Thermal images of alligators taken by a team of scientists led by Casey Holliday, a professor of anatomy at the University of Missouri School of Medicine, suggest that the Tyrannosaurus rex may have had blood vessels in its skull that helped the giant predator to regulate its body temperature – in other words, a biological air conditioner.
One of the most iconic dinosaurs, the T. rex lived during the Cretaceous Period about 68 to 66 million years ago. Measuring up to 12.3 m (40 ft) long and weighing in at up to 14 tonnes, it has fascinated everyone from paleontologists to school children ever since the first skeleton was discovered in 1900 and has been the subject of many studies seeking to learn more about its anatomy, physiology, and habits.
However, despite over a century of work, the T. rex still has many mysteries clutched in its tiny forelegs. One of these is how it regulated its body temperature. Up until the 1960s, the Tyrannosaurus was assumed, because it was a reptile, to be cold-blooded. But some scientists saw evidence indicating that the T. rex, and other dinosaurs, may have actually been warm-blooded.
Today, the jury is still out. We're not sure if the T. rex was warm-blooded, cold-blooded, or something in between. However, we do know that it did have some ways of not getting too hot or too cold – if only by being so big that its temperature couldn't change very fast.
Now, Holliday and his team, which includes researchers from the University of Missouri, Ohio University, and the University of Florida, say that cranial blood vessels in two large holes, the dorsotemporal fenestra, in the head of the T. rex acted as an internal thermostat, though the conventional view is that these holes were to make space for jaw muscles.
"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," says Holliday. "Yet, we now have a lot of compelling evidence for blood vessels in this area, based on our work with alligators and other reptiles."
To learn more about what these holes were for, the team used thermal imaging cameras to study alligators at the St. Augustine Alligator Farm Zoological Park in Florida.
"An alligator's body heat depends on its environment," says Kent Vliet, coordinator of laboratories at the University of Florida's Department of Biology. "Therefore, 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. 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."
The thermal imaging indicated that similarity between the T. rex and alligator anatomies suggests a similarity in function. What's interesting that this doesn't speak to whether T. rex was warm-blooded or cold-blooded. Alligators use the blood vessels in their skulls to both warm and cool it, but some mammals like dogs have cranial blood vessels, too, to keep their brains cool by acting as a heat exchanger – an important function in a warm-blooded predator that needs to be able to run down prey. This may indicate that the T. rex, like a dog, may have been able to chase its dinner until it died of heat exhaustion – although another study from 2017 suggests it might have been a slow chase.
"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 Larry Witmer, professor of anatomy at Ohio University's Heritage College of Osteopathic Medicine. "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."
Source: University of Missouri
