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

摘要: Abstract A new polyconitid rudist Shajia tibetica gen. et sp. nov., of late Aptian to Albian age, is described from the Langshan Formation Nyima County, northern Lhasa Block, Tibet. Though comparable in size and external morphology with Horiopleura haydeni Douville, which a common endemic species southwestern Asia, differs latter its possession an inwardly inclined, instead outwardly facing, posterior myophore right valve. In addition, single specimen Ladakh, was previously assigned Polyconites? sp., on account similar myophoral distinction H. haydeni, transferred genus. considered most likely have been derived one group that lived southern margin Mediterranean Tethys. The so-called ‘Yasin fauna’ represented by haydeni/Auroradiolites gilgitensis association, be restricted including Afghanistan, Kohistan Pakistan Ladakh India, though those two particular not so far recorded Block Nevertheless, S. tibetica co-occurs Auroradiolites biconvexus (Yang et al.), probably evolved directly A. gilgitensis (Douville), age association accordance generally accepted Yasin fauna as Albian. Hence A. biconvexus can regional variant fauna, had evidently already dispersed Aptian. So palaeogeographically linked other mid-Cretaceous shallow-marine carbonate deposits adjacent Asian regions. These findings also provide evidence assemblage Albian, although slightly younger cannot excluded.