Lizards - How They Evolved and Lost Their Limbs

Points covered:

• Why are the Lerista lizards the world's best example of limb reduction?
• How does limb loss affect lizards?
• What are the evolutionary advantages of limb loss in lizards?
• What is the evolutionary origin of snakes?

During the course of evolution, lizards have undergone limb reduction and loss many times. The lizards that best show the various stages of limb reduction are the Australian skinks in the genus Lerista, commonly known as sand-swimmers. They are the world's best example of limb reduction because there are many closely related species and, depending on the species, they range from having large limbs with five fingers and five toes to having no limbs at all. By placing animals with different degrees of limb reduction side by side, we can imagine how evolution might have worked. Studies of sand-swimmers and other limb-reduced lizards show three general principles:

• The front limbs are usually reduced and lost before the rear limbs. This is probably because the rear limbs are more important for locomotion than the front limbs.
• As species lose their limbs, they generally become longer and thinner. This is because the body is more important for locomotion than the limbs and a long, thin body provides more "wriggle" than a short stumpy one.
• Most limb reduction seems to be associated with life in tight places, such as crevices, soil and sand. These places are generally unavailable to lizards with large limbs and yet offer the advantages of shelter and new food resources to species with reduced limbs.

We sometimes think that evolution leads to ever-greater degrees of complexity. But limb reduction shows how animal structure can become less complex even though it becomes more specialised. The more limb-reduced a species is, the more specialised it is, but at the same time, it is less complex than its more fully-limbed ancestors. Snakes are another group of lizards. They probably evolved from an ancestor similar to a goanna and, like some other lizards, are almost limbless. However, not all snakes are limbless. Some species, like our pythons, retain the trace of a rear limb, noticeable as a small claw near the anal opening which they use in courtship.

Allen E Greer
Herpetology
Australian Museum