|Model of a Dimetrodon
photo credit: kaurjmeb
Our new Fossil Hall will feature a trio of fin-backed predators from 285 million years ago, dug from the famous Red Beds of north central Texas. (If you would like to learn more about our expansion and how you can donate to the HMNS’s second century of science, click here.)
One of our Dimetrodons is a monster – as big as they get – with a live weight as big as a Siberian Tiger. That’s 500 pounds.
We’ve got parts from the head end, the rump, the shoulders. But up till now we didn’t have a perfect head-neck swivel joint, known technically as the atlas-axis complex. This is where the skull meets the neck vertebrae, and it’s the most complicated anatomical unit in the entire backbone.
Just a few week ago we scored the entire swivel apparatus. The bones come from a brand-new site where a single Dimetrodon was buried by a spring flood. The bones are beautiful. Each vertebra is complete and the rock is so soft it can be removed with the judicious use of a fingernail.
The head-neck bones tell us secrets about how Dimetrodon acted in life. Here’s a basic diagram for a giant D’don, taken from the superb skeleton at the Smithsonian. The head-neck swivel complex in red. Check out the big prong sticking straight up – that’s the neural spine of the axis. And here’s a close-up of the bones. The neural spine of the axis is the biggest component.
There’s a thick muscle attached to the neural spine that runs forward to connect to the back of the skull.
Reach around and touch the back of your own neck – you’ll feel the muscle. Technical name: rectus capitis posterior (meaning: “pulls the neck straight back”). The great height of the neural spine in our D’don means an exceptionally powerful action in pulling the head back and up. That would be useful when grabbing big, struggling prey.
There is a second set of thick muscles that’s attached to the neural spine and runs outward to the outer corner of the back of the skull. Technical label: obliquus capitis. Meaning: “Muscle that pulls the head obliquely.” This muscle turns the head sideways – also useful when wresting with prey.
Now let’s investigate how the joints operate. The first vertebra behind the head is the atlas. The name comes from a fancied resemblance to the Greek hero, Atlas, who held the globe in his muscular arms. The atlas vertebra has two parts, one above the other, that together make a cup. Into the cup fits a ball on the rear of the skull.
Check out the cup in our specimen. We’ll exhibit the neck bones in a special case, so visitors can get a sense of how the living D’don was an active, dynamic bio-machine.
D’don is very close to the direct ancestry of warm-blooded, hairy mammals, including us humans (we are all warm-blooded, though some of us are hairier than others). We will exhibit a cast of a human neck joint next to the D’don to demonstrate how much evolution has taken lace since Red Beds times, nearly 300 million years ago.