Talochlamys articulata (J. de C. Sowerby, 1840)
SOWERBY J. DE C. 1840. Description of fossils from the Upper Secondary Formation of Cutch collected by C. W. Grant. [include descriptions in "Explanations of the plates and Wood-Cuts. Plates XX to XXVI. To illustrate capt. Grant’s Memoir on Cutch", not paginated]. Transactions of the Geological Society of London, Second Series, volume 5, No. 2 [p. 328, pl. 25, fig. 15]
1840 Pecten articulatus J. de C. Sowerby, 1840
1853 Pecten bouey Archiac, 1850
1853 Pecten hopkinsi Archiac in Archiac & Haime, 1853 [partim]
1853 Pecten halaensis Frauscher, 1886
1901 Pecten kokenianus Noetling, 1901
1853 Pecten bouey Archiac, 1850
1853 Pecten hopkinsi Archiac in Archiac & Haime, 1853 [partim]
1853 Pecten halaensis Frauscher, 1886
1901 Pecten kokenianus Noetling, 1901
J. de C. Sowerby, 1840, plate 25.
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«Pecten articulatus. Orbicular, with pointed beaks, depressed, radiated; radii about 28, simple, rounded, crossed by distant scales; ears large, striated, and squamose. Length in an old specimen about 1 inch 8 lines, width 1 inch 6 lines. We have figured a small specimen.
The radii when the scales are worn off appear jointed. Loc. Bank of the Runn.» JAMES DE CARLE SOWERBY, 1840
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«DESCRIPTION: The shell is rather large, moderately thick walled, aequipectinoid shaped, rounded in outline, somewhat higher than long, with obtuse umbonal angle of about 105º in mature individuals, low to moderately biconvex with about 26 prominent ribs fixed early in ontogeny and moderately sized, and with anteriorly inclined ears bearing scaly riblets. The mature individuals in the studied material attain a maximum disc height of 76 mm, whereas the usual height ranges between 50 and 70 mm.
Disc outline is roughly equilateral to slightly inequilateral due to postero-ventral elongation, with umbonal angle less obtuse in smaller individuals and stronger obtuse in adult specimens. That is reflected in the shape of the dorsal margins becoming progressively concave during ontogeny. Beak does not project over the hinge line. convexity is low to moderate with a mean of about 13 % of disc height. Point of maximum convexity is placed in the dorsal disc portion at second quarter of height. Disc flanks are flattened to slightly concave posteriorly and anteriorly with low sigmoid transitions to ear planes in cross section. Disc gapes are absent. Disc sculpture comprises 23 to 27 primary ribs fixed early in ontogeny. At lateral margins one or two additional ribs can subsequently develop. Ribs are prominent, moderately convex, and slightly top-flattened in cross section with narrower, concave interspaces. several riblets are introduced in interspaces in posterior and anterior disc exterior, sometimes developing the prominence of the primary rib. Riblets at the flanks are rare in the studied material, instead projecting scales commonly occur in the corresponding rib flank position. Shagreen microsculpture is developed in rib interspaces at least proximally although it can extend distally as well. disc interior shows plicae that are largely shallow except for the very distal margin where they suddenly broaden and deepen. Although internal rib tops flatten marginally getting quadrangular in cross section no carinae seems to develop. Ears are moderate in size, with height that does not reach 25 % of disc height and length attaining about 55 % of disc length. Their dorsal margins are straight; the lateral margins dip posteriorly by about 60º. Whereas the byssal notch is rather deep, the byssal sinus is moderately deep. Ear surface comprises up to 5 more or less prominent scaly riblets best developed at the right anterior ear. Whereas the posterior dorsal margins are rather smooth, the right anterior one bears scaly riblets as well. Byssal fasciole is broad. Hinge shows short but prominent resiliar tooth pair and equally prominent, strongly elongated dorsal tooth pair starting somewhat distally from the origin of growth. VARIATION: the single specimen from sample 14 (table 3) is distinctly smaller than any specimen of the other sample investigated and we interpret it as young adult individual. The specimen is more convex, has a lower number of primary and secondary ribs and larger ears, but its sculpture and outline agree well with the specimens described above. DISCUSSION: Douglas (1928) was the first author to report the occurrence of T. articulata (as Chlamys senatoria) in the Fars Group. C. senatoria, however, is a younger synonym of Mimachlamys sanguinea (Linnaeus, 1758) (Dijkstra 1999) and lacks shagreen microsculpture. In the Mishan Formation material, in contrast, this microsculpture is well developed (text-fig. 11 and Douglas 1928). Its presence indicates that the Mishan material is not related to Mimachlamys. According to Waller (1991, 1993, 2006), shagreen microsculpture represents a synapomorphic character for unrelated genera of the Chlamydini. The apparent similarity with Mimachlamys sanguinea must thus be disregarded at present and interpreted as representing a product of homeomorphic evolution in consequence of adaptation to a similar habitat type. Modern Talochlamys representatives show shagreen microsculpture at least early in ontogeny. Some Middle Miocene specimens from Australia regarded by Beu and Darragh (2001) as belonging to Talochlamys show apparent similarity with our material. Talochlamys keiloriana shows primary ribs starting at the central disc portion early in ontogeny and a large number of prominent ribs. We follow these authors and place our species into Talochlamys, albeit this genus is usually characterized by intercalations of secondary ribs within the rib interspaces at the disc exterior. In our material, as well as in the Australian specimens, such intercalations became apparently restricted to the very marginal posterior and anterior disc areas, producing the erroneous impression that all ribs start within the proximal disc portion. From the numerous nominal species names available for this species, Pecten articulatus J. de C. Sowerby represents the most senior, valid designation. Vredenburg (1928) was the first to point out the presence of a shagreen microsculpture in that species, enabling differentiation from the otherwise quite similar Pecten somrowensis J. de C. Sowerby. DISTRIBUTION: Miocene of western proto-Indopacific with apparent acme in the lower Miocene. Specimens with shagreen microsculpture have been observed and described from the lower and Middle Miocene Fars Formation in southwest Iran (Douglas 1928; Cox 1936; Eames and Cox 1956), the lower Miocene Gaj Formation, including the species type locality, in west India and southeast Pakistan (Vredenburg 1928), and from the lower Miocene of Burma (Noetling 1901). Morphologically similar specimens without positive evidence for a shagreen microsculpture were documented from the lower Miocene of Tanzania (Cox 1927; Eames and Cox 1956), Madagascar (Collignon and Cottreau 1927), Kenya (Cox 1930), Mozambique (Cox 1939), South Africa (King 1953), Pakistan (d’Archiac and Haime 1853; Vredenburg 1928) and India (Sowerby 1840; Vredenburg 1928).» KROH, A., H. GHOLAMALIAN, O. MANDIC, S. CORIC, M. HARZHAUSER, M. REUTER & W. E. PILLER. 2011. Echinoids and pectinid bivalves from the Early Miocene Mishan Formation of Iran. Acta Geologica Polonica, 61 (4): 419–439. [p. 434, 436]
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Talochlamys articulata (J. de C. Sowerby, 1840); A. Kroh et al., 2011, Echinoids and pectinid bivalves from the Early Miocene Mishan Formation of Iran, text-figure 18.
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