Traditionally, reptiles, mammals, and birds were classified as separate classes due to their distinct characteristics such as scales, fur, feathers, and differences in their thermoregulation (cold-bloodedness vs. warm-bloodedness). When evolutionary principles began to shape taxonomy, “reptiles” were defined as a paraphyletic group. This means that it included some, but not all, descendants of a single common ancestor. Certain branches of the evolutionary tree were separated into distinct classes. “Reptiles” referred to all amniotes (animals with eggs protected by membranes) that were not warm-blooded.

With the rise of cladistics, which emphasizes that all descendants of a group should remain within that group, the term “reptile” became synonymous with “amniote” and somewhat redundant. To address this, one of the two main branches of amniotes, the synapsids (including mammals), was declared not to be reptiles. On the other hand, the other branch, sauropsids, became more closely associated with the term “reptiles.” This classification implies that birds are considered reptiles, while certain creatures like the Permian Dimetrodon, with its sail-backed appearance, sprawling legs, and leathery egg-laying, are no longer classified as reptiles. The same applies to their close relatives, which include our ancestors.

In essence, the pre-cladistic rationale was that mammals had undergone significant evolutionary changes, distancing them from being classified as reptiles. However, from a cladistic perspective, we are now recognized as synapsids rather than reptiles, as all our ancestors, tracing back to the earliest split in amniotes, fall within the synapsid group.