A is for Antorbital Fenestra

Albuquerque museum, photo by kajmeister.

Dinosaurs had an extra hole in their head.

As we start this journey of 26 posts all about dinosaurs, you may have noticed that A does not start with a kind of dinosaur. This is not going to be about 26 different dinosaurs, although I promise I will throw in a few. So A is not for Apatosaurus, Ankylosaurus, Albertosaurus, or even Archosaur. This A is about how dinosaurs were grouped and identified as dinosaurs.

So one thing to know, even before I say more about what the ant-orbit-a-whatchamacallit, is that these posts are going to wrestle with questions about dinosaurs, such as:

  • What made a dinosaur a dinosaur?
  • Where did the dinosaurs come from? And where did they go?
  • What did they look like?
  • How did they behave?
  • And, most of all, how do we know?

In other words, I’m going to talk about the things that dinosaurs did. Their habits. Their loves and losses.. well, maybe not that. But the dinosaur ouvre, so to speak (i.e. their “body of work.” Body get it?) Since they lived 200 million years ago, it gets a little tricky trying to guess. But you would be surprised at what those clever scientists who study bones can figure out, just from the bones.

What Does antorbital fenestra Actually Mean?

Back to A is for Antorbital fenestra. Let’s start with terminology. These are words from Latin, which biologists and paleontologists–people who study bones– especially love to use.

ant (before) + orbital (eye socket) + fenestra (window)

An antorbital fenestra refers to an extra hole, or window, in the skull of an animal. Fenestra, that Latin word for window, is so great. In German, it’s Fenster; in Spanish, it’s ventana. One of my favorite words is “defenestrate,” as in “The loan shark threatened to defenestrate the guy who couldn’t pay his debt.” It’s what happens to Alan Rickman in “Die Hard.” Sorry for the interruption.

Dinosaurs were land creatures, so they had eyes and noses for breathing air. But some of them–not all, but a big portion of them–also had this extra hole in their skull between the eye socket and the nose opening. The eye socket is the orbit and the nose opening is the naris. (I just took a side journey to check out why fish have nostrils, if they don’t breathe air, and the answer is that they use their nostrils for smelling.)

How to Read Science Terminology

Here’s a pro-tip about sciency language. Sciency people tend to talk like this:

In theropod dinosaurs, the antorbital fenestra is the largest opening in the skull. Systematically, the presence of the antorbital fenestra is considered a synapomorphy that unites tetanurantheropods as a clade. In contrast, most ornithischian dinosaurs reduce and even close their antorbital fenestrae such as in hadrosaurs and the dinosaur genus Protoceratops. This closure distinguishes Protoceratops from other ceratopsian dinosaurs.

from Wikipedia’s discussion of antoribtal fenestra

I was teaching high school kids how to handle standardized reading passages on the SAT a few years back, and I would tell them, for the science passages, just replace every word you don’t know with “blah blah blah” or “technical term” and see how far you get. Then, if you can substitute in ordinary words for some of the terms, do that. You really don’t need to look up all the terms to figure out what that says, even though wikipedia gives you all these links. That will take you an hour. Meanwhile, do the simpler thing:

In some kind of dinosaurs, the extra window is the largest opening in the skull. Pretty much all the time, the presence of this extra window is a blah blah blah that unites this group into another technical term that probably means sub-group…. in other words, this extra window is a way to group a bunch of dinosaurs together to distinguish them from other dinosaurs…. In contrast, in the other kind of dinosaurs, that extra window in the skull gets closed off, for example these other sub-groups, and that closure is how you can tell one group from another.

This is how I, not remotely a paleontologist, can learn a lot about dinosaurs without getting too confused and without really knowing all that much. Some dinosaurs had this extra window thing, and some did not. FYI, in 1981, when I took a paleontology class in college for my GEs, they told us that all dinosaurs had them. Now, 40 years later, they’re changing their tune a little. Can’t use that on the final exam anymore.

Of course, if you really are going to study paleontology or biology, then you need to learn the terminology. I’m going to lean mostly on translating everything into ordinary language for you. But this term is one you can remember. Just remember Alan Rickman.

How to Get Ahead

What else has these extra windows in the head? Fish don’t. Humans don’t. Dogs don’t — ah, mammals don’t! We can start use this window-in-the-head thing to group similar animals together. It’s a handy reference because it’s based on their bones which might, in fact, still be around 200 million years later.

This chart is a way of showing how animal skulls look different, depending on what kind of holes they have. Animals have eyes and most have nostrils. But only some have the extra hole. The ones at the top are mostly reptiles. Not all reptiles have the antorbital thing. Down at the bottom towards the left are the mammals, no window. Birds now, most birds have a trace of an antorbital fenestra. They are related to the group of dinosaurs that had the extra hole in the head, which happened to be called the archosaurs.

What’s the Hole for?

That is a good question. The function of an extra hole is one which requires speculation, since we don’t entirely know. There are a few good guesses. For one thing, these dinosaurs got really big, and had those massive heads–think of the T rex head. Just having a little less bone to carry around was helpful. Scientists think they also might have been air-filled because some birds have air-filled sacs in their antorbital fenestrae (that’s the plural of fenestra). Carrying around a head full of air was lighter. Cooler, too, because if you remember 200 million years ago, the earth was kind of swampy and humid.

A Is Also for Ankle

The other thing that started to define dinosaurs and separate them from other reptiles of their day were the ankle bones. Crocodiles–y’all know that crocodiles are 150 million years old, right? and that some were 50 feet long? — crocodiles can move pretty quickly but they can’t really run. They’re not upright. What allowed the dinosaurs to be effective and flourish as an animal type was that they went upright.

One of the reasons they could do this was the difference in their ankles. The dudes on the right had a simple hinge joint in their ankle, so they could stand upright. Lots of advantages in that–visibility and speed in particular. Crocodiles and their twisty ankle friends can’t support their weight, but those ankles are stable, and they can turn and move in different directions better. Different advantages. Not easy to topple over a crocodile, whereas maybe you could have toppled over a hadrosaur, kind of like the dinosaur horse. Dinosaurs developed heavy-duty bones to counteract that. Big bones. Stay tuned for that one, which will be coming down the pike.

A Few More Windows

If you’ve ever seen a dinosaur skull up close (like the picture at the very top, which I took at the fantastic dino museum in Albuquerque), you notice there’s a lot of holes. It turns out that they’re not there just because scientists couldn’t find all the bones. All the holes had a purpose. You can see in the lovely, color-coded head that behind and around the eye are a lot of extra holes, extra fenestrae. Temporal and mandibular fenestra in particular. What are those for? Those were places where the head and jaw muscles got extra purchase.

A T rex had jaw muscles that could have crushed a car. Their head was so strong and so moveable that it acted like arms, which is why it’s thought they didn’t really need their arms. Yeah, I wrote about that before. Their head and jaws were doing all the work. By 65 million years ago, they were practically all head and teeth.

After all, who needs arms, when you have all those extra windows in your skull?

5 Replies to “A is for Antorbital Fenestra”

  1. [Why don’t I see a comment button? It seems I can only reply to previous comments]
    This is interesting. So we don’t really know what the purpose of the antorbital fenestra is, except to assist in classifying fossils!?
    BTW: Your images are so small that one can’t really read the content.

    1. Thanks for the comment — I think you did the hit the comment button as far as I can tell. The AF might have been simply to have less bone in those heavy dino-heads or to contain an air-filled sac like birds have, which also just made their head lighter. Re: small images –that’s interesting — are you talking about the chart that had all the comparative heads on it? Hm. I’ll have to figure out why it did that, blurred the words. When I originally pulled it in, they were readable. That WordPress!!!! Anyway, thanks for calling that out!

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