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Did synapsid skulls evolve from diapsid skulls or vice versa?

Aditionally, do birds have diapsid skulls? The research I have done has yielded some mixed results.

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It is believed that originally the holes evolved to give the jaw muscles a place to expand into as they contract. They evolved as the the otherwise larger jaw muscles are trapped between the bones and cannot contract properly. I say "believe" because the original function is still debated, and it is not something that can be easily tested.

Diapsids likely evolved from early synapsids, however the evidence is tentative at best and it is just as likely that they both evolved independently from anapsids.

Birds are diapsids. However many modern birds have lost the bony partition between the two holes, separating them by cartilage and soft tissue only. Just as some mammals have lost the bony partition between the jaw muscles and the orbit. Additionally in many mammals and birds the expansion of the braincase makes it harder to identify the holes for the laymen as they are pushed down and to the side.

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  • $\begingroup$ thanks for the helpful answer! One more thing for clarification...so birds, dinosaurs, and most reptiles (except for turtles) are diapsids, but are amphibians, and fish diapsids as well? $\endgroup$ – Vanessa Aug 16 '17 at 18:10
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    $\begingroup$ turtle are sort a diapsids, they closed the holes after they had them you can see this is the early fossil turtles, they actually end up closer to crocodiles and dinosaurs than other lizards. euryapsids (extinct) did something similar . the ___apsids are all amniotes and are defined as such, so amphibians and fish which do not have the holes and are not diapsids but are not anapsids either becasue they are not amniotes. $\endgroup$ – John Aug 17 '17 at 4:17

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