The Cretaceous Langshan Formation, which crops out along the northern portion of the Lhasa 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 Langshan Formation, four rudist taxa are recognized: (1) Auroradiolites gen. nov.; (2) Eoradiolites cf. davidsoni; (3) Magallanesia rutogensis sp. nov.; and (4) Sellaea sp. We propose Auroradiolites gen. nov. for the grouping of SW Asian to Pacific radiolitid species formerly attributed to Eoradiolites and characterized by a compact (non-celluloprismatic) outer shell layer. The new genus is represented in the Langshan Formation by A. biconvexus (Yang 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 Eoradiolites cf. davidsoni is of smaller size and, by contrast, shows the celluloprismatic structure of the outer shell layer. The recently described canaliculate polyconitid genus, Magallanesia Sano 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 Horiopleura, but containing tabulae. A probable Sellaea 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 Magallanesia and the orbitolinid record, this rudist fauna is probably no older than late Albian, while a younger age cannot be excluded. Both Magallanesia and Auroradiolites are characteristic of a SW Asian/central Pacific faunal province, while the other two genera are more widely distributed.
The phylogenetic origin and the timing of origination of the Decabrachia are controversial. This is due to a poor understanding of character complexes relating to the shell, which causes difficulties in establishing homologies among different taxa. One central problem concerns a clear differentiation between belemnoids and early spirulids.
Belemnites (order Belemnitida), a very successful group of Mesozoic cephalopods, provide an important clue for understanding Mesozoic marine ecosystems and the origin of modern cephalopods. Following current hypotheses, belemnites originated in the earliest Jurassic (Hettangian, 201.6-197 Ma) with very small forms. According to this view their paleobiogeographic distribution was restricted to northern Europe until the Pliensbachian (190-183 Ma). The fossil record is, however, biased by the fact that all the previous studies on belemnites focused on Europe. Here we report two belemnite taxa from the Hettangian of Japan: a new species of the Sinobelemnitidae and a large taxon of the suborder Belemnitina. The Sinobelemnitidae, which may be included in the future in a new suborder, have also been recorded from the Triassic of China, specimens so far poorly understood. The presence of a very large rostrum attributed to the Belemnitina suggests in addition that a diverse belemnite fauna evolved earlier than previously thought. Our new fi ndings therefore (1) extend the origin of the belemnites back by ~33 m.y. into the Triassic, (2) suggest that this group did not necessarily originate in northern Europe, and (3) imply that belemnites survived the Triassic-Jurassic extinction, one of the fi ve big mass extinctions in the Phanerozoic. Since belemnites provided a considerable amount of food as prey, the origination of belemnites is probably an important event also for the evolution of their predators, such as marine reptiles and sharks.
Carbonate platforms consisting of hermatypic coral, rudists and larger foraminifers formed along the margin of the NE Japanese islands during the mid-Late Aptian. The geographical northern limit of the carbonate platforms extended to c . 36°N at that time, which is the highest latitude for coral and/or rudist formation in the North Pacific region. This geographical extension of the carbonate platform in the NW Pacific indicates extreme climatic warmth in the mid-latitudes and strong poleward heat transport systems. Contemporaneous warming has been detected in European and Australian regions, as represented by the migration of Tethyan fauna toward the boreal realm and the positive excursion of δ 13 C and negative excursion of δ 18 O in deep-sea carbonates. Our new constraint on the northern limit of ‘reef’ growth reinforces the view that an interval of greenhouse-like warmth punctuated the Late Aptian climate.
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