''Entradasuchus'' (meaning "Entrada andstone and Ranchcrocodile") is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

crocodyliform Crocodyliformes is a clade of Crurotarsi, crurotarsan archosaurs, the group often traditionally referred to as "crocodilians". They are the first members of Crocodylomorpha to possess many of the features that define later relatives. They are the ...

, an early member of the group including

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 ...

ns. The only known specimen was found in rocks of the

Middle Jurassic The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...

Entrada Sandstone The Entrada Sandstone is a formation (geology), formation in the San Rafael Group found in the U.S. states of Wyoming, Colorado, northwest New Mexico, northeast Arizona, and southeast Utah. Part of the Colorado Plateau, this formation was deposi ...

of Entrada Ranch, Grand County,

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

. Middle Jurassic terrestrial

tetrapods A tetrapod (; from Ancient Greek τετρα- ''(tetra-)'' 'four' and πούς ''(poús)'' 'foot') is any four- limbed vertebrate animal of the clade Tetrapoda (). Tetrapods include all extant and extinct amphibians and amniotes, with the lat ...

are very rare, and ''Entradasuchus'' was the first unequivocal North American Middle Jurassic nonmarine tetrapod known from body fossils when it was described (1995).

Description

''Entradasuchus'' is based on CU-MWC 183-11, a skeleton including at least the upper surfaces of the skull, articulated armor scutes from the neck to the tail, and some limb bones, all preserved in top (

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage The fus ...

) view. The exact composition of the specimen was unknown at the time of its description, because it is preserved in

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

that is harder than the

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

that has replaced the bones, which makes preparation difficult. Additionally, much of the visible part of the skeleton is represented by mold impressions of eroded bones. The block containing the specimen weathered from canyon walls and probably came from the middle Moab Member of the Entrada Sandstone, dated to the very late middle

Callovian In the geologic timescale, the Callovian is an age and stage in the Middle Jurassic, lasting between 165.3 ± 1.1 Ma (million years ago) and 161.5 ± 1.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the ...

of the Middle Jurassic. The Moab Member of the Entrada Sandstone represents an inland dune ( eolian) setting. The skull of CU-MWC 183-11 is distinguished by the width of the posterior portion. Approximately 42 pairs of scutes ran down the midline of the back. The first pair behind the skull were square, but those on the back and tail were rectangular and had short spiny projections on their posterior outside corners. These spines inspired the name of the

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

: ''E. spinosus''. CU-MWC 183-11 was not a large animal, with a skull only a few centimeters or inches long. This small size and the form of the skull could indicate it was a juvenile, but the fusion of the parietals in the skull and the generally small size of animals found in settings like the upper Entrada Sandstone suggest it was an adult. Adrian Hunt and

Martin Lockley Martin G. Lockley (17 March 1950 – 25 November 2023) was a Welsh palaeontologist. He was educated in the United Kingdom where he obtained degrees (BSc and PhD) and post-doctoral experience in Geology in the 1970s. Since 1980 he has been a pr ...

, who described ''Entradasuchus'' in 1995, classified it as Crocodyliformes ''

incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

''. The type specimen is curated at the Museum of Western Colorado in Fruita, Colorado.

References

{{Taxonbar, from=Q5380653 Crocodyliformes Middle Jurassic reptiles of North America Middle Jurassic crocodylomorphs Prehistoric pseudosuchian genera