Home > Science Sidebar > Science Sidebar 4: Skulls and Gastralia

Science Sidebar 4: Skulls and Gastralia

While visiting Chicago for a wedding (congrats to Amanda and Nick!), I had a chance to visit the Field Museum.  It’s epic, and the group I was with only able to go through a small section in the time we had.

But!  While we were there, I learned a few things I hadn’t previously known or realized about tetrapods.  Which I will now inflict on you.


Your skull, less several hundred million years of modifications.  (via Wikipedia, synapsid skull)

Your skull, less several hundred million years of modifications. (via Wikipedia, synapsid skull)

A Tale of Skulls

Paleontologists like skulls.  They can tell you a lot about a critter — what kinds of things it ate based on its teeth, how big its brain was, how strong its jaw muscles were, and so forth.

But they can also make some major distinctions between different kinds of animals.

Way back in the Devonian period, just under 400 million years ago, the first tetrapods showed up on land.

Within the next 100 million years, they diversified into two major groups that survived the mass extinctions that followed.  The synapsids were distinguished by having a single hole in the skull behind the eye, called a temporal fenestra (as in the diagram above), and are sometimes called “mammal-like reptiles”.  Mammals are their modern descendants.   In modern mammals, this hole has been closed — other bones grew to fill in the gap.  (In humans, this is the sphenoid bone — there’s a little dent in your skull, below the temple and behind the eye, where you can find what’s left of the hole.)

The diapsids, by contrast, had two such holes.  Their descendents include all lizards, snakes, crocodiles, dinosaurs, and birds.  You can see those holes in, for instance, a T. rex skull.  (There are also anapsids, which includes turtles, but based on the Wikipedia discussion, there’s some uncertainty about whether they should be instead classed as diapsids that have lost the holes.  The amphibians branched off earlier, so they don’t quite fall into this scheme.)  In modern birds, the skull has changed substantially from this earlier form, and one or both holes may have fused in other critters (like snakes), but the basic idea is still there.

Regardless of the details, this diapsid/anapsid distinction is very useful to scientists trying to learn things about ancient tetrapods — as a straightforward way to distinguish mammal and reptile ancestors.  They also think that they holes may have been use to anchor stronger jaw muscles.

A related feature is that mammals have a roof in the mouth that separates the mouth from the nose.  So we can breathe and chew at the same time, but reptiles and birds can’t.  Well, at least, not without having a bunch of air in their mouths.

And then there are stranger things.  For instance, I had not previously been aware that there was such a thing as…


Yes, I learned a new word on that trip.  Gastralia (or gastralium in the singular) are bones that run along an animal’s stomach, and look kind of like ribs, except that they aren’t connected to the ribs by any bones.

Gastralia look kind of like this.  This is a T. rex example, at the Royal Belgian Institute of Natural Sciences.  (via Wikipedia)

Gastralia look kind of like this. This is a T. rex example, at the Royal Belgian Institute of Natural Sciences. (via Wikipedia)

Some modern animals, like crocodiles, have these… but so did Tyrannosaurus rex.  The Field Museum doesn’t display the grastralia along with the main skeleton — they’re housed in a separate display case, which says that they’re not sure exactly how they’re supposed to fit into the skeleton.

It’s also uncertain what purpose the gastralia serve.  In modern animals, they may have formed the basis for what became turtles’ lower shell.  For crocodiles, it sounds like they’re used to help anchor abdominal muscles (though Wikipedia doesn’t have a source on that).   For dinosaurs that had them, it’s been suggested that maybe the grastralia were helpful for breathing… but that’s not certain.

  1. michaelbusch
    2014/09/14 at 11:01 am

    Additional information on various items in the Field Museum’s collections is provided by Emily Graslie, host of the online show The Brain Scoop. Some episodes deal with far more recent animal remains than T. Rex bones, and so are rather heavy on blood and guts: http://www.youtube.com/user/thebrainscoop .

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