Evolution and systematics

With well over 100 genera and more than 1,400 species, skinks are by far the largest family of lizards, an exceedingly diverse group. Their diversity is evident in all aspects of their biology. Terrestrial, arboreal, fossorial, and even semiaquatic skinks exist. Skinks have radiated to fill niches in all types of environments, including arid deserts, savannas, lowland rainforests, temperate forests, and cool montane habitats. Some skinks are diminutive, but others are large. These lizards vary in morphologic characteristics from short and robust with strong, well-developed limbs to elongated and fragile with tiny or no vestigial limbs. In some arid regions (e.g., Australia), they dominate the lizard fauna; in others, such as the Sonoran and Great Basin Deserts, they are essentially absent. Skinks have dispersed widely, many having rafted across oceans to colonize other continents and even remote islands in the Pacific. Four subfamilies are recognized; one (Scincinae) is probably not monophyletic but rather a paraphyletic group:

Acontinae

This is a subfamily of moderately large, limbless, fossorial skinks found in South Africa. There are three genera and 17 species. They are derived from unknown scincine ancestors.

Feylininae

This subfamily comprises moderate to large limbless skinks that inhabit tropical western and central Africa. There is one genus with six species. They are derived from unknown scincine ancestors.

Lygosominae

These skinks are very diverse, ranging from small to large, advanced skinks. They are found worldwide, with more than 82 genera and about 900 species.

Scincinae

This diverse subfamily occurs in North America, Africa, southwest Asia, southern Asia, eastern Asia, and the Philippines. There are more than 30 genera, with more than 300 species.

Scincinae is not a natural group but is based on shared ancestral characters and probably does not contain all descendents of a common ancestor (i.e., it is paraphyletic). Scincines are primitive skinks with smooth cylindrical bodies and small legs. The scincine genus Eumeces displays several important ancestral character states, which could resemble the common ancestral state for all skinks. Other candidates for basal scincids include certain African and Asian scincines (Brachymeles and Chalcides).

Both acontines and feylinines appear to be derived from scincine ancestors. Acontines (Acontias, Acontophiops, and Ty-phlosaurus), found in Africa, are specialized legless burrowing skinks found only in leaf litter and loose, sandy soil or underneath logs. The small subfamily Feylininae contains a single genus, Feylinia, consisting of six species, also burrowers, from central tropical Africa.

By far the largest subfamily is the Lygosominae, with more than 80 genera and more than 900 species. Lygosomines are derived, highly advanced skinks, with five distinct and presumably monophyletic lineages: the Egernia group, the Lygo-soma group, the Mabuya group, the Sphenomorphus group, and the Eugongylus group. Most of these groups occur in Australia (a major center for skink diversity). Some members of each group also occur outside Australia, especially in New Guinea and New Caledonia. Exactly where Lygosoma and Mabuya (a paraphyletic genus) and several other Old World lygosomines

A female broad-headed skink (Eumeces laticeps) guards her eggs. (Photo by Larry L. Miller/Photo Researchers, Inc. Reproduced by permission.)

should be placed within Lygosominae remains uncertain. New skink genera are still being described.

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