''Pachycheilosuchus'' (meaning "thick lipped crocodile") is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

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 crocodylomorphs more closely related to ''Crocodylus niloticus' ...

n from the

Early Cretaceous The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma. Geology Pro ...

Texas Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

. 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. During Mesozoic and early Cenozoic times, cro ...

is notable for its procoelous

vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

e, otherwise found only in derived

eusuchia Eusuchia is a clade of crocodylomorphs that first appeared in the Early Cretaceous with ''Hylaeochampsa''. Along with Dyrosauridae and Sebecosuchia, they were the only crocodyliformes who survived the K-T extinction. Since the other two clades d ...

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 living ...

ns (the vertebrae articulate with a cup on the

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

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

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates 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 t ...

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

armor Armour (British English) or armor (American English; see spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, especially direct contact weapons or projectiles during combat, or fr ...

on the neck formed by the fusion of six individual

scute A scute or scutum (Latin: ''scutum''; plural: ''scuta'' "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 birds. The term is also used to describe the anterior po ...

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 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. The county is named for George Bernard Erath, an early surveyor a ...

in central Texas, in rocks dating from the early

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0.9 M ...

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 conventi ...

. 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 preserved ...

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 Lagerstätt ...

in a

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

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 (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

, 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 estua ...

setting. 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 specimen ...

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 (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates with t ...

s, the longest measured only 91.2 millimeters long (3.59 in). Its body length is estimated as 63.5 centimeters (2.08 ft) to 80 centimeters (2.6 ft), 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 a ...

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. 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 The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

. 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 Derive may refer to: * Derive (computer algebra system), a commercial system made by Texas Instruments * ''Dérive'' (magazine), an Austrian science magazine on urbanism *Dérive, a psychogeographical concept See also * *Derivation (disambiguatio ...

eusuchian crocodilians. Because of this difference, and because ''Pachycheilosuchus'' did not have many of the other features of eusuchians, it probably

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation t ...

procoely independently. The

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

, 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 (; ) 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 typically shared derived chara ...

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

Atoposauridae Atoposauridae is a Family (biology), family of crocodile-line archosaurs belonging to Neosuchia. The majority of the family are known from Late Jurassic to Early Cretaceous marine deposits in France, Portugal, and Bavaria in southern Germany. The ...

. This

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

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 (biology), family of basal (phylogenetics), basal eusuchian Crocodylomorpha, crocodylomorphs thought to be closely related to the order Crocodylia. Classification Hylaeochampsidae was first constructed by Ch ...

, a group of basal

ns, as shown in the

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...

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 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. In 2021, with 7 ...

and two partial skeletons at the

Texas Memorial Museum The Texas Memorial Museum, located on the campus of the University of Texas at Austin in Austin, Texas, USA, was created during preparations for the Texas Centennial Exposition held in 1936. The museum's focus is on natural history, including pal ...

, 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 Neosuchians Prehistoric pseudosuchian genera