New Chroniosuchians (Chroniosuchia: Bystrowianidae)from The Middle Permian In Yumen, Gansu And The Related Stratigraphy

Chroniosuchians are non-amniote tetrapods from the Permian and Triassic of Eurasia, they are subdivided into two families: Chroniosuchidae and Bystrowianidae. Up to now, fossil chroniosuchians have been found from Russia, China, Germany and Kyrgyzstan. Despite their extended range, chroniosuchians are still poorly known and most taxa are largely defined by their osteoderm morphology. In the bystrowianids, the skull is only known from fragment.This paper presents a detailed description of the new chroniosuchian (Bystrowianidae) material from Middle Permian Dashankou Fauna, Yumen, Gansu, China, including5genera and5species (3new genera and3new species). A well preserved anterior part of a skull is assigned to Bystrowianidae for the lack of a preorbital fenestra, which is a synapomorphy shared with all other bystrowianids. The new taxon is distinguished from other chroniosuchians by the small and separate premaxillary fontanellas and marked maxillary bulges. According to these different features of this specimen, it is assigned to a new genus and a new species: Qiliansuchus chengi gen. et sp. nov. A scute with an associated sacral vertebra and two articulated scutes associated with two dorsal vertebrae are respectively assigned to be the holotype and paratype of Yumenerpeton yangi gen. et sp. nov. A small scute, due to its special structure, is named as a new genus and species: Baiyangherpeton elegans ge. et sp. nov. The specimen GS-DSK-013belongs to Bystrowiana permira and GS-DSK-014is assigned to Dromotectum cf. D. spinosum. The present study extends both the geographical and stratigraphical ranges of the Chroniosuchia. The new fossil material from China is well preserved and reveals many features formerly not known in chroniosuchians, prompting anatomical comparison across the group and a phylogenetic analysis of a broader range of chroniosuchian and other early tetrapod taxa.Combined with previous research results and new material on observation and comparison, this paper considers chroniosuchians from Dashankou Fauna represent terrestrial or semi-terrestrial tetrapod.The fossil comes from fluvial floodplain deposits of the upper part of the Xidagou formation. As most of Dashankou Fauna fossil material is scattered bones and skulls, with a degree of wore, which indicates that the fossil material is heterochthonous burial, transportated for a certain distance, and then deposited in the river. Finally, because water level rose suddenly and they are washed to deposit, on both sides of the river floodplain mudstone, buried and turned into fossil.