Pinacosuchus is an extinct genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of crocodylomorph

Crocodylomorpha

The Crocodylomorpha are an important group of archosaurs that include the crocodilians and their extinct relatives.During Mesozoic and early Tertiary times the Crocodylomorpha were far more diverse than they are now. Triassic forms were small, lightly built, active terrestrial animals. These were...

. Its fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s were found in the Late Cretaceous

Late Cretaceous

The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...

-age North Horn Formation

North Horn Formation

The North Horn Formation is a geological formation in Utah whose strata date back to the Late Cretaceous. Dinosaur remains are among the fossils that have been recovered from the formation.-Vertebrate paleofauna:...

 of Utah

Utah

Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...

 (United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

). Pinacosuchus was a diminutive crocodylomorph with spiky armor, and is known only from meagre remains.

History and description

Charles W. Gilmore

Charles W. Gilmore

Charles Whitney Gilmore was an American paleontologist, who named dinosaurs in North America and Mongolia, including the Cretaceous sauropod Alamosaurus, Alectrosaurus, Archaeornithomimus, Bactrosaurus, Brachyceratops, Chirostenotes, Mongolosaurus, Parrosaurus, Pinacosaurus, Styracosaurus and...

 named Pinacosuchus in 1942 for USNM

National Museum of Natural History

The National Museum of Natural History is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. Admission is free and the museum is open 364 days a year....

 16592, consisting of a fragment of upper jaw

Maxilla

The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...

, seven partial vertebrae, a partial coracoid, a partial thigh bone

Femur

The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

, numerous pieces of bony armor

Osteoderm

Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the skin. Osteoderms are found in many groups of extant and extinct reptiles, including lizards, various groups of dinosaurs , crocodilians, phytosaurs, aetosaurs, placodonts, and hupehsuchians...

, and other fragments. This specimen was discovered at the "Lizard Locality" in the Manti National Forest

Manti National Forest

Manti National Forest was established as the Manti Forest Reserve by the General Land Office in Colorado and Utah on May 29, 1903 with . After the transfer of federal forests to the U.S. Forest Service in 1905, it became a National Forest on March 4, 1907. On July 1, 1915 Nebo National Forest was...

, Emery County

Emery County, Utah

Emery County is a county located in the U.S. state of Utah. As of 2000 the population was 10,860, and by 2009 had been estimated to decrease to 10,629. It was named for George W. Emery, governor of the Utah Territory in 1875...

, Utah. The type species

Type species

In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...

 is P. mantiensis. Gilmore had difficulty classifying the specimen, due to its fragmentary nature. He ruled out all then-known orders

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 of reptiles on anatomical ground except for Crocodilia

Crocodilia

Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...

 (which had a more expansive definition at the time) and the nebulous Thecodont

Thecodont

Thecodont , now considered an obsolete term, was formerly used to describe a diverse range of early archosaurs that first appeared in the Latest Permian and flourished until the end of the Triassic period...

ia. He considered a pseudosuchia

Pseudosuchia

Pseudosuchia is the name originally given to a group of prehistoric reptiles from the Triassic period. The name has been variously interpreted, and it is still sometimes, if infrequently, used in scientific literature today. A more commonly used name, Crurotarsi, is often substituted for...

n thecodont identity, but eliminated it because all members were of Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 age or older. Pinacosuchus would have been an archaic crocodilian because it lacked the procoelous vertebrae (concave articulation surface on the anterior

Anatomical terms of location

Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...

 face of the bone, and convex articulation on the posterior face) of more derived

Derived

In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...

 crocodilians, instead having amphicoelous vertebrae (concave articulations on both faces).

Pinacosuchus would have been a very small crocodylomorph. The sacrum

Sacrum

In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...

 (the vertebra supporting the hips) of USNM 16592 was only 14.3 millimetre (0.562992125984252 in) long. Gilmore regarded the specimen as an adult because the vertebrae lacked sutures for their neural arches

Vertebral arch

The vertebral arch is the posterior part of a vertebra.It consists of a pair of pedicles and a pair of laminae, and supports seven processes:* four articular processes* two transverse processes* one spinous process...

, a sign of maturity. This assessment was supported by O'Neill and colleagues, comparing Pinacosuchus to their new spiked crocodilian Akanthosuchus

Akanthosuchus

Akanthosuchus is an extinct genus of alligatoroid crocodilian, possibly an alligatorine. Its fossils are found in the Paleocene-age Nacimiento Formation of the San Juan Basin, New Mexico, USA. It is known from a partial skeleton and isolated bony armor...

. The armor of Pinacosuchus was found disarticulated. Gilmore divided the armor into five categories: simple rectangular scutes; ridged rooflike scutes; thickened rectangular scutes with asymmetrically-positioned sharp spines overhanging the borders; small pointed spines with thickened bases; and more elongate pointed spines with thickened bases.