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

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

crocodyliform Crocodyliformes is a clade of 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 only pseudo ...

, an early member of the group including

crocodilia Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest livi ...

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

Entrada Sandstone The Entrada Sandstone is a 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 deposited during the Jur ...

of Entrada Ranch, Grand County,

Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...

. Middle Jurassic terrestrial

tetrapods Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct therapsi ...

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 * Dorsal c ...

) 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 sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

that is harder than the calcite 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 166.1 ± 4.0 Ma (million years ago) and 163.5 ± 4.0 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 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 species that contains the biological type specime ...

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

incertae sedis ' () or ''problematica'' is a term used for a 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, uncertain ...

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

References

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