Link between amphibians and reptiles.
Seymouria belong to a group of vertebrates called Seymouriamorpha, a small but widespread group of limbed tetrapods. Many were terrestrial or semi-aquatic. However, they have aquatic larvae bearing external gills and grooves from the lateral line system have been found, making them unquestionably amphibians. The adults were terrestrial. They ranged from lizard-sized creatures (30 centimeters) to crocodile-sized 150 centimeter long animals. They were reptile-like. Seymouriamorphs were the distant relatives of amniotes. Amniotes are tetrapods (descendants of four-limbed and backboned animals) that are characterised by having an egg equipped with an adaptation to lay eggs on land rather than in water form into three main groups, one group that includes the best known genus, Seymouria.