Lizards & snakes: a Museum guide
Lizards and snakes are fairly familiar animals, though only a few species of each live in Great Britain. This exhibit shows some of the diversity in each group, using the breadth of the Museum’s collection for illustration. During the last century, expeditions to far-flung parts of the world collected all kinds of animals that form the basis of our collections today.
There are about 16 families of lizard, (depending on which classification is used) and our collections represent 13 of them. To British people, the lacertids are probably the most familiar, including the sand, wall and common lizards. The anguids include legless and superficially snake-like forms such as the slow-worm. Most of the native reptile species are threatened in the UK by habitat destruction and disturbance.
Chameleons are also relatively well-known, and their use of grasping feet with two toes pointing forward and two back, swivel-turret eyes, and projectile tongues for catching prey are illustrated in these spirit-preserved and freeze-dried specimens. The geckos are notable for their ability to cling to vertical surfaces including glass, due to the intricate construction of the pads on their toes. The helodermatids include only two species, both from Central America, but they are the only lizards that produce poison. Many lizard species show brightly coloured patches for display, others can be highly camouflaged. The agamid family members show both kinds of adaptation.
The varanid family includes some of the largest known lizards such as the Nile monitor whose skeleton you can see here, and the Komodo dragon, whose skeleton is mounted in a separate case. Closely related to the caranids are the mosasaurs, giant extinct lizards from the Mesozoic era (about 25 – 65 million years ago). An example of a mososaur skull is seen in the lower case. Notice how their lower jaws have a joint half way along to increase the size of the gape and power of the mouth. Some people think that snakes evolved from a group of lizards closely related to mosasaurs.
It is easier to describe what features snakes lack than what they have. They lack legs – except for rudiments remaining in most primitive forms such as boas and pythons – as well as ears and eye-lids. Most forms only have one lung. The colubrid, elapid and viperid families make up the most specialised snakes, and include by far the majority of species and all the poisonous ones. In fact, most snakes are neither poisonous nor dangerous to humans.
Boas and pythons are the most primitive of snake families, and as well as having traces of hind limbs and pelvic/shoulder girdles, they have the least specialised skull structure. Even so, like all snake skulls, those of boas and pythons are greatly modified and consist essentially of rods and struts for working the jaws and teeth, and for allowing expansion of the mouth when they swallow large prey. In the case below is the skeleton of a python showing the large number of vertebrae and ribs, all of which look much alike. The skull of a python has been dissected to show the separate parts and how they fit together.
In Great Britain, there are only two species of snake – the grass snake, belonging to the anguid family, and the adder, one of the viperids. In other parts of the world, the viperids produce much more venomous snakes, such as rattlesnakes, bushmasters and pit-vipers. The elapids include some of the most venomous of all, such as the cobra, taipan, sea snakes and Dendroaspis jamesoni, the famous black mamba.
The amphisbaenids or ‘worm-lizards’ are a group of elongate, snake-like creatures, but it is not clear whether they are closely related to snakes or to lizards or are a third, separate, group. Some, such as Bipes, retain rudimentary front limbs. They also bear a superficial resemblance to the limbless amphibians called caecilians, but the resemblance results from similarities of life-style, not of relationship. Worm lizards are found in South America, sub-Saharan Africa, and in a few Mediterranean and Middle Eastern countries.
The snakes and lizards display shows specimens preserved in a variety of ways. Most of them are ‘spirit-preserved’. When originally prepared, they would have been stored in formaldehyde or ‘spirit’ – a special formulation of alcohol. Formaldehyde is no longer used as it is considered dangerous to health. Alcohol is still used, but tends to bleach out colour from specimens. Most of those shown here have been transferred to propylene phenoxytol, which arrests the loss of colour.
Skeletal preparations are made by removing the flesh either by dissection or, in some case, by use of insects. They are then de-fatted, sometimes by use of biological detergents, and may be mounted using wires to tie the bones together. Try to imagine the skill and patience it needed to mount up the python skeleton, and how difficult it would have been not to mix up the bones!
Whole dried mounts have been made by two main techniques. Wax impregnation was used up to about 20 years ago, but freeze-drying is more common today. The specimen is mounted in position, deep-frozen and then put in a vacuum-chamber for several weeks. The ice is converted directly to water-vapour and is removed gradually, preserving tissues and colour in an almost natural state.