The Squamates are definitely a monophyletic group; they are a sister group to the Tuatara. Judged by their fossil record, the Squamates were present in the Mesozoic, but occupied a minor place in the land ecology. Three of the six lines are recorded first in the Upper Jurassic, the others in the Cretaceous. The Mosasaurs of the Upper Cretaceous were by far the most successful of all the lizards, becoming the top predator in their ecosystem. 