CT-scan description of Alexandronectes zealandiensis (Elasmosauridae, Aristonectinae), with comments on the elasmosaurid internal cranial features

Abstract

The holotype specimen of Alexandronectes zealandiensis is analyzed using digital reconstruction based on CT scans. Additional information regarding internal anatomy or obscured details are added. Additional features include: ectopterygoid rhombic in shape with posterior end pointed, a feature shared only with Aristonectes quiriquinensis; pterygoid shows a high dorsal crest located anteriorly and laterally to the level of the basipterygoid process; posterior margin of parabasisphenoid medially notched in ventral view and surrounding anteriorly and laterally a midline pit. Two canals for XII nerve are present in the right exoccipital-opisthothic and only one in the left one. Supraoccipital with two medially curved ridges on its posterior surface. Additionally, the presence of a stapes is described for the first time in an aristonectine elasmosaurid. The inner ear labyrinth is described and compared with that of other plesiosaurs, and the floccular recess (osseous correlate of the floccular lobe of the cerebellum) is described for first time among elasmosaurids. This feature is probably related to the presence of a long neck and with predatory behavior as the floccular lobe (housed in the floccular recess) stabilized the head via the cervical musculature, and stabilized the retinal image during rotational head movements.