Stenopterygius (Reptilia, Ichthyosauria) in Italian museums: morphometric and palaeoecological considerations

Stenopterygius is a genus of parvipelvian ichthyosaurs whose fossil remains are mostly found in Lower Jurassic strata of the Württemberg area, Germany. Large amounts of complete, exquisitely preserved specimens of Stenopterygius have been recovered from the famous locality of Holzmaden (Württemberg) and sold to museum collections throughout Europe since the early 19th century. Many Stenopterygius skeletons found their way to Italian natural history collections, most of which have never been properly studied. Stenopterygius fossils are widespread across museum collections, thus comprising an ideal statistical sample for investigating the inter- and intraspecific morphological variability of this genus. The study pf this sample could in turn shed light on the diversity and disparity of large-sized marine predators in coastal and shelfal environments of the Early Jurassic. Here, we report on nineteen ichthyosaur skeletons from five Italian museums (Museo Geologico Giovanni Capellini, Museo Geologico Paleontologico dell'Università di Padova, Museo di Paleontologia di Napoli, Museo di Storia Naturale di Milano, Museo di Storia Naturale dell'Università di Pisa) identified as Stenopterygius by using modern taxonomic schemes. Updated species-level identifications of these fossils were also attempted by relying on the linear morphometric analyses proposed by Maxwell (2012) as well as on the phylogenetic matrix elaborated by Maxwell & Cortés (2020). Out of nineteen studied specimens, eight were assigned to Stenopterygius quadriscissus, three to S. triscissus, with all other examples being referred to Stenopterygius cf. quadriscissus, Stenopterygius cf. triscissus or Stenopterygius sp. The inclusion of the aforementioned 19 specimens in morphometric analyses would likely result in improving the precision and resolution of these methodologies by increasing the statistical sample size. The studied specimens were documented by means of high-resolution photographs and the acquisition of 3D models via structured-light scanning and photogrammetry. Two skeletons kept at the Museo di Storia Naturale dell’Università di Pisa (MSNUP) were CT-scanned, revealing interesting anatomical elements, such as the hyoid bones and ischiopubis, which would have been otherwise hidden beneath the embedding limestone. Moreover, the CT-scans resulted in the reconstruction of the tridimensional rostral morphology of one of the MSNUP specimens, allowing for some morphofunctional inferences on the feeding behavior of Stenopterygius. According to the metrics reported by McCurry & Pyenson (2019), the rostra of some extinct and extant long-snouted odontocetes (toothed whales) and istiophoriforms (billfishes) may represent good functional analogues of those of parvipelvian ichthyosaurs.