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Mid‐ C retaceous rudists ( B ivalvia: H ippuritida) from the L angshan F ormation, L hasa block, T ibet
Author(s) -
Rao Xin,
Skelton Peter W.,
Sha Jingeng,
Cai Huawei,
Iba Yasuhiro
Publication year - 2015
Publication title -
papers in palaeontology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.827
H-Index - 15
ISSN - 2056-2802
DOI - 10.1002/spp2.1019
Subject(s) - confusion , taxon , genus , block (permutation group theory) , chemistry , paleontology , geology , stereochemistry , combinatorics , biology , botany , mathematics , psychology , psychoanalysis
The C retaceous L angshan F ormation, which crops out along the northern portion of the L hasa block, is composed mainly of grey or grey‐black fine‐grained limestones, argillaceous limestones and a few thin‐bedded siltstones and silty mudstones. From specimens collected and/or photographed in the field, combined with a revision of taxa previously recorded from the L angshan F ormation, four rudist taxa are recognized: (1) A uroradiolites gen. nov.; (2) E oradiolites cf. davidsoni ; (3) M agallanesia rutogensis sp. nov.; and (4) S ellaea sp. We propose A uroradiolites gen. nov. for the grouping of SW A sian to P acific radiolitid species formerly attributed to E oradiolites and characterized by a compact (non‐celluloprismatic) outer shell layer. The new genus is represented in the L angshan F ormation by A . biconvexus ( Y ang et al ., 1982), including several other synonymized taxa. Taxonomic confusion in the literature has resulted from the varied appearance of the strongly developed ligamentary infolding in such forms: in the right valve, it has a forked inner termination, but common transverse sections through both valves show the latter juxtaposed against its broad, rounded counterpart in the left valve, giving the combined terminations a bulb‐shaped appearance. Associated E oradiolites cf. davidsoni is of smaller size and, by contrast, shows the celluloprismatic structure of the outer shell layer. The recently described canaliculate polyconitid genus, M agallanesia S ano et al ., 2014, is represented by M . rutogensis sp. nov., which differs from the type species in possessing more numerous and ventrally extended canals in the left valve. Possibly belonging with the latter species are isolated right valves, similar to H oriopleura , but containing tabulae. A probable S ellaea sp. is represented by poorly preserved right valves nevertheless showing typical features, including a very thin outer shell layer and an erect posterior myophoral wall, separating off a broad posterior ectomyophoral cavity. Based on the presence of M agallanesia and the orbitolinid record, this rudist fauna is probably no older than late A lbian, while a younger age cannot be excluded. Both M agallanesia and A uroradiolites are characteristic of a SW A sian/central P acific faunal province, while the other two genera are more widely distributed.