Ontogenetic character analysis of the Middle Triassic Lariosaurus youngi (Reptilia: Sauropterygia) from Guizhou
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Abstract:
In this study, the key diagnostic characters of the Middle Triassic Lariosaurus youngi are re-evaluated based on comprehensive morphometric and osteological analyses of four newly discovered fossil specimens from the Xingyi fauna of Guizhou, together with previously described materials. The diagnostic characters reassessed include: (1) a pineal foramen positioned within a parietal trough; (2) the presence of a jugal; (3) an upper temporal fenestra with a longitudinal diameter 2 to 2.4 times that of the orbit; and (4) an interclavicle lacking a posterior stem. Ontogenetic stages of L. youngi—juvenile, subadult, and adult—are reconstructed. Adult individuals are characterized by a snout-vent length (SVL) exceeding 1000 mm and extensive skull fusion. Juvenile individuals (SVL ＜ 500 mm) exhibit reduced skull fusion, incomplete limb ossification, and comparatively simplified skeletal structures. Subadult individuals display intermediate morphological conditions, with skeletal features transitional between those of juveniles and adults. The relative lengths of the humerus and femur, along with the degree of epiphyseal expansion, may serve as reliable indicators of sexual dimorphism in L. youngi. Furthermore, the progressively higher humerus-to-femur length ratios observed from juveniles to adults suggest an ontogenetic enhancement of swimming capability.
