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Basal amniotes are the earliest members of the amniote evolutionary tree, which includes reptiles and mammals. They evolved from reptiliomorphs and were the first animals to colonize the continental interior. Casineria, a small animal resembling a lizard, is one of the earliest known basal amniotes and pushed the known origin of amniotes back several million years. True amniotes lack a larval stage and have complex eggs capable of circulating their own waste. Reptiliomorphs have thrived alongside true reptiles for millions of years.
Basal amniotes are the stem members of the amniote evolutionary tree. The amniote is a terrestrial tetrapod (four-legged animal) that reproduces by means of sophisticated eggs protected by multiple layers of embryos, including a hard shell, which prevents them from drying out and allows them to lay their eggs in places other than the waterfall. Amniotes are the first animals to leave wet, swampy regions and begin colonizing the continental interior 340 million years ago.
After a few million years, basal amniotes split 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 eyehole, while sauropsids have two. Anapsids (turtles, tortoises, and tortoises) have none, sparking debate as to whether this group is descended from sauropsids or whether their common ancestor was among basal amniotes.
Basal amniotes evolved from reptiliomorphs, amphibians that lived about 340 million years ago that resembled reptiles. Although called “reptiliomorphs,” this group is the common ancestor of both mammals and reptiles. One of the earliest known basal amniotes is Casineria, a small (15 cm, 6 in) animal that superficially resembled a tiny lizard, discovered in 1992 but only described in 1999. Casineria has been found to be one of the oldest known animals to live in a relatively dry environment, resulting in being among the first amniotes. Casineria is one of the few tetrapod fossils found at the end of the Romner gap, an absence of fossils ranging in age from about 360 to 340 million years. When Casineria’s significance was understood, it pushed the known origin of amniotes back several million years.
Casineria is one of the few tetrapod fossils dating before the synapsid/sauropsid split. To survive where it did, Casineria needed to have complex eggs capable of circulating their own waste inside, which amphibian eggs cannot do. There are some frogs that can spawn directly on a very moist forest floor, but this is only 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 that only looked like reptiles) have thrived alongside true reptiles for millions of years. Some of them, like Discosauriscus, had a strange and eerie appearance unlike anything alive today.
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