Eusuchia

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Eusuchia
Temporal range: Early Cretaceous - Recent, 130–0 Ma
Comparison - Crocodilia.jpg
Three species of living eusuchian: gharial (left), American alligator (center), and American crocodile (right).
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Pseudosuchia
Superorder: Crocodylomorpha
Clade: Neosuchia
Clade: Eusuchia
Huxley, 1875
Subgroups

The Eusuchia ("true crocodiles") are a clade of crocodylomorphs that first appeared in the Early Cretaceous with Hylaeochampsa.[3] Along with Dyrosauridae and Sebecosuchia, they were the only crocodyliformes who survived the K-T extinction. Since the other two clades died out 47 and 11 million years ago respectively, all living crocodilian species are eusuchians, as are many extinct forms.

Definition[]

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".[4] 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.

Description[]

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.

Phylogeny[]

A 2018 tip dating study by Lee & Yates using both molecular, morphological and stratigraphic data recovered the cladogram below:[2]

Goniopholis

Bernissartia

Shamosuchus

Eusuchia

Acynodon

Hylaeochampsa

Allodaposuchus

Lohuecosuchus

Borealosuchus

"Thoracosaurs"

Eothoracosaurus

Thoracosaurus

Argochampsa

Eogavialis

Eosuchus

Planocraniidae

Crocodylia (crown)

A 2021 Rio & Mannion phylogenetic study using a heavily modified morphological data set recovered the cladogram below:[5]

Bernissartia fagesii

Isisfordia duncani

Theriosuchus pusillus

Paralligatoridae

Shamosuchus djadochtaensis

Glen Rose Form

Wannchampsus kirpachi

Eusuchia

Acynodon iberoccitanus

Hylaeochampsa vectiana

Iharkutosuchus makadii

Allodaposuchidae

Borealosuchus

Crocodylia

Alligatoroidea

Planocraniidae

Longirostres

Gavialoidea

Crocodyloidea

(crown)

References[]

  1. ^ Casey M. Holliday & Nicholas M. Gardner (2012). "A New Eusuchian Crocodyliform with Novel Cranial Integument and Its Significance for the Origin and Evolution of Crocodylia". PLOS ONE. 7 (1): e30471. Bibcode:2012PLoSO...730471H. doi:10.1371/journal.pone.0030471. PMC 3269432. PMID 22303441. open access
  2. ^ a b Michael S. Y. Lee; Adam M. Yates (27 June 2018). "Tip-dating and homoplasy: reconciling the shallow molecular divergences of modern gharials with their long fossil". Proceedings of the Royal Society B. 285 (1881). doi:10.1098/rspb.2018.1071. PMC 6030529. PMID 30051855.
  3. ^ Benton, Michael J.; Sibbick, John (2000). Vertebrate Palaeontology. Blackwell Publishing. p. 233. ISBN 0-632-05614-2.
  4. ^ Brochu, Christopher A. (1999). "Phylogenetics, Taxonomy, and Historical Biogeography of Alligatoroidea". Journal of Vertebrate Paleontology. 19: 9–100. doi:10.1080/02724634.1999.10011201. JSTOR 3889340.
  5. ^ Rio, Jonathan P.; Mannion, Philip D. (6 September 2021). "Phylogenetic analysis of a new morphological dataset elucidates the evolutionary history of Crocodylia and resolves the long-standing gharial problem". PeerJ. 9: e12094. doi:10.7717/peerj.12094. PMC 8428266. PMID 34567843.


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