Planocraniidae

Planocraniidae Temporal range: Paleocene - Eocene, 61.6–40 Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N
Skeleton of Boverisuchus magnifrons
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauria
Clade:	Pseudosuchia
Clade:	Crocodylomorpha
Clade:	Crocodyliformes
Clade:	Neosuchia
Clade:	Eusuchia
Family:	†Planocraniidae Li, 1976
Type genus
Planocrania Li, 1976
Genera
†Boverisuchus †Duerosuchus †Planocrania

Planocraniidae is an extinct family of eusuchian crocodyliforms known from the Paleogene of Asia, Europe and North America. The family was coined by Li in 1976, and contains three genera, Boverisuchus, Duerosuchus and Planocrania.^[2][3] Planocraniids were highly specialized crocodyliforms that were adapted to living on land. They had extensive body armor, long legs, and blunt claws resembling hooves, and are sometimes informally called "hoofed crocodiles".^[4]

Classification

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.^[2] As such, Brochu (2013) transferred the other species placed in Pristichampsus to Boverisuchus, and resurrected Planocraniidae to replace Pristichampsidae/Pristichampsinae as the name for the clade.^[2] Brochu cladistically defined Planocraniidae as Planocrania hengdongensis and crocodyliforms more closely related to it than to Alligator mississippiensis (American alligator), Crocodylus niloticus (Nile crocodile), Gavialis gangeticus (gharial), Thoracosaurus macrorhynchus, Allodaposuchus precedens, or Hylaeochampsa vectiana.

Phylogenetic analyses based purely on morphological data have generally placed planocraniids in a basal position within the crocodilian crown group. Some of these analyses have found that planocraniids lie just outside Brevirostres, a group that includes alligators, caimans, and crocodiles but not gharials.^[2][5][6] However, molecular studies using DNA sequencing have found the group Brevirostres to be invalid upon finding that crocodiles and gavialids are more closely related than alligators.^{[7][8][9][10][11]}

A 2018 tip dating study by Lee & Yates using both molecular, morphological and stratigraphic data instead recovered the planocraniids outside crown group Crocodylia. Below is a cladogram from that study:^[10]

	Goniopholis† Bernissartia† Shamosuchus† Eusuchia Acynodon† Hylaeochampsidae† Hylaeochampsa† Allodaposuchidae† Allodaposuchus† Lohuecosuchus† Borealosuchus† "Thoracosaurs" Eothoracosaurus† Thoracosaurus† Argochampsa† Eogavialis† Eosuchus† Planocraniidae† Planocrania hengdongensis† Planocrania datangensis† Boverisuchus† Crocodylia

In 2021, Rio & Mannion conducted a new phylogenetic study using a heavily modified morphological data set, and also noted the lack of consensus and difficulty in placing Planocraniidae. In their study, they recovered Planocraniidae within Crocodylia, as the sister group to Longirostres, as shown in the cladogram below:^[1]

Eusuchia	Hylaeochampsidae† Allodaposuchidae† Borealosuchus† Crocodylia Alligatoroidea Planocraniidae† Planocrania datangensis† Planocrania hengdongensis† Boverisuchus magnifrons† Boverisuchus vorax† Longirostres Gavialoidea Crocodyloidea

Description

Skull of holotype of the extinct crocodile “Weigeltisuchus geiseltalensis” KUHN, 1938 (specimen no. GMH Leo X 8001; subsequently “Pristichampsus geiseltalensis”, now referred to Boverisuchus magnifrons KUHN, 1938)

Planocraniids were land-living (terrestrial) crocodyliforms with longer legs than living crocodilians. They grew to a maximum size of 2 to 3 metres (6.6 to 9.8 ft) in length.^[12] Nearly complete skeletons of Boverisuchus indicate that planocraniids 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 were blunt and have been described as hoof-like in shape,^[13] suggesting that planocraniids may have been unguligrade,^[14] walking on the tips of their toes like mammalian ungulates. The areas on the leg bones where muscles attach were in different positions in planocraniids than they are in living crocodilians, possibly as an adaptation to walking on land.^[12]

While most crocodilians have flattened skulls, planocraniids had tall and narrow (or laterally compressed) skulls. Their teeth were also laterally compressed and not conical like those of modern crocodilians. The combination of a laterally compressed skull and laterally compressed teeth is called the "ziphodont" condition.^[12]

The teeth of the upper jaw completely overlapped the teeth of the lower jaw when the mouth was closed, giving planocraniids an alligator-like overbite. Planocraniids had a notch between the premaxilla bone at the tip of the upper jaw and the maxilla behind it. Living crocodiles also have this notch, which provides room for the enlarged fourth tooth of the lower jaw when the mouth is closed. In planocraniids the fourth tooth was small and did not fit into the notch.^[12]

Evolution

The evolution of Planocraniidae occurred after the K-T extinction when niches were open. Because of the lack of competition, the Planocraniidae evolved to hunt on land, which would have been almost impossible with the dinosaur clades existing.^[15]

References

1. Rio, Jonathan P.; Mannion, Philip D. (6 September 2021). "Phylogenetic analysis of a new morphological dataset elucidates the evolutionary history of Crocodylia and resolves the long-standing gharial problem". PeerJ. 9: e12094. doi:10.7717/peerj.12094. PMC 8428266. PMID 34567843.
2. Brochu, C. A. (2013). "Phylogenetic relationships of Palaeogene ziphodont eusuchians and the status of Pristichampsus Gervais, 1853". Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 103 (3–4): 521–550. doi:10.1017/S1755691013000200. S2CID 128920027.
3. Narváez, I.; de Celis, A.; Escaso, F.; De Jesús, S. M.; Pérez-García, A.; Rodríguez, A.; Ortega, F. (2021). "Redescription and phylogenetic placement of the Spanish middle Eocene eusuchian Duerosuchus piscator (Crocodylia, Planocraniidae)". Journal of Vertebrate Paleontology. 41 (3): e1974868. Bibcode:2021JVPal..41E4868N. doi:10.1080/02724634.2021.1974868. S2CID 242094589.
4. 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.
5. Adam P. Cossette; Christopher A. Brochu (2020). "A systematic review of the giant alligatoroid Deinosuchus from the Campanian of North America and its implications for the relationships at the root of Crocodylia". Journal of Vertebrate Paleontology. 40: e1767638. Bibcode:2020JVPal..40E7638C. doi:10.1080/02724634.2020.1767638.
6. Blanco, A. (2021). "Importance of the postcranial skeleton in eusuchian phylogeny: Reassessing the systematics of allodaposuchid crocodylians". PLoS ONE. 16 (6): e0251900. Bibcode:2021PLoSO..1651900B. doi:10.1371/journal.pone.0251900. PMC 8189472. PMID 34106925.
7. Harshman, J.; Huddleston, C. J.; Bollback, J. P.; Parsons, T. J.; Braun, M. J. (2003). "True and false gharials: A nuclear gene phylogeny of crocodylia". Systematic Biology. 52 (3): 386–402. doi:10.1080/10635150309323. PMID 12775527.
8. Gatesy, J.; Amato, G. (2008). "The rapid accumulation of consistent molecular support for intergeneric crocodylian relationships". Molecular Phylogenetics and Evolution. 48 (3): 1232–1237. doi:10.1016/j.ympev.2008.02.009. PMID 18372192.
9. Erickson, G. M.; Gignac, P. M.; Steppan, S. J.; Lappin, A. K.; Vliet, K. A.; Brueggen, J. A.; Inouye, B. D.; Kledzik, D.; Webb, G. J. W. (2012). Claessens, Leon (ed.). "Insights into the ecology and evolutionary success of crocodilians revealed through bite-force and tooth-pressure experimentation". PLOS ONE. 7 (3): e31781. Bibcode:2012PLoSO...731781E. doi:10.1371/journal.pone.0031781. PMC 3303775. PMID 22431965.
10. Michael S. Y. Lee; Adam M. Yates (27 June 2018). "Tip-dating and homoplasy: reconciling the shallow molecular divergences of modern gharials with their long fossil". Proceedings of the Royal Society B. 285 (1881). doi:10.1098/rspb.2018.1071. PMC 6030529. PMID 30051855.
11. Hekkala, E.; Gatesy, J.; Narechania, A.; Meredith, R.; Russello, M.; Aardema, M. L.; Jensen, E.; Montanari, S.; Brochu, C.; Norell, M.; Amato, G. (2021-04-27). "Paleogenomics illuminates the evolutionary history of the extinct Holocene "horned" crocodile of Madagascar, Voay robustus". Communications Biology. 4 (1): 505. doi:10.1038/s42003-021-02017-0. ISSN 2399-3642. PMC 8079395. PMID 33907305.
12. Brochu, C. A. (2003). "Phylogenetic approaches toward crocodylian history" (PDF). Annual Review of Earth and Planetary Sciences. 31 (31): 357–97. Bibcode:2003AREPS..31..357B. doi:10.1146/annurev.earth.31.100901.141308. Archived from the original (PDF) on 2015-04-02. Retrieved 2019-05-10.
13. Langston, W. (1956). "The Sebecosuchia; cosmopolitan crocodilians?". American Journal of Science. 254 (10): 605–614. Bibcode:1956AmJS..254..605L. doi:10.2475/ajs.254.10.605.
14. Young, M. T.; Bell, M. A.; 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.
15. Why do Animals Look so Strange After Mass Extinctions, retrieved 2021-12-14