Evolution
Monitor lizards differ greatly from other lizards in several ways, possessing a relatively high metabolic rate for reptiles, and several sensory adaptations that benefit the hunting of live prey. Recent research indicates the varanid lizards may have some venom. This discovery of venom in monitor lizards, as well as in agamid lizards, led to the Toxicofera hypothesis: that all venomous lizards and snakes share a common venomous ancestor.
During the late Cretaceous era, monitor lizards or their close relatives are believed to have evolved into amphibious and then fully marine forms, the mosasaurs, which reached lengths of up to 17 m.
Snakes were believed to be more closely related to monitor lizards than any other type of extant reptile, however, it has been more recently proposed that snakes are the sister group of the clade of iguanians and anguimorphs.
During the Pleistocene epoch, giant monitor lizards lived in Southeast Asia and Australasia, the best known fossil being the megalania (Varanus priscus unless it falls in its own genus, in which case it is Megalania prisca). This species is an iconic member of the Pleistocene megafauna of Australia.
Some monitor lizards, including the Komodo dragon, are capable of parthenogenesis.
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