Planocraniidae

Planocraniidae is an extinct family of basal crocodylians known from the Paleogene of Asia, Europe and North America. The family was coined by Li in 1976, and contains two genera, Boverisuchus and Planocrania. Planocraniids were highly specialized crocodylians that were adapted to living on land. They have extensive body armor, long legs, and blunt claws resembling hooves, and are sometimes informally called "hoofed crocodiles". Most phylogenetic analyses (analyses of evolutionary relationships) place planocraniids in a basal position within Crocodylia. Some of these analyses find that planocraniids lie just outside Brevirostres, the group of crocodilians that includes alligators, caimans, and crocodiles but not gharials. Planocraniids are inferred to have first appeared in the Late Cretaceous, several tens of millions of years before they actually occur in the fossil record. This is because the earliest members of Brevirostres appear in the Campanian stage of the Late Cretaceous, and Planocraniidae, being an outgroup to Brevirostres,  must have branched off before this time.

Pristichampsidae
Prior to 2013 the term Pristichampsidae/Pristichampsinae was used for this group. However, the type specimen of Pristichampsus was found to be undiagnostic, and considered to be a nomen dubium. As such, Broch (2013) transferred the other species placed in Pristichampsus to Boverisuchus, and resurrected Planocraniidae to replace Pristichampsidae/Pristichampsinae as the name for the clade.

Description
Planocraniids were land-living (terrestrial) crocodylians with longer legs than living species of crocodylians. They grew to a maximum size of  in length. Nearly complete skeletons of Boversuchus indicate that planocraniids were more heavily armored than living crocodylians, with bony plates called osteoderms tightly interlocking along the back, completely encasing the tail, and extending down the legs. The claws are blunt and have been described as hoof-like in shape, suggesting that planocraniids may have been unguligrade, walking on the tips of their toes like mammalian ungulates. The areas on the leg bones where muscles attach are in different positions in planocraniids than they are in living crocodylians, possibly as an adaptation to walking on land.

While most crocodilians have flattened skulls, planocraniids had tall and narrow (or laterally compressed) skulls. Their teeth are also laterally compressed and not conical like those of other crocodilians. The combination of a laterally compressed skull and laterally compressed teeth is called the "ziphodont" condition. The ziphodont condition is common among terrestrial non-crocodylians crocodylomorphs that lived in the Mesozoic, but among crocodylians it is unique to planocraniids and the extinct Australian crocodile Quinkana (which is also thought to have been terrestrial).

The teeth of the upper jaw completely overlap the teeth of the lower jaw when the mouth is closed, giving planocraniids an alligator-like overbite. Planocraniids also have a notch between the premaxilla bone at the tip of the upper jaw and the maxilla behind it. Living crocodiles have this notch, which provides room for the enlarged fourth tooth of the lower jaw when the mouth is closed. In planocraniids and other basal crocodyloids, the fourth tooth is small and does not fit into the notch.

References

Category:Paleocene crocodylomorphs
Category:Terrestrial crocodylomorphs
Category:Paleocene first appearances
Category:Eocene extinctions
Category:Eocene crocodylomorphs
Category:Eocene reptiles of Asia
Category:Eocene reptiles of Europe
Category:Eocene reptiles of North America