The skull morphology of <i>Mioproteus wezei</i> (Caudata: Proteidae): first complete cranial material on fossil proteid salamanders

Proteidae comprise two extant and several extinct genera of paedomorphic salamanders. The extinct proteids are diagnosed based on the vertebral morphology, with only a few isolated skull bones known to date. New cranial material of Mioproteus wezei from the Late Pliocene of Northern Caucasus (Russia), includes articulated skull fragments, numerous isolated bones, and elements of the hyobranchial apparatus. This cranial material, so far the most complete for any fossil proteid, allowed reconstruction of the entire skull of M. wezei. The skull is heavily ossified with a wide orbital region, characterizing a typically aquatic, paedomorphic animal with functional eyes. It shows greater similarity to that of the black olm, Proteus anguinus parkelj, than to the white olm, P. anguinus anguis, supporting the plesiomorphic status of the former. The results provide first insights into skull evolution within the Mioproteus-Proteus lineage, indicating a gradual decrease in orbit diameter, narrowing and elongation of the rostral region and increase in tooth count.