Think Dimetrodon Was a Dinosaur? Think Again

"Dimetrodon is what we call a 'synapsid,'" explains University of Chicago paleontologist Caroline Abbott in an email.

"About 310 million years ago, the first amniotes (vertebrates able to lay eggs on dry land) split into these two separate lineages, the synapsids and the reptiles, and the two groups have had separate evolutionary histories ever since," Abbott says. "Dimetrodon is one of the earlier synapsids."

Mammals are the only synapsids that are still around today.

The first true mammals wouldn't arise until sometime between 178 and 208 million years ago; long after Dimetrodon died out. Still, being a synapsid, the old fin-back had closer evolutionary ties to humans than it did to any modern reptiles — or to the dinosaurs, as we'll discuss later on.

How do we know the Dimetrodon was a synapsid? Well, there were a few clues that tipped fossil-hunters off after the creature was discovered in the 19th century.

"The key feature of all animals in the evolutionary lineage leading to mammals is the presence of a large opening behind the eye socket on the skull," says Hans Sues, curator of vertebrate paleontology at the Smithsonian National Museum of Natural History, in another email exchange.

"This feature becomes much larger in more advanced species and houses the jaw-closing muscles. You can feel those muscles if you put your fingers on your temples and clench your jaws," he adds.

Also, just like a lot of mammals, Dimetrodon was a heterodont. That means the creature's teeth didn't all look the same. On the contrary, its pearly whites came in a variety of shapes and held a variety of functions. According to Sues, Dimetrodon had "incisor-like front teeth, a large canine (eye tooth), and smaller teeth behind the canine."

Now, about that sail ...

Backbones, or "vertebrae," are topped with knobby columns of bone called "neural spines." These are oriented vertically in animals that walk around on all fours. In humans, the neural spines point backward; they're the little bumps you can feel protruding underneath the skin when you rub the back of your neck (or your spinal column).

Dimetrodon's iconic sail was made up of extremely long, rod-like neural spines. The tallest ones occurred in the middle of the creature's back, between the shoulder and hips, giving the sail as a whole something of a "dumbbell" shape. In the largest Dimetrodon, which reached lengths of more than 15 feet (4.6 meters) and might've weighed 550 pounds (250 kilograms), the sail's tip would have stood at least 5 feet (1.5 meters) off the ground.

That's slightly taller than a typical sedan car.

If you want to know what the heck Dimetrodon's sail was for, join the club.

"Nobody really knows because there are no living animals with such 'sails' that could be used for comparison," explains Sues.

Today's mammals, by and large, keep a constant internal body temperature. It's possible that Dimetrodon lacked this ability and would've had to rely on its environment to warm itself up — or cool down.

"Having tall neural spines with tissue and blood vessels in between would provide a lot of surface area to help with thermoregulation, or how an animal maintains ideal body temperatures," says Abbott. "The 'sail' of Dimetrodon might have been essentially a giant solar panel that allowed it to get going earlier and keep moving longer in the day. That's a huge leg up if you're a predator!"

Researchers have their doubts, however.

Dimetrodon lived in the early part of Earth's Permian Period, which lasted from around 298 to 251 million years ago. One of its closest relatives was Sphenacodon, another carnivore with a similar build overall. Like Dimetrodon, Sphenacodon was a beast from the early Permian. But unlike Dimetrodon, it didn't have a massive sail on its back. So why would one of those creatures need a personal "solar panel" if the other got along fine without it?

Sexual selection could be the X-factor here. If the sail wasn't used for thermoregulation, then maybe it acted as a prehistoric babe magnet.

Abbott notes that, sometimes, "ornamental structures evolve due to mate preferences, such as in bright bird feathers or deer antlers. In that sense, a 'sail' for sexual display would have come about because other Dimetrodon found it appealing for mate choice."

Whatever the structure's real purpose may have been, this much we know: It wasn't unique to Dimetrodon. The vegetarian Edaphosaurus, another synapsid found in Permian rock deposits, had a prominent back sail of its very own. So did the prehistoric amphibian Platyhystrix. And long, long after all those critters went extinct, a few dinosaurs independently evolved the same basic feature.

Far and away, the most famous sail-backed dinosaur of them all is Spinosaurus. It lived in the Cretaceous Period some 97 million years ago. With an estimated length of almost 50 feet (15 meters), this was quite possibly the longest carnivore to ever walk on dry land — although it might've preferred hunting in rivers.

That leads us to one of the scientific community's biggest pet peeves about our friend, Dimetrodon. Since it was a forerunner of mammals, Dimetrodon had nothing whatsoever to do with the likes of T. rex, Triceratops or Spinosaurus. Yet the poor beast is commonly mistaken for a dinosaur.

Toy companies deserve some of the blame. Dimetrodon is very frequently mislabeled as a "dinosaur" in playsets and bags of plastic figurines. Hollywood is no help; watch "Fantasia" or "The Land Before Time" and you'll see this Permian synapsid brushing shoulders with actual dinosaurs.

The sad irony is that Dimetrodon died out tens of millions of years before the first dinosaurs even showed up. "We're closer in time to Spinosaurus than Spinosaurus is to Dimetrodon!" says Abbott.

Unlike the bipedal Spinosaurus, Dimetrodon trotted around on four legs. The fossil evidence has made that clear. Other aspects of its appearance and behavior are more mysterious.

"We've never found skin impressions associated with the bones of Dimetrodon, but ... the animal more than likely had some scales and no hair, both because of what we vaguely know of when hair may have evolved in synapsids, and indirect evidence from trackways," Abbot tells us.

In 2012, scientists described the fossilized impression of the feet and underbelly from an early Permian synapsid. It's thought the animal who made them was akin to Dimetrodon. Whatever it was, the animal left behind the traces of prominent stomach scales.

As for synapsid hair, we know it must have evolved by 164 million years ago. That's when the first definitive mammal hair imprints show up in the fossil record. "Tentative hair-like structures have been found in coprolites (fossil poop) from the Late Permian, which is 10 to 20 million years after the last Dimetrodon," explains Abbott.

Aspiring paleoartists will want to keep all this in mind.