Squamates (Squamata) are the most diverse of all the reptile groups, with approximately 7400 living species. Squamates include lizards, snakes, and worm-lizards.
Two characteristics that unite the squamates. The first is that they shed their skin periodically. Some squamates, such as snakes, shed their skin in one piece. Other squamates, such as many lizards, shed their skin in patches. In contrast, non-squamate reptiles regenerate their scales by other means—for example crocodiles shed a single scale at a time while turtles do not shed the scales that cover their carapace and instead add new layers from beneath.
The second characteristic shared by squamates is their uniquely jointed skulls and jaws, which are both strong and flexible. The extraordinary jaw mobility of squamates enables them to open their mouths very wide and in doing so, consume large prey. Additionally, the strength of their skull and jaws provides squamates with a powerful bite grip.
Squamates first appeared in the fossil record during the mid Jurassic and probably existed before that time. The fossil record for squamates is rather sparse. Modern squamates arose about 160 million years ago, during the late Jurassic. The earliest lizard fossils are between 185 and 165 million years old.
The closest living relatives of the squamates are the tuatara, followed by the crocodiles and birds. Of all living reptiles, turtles are the most distant relatives of the squamates. Like crocodilians, squamates are diapsids, a group of reptiles that possess two holes (or temporal fenestra) on each side of their skull.
The key characteristics of squamates include:
- most diverse group of reptiles
- exceptional skull mobility
Squamates are classified within the following taxonomic hierarchy:
Squamates are divided into the following taxonomic groups:
- Lizards (Lacertilia) - There are more than 4,500 species of lizards alive today, making them the most diverse group of all squamates. Members of this group include iguanas, chameleons, geckos, night lizards, blind lizards, skinks, anguids, beaded lizards and many others.
- Snakes (Serpentes) - There are about 2,900 species of snakes alive today. Members of this group include boas, colubrids, pythons, vipers, blind snakes, mole vipers and sunbeam snakes. Snakes have no limbs but their legless nature doesn't stop them from being among the world's most formidable reptilian predators.
- Worm-lizards (Amphisbaenia) - There are about 130 species of worm-lizards alive today. Members of this group are burrowing reptiles that spend most of their life underground. Worm lizards have sturdy skulls that are well suited for digging tunnels.