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There’s been a lot of talk recently on David Peters’ blog “The Pterosaur Heresies” (http://pterosaurheresies.wordpress.com/) on early synapsids – vertebrate tetrapods with one hole in the skull behind the eye. I find this interesting since, as you can tell from the website’s title, Mr. Peters has a particular interest in pterosaurs – those flying Mesozoic reptiles that are often mistaken for dinosaurs.
Another creature that’s often mistaken for a dinosaur is Dimetrodon. If you’re an adult reading this, you may not recognize the name, but you’ll probably know it when you see it. Ask any 6 year old child, and he or she will immediately tell you what one looks like. It measured ten feet long, walked on four legs, had a distinctively-shaped head with almost S-shaped jaws filled with an impressive array of teeth, and its most obvious feature was that it had a huge sail on its back, probably for regulating temperature.
If you want some more detailed info, Dimetrodon, meaning “two long teeth”, was a multi-speciate genus of carnivorous terrestrial synapsid amniote tetrapod, specifically a member of Spenacodontidae, which lived during the early Permian Period.
I can already tell some of you are going “Huh???” Let me see if I can explain this in regular English.
The name Dimetrodon means “two long teeth” in ancient Greek; it does NOT mean “two kinds of teeth” as you will sometimes see in books (then the name would be Dimorphodon, a name which is already used for a pterosaur – a nice David Peters segway there). The name comes from the two impressively long canines that it had on each side of its upper jaw.
Dimetrodon is the genus name; a genus is a group of related species. To be “multi-speciate” means that this particular genus contains many species. Examples of Dimetrodon species include D. grandis, D. limbatus (probably the most common species), and D. milleri. The exact number of Dimetrodon species varies, depending upon who you ask, because some people claim that certain names are invalid. The last time I checked, there were about fifteen or so different species spread out across the Northern Hemisphere from Texas, USA to Germany.
“Carnivorous” means “meat-eating”, and “terrestrial” means “lives on the land”, but I think nearly everybody knew that already.
As stated earlier, a “synapsid” is a group within a larger group of animals called terapods. The word “tetrapod” is Greek for “four feet” – tetrapods are animals which have or at one time had four limbs. These include amphibians, reptiles, mammals, and birds – wings count as limbs. Even whales and snakes are considered tetrapods because they are descended from animals which DID have four legs. Tetrapods are further sub-divided into amphibians (frogs, toads, newts, slamanders, etc.) and amniotes. An “amniote” is an animal which reproduces by laying eggs with hard shells which retain moisture. Inside the egg is a fluid-filled membrane called the “amnion” or “amnios”, which helps keep the developing embryo inside hydrated. While amphibians must lay their eggs in water to keep them moist, amniotes have the freedom of laying their eggs on dry land.
Amniotes are broadly catagorized into four groups depending upon how many holes they have in their skulls behind the eye socket and where those holes are located.
1) Anapsid – Greek for “no opening”; aside from the nostril and the eye socket, there are no other holes in the skull.
2) Synapsid – “fused opening”; one hole behind each eye, positioned low. All mammals, including humans, are synapsids.
3) Euryapsid – “wide opening”, one hole behind each eye, positioned high. Includes marine reptiles like nothosaurs, plesiosaurs, and ichthyosaurs. All euryapsids became extinct at the end of the Mesozoic Era along with the dinosaurs. NOTE: I’ve heard that the term “euryapsid” is paraphyletic, meaning that it’s an artificial group composed of many different kinds of animals that aren’t actually related to each other. But determining whether or not a group is paraphyletic is a subject for another day, and I won’t get into it here.
4) Diapsid – “two openings”; there are two holes in the skull behind each eye socket. Dinosaurs were diapsids.
Dimetrodon belongs to the synapsid group of amniotes, the one which includes mammals and the ancestors of mammals. So, in an extreme way, despite its lizard-like appearance, Dimetrodon is our great-great-great-great-great-great-great (and so on) ancestor!
In terms of how many groups of synapsids there were/are, there were/are many. One of them was a group called the “pelycosaurs” – you will often see Dimetrodon being referred to as a member of this group. The pelycosaurs first appeared at the end of the Carboniferous Period about 300 million years ago, but they really became dominant during the early part of the Permian Period, about 280 million years ago. There were many different species, but the ones which grab everyone’s attention are the sail-backed ones like Dimetrodon and Edaphosaurus.
The pelycosaurs were divided into four families, and one of them was called Sphenacodontidae, defined as a family composed of the genus Sphenacodon and all other animals that are more closely related to Sphenacodon than to other animals. The various genera that compose this group are generically referred to as “sphenacodonts”. Dimetrodon was a sphenacodont.
There, understand now?
Dimetrodon is often mistaken for a dinosaur merely because it’s prehistoric and looks dinosaur-ish. I want to show you a picture to see what I mean – it’s a pencil drawing by Vladimir Nikolov made in 2010. I’m not going to put the picture here because I don’t have permission from the original artist to publish it here. The picture is found on DeviantArt, but a word of caution – DeviantArt is known to have viruses, so I would highly suggest that instead of clicking on the link, you would instead google the image “Dimetrodon grandis Vladimir Nikolov”, and see what picture pops up in your search results. It’s a very good picture, nicely done from an artistic standpoint. I like the fact that, unlike the vast majority of paleo-artists out there, this person chose to show Dimetrodon in an active Komodo Dragon style way, rather than dragging its belly using four very scrawney limp legs. I also like the color scheme.
But there are three problems with this picture. First, I don’t think that Dimetrodon had scales of any sort. I read somewhere that preserved skin specimens actually show that it had tough leathery hide. But then again, if Dimetrodon really was a very ancient proto-mammal, you wouldn’t expect it to have scales, right? Second, he says it’s supposed to be Dimetrodon grandis, but based upon the shape of its skull, it looks more like a Dimetrodon limbatus to me – that’s the species that you’re going to see on display in most museums. Third, which is I think the most serious, Dimetrodon didn’t actually have a neck! If you were to look at any of the hundreds of Dimetrodon skeletons which are on display all over the world, you will very quickly notice that their skulls seem to be bolted directly onto their shoulders with no neck in between.
Now that I think about it, there are many examples of land-dwelling tetrapods from the Carboniferous and Permian Periods which do not have necks, especially the pelycosaurs. I’ve seen skeletons of many pelycosaurs, either in real life or images in books and on websites, and I can safely say that NONE OF THEM HAD NECKS!!!
The absence of a neck must have had some very peculiar effects on how these animals moved and especially how they ate. A neck is a wonderful thing. A neck is a flexible apparatus which enables the creature that has it to do several things:
1) The neck contains the vocal cords. The vocal cords attached to the larynx of a longer neck will likely produce different sounds than a shorter neck.
2) The neck enables the head to move without moving the body. A head can twist and turn to see and to find food while the body can remain motionless.
3) Connected to #2, a neck enables the head to have a wide range of movement during the actual process of feeding. A carnivorous animal can chomp down on a carcass, and then twist and turn its neck so that its head can wrench off a large chunk of meat.
There’s no denying that Dimetrodon had some very impressive choppers – after all, it’s named after its teeth. It also had a very large solidly-built head in proportion to its body size, which must have given it a very hefty bite. However, the absence of a neck must have meant that it fed in a certain way. It could not attack the way that meat-eating theropod dinosaurs could with their elegant S-shaped necks, nor could they even attack the way that lizards or crocodilians (who also have short necks) could. Even crocodilians have longer necks than Dimetrodon and its Paleozoic contemporaries. Due to its large sail, a Dimetrodon certainly couldn’t “death roll” the way that a crocodile can (watch any episode of The Crocodile Hunter to find out what a “death roll” is).
Dimetrodon could probaby swing its head from side-to-side reasonably well to a certain degree, but it would have had limited up-down flexibility, maybe just enough to flick its head back a bit and gulp a piece of meat down. If Dimetrodon had a free fully-mobile tongue the way that some reptiles and mammals do like us, then swallowing food would be easier. However, if it had a fixed tongue like fish, amphibians, and crocodilians, then it would have to rely on flinging the food into the back of the mouth in order to swallow it.
Here’s how I hypothesize Dimetrodon fed. I think that once it got its jaws firmly into something, it either shook its head from side-to-side like a shark, or it could have braced the meat with its front paws and then pulled its entire body backwards, thus pulling a piece of flesh off of its prey. This would be an example of “puncture and pull” feeding. It certainly did not have the necessary head mobility or the height to enable it to do “hatchet attack” feeding, where the jaws are stretched as wide open as they can possibly get and then the upper jaw is literally slammed downwards onto the prey like a guillotine.
I am not particularly inclined to engineering, physics, or mathematics, so somebody out there has to do a 3D computer model study on skull and neck mechanics in order to replicate how Dimetrodon and its neckless ilk might have eaten. However, based upon observations of the bones alone and looking at how modern animals feed, I’m pretty sure that this is how the neckless wonders of the late Paleozoic would have eaten.