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Why Dinosaurs Have Holes in Their Skull

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

why-dinosaurs-have-holes-in-their-skull

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."

Comments

Mamerto Adan (author) from Cabuyao on August 15, 2020:

Thanks! I'll check this one out.

The Sampsons from The Ozarks, Missouri on August 14, 2020:

I LOVE dinosaurs...since 3rd grade. Look:

https://books.apple.com/us/book/sketchlings/id1526...

https://books.apple.com/us/book/sketchlings/id1526...

Coelophysis on April 17, 2020:

Mammal have more hole in skull than reptile only two reptile match mammal t.rex and the gator both are tyrannosaur it’s like that because tissue goes through those holes in tetanuran dinosaur gator and t.rex have wider tetanuran skull the holes you see the main holes it’s to lighten the skull by bite force test dinosaur eat like birds with weaker bite force the holes are bigger the advance kind the hole started to reduce Early dinosaur eat up and down a bird feature the more advance kind with stronger bite force like t.rex orbital start reduce and temporal fenestra t.rex even twist it’s food like a gator and have full palate but did not have fully secondary bony palate like only dinosaur are alive the gator these superior dinosaur spinosauridae had stronger bite force the mesoeucrocodylia dinosaur have more stress on the skull when doIng death roll lot of twisting the skull it’s to prevent skull from breaking the weaker bite force dinosaur like t.rex and others who are not tyrannosaur nose hole are big and antorbital allso are big the first mesoeucrocodylia spinosauridae they started reduce antorbital and nose the more advance eusuchian mesoeucrocodylia Nile crocodile there no antorbital fenestra Nile croc is ancestor all modern crocodilian t.rex orbital is reduce like most advance bird eating dinosaur the gator is not a primitive feature mite be do to night hunting most modern crocodilian hunt at night or sclerotic ring the early gator type mesoeucrocodylia had sclerotic ring through evolution the trend in dinosaur is to make skull stronger that why modern crocodilian lost some dinosaur feature in skull some were found in early gator type mesoeucrocodylia in Nile crocodile caiman nose hole is not fuse just one hole a advance feature many advance animal have this feature to get more oxygen but advance modern crocodilian gator dwarf crocodile it is fuse a primitive feature but it has advitage it will make skull stronger dwarf crocodile mostly a land crocodile .spinosauridae temporal fenestra is like most dinosaur primitive and weak it is not fuse the gator like most modern crocodilian is fuse the dwarf caiman it is not fuse a weak feature another advance kind and it is more like land crocodilian no web toe small snout another weak feature but it’s won of stronger bite force in modern crocodilian the reason for that it’s skull like spinosaurus t.rex it has fuse nasal a land feature to protect skull from animal with legs the prey animal is alive in t.rex mouth before it kill it it’s to prevent cheap shot from prey animal kicking the skull but allso make skull stronger because skull bone are fuse baryonyx does not have fuse nasal this is line Nile crocodile came from fuse nasal are rare in mesoeucrocodylia only see that in land mesoeucrocodylia spinosauridae like early gator type mesoeucrocodylia nose hole is fuse .one dwarf caiman Chinese t.rex the temporal fenestra is fuse when juvnile and changes when adult .nile crocodile nose is before kink in spinosauridae the nose is behind kink a aquatic feature a whale feature there stress before kink and behind kink allso top temporal fenestra gharial is bigger than all modern crocodilian a aquatic feature other very aquatic fossil mesoeucrocodylia have simular feature .bird are not dinosaur they are not thecodont like the gator gator has first finger and toe and occified tendon is dinosaur these tetanuran have the most sensoritive face out any animal.gator is warm blooded by heart spleen kidney and blood warm blooded animal need lot oxygen and very fast animal that why gator kidney is more advance than mammal and flow through lungs allso help not found in mammals .supercroc and spinosaurus is size of t.rex .hole in skull a scientist say one of hole is change skin color in dinosaur if that true they need test common caiman for that .the hole that is mistaken for antorbital a dinosaur feature is missing in gator type mesoeucrocodylia

Umesh Chandra Bhatt from Kharghar, Navi Mumbai, India on January 26, 2020:

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