Dimetrodon Gives us a Nod

Model of a Dimetrodon
Creative Commons License 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.

Head-Neck Muscles

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.

Joint Mechanics

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.

If you enjoyed my post on Dimetrodon bones, be sure to check out my recent posts on the skull of the Ceratosaurus and Archosaurus.

Skull Slushies – What’s inside a dinosaur’s skull?

We get so many great questions through our blog, and every now and then we can turn those responses into a blog post. One our readers favorite posts is “What would YOU ask a paleontologist?”

Last week we got this question from Britt:

“ok so, if dinosaurs, for the most part had tiny little brains, and giant heads, what filled up the rest of their head if not brain? like some kind of brain slushie or what?”

Dr. Bakker, curator of paleontology here at the museum wrote this in response:

Skull Slushies – What’s inside a dinosaur’s skull?

Creative Commons License photo credit: Lord_Alex

Darn good question. You’re asking about the fundamental architecture of a vertebrate head. And to understand the skull, you must discover that there are really two skulls in your head, one inside the other.

The outer skull is a shell of bone that makes your eye-socket, the hole for your nostrils, your cheek and upper jaw with its row of teeth. Same for T. rex or a Triceratops or a cocker spaniel…….  Komodo Dragon, man-eating Nile Crocodile, etc., etc.

Inside the outer shell of bone is the cranial inner sanctum – the braincase. Yep – the braincase houses the brain. There are holes in the braincase to let out nerves and sense organs. The eyeball really is a big nerve. It comes out a hole in the front of the braincase and then goes into the eyesocket. The nerve for your sense of hearing goes out a hole in the braincase located further aft. This auditory nerve goes out sideways and ends up inside a ball of bone at the base of your ear.

The spinal cord is a huge bundle of nerves that goes out the biggest hole – we label it the foramen magnum. The spinal cord continues to the rear through holes in the vertebrae.

The outer skull and the braincase are attached to each other a couple of places at the top, sides and rear of the head.

Brains and only brains fill the braincase. Fine. Now what is between the outer skull and the braincase?  Slushies? Packing peanuts? Old newspapers wadded up? Receipts from Buckey’s?

Nope. More important stuff – muscles. The muscles you use for chewing are packed between the outer skull and braincase. Try this: get some tasty beef jerky and chew. Put your fore-finger on the side of your head, just behind your eye-socket. There’s a hole in the outer skull here. You can feel your jaw muscle bulging as it contracts each time you chew.

That chewing muscle is your temporal muscle. The hole in your outer skull is a temporal fenestra (temporal “window”). Now trot out to our dinosaur display and check out the T. rex. skull. There are lots of holes in the outer skull. The tall oval hole is for the eye. To the rear it has a hole shaped like a w turned on edge. That’s a temporal fenestra. Look through this hole, You’ll see the braincase.

In animals with mid-sized brains, like T. rex, there’s a lot of space between the braincase and the outer skull at the temporal window. All the space was filled with muscles. So the jaw muscles were thick and strong.

We humans are the opposite of a rex. We have a giant, bulgy braincase chock full of brain. We’re the thinkiest species on land (porpoises give us competition in water).  But we are wussies when it comes to chewing. There’s only a thin space between braincase and outer skull. Check out a human skull. It’s humbling. We just can’t chew hard.

Now, every time you see a skull on exhibit, try to judge how much room there was for chewing muscle between the outer skull and the braincase. Hyenas are particularly intriguing……

If you have any questions you would like to ask any of our bloggers or curators, send us an email at blogadmin@hmns.org.

Hittin’ the road with the HMNS Paleo crew!

BB describing boomerhead

I got the chance to travel from Houston to Seymour, TX and explore the Texas Redbeds in search of fossils with David and the HMNS Paleo Program. HMNS staff and volunteers have been making these trips for four years now. They have found several excellent specimens and brought them here to prepare for our new and improved Paleontology Hall. I’d had some experience looking at the bones and things that the crew had been bringing back to the Museum but this was my first experience actually in the field – and I was pretty excited!

Drawing of a Diplocaulus

The first morning we arrived at the site and looked around at a few different locations before settling down in the “pit” to dig. I got to spend a little time training my eyes to see fossilized bone, teeth, cartilage and coprolites among the rocks at the “spoil pile” which is a great experience because the ratio of fossils to rocks on the surface is such that you have a pretty good chance of closing your eyes and picking up a fossil! Then we moved over to learn the digging technique where fossils were a bit more hidden in the pit; it took a few minutes to get the hang of how to hold the tools and make sure that you are using enough force to move the dirt but not so much that you break a hidden bone. All and all it was really enjoyable first day at the site.

Over the next two days after Dr. Bakker arrived we visited several other sites on the property and I got a chance to work on excavating a dimetrodon spine, map some dig sites (here’s a fun school dig site mapping activity), learn about other findings like the diplocaulus or “boomerang head” skull we’re looking at in the photo above. I enjoyed the opportunity to work alongside the experts and learn about all of the preparation work that is required for each and every specimen that will be in the new Paleontology hall (coming soon!) here at the Houston Museum of Natural Science. I can’t wait to see everything on display in the new wing of the Museum – it’s going to be so exciting!

For more information about what fossils are found at the dig site in Seymour check out some of the entries on the Prehistoric CSI blog, you can also find some really awesome illustrations on that site to bring the animals to life!