Research

Limb reduction in Australian lizards

Six Lerista species showing some of the variation in the number and configuration of bones in the front foot.

Six Lerista species showing some of the variation in the number and configuration of bones in the rear foot.

Cleared and stained specimens of Lerista showing the variation in limb formation.

Lerista species showing some of the variation in limb development between species in the same genus.

The evolution of limbs was one of the major features accompanying the movement of vertebrates from the water to the land. We humans are only one species among the many vertebrates that have found front and rear limbs useful in making our way on the land.

There are, however, three groups of terrestrial vertebrates that have subsequently reduced and even lost their limbs in evolution: all caecilians and certain salamanders among the amphibians, and certain lizards among reptiles. Australia lacks the first two groups but has the latter group in abundance. In fact the best case of limb reduction among vertebrates worldwide occurs in one group of Australian lizards, the skink genus Lerista. Australian Museum scientists are studying this group to see what they tell us about what happens when lizards lose their limbs.

The genus Lerista consists of about 80 described species of small to medium sized lizards (maximum length from the tip of the snout to the base of the tail ranges from about 32 mm to 103 mm). The genus occurs throughout Australia except for the wetter and cooler south-eastern parts. All species are associated with loose, sandy substrates, one of the most common substrate types in Australia.

Three features of the genus stake its claim as the best case of limb reduction among vertebrates. First, the limbs range across the entire spectrum of possible morphologies, from well developed with five digits on each limb to totally absent. Second, there are many intermediate cases among the many species. And third, the species are all closely related which means that differences in limb morphology are not confounded by different evolutionary histories. That is, we are comparing apples with apples.

Study of the genus to date shows that as the limbs are reduced both in length and in the number of bones, the trunk increases in both length and the number of bones (Fig. 1). This general change is due to the fact that as the limbs are reduced, walking and running become less important and slithering with the body becomes more important. In the case of Lerista, this means a shift from moving on the surface to "swimming" beneath the surface. The big change comes at a stage of about three digits on each limb. Above this point, surface locomotion with limbs is paramount and below this point, subsurface locomotion with the body becomes increasingly important.

Limb reduction has occurred many times in lizards. The Australian family of "legless" lizards called the Pygopodidae is very closely related to the Australian geckos, all of which have well developed limbs. It is almost as if pygopodids were just "stretched" version of geckos. But in this case there are no intermediates, raising the interesting questions of if intermediates did exist what happened to them and if they did not exist, is this a case of evolution acting in leaps instead of gradually.

One of the world's best know groups of reptiles are also "legless" lizards. This well-known group is relatively closely related to goannas and can almost be thought of as "stretched" goannas. The name of this group? Snakes.

Allen Greer
Herpetology
Australian Museum

Copyright © Australian Museum, 2002