The paleoneurology of Ichthyopterygia and Sauropterygia: Diverse Endocranial Anatomies of Secondarily Aquatic Diapsids

Most meso- and megapredatory niches across Mesozoic marine ecosystems were gradually occupied by the secondarily aquatic Ichthyopterygia and Sauropterygia. Although their presumed Early Triassic origins remain obscured, the subsequent radiation and diversification of these diapsid superorders is reasonably well documented in the fossil record. In the first quarter of the twentieth century, the Triassic sauropterygian genera Nothosaurus and Placodus were among the first taxa for which the cranial endocast was extracted and described. The advent of computed tomography, circa 50 years after these pivotal paleoneurological investigations, has led to its adoption as the tool of choice for the non-destructive assessment of neurosensory adaptations in extinct vertebrates. Despite the increasing availability of paleoneurological data on extinct diapsids, Ichthyopterygia and Sauropterygia have received relatively modest, albeit growing, attention since. Here we review paleoneurological insights gleaned from these fossils to date and identify endocranial and neurosensory modifications associated with increasingly pelagic lifestyles and ecological specializations. Nevertheless, a broader ichthyopterygian and sauropterygian sample will be required to facilitate high-resolution taxon-wide comparisons and discern between endocranial changes accompanying progressive adaptation to aquatic niches and conservative features informing on phylogenetic identity. Systematic evaluation will reveal the neurosensory developments that facilitated these Mesozoic ecological success stories in aquatic environments.