Late Aptian Albian Yasin type rudist assemblage in the Himalayas: palaeobiogeographic implications
Shallow water rudist bivalves are a very suitable tool for palaeoenvironmental and palaeobiogeographic reconstructions of the Cretaceous Tethys. For this reason, we have reviewed fossil localities bearing polyconitid rudists of the late Aptian and Albian in the Himalayan area and in other Tethyan sites. The well preserved Rossi Ronchetti rudist collection of Yasin (Kohistan Himalayas) has been reinspected as a reference for the regionally distinctive Horiopleura haydeni biofacies. Two morphotypes of the genus related to different sedimentary settings were distinguished on the basis of biometric measurements. Assemblages characterized by benthic organisms of Mesogean affinity dominated by polyconitid and radiolitid rudists, together with nerineid gastropods, corals and orbitolinid forams have a distinct provincial significance. Today, various polyconitid bearing biofacies can be recognized in many sedimentary successions of the Himalayas, westwards to the Iberia region and even eastwards to the Pacific area. Their distribution allows recognition of a palaeobiogeographic pattern influenced by a thermal barrier. It also helps to understand how the accretion of the Lhasa terrane and the Kohistan Dras intra oceanic volcanic arc could represent a spreading centre for the "Yasin type" Horiopleura haydeni biofacies (marked by the absence of Polyconites) in the South West Asian Province.