Mosasauroids are conventionally conceived of as gigantic, obligatorily aquatic marine
lizards (1000s of specimens from marine deposited rocks) with a cosmopolitan distribution
in the Late Cretaceous (90-65 million years ago [mya]) oceans and seas of the world.
Here we report on the fossilized remains of numerous individuals (small juveniles
to large adults) of a new taxon, Pannoniasaurus inexpectatus gen. et sp. nov. from
the Csehbanya Formation, Hungary (Santonian, Upper Cretaceous, 85.3-83.5 mya) that
represent the first known mosasauroid that lived in freshwater environments. Previous
to this find, only one specimen of a marine mosasauroid, cf. Plioplatecarpus sp.,
is known from non-marine rocks in Western Canada. Pannoniasaurus inexpectatus gen.
et sp. nov. uniquely possesses a plesiomorphic pelvic anatomy, a non-mosasauroid but
pontosaur-like tail osteology, possibly limbs like a terrestrial lizard, and a flattened,
crocodile-like skull. Cladistic analysis reconstructs P. inexpectatus in a new clade
of mosasauroids: (Pannoniasaurus (Tethysaurus (Yaguarasaurus, Russellosaurus))). P.
inexpectatus is part of a mixed terrestrial and freshwater faunal assemblage that
includes fishes, amphibians turtles, terrestrial lizards, crocodiles, pterosaurs,
dinosaurs and birds.
