Austrochlamys heardensis (Fleming, 1957)
FLEMING, C. A. 1957. A new species of fossil Chlamys from the Drygalski Agglomerate of Heard Island, Indian Ocean. Journal of the Geological Society of Australia, 4: 13-19. [p. 16, pl. 1, figs. 1,3; text-fig. 1D]
1957 Chlamys (Zygochlamys) heardensis Fleming, 1957
C. A. Fleming, 1957, plste 1.
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«Material: A block of hard cemented volcanic conglomerate (F15297) bearing a right valve, part impression, part shelly (holotype), the beak portion of another right valve and a fragment of sculpture, and two similar blocks (F15298, F15299) bearing three right valves (paratypes), in the Department of Geology, University of Adelaide. Rubber casts made from the holotype show encrusting Polyzoa (indeterminable Cheilostomata Ascophora, Dr. D. A. Brown oral comm.), and serpulid worms (Spirorbis).
Description: Right valve large, solid, slightly inflated, circular in outline, height and length equal. Hinge line straight, anterior ears [auricles] slightly longer than posterior. Beak angle about 115°. Sculpture of strong flatly rounded radial ribs, mostly grouped in twos, owing to the gemination of each rib by the appearance of a groove on its summit. Fifteen ribs at 10 mm. from the beak already include one or two which have arisen by development of a median groove. At greater diameters, more and more ribs subdivide at irregular intervals and become as strong as undivided ribs so that at 40 mm from the beak there are about 9 double ribs and 15 undivided ribs, a total of about 33. At the ventral margin, about 40 ribs are mostly grouped in pairs, with interspaces between each pair about the width of a rib, those between members of a pair about half as wide. A few ribs at the margin bear faint grooves showing the beginning of a secondary splitting. Concentric sculpture of strong imbricate incremental lamellae, free distally, about 2.5 per mm. The paratypes agree remarkably well with the holotype in the arrangement and number of ribs, as far as can be determined from incomplete specimens. Posterior ear obtusely angled, sculptured by strong incremental lamellae crossing 4-5 faint radial ribs. Anterior ear elongate, narrow, with a few (2-3) obsolete radial ribs. Byssal notch and fasciole not preserved in holotype. Sub-margins obscurely preserved but apparently well defined. In a paratype (F15299) the byssal notch is seen to be shallow. Dimensions: Height 85 mm., length 84 mm., inflation 15 mm. (right valve, holotype, F15297). Locality: Boulder at foot of scree slopes, west end of West Bay, south-west coast of Laurens Peninsula, Heard Island, apparently fallen from cliffs rising 500-750 ft. above sea level. Mr. Stinear states that the specimen is from the Drygalski Agglomerate of Lambeth (1952) but that its horizon within that formation cannot be determined. Comparisons: Of the species of Zygochlamys listed above, C. heardensis is most like C. anderssoni Hennig from the Pliocene or Pleistocene Pecten Conglomerate of Cockburn Island, Grahamsland, agreeing in shape and size, and in the initial number of primary ribs, which multiply by dichotomy, but differing in its fewer and broader ribs in the adult. In C. anderssoni, the ribs increase not only by dichotomy but also by the intercalation of secondary and tertiary intercostal riblets, so that a shell 73 mm. in height (Hennig, 1911, pp. 13-14, pl. 1, fig. 4) has about 70 ribs, much narrower and more closely packed than in C. heardensis, and in larger specimens the ribs no longer appear clearly grouped in pairs. Chlamys anderssoni has been classed by Hennig as an evolutionary development of the Patagonian (Miocene) C. geminata (Sow.), with which it agrees in young stages. In C. geminata, however, 4 to 5 secondary ribs generally arise by intercalation between the dichotomous primary ribs. The tendency to increase ribs by intercalation is accentuated in C. actinodes (Sow.) (which largely lacks dichotomous ribs) and is also characteristic of the New Zealand forms. Specimens of C. geminata with rather broad dichotomous ribs and few intercalated secondaries, i.e. with the appearance of intercostals delayed or suppressed (e.g. Ortmann, PL 23, Fig. 2d), resemble C. heardensis quite closely. Chlamys mawsoni Fletcher from Kerguelen is geographically the closest Zygochlamys to C. heardensis, but differs greatly in specific characters, so far as can be judged by comparison of the left valve of mawsoni with right valves of heardensis. C. mawsoni is higher, more oval in outline, with many close-set subequal ribs at the ventral margin, whereas the corresponding left valve of C. heardensis probably has fewer ribs and wider interspaces bearing secondaries. Age: Chlamys heardensis is from a marine conglomerate that is presumably interbedded in the Drygalski Agglomerate, above the Laurens Peninsula Limestone of Laurens Peninsula, which is attributed to the Palaeogene (Lambeth, 1952). No Eogene species of Zygochlamys have yet been recognised, so C. heardensis is almost certainly post-Oligocene. Apparently, Zygochlamys arose in the Southern Ocean, and spread north into Subantarctic latitudes during the Neogene, helped by the northward movement of climatic belts during the late Tertiary cooling. Such a history is supported by the stratigraphic and geographic distribution of Zygochlamys in New Zealand, where the subgenus is first known in the Uppermost Miocene to earliest Pliocene of Campbell Island (Fleming, 1950) and the Chatham Islands (Fleming, 1944; Marwick, 1928) and reached north to latitude 40° S. in the early Pleistocene. The geographically separated races of Zygochlamys in South America, Grahamsland, Kerguelen, Heard Island and New Zealand differ from each other quite as much as do successive forms in New Zealand and Patagonia, so it is difficult to judge the age of C. heardensis solely from its morphology, in the present state of knowledge. Likewise, the morphological differences between C. heardensis and C. mawsoni do not necessarily imply differences in age. It seems probable, however, that C. heardensis could have developed, like the Cockburn Island C. anderssoni (Hennig) which it most resembles, from ancestors resembling the Patagonian Miocene C. geminata (Sow.). No living species of Zygochlamys is known closer to the Antarctic than Macquarie Island. Indeed, Chlamys is absent from the Antarctic fauna (Soot-Ryen, 1951); probably no scallop of this genus could exist on the shelf surrounding a fully glaciated land. This suggests that C. heardensis and C. anderssoni are either pre-glacial (as suggested by Soot-Ryen, 1952, for C. anderssoni) or less probably interglacial. All told, a Pliocene age seems most likely, but upper Miocene or interglacial Pleistocene are not ruled out by the scanty facts available.» CHARLES ALEXANDER FLEMING, 1957
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«Fleming (1957) had only the holotype (right valve partly decorticated), another specimen of the beak area and three other right valves. The new material provides a much larger population and there is variability not possible for Fleming (1957) to see. While most right valves recovered agree precisely with his description (supporting his statement of remarkable agreement of paratypes with the holotype), there is some variation. For example, one specimen has a beak angle of only 90° but the anterior and posterior margins of the disc are gently concave so that the margins ultimately diverge at 120° and there are more and finer posterior ribs than on the holotype. Another has only the posterior margin concave.
On most right valves, the main body of the disc has only double ribs as obvious on the holotype, but in the left valve described, these commonly are separated by an undivided rib arising close to the umbo by intercalation.» QUILTY, P. G., C. V. MURRAY-WALLACE & J. M. WHITEHEAD. 2004. Austrochlamys heardensis (Fleming, 1957) (Bivalvia: Pectinidae) from Central Kerguelen Plateau, Indian Ocean: palaeontology and possible tectonic significance. Antarctic Science, 16 (3): 329-338. [p. 335, 336]
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Austrochlamys heardensis Fleming; P. G. Quilty, C. V. Murray-Wallace & J. M. Whitehead, 2004, Austrochlamys heardensis (Fleming, 1957) from Central Kerguelen Plateau, figure 3.
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«Stratigraphical age and distribution. ?Late Miocene (<9.4 ±
2.4 Ma; see 'Locality information'; Fig.25). Known only from Heard Island (Figs. 3, 11).
Remarks. Austrochlamys heardensis is known from a limited number of right valves only; as in A. anderssoni, to which it bears a strong resemblance, left valves are probably thinner and more prone to destruction by hydrodynamic (and periglacial?) processes. Similarities berween the two species are the rather large valves and the prominent commarginals (which, however, are rather more closely spaced in A. heardensis); differences are a more circular disc in A. heardensis (Fig. 27), a higher umbonal angle, a less complex costation pattern, with rib interspaces not occupied by secondary and tertiary riblets, a higher auricular symmetry, a lower RV anterior auricle, and a relatively deeper byssal notch than in A. anderssoni (Fig. 4l). A. heardensis differs from A. natans walosseki in having a relatively longer outer ligament, and a much deepet acute byssal notch (Fig. 41, Plate 12, fig. e).» JONKERS, H. A. 2003. Late Cenozoic-Recent Pectinidae (Mollusca: Bivalvia) of the Southern Ocean and neighbouring regions. Monographs of Marine Mollusca, 5: i-viii + 1-125 pp, 17pls. Backhuys Publishers, Leiden [p. 66]
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Austrochlamys heardensis (Fleming); H. A. Jonkers, 2003, Late Cenozoic-Recent Pectinidae (Mollusca: Bivalvia) of the Southern Ocean and neighbouring regions, plate 12, figure e.
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