Abstract

Lotosaurus adentus is an unusual sail-backed, edentulous poposauroid pseudosuchian primarily known from a single, nearly monospecific bonebed discovered and excavated in the 1970s in the Middle-Upper Triassic Badong Formation of Sangzhi County, Hunan Province, South China. Renewed interest in this unique taxon prompted exposure of an additional 90 m2 of the bonebed. Almost 1000 new L. adentus bones, 28% of which were articulated, were discovered during this excavation. The bones lack evidence of tooth marks, trample marks, or insect modification, and display minimal weathering. The site is reinterpreted as a pedogenically modified floodplain pond (and overlying fluvial channel) within a warm, semi-arid sub-tropical region (paleolatitude ∼ 34°N), contrasting with previous interpretations of the locality as a tidal flat deposit. The occurrence of mudcracks, conchostrachan fossils, and vertic paleosol development with calcium carbonate accumulations in both overlying and underlying facies indicates periodic aridity and ephemeral conditions. The bonebed is characterized by partial disarticulation and minor transport, and is confined to a thin, < 30 cm-thick interval. Considered together, these features are most consistent with a mass mortality event, possibly drought related considering the sedimentological context, followed by minor transport during a rapid burial event.

U-Pb detrital zircon geochronology and Lu-Hf isotope analysis were utilized to reassess the provenance and age of the deposit, and suggest that L. adentus was likely Ladinian or possibly even Carnian in age, rather than Anisian as previously reported. Paleocurrent data, detrital zircon age spectra, and Lu-Hf isotopes indicate that fluvial sediments were partially derived from sources in the North China craton and Qinling-Dabieshan tectonic belt, implying that faunal interchange between the North and South China blocks was possible by this time.

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