size similar to that of other records of the genus (65–71 mm), length and height approximately equal (Fig. 12A–E); inequivalved, right valve less inflated than left. Valves equilateral with posterior auricle smaller than anterior. Anterior margin of right auricle with distinct byssal notch with ctenolium of 14–15 teeth. Umbonal angle highly variable from 58–100°, commonly widening shortly after a lower angle initial stage. Sculpture of 5–6 strong radial primary rounded costae, with five, in some cases as few as four or as many as seven, secondaries of irregular expression between. Shell marked by fine commarginal primary growth lines that cross costae smoothly. Towards margin, primary costae commonly develop 5–6 prominent nodes with sharply defined edges. Nodes vary greatly in expression from subtle rises to very prominent. Margin slightly scalloped. Internal features not observed owing to incomplete preservation.
Life mode. Epifaunal, epibyssate, suspension feeder. Strongly auriculate, loose byssal attachment.
Dimensions Length 65–71 mm, average 69 mm; height 61–71 mm, average 66.5 mm. All measurements recorded are from adult specimens.
Discussion. Most material was incomplete as internal moulds. A few complete adult valves and a few fragments with some surface detail are well preserved.
Ruthipecten is a very characteristic genus distinguished by the presence of prominent nodes on most specimens. Until now, specimens of this genus from Marine Plain have been identified as R. tuftsensis (Turner 1967). The type unit and locality for R. tuftsensis (as Chlamys) is the Prospect Formation of Wright Valley, McMurdo Sound (Turner 1967, Vucetich & Topping 1972, Jonkers 1998b). The age of that formation is now taken to be 5.5–6 Ma (late Miocene; Beu & Taviani 2014) and thus significantly older than the Sørsdal Formation. That species was considered to be an early Pliocene index and was the initial evidence for the age of the Marine Plain deposits (Adamson & Pickard 1986). Jonkers (2003) placed Ruthipecten in Austrochlamys Jonkers, 2003 along with Austrochlamys anderssoni. This inclusion, in contrast to that of Beu (1985), was based on the shell shape, the low ratio of LV and RV convexity, sculpture, elongate outer ligament, acute byssal notch and weak dorsal dentition. Beu (1985) argued that this genus is ‘unrelated phylogenetically’ to other species in the Scallop Hill Formation, Antarctica, and Jonkers (1998b) clearly had doubts concerning the relationships of this form and noted (Jonkers 2003) that ‘‘Chlamys’ tuftsensis would appear to be an odd member of Austrochlamys’. Beu & Taviani (2014) recognized this apparent oddity and introduced the new genus Ruthipecten. The nodose appearance of the costae is the most notable feature that distinguishes R. tuftsensis from Austrochlamys anderssoni, which also occurs in this fauna. Beu & Taviani (2014) figured several of the specimens illustrated here and, in their description, provided information to suggest a specific difference between Sørsdal Formation specimens and R. tuftsensis from the type locality. Jonkers (2003) figured a specimen from Marine Plain and mentioned that costae in Marine Plain material are especially nodose. Ruthipecten campestris sp. nov. is distinguished from R. tuftsensis in the character of the costae and the prominent nodes. As shown by Jonkers (2003, pl. 12, figs f–h) and Beu & Taviani (2014, fig. 5A), as commarginal growth lines cross costae in R. tuftsensis, the costae are affected to develop irregular surfaces. This is not the case in the new species where the growth lines appear to cross smoothly without causing such irregularities; this feature is apparent in the figured specimens (Fig. 12A, D) and on fragments, the limited sources of information on the shell surface. Ruthipecten campestris appears to be less inflated than R. tuftsensis. The nodes in R. campestris are best developed close to the distal end of costae around the margin. Turner (1967) argued that ‘Chlamys’ tuftsensis might have lived in several hundred feet (ca 100 m) of water during the Pliocene but that estimate is in marked conflict with evidence for a shallow-water environment at Marine Plain.» PATRICK G. QUILTY, THOMAS A. DARRAGH, STEPHEN J. GALLAGHER & LUCY A. HARDING, 2016
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