Shajia, a new genus of polyconitid rudist from the Langshan Formation of the Lhasa Block
Recently, Dr. Xin Rao from the State Key Laboratory of Palaeobiology and Stratigraphy and her colleagues described a new polyconitid rudist Shajia Rao et al., 2019, of late Aptian to Albian age, from the Langshan Formation of Nyima County, northern Lhasa Block. This study was published online in Cretaceous Research.
Though comparable in size and external morphology with Horiopleura haydeni Douvillé, which is a common endemic species in southwestern Asia, Shajia differs from the latter species in its possession of an inwardly inclined, instead of outwardly facing, posterior myophore in the right valve. In addition, a single specimen from Ladakh, which was previously assigned to Polyconites? sp., on account of a similar myophoral distinction from H. haydeni, is transferred to the new genus. Shajia is considered most likely to have been derived from one of a group of Horiopleura species that lived on the southern margin of the Mediterranean Tethys.
The so-called ‘Yasin fauna’ represented by the late Aptian to Albian Horiopleura haydeni/Auroradiolites gilgitensis rudist association, is considered to be restricted to southwestern Asia, including Afghanistan, Kohistan in northern Pakistan and Ladakh in northern India, though those two species in particular have not so far been recorded from the Lhasa Block of Tibet. Nevertheless, S. tibetica co-occurs with Auroradiolites biconvexus (Yang et al.), which probably evolved directly from A. gilgitensis (Douvillé), and the age of the latter association is in accordance with the generally accepted age of the Yasin fauna as late Aptian to Albian. Hence the S. tibetica and A. biconvexus association can be considered a regional variant of the Yasin fauna, which had evidently already dispersed to the Lhasa Block by the late Aptian. So the Langshan Formation can be considered palaeogeographically linked with other mid-Cretaceous shallow-marine carbonate deposits in adjacent southwestern Asian regions.
This work was supported by the National Natural Science Foundation of China, the State Key Laboratory of Palaeobiology and Stratigraphy, the Strategic Priority Research Program of the Chinese Academy of Sciences, and by the China Geological Survey.
Shajia tibetica Rao et al., 2019, holotype articulated specimen, A-D are dorsal view, ventral view, anterior view and posterior view respectively
Shajia tibetica Rao et al., 2019, holotype. A-D, successively more ventrally situated antero-posterior sections of both valves