Mosasauroids were large marine Lizards that became dominant marine predators in the Late Cretaceous after the disappearance of the Ichthyosaurs. They were related to Monitor Lizards, but fully adapted to an aquatic environment, with flipperlike limbs and elongate, streamlined bodies, probably coming ashore only to lay eggs like modern marine Turtles. The smallest members of the group reached about three meters in length, the largest were over seventeen meters long.
In a paper published in the journal PLoS One on 19 December 2012, László Makádi of the Department of Paleontology and Geology at the Hungarian Natural History Museum, Michael Caldwell of the Department of Biological Sciences at the University of Alberta and Attila Ősi of the MTA-ELTE Lendület Dinosaur Research Group at the Department of Physical and Applied Geology at Eötvös University describe a new species of Mosasauroid from the Late Cretaceous Csehbánya Formation of western Hungary.
The new species is named Pannoniasaurus inexpectatus, meaning the unexpected Lizard from Pannonia; Pannonia being the Roman name for the region where the fossils were found. The species is described from a large number of disarticulated bones from a flood breccia (conglomerate deposit formed by a flash flood) exposed at the Iharkút open-pit bauxite mine in the Bakony Hills of Western Hungary since 2000. Such disarticulated remains are difficult to work with, but in this case are so numerous as to allow a reasonable reconstruction of the living animal.
Skeletal anatomy of Pannoniasaurus inexpectatus. (A) Skull drawing shows known elements in grey. (B) Skeletal reconstruction showing preserved bones in white. Scale bar 1 m. (C) Premaxilla in dorsal view. Scale bar 1 cm. (D) Right maxilla in lingual view. Scale bar 1 cm. (E) Left postorbitofrontal in dorsal view. Scale bar 1 cm. (F) Right quadrate in lateral view. Scale bar 1 cm. (G) Left dentary in lingual view. Scale bar 1 cm. (H) Isolated teeth without and with base preserved. Scale bar 1 cm. (I) Left splenial in lingual view. Scale bar 1 cm. (J) Right coronoid in lingual view. Scale bar 1 cm. (K) Left angular in labial view. Scale bar 1 cm. (L) Right surangular in labial view. Scale bar 1 cm. (M) Left articular in dorsal view. Scale bar 1 cm. (N) Mid-cervical vertebra in lateral view. Scale bar 1 cm. (O) Dorsal vertebra in dorsal view. Scale bar 1 cm. (P) First sacral vertebra in dorsal view. Scale bar 1 cm. (Q) Second sacral vertebra in dorsal view. Scale bar 1 cm. (R) Anterior caudal vertebra in lateral view. Scale bar 1 cm. (S) Rib fragment in dorsolateral view. Scale bar 1 cm. (T) Proximal end of left humerus in flexor view. Scale bar 1 cm. (U) Distal end of right humerus in flexor view. Scale bar 1 cm. (V) Left ilium in lateral view. Scale bar 1 cm. Makádi et al. (2012)
Pannoniasaurus inexpectatus appears to have been a Crocodile-like animal reaching up to seven meters in length, though most of the remains appear to come from animals of between three and four meters. It has a flattened skull, suggesting it lived as an ambush predator operating in shallow water, similar to modern Crocodiles.
This is the first known Mosasauroid that appears to have lived entirely in freshwater (some previous specimens have been found from estuarine deposits). As wells as the sedimentological evidence that this is a fluvial flood deposit Makádi et al. carried out a number of geochemical tests, all of which point to a freshwater, rather than a marine, origin for the deposits where the specimens where found.
Apart from being disarticulated the fossils show little sign of wear and tear; this implies that the specimens were not moved far in the flood, but that they were from animals that had pre-deceased the flood event (allowing them to become disarticulated), but not attacked by scavengers (which leave distinctive marks on bones. This suggests that the animals were either quickly buried or submerged in anoxic waters, then exhumed and redeposited by the flood event.