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Why Dinosaurs Have Holes in Their Skull

Updated on November 19, 2019
Mamerto profile image

Mamerto Adan is a feature writer back in college for a a school paper. Science is one of his many interests, and his favorite topic.

Apart from their huge sizes, there is something different about dinosaur skulls. I never really minded it, and I never bothered why. But just how different? Some people will point out the dagger-like teeth in some species, the strange protrusions like bumps or horns, frills, duck bills, armors, and other strange head ornaments that helped them survive the cruel age of the Jurassic period. But stripped of those contraptions and their skulls still look different.

Let me give you a clue. Humans only have holes in their skulls for the eyes and nose. The same could be said on other animals. But you never need to look closely to see that dino skulls are riddled with openings.

As I grew older, I began to wonder why they looked like that. People just cannot imagine having extra sets of holes in their head. Unless the dino species are suffering from some sort of bone disease, which they obviously didn’t, those holes certainly served some purpose.

The Holes in Archosaurus Skull

Fenestrae on the skull.
Fenestrae on the skull.

Scientifically, the holes in their skulls are known as fenestrae (singular is fenestra). And yes, I did mention that it made the dino skulls unique in some ways, but I was wrong. Because fenestrae are not unique to dinosaurs.

Dinosaurs sported several sets of extra holes in their skulls. One is the Antorbital Fenestra, the hole in the front of eye socket. The other were the infratemporal fenestra, the opening behind the eye socket. Not only that, dinosaurs also have holes in their jawbone, the mandibular fenestra, and a bizarre opening on the top of the head, the supertemporal fenestra. But creatures before them already sported these sets of holes in their heads.

The Archosaur is a diverse group where the dinosaurs belong. And yes, pterosaurs, birds and crocodilians are also included. And one of the features that united these creatures are the holes.

Firstly, let’s start with an early form of carnivorous archosauriform reptile. An aptly named creature, the Archosaurus. Life restoration will reveal a strange creature with an overbite. It predates the dinosaurs, but its skull is already riddled with holes. Millions of years later, into the modern age, later archosaurs either lost or retain some of the openings as what evolution dictated. In the case of the modern dinosaurs, the birds, it still retained the antorbital fenestra. But crocodilians already lost them. What remains in the croc skulls are the supertemporal and infratemporal fenestra, those holes on the top of their heads.

Hypothesis

Tuatara skull with a collection of fenestrae.
Tuatara skull with a collection of fenestrae.

And now came the question why these skulls have holes. One assumption is that the fenestra made the skulls lighter. This might make sense considering that dinosaurs could grow to enormous size. When you are a hulking creature bigger than a house, you need to lighten the load as much as possible. Hence as evolution dictated, any unnecessary weights must be removed. These includes portions of the skulls.

There are problems with the notion though.

If fenestrae reduced the weight of the bulky animals, then how come smaller dinosaurs still have them? Do note that although we have a collection of impressive and towering dinosaurs, a lot is still small. They could be the size of humans, or wolves. We even have dinosaurs the size of turkeys (I’m looking at you Velociraptor). If fenestrae are for weight reduction, then smaller dinosaurs should have evolved with a more solid skull, while larger beasts will retain their hole ridden skull which is not the case here.

Then there are the modern archosaurs.

Crocodylians are monstrous, but they are not large enough to reach T-rex proportion. Yet they retained the fenestrae on their heads. The same could be said to birds.

And lastly are the archosaurs that predated the dinosaurs, which sported fenestrae despite being the size of monitor lizards.

But if weight reduction is not the purpose, then what are those holes for?

Proposed functions include housing of glands, muscle attachments, or to hold air-filed sacks. The last hypothesis seems to support the weight reduction function, as this will help lighten the skull. In living birds, their antorbital fenestrae houses a large air-filled diverticulum known as the antorbital sinus. Basically, they have a large air chamber inside their heads.

And being an airhead may have helped the dinosaurs grow to massive sizes.

Because the fenestrae are connected to air chambers, which is also connected to the respiratory system, it might also help the dinosaurs cool down. The dinosaur’s large size means it generates intense heat that might overheat the brain. Hence holes in the skull containing air filled chambers could have prevented the giants from being too hot headed.

The Fenestrae Are For Cooling

The holes could have made this T-rex cool headed.
The holes could have made this T-rex cool headed.

This recent report was released by National Geographic. This seems to support that idea that fenestrae are for cooling. It was originally thought that the fenestrae on the top rear of the skull was there for muscular support. These muscles are for operating the jaw, to generate that monstrous bite force required for predation. But inside the fenestrae is the frontoparietal fossa, where there is an absence of muscle attachments. In living reptiles like the crocs, the skull lost the antorbital fenestra but still retained the fenestrae on top of their skulls, where the holes house fat and blood vessels.

And these blood vessels could serve as temperature regulators for the head.

Being cold blooded, the blood vessel filled fenestrae might come in handy when the croc’s surroundings became too cold or hot, as it helped them retain or lose heat.

And what works for crocodilians would have worked for another archosaur, the dinosaurs.

Do note that unlike gators and crocs, dinosaurs are warm blooded, and their body temperatures could be further elevated by their sizes. This made them prone to overheating. In this case, the holes would have prevented the overheating of the brain.

And added supply of blood vessels probably, made these creatures look wilder.

Near these blood vessels, where blood supplies are rich, dinosaurs would develop keratin. And these keratin could translate to extravagant head-gears, like the crests of the Parasaurolophus. Now if you saw the recent T-Rex reconstruction, we will see a beast with horny brows, in the place near the holes in the skull.

Overall

It is quite likely that the holes are there not for weight reduction, but as a cooling system. Ancient cold-blooded archosaurs could have use them to regulate the heat in their bodies, while the fenestrae helped the warm blooded dinosaurs keep cooler.

References:

1. Black, Riley. (September 4, 2019). "Special Skull Windows Helped Dinosaur Brains Keep Cool." Smithsonian.com.

2. Curie, P.J. (1987). "The Nature of the Antorbital Fossa of Archosaurs: Shifting the Null Hypothesis." Fourth Symposium on Mesozoic Terrestrial Ecosystems.

3. Pickrell, John (September 4, 2019). "New discovery shows how T. rex kept its brain cool." National Geographic.

4. Witmer, L.M. (1997). "The Evolution of the Antorbital Cavity of Archosaurs: A Study in Soft-Tissue Reconstruction in the Fossil Record with an Analysis of the Function of Pneumaticity."

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    • bhattuc profile image

      Umesh Chandra Bhatt 

      3 weeks ago from Kharghar, Navi Mumbai, India

      I have some interest in pre-historic times and theory of evolution and this article is of interest to me. Thanks.

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