Here, from Lewis I. Held, Jr., author of Quirks of Human Anatomy,:
One trait that seems to have gotten stuck at a single point in its Morphospace is reptile teeth, which remained conical (like those of crocodiles) for more than 100 million years [1357], despite the fact that dental heterogeneity (molars, canines, etc.) would have offered tremendous advantages for chewing different types of food [1587]. Did the right mutations simply never occur [1309,1484], or was it just too hard to concoct the right binding site combinations for the relevant trans-acting transcription factors [1687,2325]? No one knows.
Many other cases of stasis are just as intriguing [959,2824,2861], especially with regard to why certain crannies in “niche space” have stubbornly resisted invasion for eons immemorial [227,482]. Ecologists have toyed with these irritating riddles via game theory [2692], but the ultimate answers are hiding in the genome [793,1303].
Or not? What about the epigenome?