It's not everyday you see a phytosaur...
... yet, there it was in the Carnegie Museum of Natural History. Big, with an impressive set of teeth, and looking for all the world like a massive crocodile.
But what really shocked me was what looks just like ribs in its belly. Is that a thing? Why have I never noticed this before?
Here's a close-up shot:
See what I mean? There is the backbone above, with a lot of ribs hanging off the vertebrae. You can easily see the massive leg bones and feet.
Here's a redrawing I made of the photo, emphasizing the "belly ribs."
Note how they seem to be kind of U-shaped, with long, pointed arms wrapping up as if to encase the abdomen. Note too how the ribs hanging down from above are squared-off at the ends. The two different kinds of ribs look very, very different. These belly ribs are mostly free-floating except for the one at the left, which attaches to a kind of sternum that runs up to the phytosaur equivalent of a collar bone.
I don't know that I've seen that on any other animal. Or have I just not been paying attention?
Here are last week's two Challenges for you:
1. What do you call that thing in the chest and belly of the Redondasaurus bermani?
For something that I'd never seen before, I was pleased with my first query:
[ ribs in dinosaur belly ]
Yes, I know that technically the Redondasaurus isn't a dinosaur (it's a phytosaur), but for the purposes of searching, I figured that the term "dinosaur" would be used on the page, if only to explain that it's not a dinosaur.
This query worked remarkably well, and told me about a word I'd never read before: gastralia, the "belly bones" of dinosaurs, such as the T. rex.
Naturally I checked. Just because a therapod like T. rex (that lived during the Cretaceous period, about 70 to 65 million years ago), there's no guarantee that a phytosaur (that lived during the Triassic some 252-201 million years ago) would have the same skeletal feature. There's a decent change that something might have evolved differently over the distance of 180 million years.
A quick search for:
[ gastralia phytosaur ]
Bottom line: these additional bones are called gastralia, and were (as Regular Reader remmij pointed out), pretty clearly used as part of their breathing apparatus.
Remmij used the search [Dinosaur gastralia and their function in respiration] and found Some video that might be helpful for understanding gastralia's role in breathing.
2. Do other animals have this thing now, or was it just an Age of the Dinosaurs skeletal feature?
What about modern animals with gastralia?
The Wikipedia article about gastralia tells us that "Gastralia (singular gastralium) are dermal bones found in the ventral body wall of modern crocodilians and tuatara, and many prehistoric tetrapods." That's a complicated way of saying that there's a basket of bones in the belly of the beasts... or at least of some crocodilians (alligators and crocodiles) and tuatara (a kind of lizard from New Zealand).
But to remember the past, as we've seen already, gastralia are also part of other classic dinosaurs such as T-rex.
As you know, there's a T-rex skeleton on the Googleplex campus in Mountain View, California. (Discussed before in SRS.) Here's Stan, the Google T-rex with flamingos...
But note that Stan seems to be missing his gastralia! Here's a better image to show the missing bones. I see ribs, but no gastralia.
When I did an image search for [ t-rex skeletons ] I found this somewhat variable set of results:
As you can see, about half of the reconstructed skeletons (including the one of Stan from the Black Hills) are missing their gastralia.
A quick search for [ missing gastralia ] led me to this article from the Field Museum (Chicago) about how they recently revamped their T-rex display of Sue. (It's worth clicking on that link--it's a really nice article about how they changed their minds about Sue's abdominal bones.)
When fossil hunters first uncovered SUE, they found a set of rib-like bones that clearly belonged to the dinosaur, but no one was really sure where they fit in with the rest of the skeleton.
Now we know that the gastralia sit below the ribs and along the belly. Most likely, they helped T. rex breathe by pushing air in and out of the lungs (we humans have a diaphragm for this purpose). SUE’s gastral basket is the most complete among over 30 known T. rex specimens. It includes 26 of approximately 60 total gastral bones. We don't know exactly how many bones make up the gastralia since most—but not all—segments consist of two pieces. And there isn't yet a complete T. rex gastralia for us to check out and confirm how many segments do have two pieces.
So our understanding the appearance of a T-rex has changed over time. Now, if you see a T-rex without a gastralia, you know that someone has left out part of the skeleton!
This wasn't a difficult search--my very first search was pretty obvious [ribs in dinosaur belly], but it worked just fine, and I learned a very specific term--gastralia--that was useful in digging more deeply into the subject.
It was great fun to learn that so many dinosaurs skeletons that we see in museums (and on the silver screen) are actually missing a major part of their anatomy. But by doing quick visual comparisons, we came to learn how to see dino skeletons as part of a full ensemble.
As a reminder of the value of checking Wikipedia articles in other languages, Regular Reader Ramón points out that in the Spanish language Wikipedia article on gastralia it tells us about the origin of the term:
"Dinosaur gastralia were first described by Eudes-Deslongchamps in 1838 in Poekilopleuron bucklandii , but without having recognized them as dermal bones. Therefore, Osborn's description of the gastralia of a Tyrannosaurus rex from 1906 is often cited as the first description."
Even though T-rex gastralia were among the first described (over 100 years ago), they're STILL left off of many T-rex skeletons.