A new basal ichthyosauromorph from the Lower Triassic (Olenekian) of Zhebao, Guangxi Autonomous Region, South China

Abstract

Here we describe a newly discovered basal ichthyosauromorph from the Lower Triassic of South China, Baisesaurus robustus gen. et sp. nov. The only known specimen of this new species was collected from the Lower Triassic (Olenekian) Luolou Formation in the Zhebao region of Baise City, on the northwest margin of the Nanpanjiang Basin, and comprises a partial skeleton including the ribs, the gastralia, a limb element, 12 centra, and seven neural arches. Comparisons to a wide variety of Early Triassic marine reptiles show Baisesaurus robustus to be a basal ichthyosauromorph based on the following features: neural arches lack transverse processes; dorsal ribs are slender, and not pachyostotic even proximally; and median gastral elements have long, sharp anterior processes. The limb element is long and robust, and is most likely to be a radius. Baisesaurus robustus is large (estimated length more than 3 m) relative to early ichthyosauromorphs previously discovered in China, and shares noteworthy morphological similarities with Utatsusaurus hataii, particularly with regard to body size and the morphology of the probable radius. Baisesaurus robustus also represents the first record of an Early Triassic ichthyosauromorph from Guangxi Autonomous Region, extending the known geographic distribution of ichthyosauromorphs in South China.