Perhaps it went extinct because the jaw DID NOT give it an advantage.
If mutations occur and some result in positive adaptations for a species, then it is logical that other mutations cause negatives for a species.
I’m of the school of thinking that believes giraffes did not get taller at 1mm every 1000 years in order to better survive. They got taller and found a way to deal with it, that did not result in the end of the species.
The problem with evolutionary biologists is that they first reject a creator and then to decide that all evolutionary changes must have some “devine” purpose. They can’t have it both ways.
Evolution by mutation isn’t perfection by design, its happy accidents and getting by with what you got.
Of course a trait can be beneficial in some environments, and harmful in others.
One may also recall the fat years and the lean years from Pharoh’s dream in Genesis: During the fat years, all the mutations can survive, and life prospers, a trait that out breeds the competition provides an advantage. A trait that outbreeds the competition may have an opportunity to get more mutations: (1000 animals will see more mutations than 5 animals).
During the lean years, many die. Animals with packages of mutations or that can not adapt their behaviors to live do not survive, or perhaps even die out without issue.
Of course that requires believing in occasional global warming without insisting that humanity causes it. Since weather is chaotic and can bifurcate at any time, that is reasonable from this side of the keyboard.
Or G-d might have buried fossils in the ground to fool us, as a cunning ploy to guide us to oil deposits.