Snakes and Lizards

Lizards and snakes both belong within the order of reptiles known as the Squamata, which loosely translates as ‘the scaly ones’. Their inclusion together in one order reflects their close evolutionary relationship; snakes having evolved directly from lizards. The probable ancestors of snakes were lizards that took to life underground, where elongation of the body conferred an advantage while limbs became an impediment and thus were gradually lost. Across the world, many contemporary species of lizards that spend most of their lives underground exhibit various stages of limb degeneration and body elongation. This includes our native slow-worm in which both characteristics are at an advanced stage. Among the more primitive species of snakes that exist today, some still retain vestiges of their pelvic bones and even small spurs where the legs would once have been, revealing their lizard ancestry.

The earliest true lizards yet found are the specialised flying lizards, Kuehneosaurus, extracted from Emborough quarry in the Mendips, in deposits approximately 180 million years old. Before this there were many earlier lizard-like reptiles, but none of these were true lizards. Since the appearance of Kuehneosaurus lizards have proved to be extremely successful, diversifying and spreading across the globe, so that today they are the largest extant group of reptiles with over 6000 recognised species.

Snakes appeared considerably later than lizards. The first evidence of snakes comes from the early Cretaceous about 130 million years ago, but they do not appear to have become widespread across the world until the late Cretaceous about 65 million years ago, around the time the dinosaurs and their kin were disappearing. Now they are a major group amongst the reptiles, consisting of some 3500 species.

Anatomically the squamates have some curious adaptations, not least the male sexual organs. Males of both lizards and snakes have a pair of hemipenes; essentially a penis which has become divided longitudinally to produce two independently functional organs. The hemipenes lie within the cloaca, each inverted rather like the finger of a glove turned inside out. When needed, either of the hemipenes can engorge with blood and be everted through the vent, rather like the finger of the glove returning to its normal position. Each hemipene is covered with small barbs which, once inserted into the cloaca of the female, hold it in place until mating is complete and the hemipene shrinks.

While lizards have conspicuous ear drums and sharp eyesight, snakes lack ears and are completely deaf to airborne sounds. Additionally their eyesight is not terribly good, perceiving movement rather than focussing sharp images. However, they compensate for this by being remarkably sensitive to ground vibrations, to alert them to approaching danger, and by being in possession of a forked tongue that acts as a sensory organ. Set into the roof of the mouth are two pits which together make up the Jacobson’s organ. The forks of the tongue fit neatly, one into each of these pits, which are rich in nerve endings. When the tongue of a snake is protruded, it collects chemical particles from the air which are then transferred to the receptors within the pits for analysis; an olfactory process similar to smell or taste.

There are three native snakes in Britain, the adder, grass snake and smooth snake. Of these, only the adder and the grass snake are found in Somerset, the smooth snake being confined to warm heathland sites in the Poole Basin, the New Forest and in Surrey, although there is also now an introduced population in Devon.

Likewise, there are three native lizards in Britain although only two of these, the slow worm and the viviparous lizard are present in Somerset. The third, the sand lizard, shares its habitat preference with the smooth snake and is also confined to the south coast and Surrey as well as occupying a few coastal heathland sites in North Wales and on Merseyside. Despite the occasional seductive, and almost certainly erroneous, record of either a smooth snake or a sand lizard in Somerset, neither have ever been demonstrated to exist in the county and their presence is most improbable.