Dimetrodon: Beast of the Week

This week we have a creature that is no doubt very popular even in popular culture, but is sadly very poorly understood by the majority of people out there.  Let's get to know Dimetrodon!

Dimetrodon was a synapsid (same wide group that includes us, mammals) that lived during the early Permian Period, between 295 and 272 million years ago (Several species of Dimetrodon spanned this time range.) in what is now the Southwest United States, in Texas and Oklahoma.  Some fossils of Dimetrodon have also been unearthed in Germany.  It was a meat-eater, and the biggest individuals measured about ten feet long from snout to tail.  The name, Dimetrodon, translates to "Two Kinds of Teeth".  Despite this, Dimetrodon more accurately had three kinds of teeth. (womp-womp)

Dimetrodon grandis with the amphibian, Diplocaulus, as prey.  Reconstruction by Christopher DiPiazza.

If you ever had dinosaur toys as a child (or as a totally awesome not-boring adult, like myself) you might recognize Dimetrodon.  It stands out with it's lizard-like body and enormous fin along its back.  What many people don't realize is that Dimetrodon wasn't a dinosaur.  In fact, it was much more closely related to you, me, and my Yorkshire terrier than it was to any dinosaur.  Dimetrodon is oftern referred to as a "mammal-like reptile".  So it's still considered a reptile to a degree, but it was evolving characteristics that would eventually allow its relatives to become true milk-producing, hairy mammals by the time actual dinosaurs appeared on the earth, and eventually the many mammals we are familiar with today.

First of all, like I said above, Dimetrodon came and left (went extinct) about 40 million years before the first dinosaurs ever evolved on the planet.  In addition, Dimetrodon gives us good clues to what it really was in the bones alone.  The teeth, as the name implies, were differentiated, instead of uniform (what you would see in most other reptiles.)  As mammals, we too have different kinds of teeth in our mouths.  In the front of Dimetrodon's mouth, it had short teeth, similar to incisors.  On the sides it had long, canine-like teeth for slashing, and in the back the teeth were shorter, probably good for breaking up and processing chunks of other animals to be swallowed.  If you look at the rest of the skull, you would notice that it has nostril holes in the front, eye sockets towards the back, and another hole behind the eye sockets, called the temporal fenestra.  ("fenestra" means "window": fun fact)   This hole is present in all mammal skulls, as well.  (Yes, even yours.  We even call them "temples") Dinosaurs and their closest relatives,  have an additional hole between the eye socket and the nostril on each side, called the antorbital fenestra.

Dimetrodon grandis skull.  Note the differences in teeth and the temporal fenestra behind the eye socket.

Dimetrodon's legs were relatively short compared to its long body and large head.  The limbs could have been held erectly as it walked and ran, but it would have had to rest on its belly with them sprawled out to the sides, like a lizard or crocodilian.  Because of this, Dimetrodon probably wasn't capable of running very fast for long distances, but it wouldn't have needed to since all the animals it coexisted with were either just as slow, or slower.  When alive, Dimetrodon would have been the top predator of its environment (that we know of) and it probably would have regularly preyed upon other reptiles, amphibians, like Eryops, and even prehistoric sharks, like Orthacanthus.

Dimetrodon limbatus skeleton on display at the American Museum of Natural History in New York.

The most striking feature about Dimetrodon was its amazing fin, or sail, we call it, growing from its back.  This structure was made up of very long, thin extensions of its neural arches,  Neural Arches are the little top parts of the vertebrae bones.  You can feel your own if you run your finger down the middle of your back.  In life, these rod-like bony structures would have been covered with a thin layer of skin, giving it the sail appearance.  One reason why scientists think this structure evolved was to help regulate Dimetrodon's body temperature.  If there were blood vessels in the skin part of this sail in life, the blood would more easily be warmed when exposed to the sun, circulating to warm up the rest of the body faster, or the opposite if Dimetrodon was getting overheated and decided to go into the shade to cool off.  Since all of the other animals it coexisted with would have also been ectothermic, and would have needed time to heat up in the sun each morning, Dimetrodon's sail could have served as a good adaptation to give it a head start at becoming active each day, allowing it to hunt prey, which would have still been in an inactive state.  It's the same reason why it is always easiest to catch frogs and reptiles in the morning, when they are still warming their bodies up and can't move around as quickly as they would be able to midday. (Although I don't condone going out and trying to grab wild animals!)  Others question if the sail would really have been that effective at thermoregulation and suggest it was more likely just an adaptation for display within the species.  There are actually more than one recognized species of Dimetrodon, each sporting variances in the shapes of their sails.  That being said, there is a chance at least some of these variations, especially the ones that coexisted with one another, could have been different sexes and/or ages of the same species.

That is all for this week!  As always feel free to comment below or on our facebook page!

References

Angielczyk, K. D. (2009). "Dimetrodon is Not a Dinosaur: Using Tree Thinking to Understand the Ancient Relatives of Mammals and their Evolution". Evolution: Education and Outreach 2 (2): 257–271.

Baur, G.; Case, E.C. (1899). "The history of the Pelycosauria, with a description of the genus Dimetrodon, Cope".Transactions of the American Philosophical Society 20 (1): 5–62.

Florides, G.A.; Wrobel, L.C.; Kalogirou, S.A.; Tassou, S.A. (1999). "A Thermal Model for Reptiles and Pelycosaurs".Journal of Thermal Biology 24 (1): 1–13.

Olson, E.C. (1966). "Community evolution and the origin of mammals". Ecology 47 (2): 291–302.