The Eusuchia ("true crocodiles") are a clade of crocodylomorphs that first appears 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 died out 35 and 11 million years ago, all living crocodilian species are eusuchians, as are many extinct forms.
|Three species of living eusuchian: gharial (left), American alligator (center), and American crocodile (right).|
Eusuchia was originally defined by Thomas Henry Huxley in 1875 as an apomorphy-based group, meaning that it was defined by shared characteristics rather than relations. These characteristics include pterygoid-bounded choanae and vertebrae which are procoelous (concave from the front and convex from the back). The possibility that these traits may have been convergently evolved in different groups of neosuchians rather than one lineage spurred some modern paleontologists to revise the group's definition to make it defined solely by relations. In 1999, Christopher Brochu redefined Eusuchia as "the last common ancestor of Hylaeochampsa and Crocodylia and all of its descendants". In this definition, "Crocodylia" specifically refers to descendants of the common ancestor of the three modern lineages of eusuchians: Gavialoidea (gharials), Alligatoroidea (alligators and caimans), and Crocodyloidea (crocodiles). Whether certain families or genera qualify as basal eusuchians or non-eusuchian neosuchians is a matter of debate over which definition is used for the group. At least one genus, Isisfordia, would qualify as a eusuchian under Huxley's definition of the group but would not necessarily qualify under Brochu's definition.
Unlike primitive crocodylomorphs, crocodyliforms have secondary bony palates. This feature enables living crocodylians to safely breathe in through their nostrils while the rest of the head (including the mouth) remains submerged. This structure reaches its greatest elaboration among eusuchians, in which the internal nares are completely surrounded by the pterygoid bones.
Below is a cladogram after Puértolas, Canudo and Cruzado-Caballero, 2011:
Aegyptosuchidae is an extinct family of eusuchian crocodyliforms from the Cretaceous period of Africa. They are characterized by their large size and flat heads. The family includes two genera, Aegyptosuchus and Aegisuchus.Alligatorium
Alligatorium is an extinct genus of atoposaurid crocodylomorph from Late Jurassic marine deposits in France.Allodaposuchus
Allodaposuchus is an extinct genus of crocodyliforms that includes four species that lived in what is now southern Europe during the Campanian and Maastrichtian stages of the Late Cretaceous. Although generally classified as a non-crocodylian crocodylomorph, it is sometimes placed as one of the earliest true crocodylians. Allodaposuchus is one of the most common Late Cretaceous crocodylomorphs from Europe, with fossils known from Spain, Romania, and France.Atoposauridae
Atoposauridae is a 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 discovery of the genus Aprosuchus, however, extends the duration of the lineage to the end of the Cretaceous in Romania.Batrachomimus
Batrachomimus is an extinct genus of neosuchian crocodyliform known from the Late Jurassic of northeastern Brazil. It contains a single species, Batrachomimus pastosbonensis, which was first described and named by Felipe C. Montefeltro, Hans C. E. Larsson, Marco A. G. de França and Max C. Langer in 2013. It is known from a nearly complete skull, osteoderms and limb bones. Batrachomimus belongs to the family Paralligatoridae and predates all other members of the family and its immediate sister group, Eusuchia, by 30 million years.Brillanceausuchus
Brillanceausuchus is an extinct genus of atoposaurid crocodylomorph. Fossils have been found in Early Cretaceous–age rocks of Cameroon. The genus is notable for the position of the secondary choana within its palate. Parts of the pterygoid bones make up the rostral margin of the choana and thus separate it from the palatines, a feature also seen in the more advanced neosuchian suborder Eusuchia. This characteristic was once thought to be characteristic of Eusuchia, but its presence in Brillanceausuchus suggests that the trait is homoplasic, thus making the evolution of the position of the choana within crocodilians more complex than previously thought.Coelognathosuchia
Coelognathosuchia is an extinct clade of neosuchian crocodyliforms that includes Goniopholididae and Pholidosauridae, two families of superficially crocodile-like aquatic crocodyliforms from the Mesozoic. Martin et al. (2014) named the clade after finding goniopholidids and pholidosaurids to group together in their phylogenetic analysis of crocodyliform evolutionary relationships. In their analysis, Pholidosauridae was monophyletic and Goniopholididae was paraphyletic, being an assemblage of successively more basal taxa within Coelognathosuchia. Coelognathosuchia itself was positioned near the base of the larger clade Neosuchia as the sister group to a clade containing the Early Cretaceous neosuchian Bernissartia and Eusuchia, the group that includes all modern crocodilians and their closest extinct relatives.Martin et al. named Coelognathosuchia from the Greek κοῖλος (koĩlos, "concave"), γνάθος (gnáthos, "jaw") and σοῦχος (soũchos, "crocodile"), after a small depression on the surface of the skull between the maxilla and jugal bones in both goniopholidids and pholidosaurs. Other diagonostic features of Coelognathosuchia include orbits (eye sockets) that are narrower than the frontal bone that separates them and smaller than the supratemporal fenestrae (two holes at the back of the skull roof) behind them, a notch between the premaxilla and maxilla at the tip of the snout, and the reduction or absence of the antorbital fenestrae (a pair of holes in the snout in front of each orbit).A close relationship between Goniopholididae and Pholidosauridae conflicts with the hypothesis that pholidosaurids are instead more closely related to the family Dyrosauridae. This alternate phylogeny was found in many analyses, including Jouve et al. (2006), Pol and Gasparini (2009), and de Andrade et al. (2011), who named the clade Tethysuchia. In both studies, Goniopholididae was found to be a more distantly related clade within Neosuchia. In their description of Pholidosaurus specimens from southwestern France, Martin and colleagues reiterated their opinion that Dyrosauridae is not as closely related to Pholidosauridae as stated by previous authors by pointing to numerous shared characters between the Cherves-de-Cognac pholidosaurid remains and Goniopholididae.Dolichochampsa
Dolichochampsa is an extinct genus of eusuchian crocodylomorph. It is the type genus and only member of the family Dolichochampsidae. Fossils have been found in the Yacoraite Formation of Argentina and the El Molino Formation of Bolivia of Maastrichtian age. It had a distinctive slender snout. Because the material associated with the specimens is so fragmentary, its relationships with other eusuchians remain unknown.Gilchristosuchus
Gilchristosuchus (meaning "Gilchrist [the owners of the ranch where the type specimen was found] crocodile") is an extinct genus of neosuchian crocodyliform. Its fossils have been found in the upper Milk River Formation of Alberta, Canada, in rocks of either latest Santonian or earliest Campanian age (Late Cretaceous). Gilchristosuchus was described in 1993 by Wu and Brinkman. The type species is G. palatinus, in reference to its distinctive palatine bones.Gilchristosuchus is based on RTMP 91.101.1, a partial posterior skull and a neck vertebra. The skull would have been about 15 centimetres (5.9 in) long when complete. It represents the first articulated crocodylomorph specimen from the Milk River Formation. Isolated remains had been found earlier and assigned to Brachychampsa and Leidyosuchus; some of these fossils probably belong to Gichristosuchus. Although the specimen is not large, the ornamentation of the skull surfaces and bone fusion indicate it was an adult. Based on previous research, Wu and Brinkman suggested that their new genus was the most derived neosuchian that wasn't within Eusuchia, the group including all living crocodilians.Isisfordia
Isisfordia (named after the discoverer; former Deputy Mayor of Isisford, Ian Duncan) (holotype QM F36211) is an extinct genus of crocodyliform closely related to crocodilians that lived during the Middle Cretaceous (Albian–Cenomanian). Its fossils were discovered in the Winton Formation in Isisford, Queensland, Australia and Lightning Ridge in the mid-1990s. Most of the animal was discovered, with the exception of the front portion of the skull. On a later expedition to the location, paleontologists discovered a complete skull which differed from the original specimen in size only.The estimate of the length of Isisfordia is about 1.1 m (3.6 feet).Mesoeucrocodylia
Mesoeucrocodylia is the clade that includes Eusuchia and crocodyliforms formerly placed in the paraphyletic group Mesosuchia. The group appeared during the Early Jurassic, and continues to the present day.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 (the Nile Crocodile) than to Notosuchus terrestris. Neosuchia is very diverse and may be polyphyletic, as the clade has undergone many revisions since it was first named in 1988. Neosuchians first appear in the Early Jurassic with the earliest known goniopholid Calsoyasuchus, which lived during the Sinemurian and Pliensbachian stages.Paluxysuchus
Paluxysuchus is an extinct genus of neosuchian crocodyliform known from the Early Cretaceous Twin Mountains Formation (late Aptian stage) of north-central Texas. It contains a single species, Paluxysuchus newmani. Paluxysuchus is one of three crocodyliforms known from the Early Cretaceous of Texas, the others being Pachycheilosuchus and an unnamed species referred to as the "Glen Rose Form". Paluxysuchus has a long, flat skull that is probably transitional between the long and narrow skulls of many early neosuchians and the short and flat skulls of later neosuchians.Paralligatoridae
Paralligatoridae is an extinct family of neosuchian crocodyliforms that existed during the Jurassic and Cretaceous periods. It includes the genera Paralligator, Brillanceausuchus, Kansajsuchus, Shamosuchus, Scolomastax, Sabresuchus, Rugosuchus, Batrachomimus and Wannchampsus, as well as the yet-unnamed "Glen Rose form".Sabresuchus
Sabresuchus is an extinct genus of neosuchian crocodyliform from the Cretaceous of Europe. The name is derived from 'Sabre' in reference to the enlarged and curved fifth maxillary tooth, and 'suchus' from the Ancient Greek for crocodile.Susisuchus
Susisuchus is an extinct genus of neosuchian mesoeucrocodylian crocodyliform from the Early Cretaceous of Brazil. Fossils have been found from the Nova Olinda Member of the Aptian-age Crato Formation in the Araripe and Lima Campos Basins of northeastern Brazil. Named in 2003, Susisuchus is the sole member of the family Susisuchidae, and is closely related to the clade Eusuchia, which includes living crocodilians. The type species is S. anatoceps, known from a single partial articulated skeleton that preserves some soft tissue. A second species, S. jaguaribensis, was named in 2009 from fragmentary remains.Terminonaris
Terminonaris is a genus of extinct pholidosaurid crocodyliforms that lived in the Late Cretaceous (Cenomanian and Turonian). The name means: “enlarged snout or nose” at the front of the skull. Terminonaris is an early crocodile, within a subgroup called Mesoeucrocodylia. Its remains have only been found in North America and Europe. Originally known under the generic name Teleorhinus, it was once believed to be a teleosaurid (a family of marine gavial-like thalattosuchians). Both, prehistoric crocodiles such as Terminonaris as well as modern crocodiles belong to the same group called crocodyliformes, although modern crocodiles have specific features that show indicate distant relatives of this species and in the subgroup Eusuchia.Theriosuchus
Theriosuchus is an extinct genus of atoposaurid mesoeucrocodylian from Late Jurassic to Early Cretaceous of Europe (southern England), Southeast Asia (Thailand) and western North America (Wyoming), with fragmentary records from Middle Jurassic and Early Cretaceous sites in China, Morocco, and Scotland.