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Their skins have overlapping horny scales. They also have movable quadrate bones, which make it possible to move the upper jaw relative to the braincase. This is particularly visible in snakes, which are able to open their mouths very wide to accommodate comparatively large prey.
Squamate fossils first appear in the early Jurassic, but a mitochondrial phylogeny suggests that they evolved in the late Permian. The evolutionary relationships within the squamates are not yet completely worked out, with the relationship of snakes to other groups being most problematic.
From morphological data, Iguanid lizards have been thought to have diverged from other squamates very early, but recent molecular phylogenies, both from mitochondrial and nuclear DNA, do not support this early divergence. Because snakes have a faster molecular clock than other squamates, and there are few early snake and snake ancestor fossils, it is difficult to resolve the relationship between snakes and other squamate groups.
Evolution of venom
Venom in the families Caenophidia, Anguimorpha, and Iguania evolved once, then the three families diverged. All three lineages share nine common toxins. The fossil record shows that the divergence between anguimorphs, iguanians, and advanced snakes dates back roughly 200 MYA to the Upper Triassic/Lower Jurassic.
Different toxins have been recruited from different proteins and are diverse as the functions themselves.
Natural selection has driven the diversification of the toxins to counter the defences of the prey. Venom genes form large multigene families, and evolve by protein evolution. This leads to a diversification of toxins that let sit-and-wait predators attack a wide range of prey.
Classically, the order is divided into three suborders:
- Suborder Iguania (the iguanas and chameleons)
- Suborder Scleroglossa
- Suborder Serpentes (the snakes)
- Suborder Amphisbaenia
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