Dinosaur skulls have something in common with Swiss cheese—they’re both full of holes. From Tyrannosaurus to Triceratops, the skulls of the terrible lizards have the same eye and nasal passages common in many vertebrates, as well as additional pockets unique to reptiles. For example, a hole at the top and rear of the skull anchored jaw muscles for chomping through the Mesozoic world—and a new study suggests there was more to this peculiar anatomical window than just biting. The prehistoric skulls of dinos may have held special blood vessels that allowed the animals to keep their brains at just the right temperature.
The specialized pocket sits within an opening on the top rear of the skull called the dorsotemporal fenestra. This opening plays a role in jaw muscle attachment, and scientists previously thought it was filled with the fibrous tissues that allowed dinosaurs to bite. But a curious pocket within the larger hole, called the frontoparietal fossa, seems to have served a different purpose.
While researching jaw muscle attachments in reptiles, University of Missouri anatomist Casey Holliday noticed that the frontoparietal fossa didn’t seem to have much to do with chomping. “I wasn’t finding any evidence to support the presence of a muscle and so had to start figuring out what else it could be,” Holliday says. Among living reptiles, Holliday and colleagues found the area in question houses fat and blood vessels.
Digging in further, the research team compared the past to the present. In addition to detailed examinations of fossil dinosaur skulls, the paleontologists also looked to some of dinosaurs' closest living relatives—American alligators and wild turkeys—to see if there might be unidentified soft tissues in this skeletal divot. Their results were published today in The Anatomical Record.
The researchers found that the skull pocket had little to do with chewing. Both living birds and crocodylians have a pad of blood vessels and fat sitting in the cavity atop the underlying musculature. The skull pocket isn’t jam-packed with muscle fibers, as it would be if the feature’s only function were to increase the force of a bite. Instead, quite a bit of vasculature runs just below the skin, suggesting blood vessels could serve as temperature regulators in the head, and non-avian dinosaurs like Velociraptor show the same anatomical hallmarks.
“I think the authors present a strong case for the presence of specialized vascular features in the frontoparietal fossa,” says University of Florida anatomist Catherine Early. Additional work may revise the details, Early notes, but the new research changes what scientists previously expected of reptile anatomy.
Looking at temperature profiles of modern alligators helped explain why ancient reptiles might have needed a network of blood vessels in the back of their heads. Holliday and colleagues took thermographic images of alligators at Florida’s St. Augustine Alligator Farm and Zoological Park at different times of day. In the cool of the morning, the area with the relevant skull opening was relatively warm compared to the rest of the reptiles. The alligators had warm brains even as their bodies hadn’t yet caught up.
In the heat of the afternoon, the pattern reversed. The alligator brains were relatively as ambient conditions raised the reptiles’ body temperatures. “Thermography revealed that this pocket of vessels was identifiable via heat signatures, and that it seems to vary in temperatures relative to body surface temperature throughout the day.” Taking thermal images of wild and captive alligators was a treat, Holliday says, but he notes that the process can be challenging and even dangerous up close. “Next come the drones” to get such images from a distance.
While we can’t directly observe Allosaurus without a time machine, the crocodylian clues suggest that ancient dinosaurs had this network of blood vessels for much the same reason as their modern relatives. “The vasculature of the frontoparietal fossa fits into a network of vessels that help animals regulate brain, eye and body temperature,” Holliday says. Shunting blood through this skull network could keep their brains warm or cool, which would have been particularly important given that dinosaurs likely had elevated body temperatures and similar metabolisms to birds and mammals rather than lizards. Dinosaurs may have been prone to overheating, and so conveniently located blood vessels would have allowed cooler heads to prevail.
“What stands out to me is that this paper synthesizes evidence on how archosaurs in general are using cranial vasculature to thermoregulate,” Early says. What alligator heads are doing today provides us with a window to better understand creatures that lived millions of years ago.
The blood vessel networks may have had another advantage as well. Dinosaurs like the triple-horned carnivore Ceratosaurus and many tyrannosaurs had skull ornaments, or lavish head displays, in the vicinity of these blood vessels. The bone would have been covered in keratin in life, nourished by the same cardiovascular system that controls temperature. The blood vessels that helped dino brains warm or cool could have also allowed these animals to develop extravagant headgear, co-opting a temperature function into something flashy, such as the tube-like crest of Parasaurolophus or the long brow horns of Pentaceratops.
Additional research is required, Holliday cautions, but notes that “if you’re going to have a giant structure growing off your head like dinosaur frills and horns, or duckbill crests, or tyrannosaur excrescences, there needs to be blood to supply it.” The blood vessels might have even nourished as-yet-undiscovered head features, like combs, wattles, snoods or crests. A newly analyzed hole in the back of the skull might provide clues that dinosaurs were even flashier than ever expected.