Planocraniidae is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

of eusuchian crocodyliforms known from the Paleogene of

Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

and

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

. The family was coined by Li in 1976, and contains three

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

, '' Boverisuchus'', '' Duerosuchus'' and '' Planocrania''. 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".

Classification

Prior to 2013, the term Pristichampsidae/Pristichampsinae was used for this group. However, the

type specimen In biology, a type is a particular wikt:en:specimen, specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally associated. In other words, a type is an example that serves to ancho ...

of '' Pristichampsus'' was found to be undiagnostic, and considered to be a ''

nomen dubium In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application. Zoology In case of a ''nomen dubium,'' it may be impossible to determine whether a ...

''. 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 In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

. 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 In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

analyses based purely on morphological data have generally placed planocraniids in a basal position within the

crocodilia Crocodilia () is an order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchia ...

n 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. However, molecular studies using

DNA sequencing DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, thymine, cytosine, and guanine. The ...

have found the group Brevirostres to be invalid upon finding that crocodiles and gavialids are more closely related than alligators. 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 A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

from that study: In 2021, Rio & Mannion conducted a new

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

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 In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

to Longirostres, as shown in the cladogram below:

Description

Planocraniids were land-living ( terrestrial) crocodyliforms with longer legs than living crocodilians. They grew to a maximum size of in length. 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, suggesting that planocraniids may have been unguligrade, walking on the tips of their toes like mammalian

ungulate Ungulates ( ) are members of the diverse clade Euungulata ("true ungulates"), which primarily consists of large mammals with Hoof, hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined ...

s. 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. 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. 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.

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 Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

clades existing.

References

{{Taxonbar, from=Q15076991 Neosuchia Paleocene crocodylomorphs Terrestrial crocodylomorphs Paleocene first appearances Eocene extinctions Eocene crocodylomorphs Eocene reptiles of Asia Eocene reptiles of Europe Eocene reptiles of North America Pseudosuchian families