Seymouria

Seymouria is an extinct genus of seymouriamorph from the Early Permian of North America and Europe. Although they were amphibians (in a biological sense), Seymouria were well-adapted to life on land, with many reptilian features—so many, in fact, that Seymouria was first thought to be a primitive reptile. It is primarily known from two species, Seymouria baylorensis and Seymouria sanjuanensis. The type species, S. baylorensis, is more robust and specialized, though its fossils have only been found in Texas. On the other hand, S. sanjuanensis is more abundant and widespread. This smaller species is known from multiple well-preserved fossils, including a block of six skeletons found in the Cutler Formation of New Mexico, and a pair of fully grown skeletons from the Tambach Formation of Germany, which were fossilized lying next to each other.

For the first half of the 20th century, Seymouria was considered one of the oldest and most "primitive" known reptiles. Paleontologists noted how the general body shape resembled that of early reptiles such as captorhinids, and that certain adaptations of the limbs, hip, and skull were also similar to that of early reptiles, rather than any species of modern or extinct amphibians known at the time. The strongly-built limbs and backbone also supported the idea that Seymouria was primarily terrestrial, spending very little time in the water. However, in the 1950s, fossilized tadpoles were discovered in Discosauriscus, which was a close relative of Seymouria in the group Seymouriamorpha. This shows that seymouriamorphs (including Seymouria) had a larval stage which lived in the water, therefore making Seymouria not a true reptile, but rather an amphibian (in the traditional, paraphyletic sense of the term). At that time, it was still thought to be closely related to reptiles, and many recent studies still support this hypothesis. If this hypothesis is correct, Seymouria is still an important transitional fossil documenting the acquisition of reptile-like skeletal features prior to the evolution of the amniotic egg, which characterizes amniotes (reptiles, mammals, and birds). However, under the alternative hypothesis that Seymouria is a stem-tetrapod, it has little relevance to the origin of amniotes.