A New Sauropterygia from Eastern Tethys (middle Triassic): Morphological Characteristics and Implications Paleobiogeography

Following the end-Permian mass extinction, one of the most severe biotic crises in Earth’s history, the global ecosystem started recovery during the Early Triassic. During this period, the reptiles re-invaded the ocean and became the most diverse group of the Mesozoic marine ecosystem. Sauropterygia is a diverse group of diapsid reptiles that inhabited the Mesozoic oceans. Fossil records of Sauropterygia are widely documented along the eastern and western Tethys Ocean. Consequently, the geographic origin and early evolutionary radiation of Sauropterygia remain intensely debated, largely due to inferred limited transoceanic dispersal capabilities among Triassic taxa, together with the incompleteness of the fossil record and spatiotemporal uncertainties of fossil-bearing strata. In recent years, intensive new methods and fossil discoveries challenged the long-standing hypotheses regarding sauropterygian origins and dispersal. Here we describe a new Sauropterygia, namely Spatulidentatusbreviceps gen. et sp. nov., from the upper member of the Middle Triassic Guanling Formation in the border area between Guizhou and Yunnan, south China. S. breviceps is characterized by a short rostrum, spatulate premaxillary teeth, a subtriangular maxilla, and a robust trunk, all of which align closely with basal Sauropterygia of the contemporaneous strata. The new species expands the paleogeographic distribution and provides comprehensive morphological data for understanding the early evolution of basal Sauropterygia from the eastern Tethys. Notably, the elongated dorsal transverse processes exhibit morphological affinities to the enigmatic Saurosphargidae and Placodontia. This mosaic of plesiomorphic and derived features reflects a gradual adaptive transition toward obligate aquatic lifestyles. Moreover, this taxon offers novel insights into Triassic marine paleobiogeography and faunal recovery dynamics after the end-Permian extinction.