(genus: Homo) are, of course mammals, and basal mammals lose the postorbital bar found in cynodonts like Chiniquodon. But even that postorbital bar does not include two bones found in more basal therapsids, the postfrontal and postorbital. In Chiniquodon the postorbital bar is created by a process of the frontal meeting a process of the jugal.
So I was surprised to find
what appear to be vague apparitions that look like those lost bones in the frontal of the human skull. There are 17 frames in this animation (Fig. 1). The last one holds for five seconds and reveals where I think I see vague outlines of the the prefrontal, postfrontal and postorbital bones all fused to the frontals, which are themselves fused medially.
The last part of the postorbitals
to ‘disappear’ are the posterior processes, which seem to laminate to the cranial bones in several therapsids, including dinocephalians. But there is also a vague portion that appears on the duckbill platypus, Ornithorhynchus.
were the last to fuse and so appear to be the easiest to see now. Like gills and a tail, humans retain the genes for these, but they get resorbed or fused during embryonic ontogeny.
There is a difference between losing a bone
and fusing a bone. Phylogenetically losing a bone is usually marked by a reduction to disappearance. Fusion is, well, fusion. And that can be more readily reversed.
The pterygoid and lateral sphenoid
are both bones typically seen in palatal view, but in mammals and humans they rise, tent-like, to appear in the nasal and orbital cavities.
In mammals the bones sometimes get new names
but they’re still the same bones. The jugal becomes the zygomatic arch, for instance. The bones that make up the occiput fuse to form the occipital.
We’ll look at the palate and occiput soon.
And a fetus.