When a Lateral Temporal Fenestra Becomes an Upper Temporal Fenestra

It’s interesting.
Take a look at a cynodont or dicynodont skull (Fig. 1) and you get the impression that those two fenestra, nearly invisible in lateral view and touching each other in dorsal view, must be upper temporal fenestrae.

Basal Therapsida.

Figure 1. Click to enlarge. Basal Therapsida. Sorry for using the same figure twice within a week. Note the convergent enlargement of the lateral temporal fenestra in dicynodonts and dinocephalians, with the subsequent reduction in Tapinocephalia due to pachyostosis (bone thickening).

Placed into a phylogenetic framework…
It’s been known for decades that the lateral temporal fenestra in basal synapsids expanded dorsally and laterally, at first squeezing the cranium between its advancing margins. Then, as the brain expanded in mammals, the cranium rose above the lateral temporal fenestrae. There is still a crest marking the old midline boundary in many mammals. In humans the crest is absent, replaced by an even larger cranium.

Essentially the skull in mammals is inside out
In most reptiles, the jaw muscles and the braincase are inside the skull. In mammals the braincase and cheekbones are all that remains of the old reptile skull. The braincase and jaw muscles are on the outside.

Sorry, not alot of jaw-dropping news lately. Must be the summer collection season…

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