Evolution and systematics

The Borneon earless monitor Lanthanotus is the sister group to all Varanus. Recent phylogenetic studies suggest that African monitors are the sister group to all other monitors. There is a large Asian clade and two clades of Australian monitor lizards, one consisting of pygmy monitors and the other of larger species. Monitors may well be more closely related to snakes than to most other lizards.

The varanid lizard body plan has been exceedingly successful as it has been around since the late Cretaceous. Ko-modo dragons are dwarfed by a closely related, extinct gigantic varanid, Megalania prisca, originally placed in the genus Varanus (to which it must be returned). This Australian Pleistocene species is estimated to have reached more than 20 ft (6 m) in total length and to have weighed over 1,320 lb (600 kg). Megalania fossils have been dated at 19,000-26,000 years before present.

Large monitor lizards evolved large body size in response to availability of very large prey (pygmy elephants in Indonesia for V. komodoensis and rhinoceros-sized diprotodont marsupials in Australia for Megalania). At the end of the Pleistocene, much of the megafauna went extinct. Without large prey to support them, large predators also died out. Fortunately, Varanus komodoensis did not die out when pygmy elephants went extinct at the end of the Pleistocene.

There two living genera: Varanus (about 60 species) and Lanthanotus (1 species). Two subfamilies are recognized: Varanines and Lanthanotines.

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