What is a Dimetrodon and why it’s wrong to call it a Dinosaur?

Dimetrodon DR

If you search through your old toy chest that your parents were careful enough to save, you might find among your dinosaur toys a small lizard-like animal with a sail on its back and a menacing grin.

This animal is called a Dimetrodon and it’s not a Dinosaur. In fact, it’s more closely related to us than it is to a T. rex or a Triceratops.

Then why do so many people confuse it with a Dinosaur? As with many misconceptions in the World of paleontology, this one has its roots in popular media, with the latest installment of the Jurassic Park franchise, Jurassic World Dominion, being the latest in a long list of movies and TV series showing this animal living alongside dinosaurs and pterosaurs (which are also not dinosaurs, but we’ll get into that in another article).

Dimetrodon was part of a group of animals called the synapsids, with its fossils being found all over the northern hemisphere from the US to Germany. Dimetrodon actually predates the dinosaurs by over 40 million years. It lived in the Permian period from 295 to 272 million years ago, going extinct even before the Great Dying, a massive extinction-level event that wiped out 90% of all life on Earth, marking the end of the Permian period and the beginning of the Triassic, 252 million years ago.

Synapsids are one of the two major groups of animals that evolved from basal amniotes, a clade of tetrapod (meaning four-legged) animals that comprise both the synapsids (mammals and their relatives) and the sauropsids (reptiles, dinosaurs, and birds).

One of the main characteristics that distinguish synapsids from other animals is that they have a temporal fenestra, an opening low in the skull roof behind each eye, leaving a bony arch beneath them. Paleontologists believe this distinctive trait developed around 318 million years ago during the late Carboniferous period when synapsids and sauropsids diverged.

(A little side note, it’s from the Carboniferous period that most of the coal used to power the industrial revolution came from. A topic for another day)

And Dimetrodon fossils have this distinct trait, making them not necessarily a mammal ancestor, but a close relative to us and to all other mammal species alive today.

Dimetrodon is actually a genus name comprising about 13 known species, the largest of which was Dimetrodon angelensis, growing to around 4 m (13 ft) in length, and the smallest being Dimetrodon teutonis with only 60 cm (24 in).

Fossils of Dimetrodon are known from the United States (Texas, Oklahoma, New Mexico, Arizona, Utah, and Ohio) and Germany, areas that were part of the supercontinent Euramerica during the Early Permian. Almost all fossils of Dimetrodon found in the US have come from three geological groups in north-central Texas and south-central Oklahoma: the Clear Fork Group, the Wichita Group, and the Pease River Group.

Most fossil finds are part of lowland ecosystems which, during the Permian, would have been vast wetlands. It lived alongside amphibians like Archeria, Diplocaulus, Eryops, and Trimerorhachis, the reptiliomorph Seymouria, the reptile Captorhinus, and the synapsids Ophiacodon and Edaphosaurus (a sailed-back herbivore).

Besides Dimetrodon, Jurassic World Dominion also features another synapsid, the Lystrosaurus. This little guy – full-grown adults reached around 1 meter (3 ft) in length – was actually one of the few lucky species to survive the Great Dying.

Lystrosaurus animatronic from the set of Jurassic World Dominion

It is found all over the world from Antarctica to South Africa and China, and its fossils were used to prove the theory of continental drift that led us to better understand plate tectonics and to recreate the supercontinent of Pangaea.

Lystrosaurus is an extinct member of herbivorous dicynodont therapsids. Therapsids are a group that includes true mammals, and dicynodonts were a family of therapsids that had a pair of tusk-like canines that serve as a tell-tale characteristic for Lystrosaurus.

It is also possible that these were amongst the first mammal-like animals to give birth to live young, although this hypothesis is only supported by the fact we have yet to find any evidence of Lystrosaurus’ eggs.

Although this animal actually lived in the Triassic it was still separated from the first dinosaur by about 20 million years. It did, however, most likely share its environment with the first dinosauromorphs, the group that would later give rise to the Dinosaurs that we all know and love.

To summarize, Dimetrodon and Lystrosaurus are a part of our own evolutionary history, albeit far in the distant past. They may not have been dinosaurs but that didn’t stop them from making their mark on our planet’s history.     

And although one cannot shake the fact that Dinosaurs dominate our collective imagination and our media landscape, we should also be aware of the amazing animals that lived long before their rise.