Alexeyisaurus

Alexeyisaurus is the stratigraphically oldest named possible plesiosaur. It was named and described by Sennikov and Arkhangelsky (2010) as an elasmosaurid plesiosaur from the Late Triassic of Franz Josef Land, Russia. This would be highly unusual because the oldest definitive elasmosaurid plesiosaurs are otherwise restricted to the Cretaceous Period. The holotype material of Alexeyisaurus is a partial scrappy skeleton, which consists of an incomplete vertebral column, ribs, coracoid, pubis, humerus, femur, and unidentifiable fragments of skull bones.

Sennikov and Arkhangelsky (2010) described one character in common with elasmosaurids: “in the complex shape of the coracoid, the new genus is most similar to elasmosaurids among advanced sauropterygians” (p. 571). However, this single similarity could be morphological convergence (Renesto and Dalla Vecchia 2018). Wintrich et al. (2017) regarded the fossil as an “undiagnostic sauropterygian skeleton”, casting doubt on whether it is a plesiosaur, and Renesto and Dalla Vecchia (2018) also questioned it’s taxonomic affinity: “we consider as dubious its attribution to the Elasmosauridae, while it may well represent a peculiar sauropterygian of uncertain affinity” (p.289). Benson et al. (2015) noted that Alexeyisaurus “might have possessed an intermediate [limb] morphology relevant to the origins of specialized flippers in Plesiosauria” (p.1).

Alexeyisaurus has not been included in any cladistic analyses of sauropterygians because it is too incomplete. It is known from the single species A. karnoushenkoi represented by a single specimen (SGU 104a/36).