Radulopecten Rollier, 1911
ROLLIER, L. 1911. Les faciès du Dogger ou Oolithique dans le Jura et les régions voisines. Mémoire publié par la Fondation Schnyder von Wartensee, 352 p. Zürich. [p. 158]
«Pecten (Radulopecten) hemicostatus Morr. a. Lyc., Schlippe, 1 valve gauche sur roche et 1 grande valve droite, Syn. P. vagans Laube Biv. Balin, Taf. 1, Fig. 10, non Sow.»
LOUIS ROLLIER, 1911
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Pecten vagans Lbe.; G. C. Laube, 1867, Die Bivalven des braunen Jura von Balin: mit Berücksichtigung ihrer geognostischen Verbreitung in Frankreich, Schwaben, England und anderen Ländern, plate 1, figure 10.
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«We propose the rank of the subfamily Radulopectininae Romanov, 1985 to be raised to the rank of family, and to consider it to be part of the superfamily Chlamydacea Teppner, 1922. Within the family Radulopectinidae Romanov, 1985, besides the nominative, we include as well the new subfamily Pseudopectininae Kasum-Zade subfam. nov., within which we include the genera Pseudopecten Bayle, 1838 and Weddelliopecten Kasum-Zade gen. nov., and “large-toothed” genera, such as Indopecten Douglas, 1929, and Praespondylopecten Romanov, 1987, considered by Romanov (1990) to be in Radulopectininae, is removed from the family Radulopectinidae and considered to be part of the family Spondylopectinidae.
Range. Jurassic – ?Lower Cretaceous (Berriasian). Worldwide. Subfamily Radulopectininae Romanov, 1985, emended Kasum-Zade, herein
Type genus. Radulopecten Rollier, 1911 [type: Pecten hemicostatus Morris et Lycett, 1853].
Diagnosis. Shell rounded or triangular-rounded in outline, inequivalved, irregularly convex, almost equilateral. Auricles unequal, anterior is larger than posterior. Hinge margin is mainly straight. Radial sculpture on valves varies and is represented by plica-like costae {ribs}, approaching in pairs or in threes and often with spines, tubercles. Generic composition. Radulopecten Rollier, 1911 (with two subgenera: Radulopecten s. s. and Fibrosopecten Romanov, 1985); Minervapecten Romanov, 1985; Pamiropecten Romanov, 1985 and Sigmaringenopecten Kasum-Zade gen. nov. Comparison. It is distinguished from subfamily Pseudopectininae Kasum-Zade subfam. nov. by the shells being inequivalved and the differing ornamentation of the valves We distinguish a new species [Genus Sigmaringenopecten Kasum-Zade gen. nov.] within the described subfamily, the description of which we give below. Range. Middle and Upper Jurassic of Eurasia.» KASUM-ZADE, A. A. 2003. Advance in research of mesozoic bivalve mollusks in Azerbaijan (Order Pectinoida: Revision and Systematics). 111 pp. Baku (in Russian; translated by Rosanne D’Aprile Johnson, VIARC, Smithsonian Institution). [p. 53, 54]
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Radulopecten vagans (J.. de C. Sowerby 1826); A. L. A. Johnson, 1984, The palaeobiology of the bivalve families Pectinidae and Propeamussiidae in the Jurassic of Europe, plate 9, figures 9-33.
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«Genus RADULOPECTEN Rollier 1911
Type species. OD; ROLLIER 1911, p. 158; Pecten hemicostatus MORRIS and LYCETT 1853, p. 10, pl. 1, fig. 16; Great Oolite (Bathonian), Minchinhampton, Gloucestershire.
DIAGNOSIS
Between 4 and 15 initial radial plicae, bearing comarginal lamellae at all stages of ontogeny on left valve but only in later stages of ontogeny or not at all on right valve.
Stratigraphic range; Jurassic (Aaleman-Tithoman). Geographic range; Europe, Asia, Africa, ?North and Central America. DISCUSSION
Since all the Jurassic pectinid species which come under the Treatise definition of Chlamys (see p. 161) but which do not belong to Ch. (Chlamys) seem to form a monophyletic group they are usefully accorded a generic distinction from Chlamys. The name Radulopecten has been previously applied at che subgeneric level (e. g. ARKELL, 1931a; COX, 1952; DUFF, 1978) to certain members of this group and it is herein adopted, with a revised diagnosis (see above), as the generic name for the complete group.
Within Radulopecten seven groups may be distinguished on the following basis:
Since the stratigraphic ranges of Groups 1 and 2 have little or no overlap it is conceivable that the supposed evolution of the latter from the former occurred without Splitting. However since the morphological distance between the groups is quite large and there appear to be no intermediates, the groups are treated as separate lineages for reasons of convenience. Similar reasoning applies to the Separation of the undoubtedly related Groups 5 and 6.
Groups 4, 5 and 7 tend to be restricted to particular facies (respectively; arenaceous, argillaceous and coralliferous) and since there is considerable overlap in their ranges of variation it may be that they represent ecophenotypes of a single species as has been suggested for similarly facies-restricted forms assigned to Chlamys (Ch.) textoria. However, unlike the latter species, the features distinguishing the groups would have to have resulted from very early ontogenetic developmental flexibility. Since the author can envisage no way of testing for such developmental flexibility it seems preferable to treat the groups as separate species. In any case, at least for Groups 4 and 5, there are considerable differences in geographic distribution which are difficult to relate to facies and each group is occasionally found in the facies apparently appropriate to the other, so it is perhaps more likely that they represent separate species. Similar reasoning applies to Groups 2 and 3 where, for example, BUVIGNIER's ( 1852) records suggest some morphological overlap. Notable differences in the mean form of the ornament between the latest and earliest populations of Group 4 are attributed to phyletic evolution within a single species since there can benodoubt of direct relationship and there is no comvincing evidence of the contemporaneous existence of two separate lineages (see p. 207). The latest (uppermost Oxfordian and Kimmeridgian) populations are characterised b\ the development of a larger umbonal angle (specimens marked with a glyph in text fig. 187) but this is clearly a simple reflection of the attainment of greater size. Differences in the mean form of the ornatnent between earlier and later populations in Groups 1 and 5 can also be interpreted as the result of phyletic evolution within single species. Since however in Group 1 the difference in mean form results from a change in the relative proportions of two distinct ('striate' and 'non-striate') morphs there is here the possibility that the difference reflects a shift in the relative abundances of two quite separate species (see p. 192 for a refutation of this argument).» JOHNSON, A. L. A. 1984. The palaeobiology of the bivalve families Pectinidae and Propeamussiidae in the Jurassic of Europe. Zitteliana, 11: 1-235, pls. 1-11. [p. 187, 188]
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Pecten hemicostatus; J. Morris & J. Lycett, 1853, A Monograph of the Mollusca from the Great Otolite chiefly from Minchinhampton and the Coast of Yorkshire. Part II. Bivalves, plate 1, figure 16.
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«Subgenus RADULOPECTEN Rollier, 1911
Type species. Original designation; Rollier 1911, p. 158; Pecten hemicostatus Morris & Lycett 1853, p. 10, pl. 1, fig. 16; Great Oolite (Bathonian) of Minchinhampton, Gloucestershire.
Diagnosis. Right valve usually more inflated than left. Ornamentation variable, typically 5-6 pairs of twinned ribs on right valve, but in many species this is faint or obsolete, in which case there are 11-12 prominent radial ribs; left valve with 5-6 radial ribs in forms with twinned ribs on right valve, and 10-11 in forms without; right valve also with faint regular concentric growth lines, left valve with prominent, regular, wire-like concentric lamellae.
Remarks. Cox's (1952) emended interpretation of the nature of ribbing in Radulopecten has been followed here, so that the subgenus includes all of the Jurassic species placed by Staesche (1926) in the "Group of Aequipecten fibrosus Sowerby". Thus forms with poorly developed paired ribs on the right valve, such as C. (R.) fibrosus and some specimens of C. (R.) scarburgensis and C. (R.) drewtonensis may be included, together with the more 'typical' members of the subgenus, such as C. (R.) hemicostatus. In essence, Radulopecten now includes most of the Jurassic species referred to Chlamys.»
DUFF, K. L. 1978. Bivalvia from the English Lower Oxford Clay (Middle Jurassic). Palaeontographical Society. Monograph 553: 1-137, pls. 1-13. [p. 70]
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