One of the frustrating things about studying long-extinct animals is how thoughtless they were. Dinosaurs — already factually* proven to be the coolest creatures to ever exist — were terribly bad at leaving us good examples of their soft tissues to study. Instead of lining up in neat orderly rows under ideal conditions for long-term fossilization, they just died everywhere. This has made it vastly more difficult to study them appropriately. In most cases, fossilization only preserves bone, though faint markings, scratches, or preserved ‘shadows’ sometimes still show where soft tissue existed.
Because we can’t examine soft tissue directly, paleontologists have to study them indirectly, by examining the bone structures that were preserved for millions of years and comparing them with creatures that still exist today. By tracing the evolutionary lineage of still-extant creatures backwards in time to when it converges with now-extinct creatures, scientists can observe how these features evolved and intuit some aspects of how older structures might have functioned.
Researchers examining the skull of Tyrannosaurus Rex have published a paper arguing that these creatures effectively had air conditioners built into their skulls. Maintaining appropriate body temperature can be a challenge in large animals, and many creatures have adapted various strategies for solving the problem. Elephants have large ears to radiate heat from blood vessels and can flap their ears to create cooling air currents. Some large animals spend a great deal of time in or near water to keep their own body temperatures regulated. Some are active mostly at night when temperatures are lower.
Tyrannosaurs, on the other hand, had holes in their skulls. These holes, known as dorsotemporal fenestrae, have long been thought to function as massive anchors for the creature’s huge jaw muscles. These muscles were thought to entirely fill the cavity when the creatures were alive. According to new research published in The Anatomical Record, however, muscles weren’t the only thing tyrannosaurids packed into the space. These fenestrae may have served a dual function by providing important cooling capability as well. They write:
[H]ere we present numerous lines of evidence which indicate that a sizable portion of the dorsotemporal fenestra in crocodylians, non-avian dinosaurs, and many other fossil archosaur lineages was not wholly muscular but instead likely housed vascular tissues. When skull roof tissues were elaborated in fossil specimens, evidence indicates that blood vessels found in the dorsal temporal fossa were often involved in supporting soft tissue cranial display structures… and possibly vascular physiological devices.
Several factors led the paleontologists to this conclusion. For one thing, the anatomical location of the holes made them a difficult attachment point for the jaw muscles. For another, the bone in this area of the skull is smooth. Attachment points for muscles typically aren’t. To test their theory, the scientists used a FLIR camera and measured the body temperatures of alligators, paying special attention to the temperatures of the dorsotemporal fenestrae. What they found is that these areas of the body are markedly hotter when the alligator is basking in the sun and cooler when it dozes in the shade.
“One of the major physiological challenges that large animals have is being able to shed heat,” Casey Holliday, the leader of the study, told National Geographic. “If big theropod dinosaurs were warm-blooded … then they too probably had challenges dissipating heat in some instances.”
Other dinosaurs, like ankylosaurs, have been found to have large, complex nasal passages filled with blood vessels as a means of dissipating heat. Tyrannosaurids lacked this adaptation, which means the creatures — which were as much as 40 feet long and 20 feet tall — had to dissipate heat through some other means. Radiating it outwards from the skull would protect the creature’s brain from overheating. National Geographic also notes that some dinosaurs had fenestrae that were close to their neck frills, which are thought to have been used in mating and threat signaling. It’s possible that Tyrannosaurus Rex or its family members may have been able to use its blood vessels for color-changing displays, though this is strictly a theory at this juncture.
* – As measured by eight-year-old me.
Top image credit: Scott Robert Anselmo/Wikimedia Commons