Squamata: the scaled reptiles — lizards, snakes and worm lizards

Key physical characteristics

Several traits characterize squamates and distinguish them from other reptiles. Prominent features include:

• Overlapping horny scales that cover the skin and are periodically shed; these scales give the group its common name and help reduce water loss and abrasion (scales).
• Movable quadrate bones, which enable the upper jaw to move relative to the skull. This mobility is especially pronounced in snakes and allows them to swallow large prey.
• Variation in limb morphology, from fully developed limbs in many lizards to reduced or absent limbs in snakes and several lizard lineages.
• Feeding and defense specializations, including constriction, venom in many snake lineages and a few lizards, and a variety of teeth shapes linked to diet.

Size range, examples and notable species

Squamates show extreme size variation. Some of the smallest known species are tiny geckos such as the 16 mm Dwarf gecko (Sphaerodactylus ariasae), while the largest are massive snakes like the Green Anaconda (Eunectes murinus), which can exceed several meters in length. Diets range from insectivory and herbivory in some lizards to the carnivorous and piscivorous habits of many snakes.

Taxonomy and evolutionary history

Modern taxonomy typically recognizes three major squamate assemblages: lizards (a paraphyletic grouping in older schemes), snakes (Serpentes) and amphisbaenians (worm lizards). Squamates first appear in the fossil record in the Mesozoic era, with diversification continuing through the Cretaceous and after the end-Cretaceous extinction. Their evolutionary success is linked to ecological versatility and a capacity for novel jaw and limb morphologies.

Reproduction and life history

Reproductive modes in squamates are unusually diverse for reptiles. Many species are oviparous (egg-laying), but numerous lineages exhibit viviparity or ovoviviparity, where eggs develop internally and give rise to live young. This diversity of reproductive strategies has allowed squamates to colonize climates where external egg incubation would be difficult (reproductive modes).

Ecological roles, human interactions and distinctions

Squamates occupy key ecological niches as predators of invertebrates and vertebrates and as prey for larger animals. They are important in pest control, ecosystem functioning and as indicators of environmental change. Humans interact with squamates through agriculture, the pet trade, culture and medicine (for example, research on venom). Notable taxonomic distinctions include the exclusion of the tuataras (Tuataras)—unique reptiles native to New Zealand—which are a sister group to squamates rather than members. More distantly related are the crocodilians (Crocodilia), which share a more ancient common ancestor with squamates.