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The Mystery Of Why T. Rex Had Two Weird Holes In Its Skull Has Been Solved

Ryan Ford 6 Sep 2019

Millions of years of extinction couldn't keep the fearsome Tyrannosaurs rex from dominating pop culture.

Humans have never seen anything quite like it, basically dagger-filled jaws on legs, stalking the land to feed a bottomless appetite, and how can you not be awed by that?

But for all their popularity, we still don't know as much about the T. rex as you might expect.


They've been studied intensely for as long as we've known about them, but the first T. rex fossil was only unearthed in 1902, and it didn't get its now-famous moniker until 1905. That's barely more than a century of study for something that we only have bones and fossils of.

Researchers have obviously done their utmost with the formidable king of the dinosaurs, but one feature in particular has had them stumped for a long time.

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The T. rex's skull has plenty of holes in it, but two of them were more mysterious than the others.

Wikimedia | https://commons.wikimedia.org/wiki/File:B-rex_skull.jpg

For a long time experts believed the holes, called the dorsotemporal fenestra, housed muscles that helped the jaw do its thing. Considering the T. rex had the strongest bite of any land animal ever to exist — a force equal to a medium-sized elephant sitting down — that wasn't an unreasonable guess.

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However, something about the placement of the holes didn't quite make sense.

Wikimedia | https://commons.wikimedia.org/wiki/File:Tyrannosaurus_Rex_Holotype.jpg

"It's really weird for a muscle to come up from the jaw, make a 90-degree turn, and go along the roof of the skull," said University of Missouri anatomist Casey Holliday.

But the evidence still suggests that the area was filled with blood vessels, just like alligators and other reptiles have.

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That alligator connection would prove to be quite informative.

Wikimedia | https://commons.wikimedia.org/wiki/File:Alligator_Cr%C3%A2ne_et_Mandibule.jpg

Not all dinosaurs had fenestra, but T. rex also wasn't the only animal to have fenestra on its head. Similar holes can be found on some birds, lizards, and yes, alligators.

They're part of a class of animals called diapsids. Looking at the skulls of diapsids, a team of researchers narrowed down which ones were most similar to T. rex's and found that alligators were a good match.

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So, to figure out what the king of the dinosaurs might have used those holes for, the researchers took a thermal camera to look at some alligators.

University of Missouri

Holliday and his co-authors, William Porter and Lawrence Witmer of Ohio University and Kent Vliet of the University of Florida, took their camera to check out the residents at the St. Augustine Alligator Farm Zoological Park.

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Right away, the trip proved fruitful.

The Anatomical Record | Holliday et al.

"We noticed when it was cooler and the alligators are trying to warm up, our thermal imaging showed big hot spots in these holes in the roof of their skull, indicating a rise in temperature," said Vliet.

"Yet, later in the day when it's warmer, the holes appear dark, like they were turned off to keep cool. This is consistent with prior evidence that alligators have a cross-current circulatory system — or an internal thermostat, so to speak."

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Of course, being endothermic, or cold-blooded, that internal thermostat is critical.

University of Missouri

Alligators don't regulate their body temperatures like we endothermic humans do. Their body temperatures depend on the environment around them. Those fenestra help alligators warm and cool the blood passing through.

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Having only the fossils to work with, nobody knows for sure whether the T. rex was warm- or cold-blooded.

University of Missouri

However, researchers can make some good guesses based on body sizes and growth rates. Experts are pretty much split, and some believe dinosaurs might have been somewhere between warm- and cold-blooded, a condition known as mesothermy.

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So, what does that all mean for the holes in T. rex's skull?

University of Missouri

Well, they definitely weren't for jaw muscles, for one thing.

It's far more likely that they were for regulating body temperature, as with modern alligators, which means the king of dinosaurs at least had some cold-blooded body structures.

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This turns a century of research on its, well, head.

Giphy | Jurassic World

"We know that, similarly to the T. rex, alligators have holes on the roof of their skulls, and they are filled with blood vessels," said Witmer. "Yet, for over 100 years we’ve been putting muscles into a similar space with dinosaurs. By using some anatomy and physiology of current animals, we can show that we can overturn those early hypotheses about the anatomy of this part of the T. rex’s skull."

h/t: University of Missouri, The Anatomical Record

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