|Three species of living eusuchian: gharial (left), American alligator (center), and American crocodile (right).|
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.
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:
- Benton, Michael J.; Sibbick, John (2000). Vertebrate Palaeontology. Blackwell Publishing. p. 233. ISBN 0-632-05614-2.
- 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.
- 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. doi:10.1371/journal.pone.0030471. PMC 3269432. PMID 22303441.
- Eduardo Puértolas; José I. Canudo & Penélope Cruzado-Caballero (2011). "A New Crocodylian from the Late Maastrichtian of Spain: Implications for the Initial Radiation of Crocodyloids". PLoS ONE. 6 (6): e20011. doi:10.1371/journal.pone.0020011. PMC 3110596. PMID 21687705.
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