I’m not sure what this actually does for some of the evolutionary theories, but apparently these lizards were introduced to this environment just a few years ago (in the 70s), and they’ve already adapted numerous biological functions (bigger heads, intestinal doo-dads, etc).
Researchers found that the lizards developed cecal valves—muscles between the large and small intestine—that slowed down food digestion in fermenting chambers, which allowed their bodies to process the vegetation’s cellulose into volatile fatty acids.
“They evolved an expanded gut to allow them to process these leaves,” Irschick said, adding it was something that had not been documented before. “This was a brand-new structure.”
Along with the ability to digest plants came the ability to bite harder, powered by a head that had grown longer and wider.
This would seem to explain an evolutionary “explosion” under new/suddenly altered environmental conditions. I believe this matches the fossil records (long periods of relative stasis punctuated by changes on otherwise short timelines). To me the most interesting adaptation is the intestinal one. A bigger head seems to be an “easy” adaptation (big guys thrive, big heads become more common), but developing internal valves that allowed for fermenting and digesting plant matter is, I believe, a much more complex adaptation.