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Concept
Eothoracosaurus
Résumé
Eothoracosaurus is an extinct monospecific genus of eusuchian crocodylomorphs found in Eastern United States which existed during the Late Cretaceous period. Eothoracosaurus is considered to belong to an informally named clade called the "thoracosaurs", named after the closely related Thoracosaurus. Thoracosaurs in general were traditionally thought to be related to the modern false gharial, largely because the nasal bones contact the premaxillae, but phylogenetic work starting in the 1990s instead supported affinities within gavialoid exclusive of such forms. Even more recent phylogenetic studies suggest that thoracosaurs might instead be non-crocodilian eusuchians.
Fossils are known from the Ripley Formation in Mississippi and date back to the early Maastrichtian stage of the Late Cretaceous. Some fragmentary material from the Coon Creek Formation of western Tennessee dating back to the late Campanian (slightly older than the specimens from Mississippi) has been referred to Eothoracosaurus as well. The holotype specimen of Eothoracosaurus (MSU 3293, a skull and associated postcrania) was originally discovered in 1931 and first described by Kenneth Carpenter in 1983 and initially referred to Thoracosaurus neocesariensis. The material was eventually reexamined by Christopher Brochu in 2004, taking note of substantial differences to other thoracosaurs and finding them severe enough to warrant a separate genus: Eothoracosaurus.
The name derives from the genus Thoracosaurus and the prefix "eos" meaning dawn, chosen to reflect the fact that Eothoracosaurus appeared earlier in the fossil record than its relative. The species name mississippiensis represents the state of Mississippi, where the holotype was discovered.
Like in the modern gharial, the skull of Eothoracosaurus is strongly elongated with the head growing notably broader further back. The external nares are entirely surrounded by the premaxilla, which extends between the maxilla as far back as the approximate position of the fourth maxillary tooth on the dorsal surface and up to the third tooth when viewed from below.
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