Depending on which taxa are included the order may have ranged from the late Carboniferous to the Eocene but the consensus is that eosuchians are confined to the Permian and Triassic[citation needed].

Eosuchia was initially defined to include all "thecodontian" reptiles which did not have an antorbital fenestra but did retain tabulars, postparietals and a large pineal foramen (Broom, 1914).

A definition for inclusion in the order is difficult: it is almost easier to list the primitively-diapsid reptiles that have not been included at one time or another.

[1] The one constant eosuchian has been Youngina, a small lizard-shaped reptile from the Upper Permian of South Africa.

[2] In some phylogenies Eosuchia has been treated (probably erroneously) as a sister lepidosaur taxon to Squamata and Rhynchocephalia.