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.

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 in the collection of the university's Dunn-Seiler Museum) was originally discovered in 1931 and first described by Kenneth Carpenter in 1983 and initially referred to Thoracosaurus neocesariensis.

[3] 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.

[4] The name derives from the genus Thoracosaurus (chest lizard) and the prefix "eos" meaning dawn, chosen to reflect the fact that Eothoracosaurus appeared earlier in the fossil record than its relative.

The interfenestral bar of the parietal bone is another key trait that differentiates Eothoracosaurus, being relatively wider (around half the length of one fenestra), while in Thoracosaurus, the width varies between less than a third or a fourth depending on the species.

[1] †Paratomistoma courti Tomistoma schlegelii, false gharial †Toyotamaphimeia machikanensis †Penghusuchus pani †"Tomistoma" cairense †Thoracosaurus isorhynchus †Eosuchus minor †Eosuchus lerichei †Portugalosuchus azenhae †Thoracosaurus neocesariensis †Eothoracosaurus mississippiensis †Tomistoma dowsoni †Eogavialis africanum †Aktiogavialis caribesi †Argochampsa krebsi †Piscogavialis jugaliperforatus †Siquisiquesuchus venezuelensis †Ikanogavialis gameroi †Dadagavialis gunai †Gryposuchus pachakamue †Gryposuchus colombianus †Gryposuchus neogaeus †Gryposuchus croizati †Gavialis lewisi †Gavialis browni Gavialis gangeticus, gharial Alternatively, recent phylogenetic studies combining morphological, molecular and stratigraphic data argue that rather than being gavialoids, thoracosaurs were basal, non-crocodylian Eusuchians,[2][5] as shown in the cladogram below:[2] Allodaposuchus palustris Lohuecosuchus megadontos Borealosuchus sternbergii Borealosuchus formidabilis Borealosuchus acutidentatus Borealosuchus wilsoni Eothoracosaurus mississippiensis Thoracosaurus neocesariensis Thoracosaurus macrorhynchus Argochampsa krebsi Eogavialis africanum Eosuchus minor Eosuchus lerichei Planocraniidae Alligatoridae Gavialidae Crocodylidae Thoracosaurs such as Eothoracosaurus are typically associated with marine environments and coastal habitats and are thought to have lived in and around shallow marine areas.