Casineria was a tetrapod which lived 340 million years ago in the Mississippian period. Casineria was very small (15 centimeters long), but its skeleton was more advanced than the primitive tetrapods, and it was more closely related to amniotes than to amphibians. Casineria may have been one of the very first true amniotes, a group which consists of synapsids and sauropsids, and it pushes back the origin of amniote lineages much farther than was previously realized. Casineria, phylogenetically, is placed by paleontologists as a basal ("primitive") member of the group Amniota, though it may lie just outside the group, as a basal member of Amniota sensu lato. Like most primitive amniotes, this creature was an insectivore, and it may have laid eggs on land. This earliest amniote had five fingers on each hand.
In the early Carboniferous period before the appearance of Casineria, vertebrates were aquatic. Casineria and its relatives were the first vertebrates to live and reproduce on land.