Pristichampsidae

Pristichampsidae Temporal range: Paleocene–Eocene PreЄ Є O S D C P T J K Pg N
Skull of Pristichampsus geiseltalensis
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	Crocodylomorpha
Order:	Crocodilia
Family:	†Pristichampsidae Kuhn, 1968
Genera
†Planocrania †Pristichampsus

Pristichampsidae is an extinct family of crocodilians that lived in Europe, Asia, and North America during the Paleogene. Two genera of pristichampsids are currently recognized, Pristichampsus and Planocrania. Pristichampsids were highly specialized crocodilians 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".^[1] Most phylogenetic analyses (analyses of evolutionary relationships) place pristichampsids in a basal position within Crocodylia. Some of these analyses find that pristichampsids lie just outside Brevirostres, the group of crocodilians that includes alligators, caimans, and crocodiles but not gharials. Pristichampsids 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 Pristichampsidae, being an outgroup to Brevirostres, must have branched off before this time.^[2]

Description

Pristichampsids were land-living (terrestrial) crocodilians with longer legs than living species of crocodilians. They grew to a maximum size of 2 to 3 metres (6.6 to 9.8 ft) in length.^[2] Nearly complete skeletons of Pristichampsus indicate that pristichampsids were more heavily armored than living crocodilians, 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,^[3] suggesting that pristichampsids may have been unguligrade,^[4] walking on the tips of their toes like mammalian ungulates. The areas on the leg bones where muscles attach are in different positions in pristichampsids than they are in living crocodilians, possibly as an adaptation to walking on land. One study suggested that these anatomical modifications may have enabled pristichampsids to walk on their hind limbs for brief periods of time, making them possible facultative bipeds.^[2]

While most crocodilians have flattened skulls, pristichampsids have 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-crocodilian crocodylomorphs that lived in the Mesozoic, but among crocodilians it is unique to pristichampsids and the extinct Australian crocodile Quinkana (which is also thought to have been terrestrial).^[2]

The teeth of the upper jaw completely overlap the teeth of the lower jaw when the mouth is closed, giving pristichampsids an alligator-like overbite. Pristichampsids 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 pristichampsids and other basal crocodyloids, the fourth tooth is small and does not fit into the notch.^[2]

Phylogeny

Below is a cladogram illustrating the phylogeny of the Pristichampsidae from Brochu et al. (2012):^[5]

	†Bernissartia fagesii †Hylaeochampsidae Crocodilia (crocodilians) Gavialoidea (gharials and extinct relatives) †Borealosuchus spp. †Pristichampsidae †Planocrania hengdongensis †Planocrania datangensis †Pristichampsus vorax †Pristichampsus geiseltalensis Brevirostres Alligatoroidea (alligators, caimans, and extinct relatives) Crocodyloidea (crocodiles and extinct relatives)

References

1. Brochu, C. (2007). "Systematics and phylogenetic relationships of hoofed crocodiles (Pristichampsinae)". Journal of Vertebrate Paleontology 27 (3, Suppl.): 53A. doi:10.1080/02724634.2007.10010458.
2. Brochu, C. A. (2003). "Phylogenetic approaches toward crocodylian history" (PDF). Annual Review of Earth and Planetary Sciences 31: 357–97. doi:10.1146/annurev.earth.31.100901.141308.
3. Langston, W. (1956). "The Sebecosuchia; cosmopolitan crocodilians?". American Journal of Science 254 (10): 605–614. doi:10.2475/ajs.254.10.605.
4. Young, M. T.; Bell, M. A.; De Andrade, M. B.; Brusatte, S. L. (2011). "Body size estimation and evolution in metriorhynchid crocodylomorphs: Implications for species diversification and niche partitioning". Zoological Journal of the Linnean Society 163 (4): 1199. doi:10.1111/j.1096-3642.2011.00734.x.
5. Brochu, C. A.; Parris, D. C.; Grandstaff, B. S.; Denton, R. K.; Gallagher, W. B. (2012). "A new species of Borealosuchus (Crocodyliformes, Eusuchia) from the Late Cretaceous–early Paleogene of New Jersey". Journal of Vertebrate Paleontology 32: 105. doi:10.1080/02724634.2012.633585.