Evolution

EVOLUTION OF LIZARDS AND SNAKES
Lizards arose from early reptiles about 200 million years ago and are found on all the continents except Antarctica. There are about 3,750 species of lizards, making them one of the largest and most successful groups of reptiles. None of Australia’s 500 species are venomous.

LIZARDS AND SNAKES
Both snakes and lizards are classified in the same order, Squamata, but lizards are separated into their own suborder, Sauria.  Lizards generally can be distinguished from snakes by the presence of two pairs of legs suited for rapid locomotion, external ear openings flush with the skin and moveable eyelids. But these convenient external diagnostic features, while absent in snakes, are also absent in some lizards. Lizards can be precisely separated from snakes, however, on the basis of certain internal characteristics. All lizards have at least a vestige of a pectoral girdle (skeletal supports for the front limbs) and sternum (breastbone). The lizard's ribs are never forked, as are one or two pairs in a snake. The lizard's brain is not totally enclosed in a bony case but has a small region at the front covered only by a membranous septum. The lizard's kidneys are positioned symmetrically and to the rear.

What makes a Lizard a LIZARD sub-order Lacertilia, and not a SNAKE sub-order Ophidia?

1.  Most lizards have short bodies and four limbs, snakes never have front limbs.
2.  The two halves of the lower jaw are united, unlike a snakes which are joined with ligament.
3.  Lizards have a flat fleshy tongue, except for goannas which have a snake like forked tongue.
4.  Most lizards have an external ear opening, snakes don’t have any sign of an external ear.
5.  Many lizards have movable eyelids, snakes do not.

Lizards belong to the scientific sub-order Lacertilia, a division of the reptile class Reptilia.
The sub-order Lacertilia is divided up into different families.
In Australia, lizards belong to the following five families: -

Gekkonidae: Geckos are usually small velvety lizards up to 10 cm long which live under bark, logs, rocks or in burrows or litter. Their main characteristics are bulging eyes with oval shaped vertical pupils for night vision, teeth on the side of the bone that supports them, skin without overlapping scales and tails which they can shed.

Scincidae: Skinks are the largest group of lizards in Australia. Their skin is usually smooth and shiny. Most have 5 toes on each of their four legs, as do other lizards. Most Skinks are ground dwellers, have immovable eyelids and can shed their tail.

Varanidae: Goannas are powerfully built with strong limbs, a pointed wedge-shaped head and a long muscular tail. The shin of goannas is tough and leathery and the scales do not overlap. Their teeth are long and backward-curving in order to grab their prey securely.

Agamidae: Dragons have dull rough scales which lie side by side and well suited to desert conditions. As with goannas, they are unable to regenerate the tail once it is broken. The eyes are small with prominent eyelids, while the tongue is broad and short.

Pygopdidae: Legless Lizards have rudimentary limbs if any and a fleshy tongue. They are long and thin and burrow under soft earth and debris.  Being of plain colours and nocturnal Legless Lizards are rarely seen.

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