Biostratigraphy and Palaeobiogeography of the Middle and Upper Devonian Brachiopod Fauna in the Qamdo Block

The Qamdo Block, located in the eastern Tibetan Plateau, represents an allochthonous terrane within the Palaeotethys orogenic belt during the Palaeozoic. Throughout the Devonian Period, this block was broadly positioned along the northern margins of Gondwana, accumulating shallowing-upwards marine deposits hosting abundant benthic faunas. From the Early to Late Devonian, the Qamdo Block transitioned from littoral sandstone to shallow marine shelf sediments that preserve diverse brachiopod assemblages. Despite the Upper Devonian cyrtospiriferids brachiopod fossils were identified during previous regional surveys, knowledge of their biostratigraphical correlations and biogeographical affinities remained sparse. Based on our investigations and fossil collections during 2021-2024, this study concentrates on the Middle to Late Devonian (Givetian- Famennian) brachiopod fossils and strata from the Jueyong, Nuoma, and Toba sections in the Qamdo region. The Devonian successions include the Dingzonglong, Zhuogedong, and Qiangge formations in ascending order. The Middle Devonian Dingzonglong Formation, consisting of fossiliferous limestones and marls, was measured at the Jueyong and Nuoma sections where it unconformably overlies the Lower Devonian Haitong Formation. It connected with the overlying bioclastic and dolomitic limestones of the Zhuogedong Formation. The Upper Devonian Qiangge Formation, characterized by deep black limestone, marls, and shales, was extensively sampled at the Toba section. Brachiopod assemblages from the Dingzonglong Formation predominantly comprise cosmopolitan species of Atrypida (e.g., Desquamatia, Pseudoatrypa, and Variatrypa). In the Jueyong section, these atrypides alongside Schizophoria excellens constitute approximately 90% of the samples within the argillaceous horizons. The Givetian assemblage maintained broad links with global faunas, representing a low-diversity but highly cosmopolitan ecological structure. In contrast, Late Devonian brachiopod faunas exhibit a pronounced shift toward highly specialized, regional assemblages. In the Frasnian within the Zhuogedong Formation at the Nuoma section, the faunal composition shifted to include cyrtospiriferid and leiorhynchid brachiopods, notably Cyrtospirifer whitneyi and Caryorhynchus nana. In the Famennian within the Qiangge Formation at the Toba section, the assemblage increasingly comprised Cathaysian and European groups, representing the Cyrtiopsis-Yunnanella-NayunnellaZone. This assemblage includes diverse cyrtospiriferids like Cyrtospirifer and Cyrtiopsis, alongside rhynchonellides such as Ptychomaletoechia and Yunnanella. In addition, the productid brachiopod Mesoplicaintermedia was noted alongside Yunnanella and Cyrtiopsis, providing further biostratigraphic and palaeobiogeographic correlation to the lower Famennian. Palaeobiogeographically, the Late Devonian was widely recognized as a period of heightened global cosmopolitanism among marine benthic faunas, driven by eustatic sea-level rises that allowed generalist taxa to expand across terranes. The presence of the Famennian Cyrtiopsis-Yunnanella-Nayunnella fauna, largely confined to the Cathaysian province, acts as robust evidence for exclusive biogeographic connections between the Qamdo Block and the South China Block. These findings strongly suggest a close proximity between the Qamdo and South China blocks during the Late Devonian, providing vital new insights into the tectonic rifting and evolutionary history of the Tibet Plateau.