Dimetrodon is #notadinosaur

I spent my Saturday perusing a nearby toy store and photographing instances in which Dimetrodon and other early Permian (298-295mya) lifeforms are depicted alongside dinosaurs (231-201mya). It’s research for an upcoming episode on The Brain Scoop (or so I tell myself). 

Dimetrodon is a genus of extinct early synapsid, and as such were some of of the earliest of their kind to be attributed with a few key characteristics of mammalian classification. That makes them ancient mammal relatives. In other words, we’re are all synapsids, too. Occasionally, Dimetrodon and co. are referred to as “mammal-like reptiles,” but this, too, is a misnomer, as they weren’t reptiles at all. In fact, humans are more closely related to Dimetrodon than Dimetrodon is related to any reptile, including dinosaurs. 

So, who cares? Why should we be bothered that toy and book companies aren’t adequately researching their imagery before capitalizing on the market appeal of dinosaurs? Does this do any harm other than annoy those of us who would prefer that our consumables be scientifically or historically accurate?

I think we’re subtly cheating ourselves, and subsequently our children, out of being able to appreciate the diversity of evolved life by clumping extinct species into easy-to-label but unrelated groups and relying on people literally buying into ill-informed products. Dimetrodon is other worldly, ancient, captivating - but so were a million other early synapsids, very few of which make it into the coloring books. The Permian was a time of abundant diversity and we owe it to our natural curiosities to explore our collective past. Doing so may grant us a greater appreciation for how we’re here today.


These photos are from the salt mines below Yekaterinburg, Russia. Mine tunnels stretch four and five km some 220m underground, and the fantastic color variation is due to the variability of salt mineralogy: halite NaCl, our common table salt, is generally white; sylvite KCl, used in bitter tasting salt substitutes for folk with high blood pressure, is more often orangey pink in part due to iron inclusions; carnellite KMgCl3.6(H2O) can range between blue and red and tan colors and is the predominate mineral in these mines. Salts are a relatively soft bunch of minerals, and when tectonic forces are applied they can flow and fold easily, creating the psychedelic patterns on these walls. Well, okay, you can also see some circular incisions on the tunnel walls added by drilling exploitation of these deposits.

These Russian deposits date to the Permian, yes, ~280 million years ago, when an entire sea dried up (called by some the Perm Sea), leaving behind a salty residue of evaporate minerals in its place. With the vicissitudes of geologic history, these became buried and forgotten until salt was mined in Russia (starting in the 2nd millennia BCE), and only now, with these incredible photos, are we able to view them.

Within these salt mines, the air is ultra-dry, filled with miniscule salt particles, corrosive (endangering electric connections in lights and cameras), and there is constant danger of a cave-in or sudden lead of toxic gasses such as methane, hydrogen sulfide, carbon dioxide. These photographs, now found scattered all over the internet, were taken by a truly intrepid Russian photographer, Mikhail Mishainik.

Annie R

Photos by M. Mishainik, distributed by Caters News Agency.

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When: Permian (300-250 million years ago)

Where: North America

What: Diplocaulus is an amphibian. It reached lengths of roughly 3 feet (~1 meter long), making it the largest of its clade. We have an exceptional fossil record of these animals, including wonderful ontogenetic series of juveniles, which show that the two-phase metamorphosis of modern amphibians was not present in these taxa. Instead juvenile Diplocaulus looked mostly like small adults. The biggest difference is that the head shield became more and more elaborate as the animal grew, juveniles barely had any ‘boomerang’ shape to their heads.  Most Nectridea had head shields, but they were among their most developed in Diplocaulus. These animals were carnivorous and lived in the many rivers, lakes, and swamps covering North America during the Permian. 

How Diplocaulus is related to extant amphibians is a topic of much debate. It, along with a number of other extinct amphibian clades, have in the past been grouped as the Lepospondyli. However, modern interpretations are divided as to if this is a real group, with all of its members descended from a common ancestor, or if it is an artificial collection of taxa. It is not likely that they are found within the Lissamphibia (the modern amphibians), but they have been proposed to be stem taxa to this clade. Some members have also been proposed to be more closely related to amniotes (reptiles - including birds, and mammals)  than to these living amphibians. Detailed cladistic studies incorporating all relevant taxa are needed for a firm understanding of this part of the evolutionary tree. 


Estemmenosuchus mirabilis was a 4-5m long Permian therapsid. It possessed large horns, and a fierce but mixed dentition, indicating that it was probably omnivorous. Fossilised skin impressions hav been discovered, and they show that this animal had smooth skin with no hair or scales. Because of its size it had a small surface to volume ratio, meaning that its body temperature remained relatively stable. This is mostly true for all other large Permian therapsids, and is key to their success.

The Prionosuchus (1948)

Phylum : Chordata
Superclass : Tetrapoda
Class : Amphibia
Order : Temnospondyli
Family : Archegosauridae
Genus : Prionosuchus
Species : P. plummeri

  • Late Permian (270 Ma)
  • 8 m long and 550 kg (size)
  • Pedra do Fogo formation, Brazil (map)

First things first: not everyone agrees that Prionosuchus deserves its own genus; some paleontologists maintain that this huge prehistoric amphibian was actually a species of Platyoposaurus. That said, Prionosuchus was a true monster among amphibians, which has inspired its inclusion in many imaginary “Who would win? Prionosuchus vs. [insert large animal here]” discussions on the internet. If you managed to get close enough–and you wouldn’t want to–Prionosuchus would probably have been indistinguishable from the large crocodiles that evolved tens of millions of years later, and were true reptiles rather than amphibians.

Prionosuchus reconstitution


Order: Trigonotarbida

…an extinct group of arachnids that lived from the late Silurian to early Permian period. Trigonotarbids superficially resembled spiders but unlike spiders they have tergites on the dorsal side of their abdomen that are divided into median and lateral plates, they also lacked silk-spinning spinnerets. Trigonotarbids were a very diverse group of animals with most species found in the Carboniferous, although some were found in the late Silurian (415 mya), making them some of the first terrestrial animals. And as such they were very important in early ecosystems.



Images: Dunlop,1996 and Tommy

Inostrancevia, a gorgonopsid from the Late Permian of Russia (260-254 mya). Growing to about 3m long (9'10"), it had massive “sabertooth” canine teeth and probably preyed upon large stocky pareiasaurs like Scutosaurus.

Gorgonopsids were synapsids, or “stem-mammals”, and were often referred to as “mammal-like reptiles” in older sources. Unlike the basal reptile-like pelycosaurs (which include the familiar Dimetrodon), the gorgonopsids were members of the more “advanced” therapsid branch of the synapsids – possessing upright limbs, differentiated teeth, and the early stages of the evolution of the mammalian ear arrangement.

It’s not known whether these animals had mammal-like hair or naked leathery skin. I’ve also chosen to depict this one with small external ear flaps.



Fossil specimen on display at the American Museum of Natural History

Reconstruction by Hirokazu Tokugawa

When: Permian (~299 to 265 million years ago)

Where: North America 

What: Cotylorhynchus is a member of one of the most basal groups of synapsids, the  Caseidae.  Cotylorhynchus was a herbivore, and reached lengths of up to 20 feet (~6 meters) long, with a massive barrel chest, putting weight estimates at around 2 tons. This animal is very large for its time… well at least its body is. Cotylorhynchus has one of the most extreme cases of ‘tiny head’ I have ever seen. Even more so than the sail-backed EdaphaosaurusWhich is closer to modern mammals than Cotylorhynchus is. It is one of the most primitive animals known that unambiguously falls on the synapsid lineage. It is so basal that it does not even have any differentiation seen in its dentition, though there are less teeth than found in the non synapsid contemporaries of this wee-headed creature. 


Here’s a large, unbroken Ichnofossil (trackway plate) from the Permian Era.The clearly defined footprints seen on this fine Coconino Sandstone slab were first attributed to animals making tracks on damp sand dunes in a desert. However, other researchers compared the Coconino footprint slabs with footprints made by actual reptiles under various conditions, and they found Coconino footprints best matched the footprints made underwater where the tail is suspended and does not make “drag marks”. 

This amazing 19” wide plate is covered with footprints, the largest ones probably made by Cheilichnus duncani - a synapsid -or mammal-like reptile. Synapsids walked with their legs more directly under it’s body, than true reptiles. They can also be referred to as “stem mammals” or “proto-mammals” The tracks are are deep, detailed prints and measure about an ¾ wide and feature clearly define the claws marks. There are two other unidentified types of vertebrate and invertebrate trackways on the plate. There has been much speculation about the deposition and nature of these enigmatic trackways. Some on this plate are tentatively thought of as being from an arthropod called Octopodichnus . 

The Coconino Sandstones cover an area of 200,000 square miles and was deposited early Permian, 280-275 million years ago. Coconino Sandstone is typically buff to white in color. It consists primarily of fine well sorted quartz grains, with minor amounts of potassium feldspar grains. 

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