Description of a New Basal Mosasauroid from the Late Cretaceous of Croatia, with Comments on the Evolution of the Mosasauroid Forelimb

A new genus and species of plesiopedal mosasauroid, Portunatasaurus krambergeri, from the Cenomanian-Turonian (Late Cretaceous) of Croatia is described. An articulated skeleton, representing an animal roughly a meter long was found in 2008 on the island of Dugi Otok. The specimen is articulated, in approximate life position, and is well represented from the anterior cervical series to the pelvis. Preserved elements include cervical and dorsal vertebrae, rib fragments, pelvic fragments, and an exquisitely preserved right forelimb. The taxon possesses plesiomorphic characters such as terrestrial limbs and an elongate body similar to that of basal mosasauroids such as Aigialosaurus or Komensaurus, but also shares derived characteristics with mosasaurine mosasaurids such as Mosasaurus. The articulated hand exhibits a unique anatomy that appears to be transitional in form between the terrestrially capable aigialosaurs and fully aquatic mosasaurines, including 10 ossified carpal elements (as in aigialosaurs), intermediately reduced pro- and epipodials, and a broad, flattened first metacarpal (as in mosasaurines). The new and unique limb anatomy contributes to a revised scenario of mosasauroid paddle evolution, whereby the abbreviation of the forelimb and the hydrofoil shape of the paddle evolves either earlier in the mosasaur lineage than previously thought or more times than previously considered. The presence of this new genus, the third and geologically youngest species of aigialosaur from Croatia, suggests an unrealized diversity and ecological importance of this family within the shallow, Late Cretaceous Tethys Sea.