Ichthyosaurs represent one of the most highly specialized lineages of marine reptiles, but our understanding of the evolution of this group is based on specimens found at a surprisingly small number of stratigraphic intervals and localities. The Lower Jurassic (Toarcian) Posidonia Shale of southwestern Germany is one of the richest ichthyosaur-bearing formations in the world and has produced thousands of skeletons, including specimens with preserved soft tissue, and fetal remains inside the body cavity. The most abundant ichthyosaur genus in the Posidonia Shale is Stenopterygius. In spite of almost 200 years of research effort, the number of species in this genus is still a point of active disagreement in the literature. Here, bivariate and multivariate analyses are used to classify both articulated and disarticulated skeletons to the level of species, using measurement data from individual cranial and postcranial elements. Unlike previous classification attempts, this technique pinpoints ontogenetically conserved differences in size and proportion between the species, and so can be applied to adult, subadult, and neonatal specimens. Using this method, three species of Stenopterygius, S. quadriscissus, S. triscissus, and S. uniter are differentiated.

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