''Pachycheilosuchus'' (meaning "thick lipped crocodile") 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 ...

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 ...

neosuchia Neosuchia is a clade within Mesoeucrocodylia that includes all modern extant crocodilians and their closest fossil relatives. It is defined as the most inclusive clade containing all Crocodylomorpha, crocodylomorphs more closely related to ''Croc ...

n from the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. Previously known, in part, as the "Glen Rose form", this

crocodylomorph Crocodylomorpha is a group of pseudosuchian archosaurs that includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction. Extinct crocodylomorphs were considerably mor ...

is notable for its procoelous

vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

e, otherwise found only in derived

eusuchia Eusuchia is a clade of neosuchian Crocodylomorpha, crocodylomorphs that first appeared in the Early Cretaceous, which includes modern Crocodilia, crocodilians. Along with Dyrosauridae and Sebecosuchia, they were the only crocodyliformes who survi ...

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 vertebrae articulate with a cup on the

anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

surface and a rounded posterior surface), a thick margin on the

maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...

e (the main tooth-bearing bones of the upper jaw; thus "thick lipped crocodile"), and a shield of

armor Armour (Commonwealth English) or armor (American English; see American and British English spelling differences#-our, -or, spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, e ...

on the neck formed by the fusion of six individual

scute A scute () or scutum (Latin: ''scutum''; plural: ''scuta'' "Scutum (shield), shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of Bird anatomy#Scales, birds. The ter ...

Description

''Pachycheilosuchus'' is based on SMU 75278, a right

, with the remains of at minimum 13 other individuals also known, representing most of the skeleton except hands, feet, and part of the skull. The remains were recovered near the top of the

Glen Rose Formation The Glen Rose Formation is a shallow marine to shoreline geological formation from the lower Cretaceous period exposed over a large area from South Central to North Central Texas. The formation is most widely known for the dinosaur footprints ...

Erath County Erath County () is a County (United States), county located in the U.S. state of Texas. According to the United States Census bureau its population was 42,545 in 2020. The county seat is Stephenville, Texas, Stephenville. The county is named fo ...

in central Texas, in rocks dating from the early

Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...

faunal stage In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by conven ...

. The

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s were found in a

bonebed A bone bed is any geological stratum or deposit that contains bones of whatever kind. Inevitably, such deposits are sedimentary in nature. Not a formal term, it tends to be used more to describe especially dense collections such as Lagerstätte. ...

in a

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

rock unit, with scattered lenses of

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

, probably deposited in a shallow, protected, nearshore

brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuary ...

setting. 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 ...

is ''P. trinquei'', in honor of Lance Trinque, a field assistant who helped discover and excavate the site where this animal's remains were found. This genus was named and described by Jack Rogers in 2003. It was not a large crocodylomorph; for example, of the collected

thigh bone The femur (; : femurs or femora ), or thigh bone is the only bone in the thigh — the region of the lower limb between the hip and the knee. In many four-legged animals the femur is the upper bone of the hindleg. The top of the femur fits in ...

s, the longest measured only 91.2 millimeters long (3.59 in). Its body length was initially estimated at , using two methods. Although the remains are small, it appears that at least some of the individuals were mature, with fusion of parts of individual vertebrae. Additionally, a 49 millimeter-long (1.9 in) crocodyloid

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the ...

was recovered with the skeletal fossils, and is of reasonable size to have come from an individual with a length of 63.5 centimeters, suggesting that some of the individuals were sexually mature. A study in 2020 suggests that it may have reached up to in total body length. The maxillae had overhanging lips along their outer margins, and the tooth row was set away from the margin. One maxilla has an oval puncture mark, 5 millimeters by 6.5 millimeters (0.2 by 0.26 in), probably made by a larger predator. The snout was short and flat. The vertebrae were procoelous in the neck, back, and part of the tail, and the procoelous shape was most strongly developed in the neck vertebrae. Procoely is a type of vertebral articulation, based on the shape of the anterior and posterior faces of the

vertebral centrum Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

. In procoelous vertebrae, the vertebrae articulate with a concave leading surface and a convex posterior surface. The vertebrae of ''Pachycheilosuchus'' had a slight dimple or concavity on the posterior surface as well, making these bones different from the procoelous vertebrae that are a hallmark of derived eusuchian crocodilians. Because of this difference, and because ''Pachycheilosuchus'' did not have many of the other features of eusuchians, it probably

evolved Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

procoely independently. The

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

, the major bone of the forearm, is strongly curved. ''Pachycheilosuchus'' had among its armor a unique shield of bony scutes for its neck, composed of six individual fused scutes.

Classification

Using a

cladistic analysis Cladistics ( ; from Ancient Greek 'branch') is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is ...

, Rogers found that ''Pachycheilosuchus'' was most likely closest to the

Atoposauridae Atoposauridae is an extinct family of neosuchian crocodyliforms, known from the Jurassic and Cretaceous of Eurasia. Characterised by their small size and their heterodont (having multiple tooth types) dentition, they are thought to have lived in ...

. This

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 ...

of small crocodylomorphs is also known from the Early Cretaceous, and includes some members with procoelous vertebrae. However, a subsequent 2011 study found ''Pachycheilosuchus'' to be a member of

Hylaeochampsidae Hylaeochampsidae is an extinct family of basal eusuchian crocodylomorphs thought to be closely related to the order Crocodylia. Classification Hylaeochampsidae was first constructed by Charles William Andrews in 1913 to include just one memb ...

, a group of basal

ns, as shown in the

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 ...

below.

The "Glen Rose Form"

Previously, Glen Rose crocodylomorph fossils had been known under the informal designation of the "Glen Rose Form". Fossils known under this name include a skull at the

National Museum of Natural History The National Museum of Natural History (NMNH) is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. With 4.4 ...

and two partial skeletons at the

Texas Memorial Museum Texas Science & Natural History Museum is located on the campus of The University of Texas at Austin in Austin, Texas, U.S. It opened as Texas Memorial Museum during preparations for the Texas Centennial Exposition held in 1936. The museum's foc ...

, along with isolated bones. With the description of ''Pachycheilosuchus'', it appears that the "Glen Rose Form" had been chimeric; the skull and the two skeletons lack the lipped jaw and procoelous vertebrae, and so are not ''Pachycheilosuchus'' but an undescribed form, but at least some of the isolated remains are ''Pachycheilosuchus''. Thus, there were at least two crocodylomorphs in the Glen Rose Formation.

References

{{Taxonbar, from=Q7122101 Early Cretaceous crocodylomorphs of North America Early Cretaceous reptiles of North America Neosuchia Prehistoric pseudosuchian genera