Tosapectinidae Trushchelev, 1984
TRUSHCHELEV, A. M. 1984. Novye taksony triasovykh dvustvorok Iakutii [The new Triassic bivalve taxons of Jakutia]. Rossiiskaya Akademiya Nauk Sibirskoe Otdelenie, Trudy. Instituta Geologii i Geofiziki, Akademiia Nauk SSSR 600: 64-72, pl. 10-11 (in Russian).
«The shell reaches large dimensions, is sometimes thick-shelled, of rounded outline, almost equilateral, equivalved or with one of the valves being strongly convex. The auricles are usually large and well delimited from the disk. The anterior auricle is somewhat larger than the posterior. A byssal notch is always present. The ends of the auricles are drawn upward from the hinge [referring to deep scrolls] in both valves. The left valve has two, more rarely, one, lateral swelling [of the external disk surface], separated from the [central part of] the disk by [radial] depressions. Sometimes there is one large lateral swelling on the right valve on the anterior part of the shell. The ornament consists of distinct radial ribs, differing on the right and left valves. Sometimes fine radial lines are observed. Concentric [commarginal] growth lines are usually distinct.
Pseudo-hinge apparatus is well developed and consists of simple cardinal crura [hinge teeth] diverging along both sides from the resilium [resilifer], the lateral bolsters [resilial teeth] on the right valve fitting into lateral sockets on the left valve, and of auricular crura [basal auricular buttresses] with distal teeth [tubercles]. Anterior distal teeth [dorsal teeth?] are absent. The inner ligament is situated in the triangular-oval resilifer. The inner surface [of the disk] is ribbed, with a large oval muscle [adductor] scar displaced toward the posterior margin. The pallial line is whole [i.e., entire, not discontinuous], becoming more deeply impressed and consisting of pits in the dorsal part of the shell.» A. M. TRUSHCHELEV, 1984
[Translation from Russian by R. D. Johnson, with additions or clarifications in brackets. From T. R. Waller, & G. D. Stanley Jr., 2005, Middle Triassic Pteriomorphian Bivalvia (Mollusca) from the New Pass Range, west-central Nevada: Systematics, biostratigraphy, paleoecology, and paleobiogeography, p. 44].
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«Genera in the Tosapectinidae display a succession of morphological changes during the Triassic that suggest a possible origin for the Neitheidae. There is a reduction in the size of the auricles relative to the size of the disk, an increase in the inequality of valve convexity (some Upper Triassic Tosapecten s.s. becoming strongly right convex), and a shallowing of the byssal notch (Waller, in Waller & Stanley, 2005: 45). The oldest tosapectinine genera, Janopecten Arkhipov & Trushchelev, 1980 (upper Anisian to lower Carnian of north-eastern Russia and central Siberia) and Nevadapecten Waller (in Waller & Stanley, 2005) (upper Ladinian of Nevada), have valves that are nearly equiconvex or left convex, auricles that are relatively large, and a relatively deep byssal notch. Tosapecten (Tosapecten) Kobayashi & Ichikawa, 1949 and Tosapecten (Indigiropecten) Trushchelev, 1984, both from the Upper Triassic, have relatively short auricles and a shallower byssal notch. In Tosapecten s.s., the right valve is deeply convex and the left valve nearly flat, at least in early ontogeny. In both Tosapecten s.s and Tosapecten (Indigiropecten), scrolling of the auricles is common and may be extreme in some species.»
WALLER, T. R. 2006. Phylogeny of families in the Pectinoidea (Mollusca: Bivalvia): importance of the fossil record. Zoological Journal of the Linnean Society, 148: 313-342, figs. 1-12. [p. 329]
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Tosapecten subpolaris Polubotko, sp. nov.; Y. M. Bychkov, A. S. Dagys, A. F. Efimova & I. V. Polubotko, 1976, Atlas of the Triassic Fauna and Flora of Northeastern USSR, plate 35, figures 5-11.
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«Subfamily TOSAPECTININAE Trushchelev, 1984 [emended]
[...]
Emended diagnosis.— Pectinoidea of medium to large size, having distinct radial plicae that lack internal carinae and begin in very early ontogeny. Left valve with length at maturity commonly exceeding height, the rapid ontogenetic increase in length producing disk flanks that are outwardly concave in plan view. Anterior and/or posterior sectors of left valve commonly flattened or inflated, the plicae on these sectors weaker and commonly more closely spaced than on the central sector. Strong valve inequality common, either the right or left being the more convex. Auricles noncostate or only weakly and sparsely costate, with free margins of auricles commonly meeting the hinge line at acute angles; anterior auricle slightly longer than posterior; dorsal margins of right auricles commonly with hypertrophied scrolls, especially on the anterior; auricles of left valve with straight or only very weakly scrolled dorsal margins; byssal notch of variable depth, commonly persisting throughout ontogeny; ctenolium absent in late ontogeny, not observed in early ontogeny. Antimarginal microornament obscure or absent; disk flanks non-costate and without cross-ridging; regular commarginal lirae commonly present on disk and auricles. Hinge dentition simple, dominated by resilial teeth and sockets; dorsal teeth weak or absent.
Discussion.— The subfamily Tosapectininae as currently understood is known only from the Middle and Upper Triassic and consists of three genus-group taxa: Janopecten Arkhipov and Trushchelev, 1980, of the upper Anisian to lower Carnian of northeastern Russia and central Siberia; Tosapecten (Tosapecten) Kobayashi and Ichikawa, 1949, of the lower Carnian to upper Norian of Asia and western North America; and Tosapecten (Indigiropecten) Trushchelev, 1984, of the early and middle Norian of central Siberia. All of these reach a large size (60 mm or larger) and are radially plicate. The main differentiating characters among the genera involve differences in convexity between the two valves, the extent of scrolling of the dorsal margins of the right auricles, the relative size of auricles relative to size of disk, and the degree to which the byssal notch becomes shallow in late ontogeny.
Arkhipov and Trushchelev (1980, p. 12) observed that early representatives of Janopecten in the Anisian have a left valve that is only slightly more convex than the mildly convex right valve. Beginning in the Ladinian and extending into the early Carnian, however, species of Janopecten developed strongly inequivalved shells with the right valve flat or even concave and the left valve strongly inflated. The auricles are relatively large and nearly equal in size, the right auricles having only weakly developed scrolls or none at all. The byssal notch remains deep throughout ontogeny. Tosapecten s.s., in contrast, has a mildly to strongly convex right valve and a flat to concave left valve. Its right anterior auricles are commonly deeply scrolled, and there is a tendency for the byssal notch to become shallow in later ontogeny. Arkhipov and Trushchelev (1980, p. 12) thought that Tosapecten subpolaris Polobotko in Bychkov et al., 1976 may exemplify a transitional state between Janopecten and Tosapecten s.s. This species occurs in the lowermost Carnian (the Protrachyceras omkutchanicum Zone) of northeastern Russia (Bychkov et al., 1976, p. 11, 74).» WALLER, T. R. & G. D. STANLEY JR. 2005. Middle Triassic Pteriomorphian Bivalvia (Mollusca) from the New Pass Range, west-central Nevada: Systematics, biostratigraphy, paleoecology, and paleobiogeography. Journal of Paleontology, [Memoir Paleontological Society 61], 79 (1): 1-59, figs. 1-14. [p. 44, 45]
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