Evolution and palaeogeographical dispersion of the radiolitid rudist genus Auroradiolites (Bivalvia, Hippuritida) with descriptions of new material from Tibet and archived specimens from Afghanistan

Abstract

Diagnostic characters of the recently established new genus of radiolitid rudist Auroradiolites include an entirely compact outer shell layer, a distinctly convex upper (left) valve and a robust myocardinal apparatus surrounding a strongly internally projected ligamentary infolding. Until now, A. biconvexus (previously considered to be exclusively late Albian in age) has been reported only from the Langshan Formation, which crops out along the northern portion of the Lhasa block, Tibet. Here, it is recorded for the first time from the Sangzugang Formation of the Xigaze Forearc Basin, situated on the southern margin of the Lhasa block. A. biconvexus differs from the south‐west Asian type species A. gilgitensis (late Aptian to Albian) by its relatively larger size, more strongly domed left valve and distinct radial undulations of the outer shell layer. The characters of A. gilgitensis are further clarified from archived material from central Afghanistan, also newly identified and described herein. All Auroradiolites records to date are revised. The recognition of examples of A. biconvexus from upper Aptian strata increases both the stratigraphical and geographical ranges of the species, indicating that it had already branched off from A. gilgitensis in the late Aptian. Some new radiolitid specimens that combine both compact and cellular calcitic outer shell layer structures are also described from the Langshan Formation, but a number of internal differences from Auroradiolites cast doubt on their constituting a sister group to the latter and we assign them to Eoradiolites cf. hedini. Rather, the evolution of Auroradiolites directly from the ancestral radiolitid genus Agriopleura is favoured on the grounds of parsimony. So far, the genus Auroradiolites has been recorded from Iran, central and eastern Afghanistan, the type locality of Yasin in north‐western Pakistan, southern and northern Ladakh, the Lhasa block, as well as Hokkaido in northern Japan. During the late Aptian to Albian interval, all of these localities were associated with terranes and blocks that were limited to the north‐eastern margin of Tethys and the western Pacific margin, making Auroradiolites an indicator of a SW Asian to Pacific faunal province.

Shajia, a new genus of polyconitid rudist from the Langshan Formation of the Lhasa block, Tibet, and its palaeogeographical implications

“…and Nagao, 1926-Magallanesia Sano et al, 2014, as well as some more widely dispersed taxa such as Eoradiolites Douvillé, and Sellaea Di Stefano, (Skelton et al, 2013;Sano et al, 2014;Rao et al, 2015). The orbitolinid assemblage associated with these rudists is likewise composed of several endemic taxa such as Mesorbitolina birmanica (Sahni, 1937), Palorbitolinoides orbiculata Zhang, 1986 and Palorbitolinoides hedini Zhang, 1986, together with cosmopolitan taxa such as Mesorbitolina texana (Roemer, 1849), M. subconcava (Leymerie,1878) and M. aperta (Erman, 1854) (Cherchi and Schroeder, 2013;Schlagintweit and Wilmsen, 2014;Rao et al, 2015Rao et al, , 2017Boudagher-Fadel et al, 2017).…”

“…Within the SW Asian/Pacific Faunal Province, the so-called 'Yasin fauna' of rudists represented by the late Aptian-Albian Horiopleura haydeni/Auroradiolites gilgitensis association had a widespread distribution in southwest Asia including Iran, Afghanistan, Kohistan, and Ladakh (Douvillé, 1926;Rossi Ronchetti, 1965;Montenat et al, 1982;Upadhyay, 2001Upadhyay, , 2014Skelton et al, 2005;Sha and Cestari, 2016). In the mid-Cretaceous, these fossil sites were all located on blocks/terranes arrayed along, or offshore from the southern Asian margin on the northern side of the eastern Neo-Tethyan Ocean (Rao et al, 2017). The Yasin fauna, as such, has not previously been recorded from the Lhasa Block, which occupied a similar geographical and tectonic position to the blocks/terranes mentioned above.…”

“…Abundant fossils have been recorded from the Langshan Formation, dominated by orbitolinids (Zhang, 1982(Zhang, , 1986(Zhang, , 1991Boudagher-Fadel et al, 2017), rudists (Yang et al, 1982;Gou andShi, 1998, Scott et al, 2010;Rao et al, 2015Rao et al, , 2017, gastropods (Yu and Xia, 1985) and corals (Löser and Liao, 2001;Deng and Wang, 2013). The age of the Langshan Formation is mainly constrained by foraminifers, especially large benthic orbitolinids.…”

“…Although the left valves of these Auroradiolites specimens were not preserved, they are most likely attributable to A. biconvexus because of the large size of the right valves. As the age range of A. biconvexus was revised to late Aptian-Albian (Rao et al, 2017), and the geological range of the upper The suprageneric classification used herein follows Skelton (2013a, b).…”

“…The mid-Cretaceous rudists of southwestern Asia and the western Pacific region show great similarity and affinity (Sano and Masse, 2013;Skelton et al, 2013). Accordingly, the Southwest Asian/Pacific Faunal Province was proposed, covering Iran, Afghanistan, Kohistan in northern Pakistan, Ladakh in northern India, Tibet in China and Japan, as well as Cebu Island and the Japanese seamounts in the Pacific region (Rao et al, 2015(Rao et al, , 2017. The rudist fauna of this province is dominated by the endemic radiolitid Auroradiolites Rao et al, 2015 and the polyconitid lineage of Horiopleura haydeni Douvillé, 1926-Praecaprotina Yabe…”