Different groups of gigantic dinosaurs had different thermoregulatory strategies to help moderate brain temperatures in the face of high heat loads, according to new research from the Ohio University’s Heritage College of Osteopathic Medicine.

“Small dinosaurs could have just run into the shade to cool off, but for giant dinosaurs, the potential for overheating was literally inescapable,” said Professor Lawrence Witmer, co-author of the study.

“They must have had special mechanisms to control brain temperature, but what were they?”

Professor Witmer and his colleague, Dr. Ruger Porter, looked to the modern-day relatives of dinosaurs, birds and reptiles, where studies indeed showed that evaporation of moisture in the nose, mouth, and eyes cooled the blood on its way to the brain.

Using a technique that allows arteries and veins to show up in CT scans, they were able to trace blood flow from the sites of evaporative cooling to the brain. They also precisely measured the bony canals and grooves that conveyed the blood vessels.

“The handy thing about blood vessels is that they basically write their presence into the bones,” said Dr. Porter, lead author of the study.

“The bony canals and grooves that we see in modern-day birds and reptiles are our link to the dinosaur fossils.”

“We can use this bony evidence to restore the patterns of blood flow in extinct dinosaurs and hopefully get a glimpse into their thermal physiology and how they dealt with heat.”

The researchers looked at bony canal sizes in the dinosaurs to assess the relative importance of the different sites of evaporative cooling based on how much blood was flowing through them.

A key factor turned out to be body size. Smaller dinosaurs such as the goat-sized pachycephalosaur Stegoceras had a very balanced vascular pattern with no single cooling region being particularly emphasized.

“That makes physiological sense because smaller dinosaurs have less of a problem with overheating,” Dr. Porter said.

“But giants like sauropods and ankylosaurs increased blood flow to particular cooling regions of the head far beyond what was necessary to simply nourish the tissues.”

This unbalanced vascular pattern allowed the thermal strategies of large dinosaurs to be more focused, emphasizing one or more cooling regions.

But although sauropods like Diplodocus and Camarasaurus and ankylosaurs like Euoplocephalus all had unbalanced vascular patterns emphasizing certain cooling regions, they still differed.

Sauropods emphasized both the nasal cavity and mouth as cooling regions whereas ankylosaurs only emphasized the nose.

“It’s possible that sauropods were so large — often weighing dozens of tons — that they needed to recruit the mouth as a cooling region in times of heat stress. Panting sauropods may have been a common sight,” Dr. Porter said.

One problem that the scientists encountered was that many of the theropod dinosaurs — such as the 10-ton T. rex — were also gigantic, but the quantitative analysis showed that they had a balanced vascular pattern, like the small-bodied dinosaurs.

“This finding had us scratching our heads until we noticed the obvious difference — theropods like Majungasaurus and T. rex had a huge air sinus in their snouts,” Professor Witmer said.

Looking closer, the team discovered bony evidence that this antorbital air sinus was richly supplied with blood vessels.

Profeesor Witmer had previously shown that air circulated through the antorbital air sinus like a bellows pump every time the animal opened and closed its mouth.

“Boom! An actively ventilated, highly vascular sinus meant that we had another potential cooling region. Theropod dinosaurs solved the same problem… but in a different way,” he said.

The findings appear in the journal Anatomical Record.

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Wm. Ruger Porter & Lawrence M. Witmer. Vascular Patterns in the Heads of Dinosaurs: Evidence for Blood Vessels, Sites of Thermal Exchange, and Their Role in Physiological Thermoregulatory Strategies. Anatomical Record, published online October 16, 2019; doi: 10.1002/ar.24234