The osteology, taxonomy, and phylogenetic affinities of the Early Jurassic plesiosaur <i>Lusonectes sauvagei</i>

The transition from the Early to the Middle Jurassic was marked by significant restructuring of plesiosaur communities. While knowledge of the earliest Middle Jurassic plesiosaurs is generally limited, Toarcian plesiosaur occurrences are abundant, though the vast majority of specimens have been unearthed in the United Kingdom and Germany. Here, we reassess <i>Lusonectes sauvagei</i>, an early-diverging plesiosaur from the lower to middle upper Toarcian of the São Gião Formation in Portugal. Originally described as <i>Plesiosaurus</i> sp., it was later established as a distinct taxon closely related to taxa currently encompassed within Microcleididae. Our firsthand examination of the holotype of <i>L. sauvagei</i> resulted in differing interpretations of certain aspects of its morphology, prompting a detailed osteological, taxonomic, and phylogenetic reevaluation. We provide a redescription of <i>L. sauvagei</i>, propose a new diagnosis, and investigate its phylogenetic affinities. Although the specimen is fragmentary and poorly preserved, our study suggests that, contrary to the original interpretation, <i>L. sauvagei</i> is not affiliated with <i>Microcleidus</i> spp. The taxon remains problematic and may represent either an early-diverging pliosaurid or a plesiosauroid. <i>Lusonectes</i> is one of the few diagnosable plesiosaurs from the upper Lower Jurassic found outside the classic British and German localities and thus offers insights into the diversity of plesiosaurs just prior to a major event in the evolutionary history of the clade.