Yesterday in my comparative osteology class I started to wonder: what is the deal with the mammalian postorbital bar? Most people reading this might simply shrug, and maybe, if I was lucky, bother to go Google “postorbital bar.” But the postorbital bar is one of those interesting anatomical traits that independently pops up in mammalian evolution again and again, and often in some strange places.
So lets back up. The vertebrate skull is made up of lots and lots of bones. In some animals, like modern teleost fish (pretty much all fish), these are fairly loosely connected, if they’re directly connected at all. The cranial bones of tetrapods (land vertebrates) are connected by sutures, which can sometimes make the separate bones a little hard to make out.
The PO bar is/was a thin bar of bone that divides the orbit (where your eye goes) and the inferior temporal fenestrum (a hole that does jaw-muscly stuff) in synapsids. It is made up of two bones: the postorbital bone (original!) and the jugal bone (which eventually becomes our zygomatic).
But now you’re wondering: what the fuck is a synapsid? Well, we are synapsids. Synapsids are an early group of tetrapods that eventually gave rise to mammals sometime in the Mesozoic. There’s lots of other early synapsids that your preschool dinosaur picture books probably told you were dinosaurs.
But anyway, sometime during early mammalian evolution, the post orbital bar and bone… disappeared, making the orbit and the inferior temporal fenestrum confluent. It’s just one of the many things that make cat skulls really creepy-looking.
Really, it’s just that the postorbital bone itself went up and left, and I can’t quite seem to find a satisfactory explanation for it yet. Sidor (2001) lumps it in with a trend of cranial simplification in synapsid evolution and it’s a hard point to ignore – recent mammals have a lot less bones in our skulls than do early synapsids. And it probably has something to do with the enlarging frontal bone (that’s our forehead) as mammal brains were expanding and also with space for burgeoning mammal jaw muscles.
And tons of mammals get along just fine without a postorbital bar, so it clearly is totally cool not to have one. But then some mammals re-evolved them (that is, if you believe in all of this evolution stuff). Important mammals, like primates. Horses. Cows. And they’re not all created equal – horses have postorbitals made up of the frontal and the squamous bone (which is in primates is absorbed into the temporal bone), while most others (including ours) consist of the frontal and zygomatic (our cheek bone, formerly called the jugal). So they clearly re-evolved a number of separate times. But why?
It’s getting pretty late, so I’m going to have to leave the awesome answers for next time. I’m sure that you all will be waiting with bated breath.