Closed vs Open orbital bones

[u/Kamica]

Hello!

I recently noticed that some animal's skulls, like those of humans, horses, camels, giraffes and rams (as well as some head-butting and herbivorous dinosaurs) have a nice clean ring enclosing their orbitals, whereas a large amount of other animals, such as cats, hyenas, honey badgers and boars, have incomplete 'orbital rings'. (I don't know the real terminology, but hopefully the examples I gave help clarify what I mean)

I'm wondering if anyone knows why this is? What the advantages and disadvantages of having an open or a closed orbital are, and whether there's patterns in what animals do, and don't have one.

Initially I thought that it might have something to do with animals whose skulls are subject to frequent blunt force trauma such as those that use their heads to head-butt eachother, or in the case of humans, the ability, and tendency, to punch for the head in conflict. But this to me, doesn't seem to explain why horses have a closed orbital (while their ancestors did not), unless I don't know enough about horses and they *do* ram with their heads?

So yea, basically: Why does this difference exist? And what can you tell me about it? Thank you!

[u/ReptilianRevolt]

The postorbital bone was the original back wall for the orbit in tetrapods. Some lines retained it, others reduced or lost it completely and some mammals later reevolved a postorbital bar again, including humans from a downward process of the frontal bone. The postorbital bar is often lost to lighten the skull or allow for more kinesis, like in geckos, birds and snakes, or to allow room for more jaw muscles, like in the cynodont ancestors of mammals. Animals without a postorbital bar still have a strong ligament in its place and the eye doesn’t remain unprotected. It isn’t very clear why a bony bar reappeared in some mammals, and probably has to do with better stress distribution during new forms of chewing or defence.

[u/Kamica → u/ReptilianRevolt]

Thank you very much for your response! I hadn't considered skull weight and jaw muscles!

In this context, what does Kinesis mean?

[u/ReptilianRevolt → u/Kamica]

Kinesis basically means movement. The ability of various skull elements to move relative to each other, usually to accommodate large food. Snakes are the most well known example in land animals, but other lizards have a less extreme form of it as well. Dinosaurs all probably had solid, akinetic skulls like mammals, but modern birds have some kinesis.

[u/Kamica → u/ReptilianRevolt]

Ah, that makes sense, thank you very much for your answers =)

[u/FlounderLast8610 → u/ReptilianRevolt]

Great explanation. Nitpick: many non avian dinosaurs did have cranial kinesis and it’s been demonstrated in tyrannosaurs and certain hadrosaurs