Varanidae is an ancient group of anguinomorph lizards, comprising about 50-60 species into the genus Varanus. Varanids are found in Africa, central and southern mainland Asia and Malaysian and Indonesian islands, Papua New Guinea, and Australia (where about half the species are found).

Monitors are robust, diurnal lizards with long, non-autotomous tails and elongate necks. Varanids have nine cervical vertebrae, a condition they share with Lanthanotus, while all other lizards have eight or fewer. They also have long forked tongues that are used for chemoreception, reminiscent of but not identical to snakes' tongues. The hemipenes of varanids are unusually ornate, and have paired apical horns. Monitors range in length and weight from the diminutive short-tailed monitor's 20 cm and 20 g to the Komodo dragon's 3 m and 54 kg. The feeding biology of monitors is also diverse, ranging from Gray's monitor, which forages and eats mollusks and fruit, to the Komodo dragon, which is a sit-and-wait predator of large mammals such as deer and buffalo. Large monitors may also eat carrion, and both the Komodo dragon and the water monitor are rumored to be adept at finding, exhuming, and eating human corpses. Monitors occupy a wide range of habitats, and different species of terrestrial monitors can be surface dwelling, burrowing, arboreal, or saxicolous. Monitors can also be primarily aquatic or marine, though like all reptiles they must lay their eggs on land.

Currently ten subgenera of Varanus are recognized, of which two (Varanus and Odatria) comprise the majority of species. Subgenus Varanus is a polyphyletic group that includes the some large-bodied Australian and Indonesian taxa, while subgenus Odatria includes the small-bodied Australian endemics. The remaining subgenera are monotypic or contain only a few species.

Varanidae is the largest group (in both species count and body size) in Varanoidea, an assemblage comprising Varanidae, Helodermatidae, and Lanthanotidae (often considered to be a subfamily (Lanthanotinae) of Varanidae). Varanoids in turn belong to the larger group Anguimorpha, which includes such forms as anguids (alligator lizards) and xenosaurids (knob-scaled lizards). Snakes are also generally thought to be related to varanids. Extinct relatives of varanids include the mosasaurs.

Monitor lizards are represented in ancient Indian and Australian art, and their presence is pervasive in the folklore and superstitions of the regions where they occur. Some species of Varanus are of economic importance to humans, as the skins are used for leather and various body parts used for folk medicines. Monitors are eaten by humans in Africa, Asia, and Australia. Some species appear to be in decline, mostly from habitat destruction rather than direct exploitation, but monitors that scavenge trash and carrion thrive in the presence of human communities. All the endemic Australian species are listed by CITES.

Varanidae is well represented in the fossil record. Pre-Miocene fossil varanids are found mostly in Asia and North America, including Telmasaurus from Cretaceous deposits of Mongolia and North America. Varanids reached had Europe, Africa, and Australia by the middle Miocene, and the fossil

Varanus marathonensis may have persisted in Europe up to the Pliocene. The enormous fossil Megalania prisca from Quaternary deposits in Australia reached a length and weight of 7 m, 600 kg, and may have become extinct only 20,000 years BP. With 2 cm curved serrated teeth, it is thought to have used a slashing bite similar to that of the Komodo dragon. Megalania is believed to have fed primarily on the giant extinct protodont marsupials; humans, almost certainly contemporary in time and space with Megalania, may have formed a part of its diet as well.

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Jennifer C. Ast (author).