A name for the Provincial Fossil of British Columbia: a strange new elasmosaur taxon from the Santonian of Vancouver Island

The first elasmosaurid skeleton from the Haslam Formation (Upper Santonian) of the Nanaimo Group (Late Cretaceous) on Vancouver Island was first described in 2002, and has recently been declared the Provincial Fossil of British Columbia. Since then, additional fossils have been recovered: an isolated right humerus and a well-preserved, osteologically immature skeleton comprising thorax, girdles and limbs. The Haslam material can now support further taxonomic assessment, and we erect the species <i>Traskasaura sandrae</i> based on it. <i>Traskasaura</i> possesses a strange mosaic of features. The mandible is plesiomorphic as seen in <i>Libonectes</i>: relatively narrow with large teeth and a broad symphysis. The skull therefore lacks any of the radical oral cavity adaptations seen in basal aristonectines. The neck of the Haslam animal is also plesiomorphic, with at least 36 preserved cervical vertebrae having vertebral length indices (VLIs) over 100 (the total number of cervicals is not known). The centra lack the anteroposterior compression and midline longitudinal constriction characteristic of derived aristonectines, yet the cervical ribs trend forward – a condition known only in derived aristonectines and <i>Vegasaurus</i>. The autapomorphic coracoid of <i>Traskasaura</i> differs greatly from any known elasmosaurid; the cardiform recess is reduced and posteriorly located, with some similarities to that of <i>Aristonectes quiriquinensis</i>. The humerus is autapomorphic, possessing a relatively straight shaft, pronounced ventral camber and an articular facet on the leading edge that makes a 90° angle with the radial facet. Taken together, these features document a new genus, with a plesiomorphic axial skeleton, but with several convergent appendicular adaptations with derived aristonectines. A revised phylogenetic analysis of Elasmosauridae recovered the new taxon in a basal position. Therefore, the postcranial adaptations shared with derived aristonectines appear to be convergent. https://zoobank.org/urn:lsid:zoobank.org:pub:CCD94C38-E0E7-43CE-B757-A387D055C229