Hot Take: I Personally Still Call Synapsids "Mammal-Like Reptiles"

Dimetrodon, the quintessential "mammal-like reptile" with its great sail and primordial mix of transitional characteristics. Now it’s not anymore officially called a reptile (like Pluto no longer officially a planet), but I personally don’t find this so simple, as we’ll shortly discuss here. Art by Dmitry Bogdanov.

These days I like to be creative with palaeontology, in the old traditional way, but still faithful to our current knowledge of the world long ago. Synapsids, the ancestors of mammals that ruled the Earth before the dinosaurs, were historically dubbed "mammal-like reptiles" by the scientific community with their notable transition, but had officially lost that title as taxonomy improved and a new definition for reptiles in cladistic (classified as "sauropsids") appeared. But I personally find not calling them reptiles entirely a bit wonky, especially with other very reptilian animals not classified as reptiles scientifically these days. Let me explain.

Sauropsids containing true reptiles and birds and synapsids including mammals are really just differentiated by the amount of fenestrae in the skull but what traditionally defines reptiles in general are scaly skin, hard-shelled eggs, cold blood, you get it. Even the most primitive amniote is basically that, so wouldn’t Casineria technically be a reptile? As for the synapsids in particular, they are more different, but they still share some ancestral traits we’d call reptilian, and Dimetrodon and Archaeothyris are especially convincing with their lizard-like appearances. In the same traditional fashion, I call tetrapodomorphs like Ichthyostega amphibians, and the situation here discussed overall is like how birds are dinosaurs and how we all tetrapods are lobe-finned fish in a sense, we talk about that.

I still wouldn’t refer as far as referring to mammals as reptiles traditionally, just like we don’t typically call ourselves fish or birds reptiles, it’s just that "mammal-like reptile" is a simpler term that fits with how we traditionally classify different animals. It’s just paraphyletic. I call amniotes from the base reptiles, basically, and synapsidan mammal-like reptiles are a transition from reptile to mammal. I personally wouldn’t equate naming an animal by cladistics than tradition too much.