Basal amniotes are the stem members of the amniote evolutionary tree. An amniote is a terrestrial tetrapod (four-legged animal) that reproduces by means of sophisticated eggs protected by several layers of embryos, including a hard shell, preventing them from drying out, and allowing them to lay their eggs in places other than water. Amniotes are the first animals that left wet, swampy regions and began colonizing continental interiors 340 million years ago.
After just a few million years, basal amniotes diverged into synapsids (primitive relatives of mammals) and sauropsids (reptiles). These two groups are distinguished by the number of gaps they have in their skulls — synapsids have one gap behind each eye hole, while sauropsids have two. Anapsids (turtles, tortoises, and terrapins) have none, sparking debate about whether this group descended from sauropsids or if their common ancestor was among the basal amniotes.
Basal amniotes evolved from reptiliomorphs, amphibians living approximately 340 million years ago which resembled reptiles. Although called "reptiliomorphs," this group is the common ancestor of both mammals and reptiles. One of the first known basal amniotes is Casineria, a small (15 cm, 6 in) animal which superficially resembled a tiny lizard, discovered in 1992 but only described in 1999. It was found that Casineria is one of the oldest known animals to live in a relatively dry environment, making it among the first amniotes. Casineria is one of few tetrapod fossils found at the end of Romner's gap, an absence of fossils with an age between roughly 360 and 340 million years. When the significance of Casineria was realized, it pushed back the known origin of amniotes several million years.
Casineria is one of the only tetrapod fossils dating to before the synapsid/sauropsid split. To survive where it did, Casineria must have had complex eggs able to circulate their own waste internally, something amphibian eggs cannot do. There are some frogs that can lay their eggs directly onto a very moist forest floor, but this only is possible in a narrow range of tropical environments and is not indicative of true amniotes. True amniotes lack a larval stage.
Despite the appearance of amniotes, reptiliomorphs (amphibians which just looked like reptiles), thrived alongside true reptiles for millions of years. Some of them, like Discosauriscus, had an odd and creepy look, unlike anything alive today.
