Monitor lizards or goannas (family Varanidae) are common in tropical and subtropical areas of the globe today, consisting of almost three dozen species spread across Africa, Asia, the East Indies, and Australia. The largest is the ferocious, 3-meter Komodo dragon (Varanus komodoensis) which will eat livestock and humans; most varanids are smaller (though still fairly large as lizards go -- a meter or two long), and feed on small animals, molluscs, insects, and eggs. The first fossils of the genus Varanus are found in Africa in the Early Miocene, while the family Varanidae has a fossil history extending back into the Cretaceous of Mongolia.
The nearest living relatives of modern goannas are the earless monitor Lanthanotus, an obscure southeast Asian lizard, and the gila monster and beaded lizard of the genus Heloderma. The true monitor lizards form a cohesive group that is easily identifiable by their streamlined shape, elongated neck, semi-erect posture, and forked tongue. They inhabit a variety of econiches ranging from savannah to woodland to riparian habitats. Those monitors which spend a lot of time in the water have tall neural spines on their tail vertebrae. This makes the tail tall and flat so that it is useful as an organ of propulsion. The earliest known member of the genus, Varanus rusingensis from the Miocene of Kenya, also had such a tail and was probably semiaquatic like the Nile monitor (V. niloticus) of today. Some of the Asian monitors, e.g. the water monitor V. salvator, also spend a lot of time in rivers. Freshwater molluscs form a significant portion of the diet of the Nile monitor and Gray's monitor (V. olivaceus), and probably also did for the 18-million-year-old Rusinga monitor (V. rusingensis), as evidenced by the appearance in adulthood of a durophagous dentition (blunted, crushing-type teeth). Features of the skull bones show the Rusinga monitor to have been less specialized for such a diet than the modern molluscivorous lizards, however.
Most other modern monitor lizards are active predators with sharp, trenchant teeth. They pursue such agile prey as birds and squirrels, which they find by tongue-flicking in the manner of snakes. Most fossil monitors appear to have been of this type, as shown by their similar skeletal features and teeth. Aside from the Rusinga monitor, no fossil species of the genus Varanusis known with much completeness: there are several European species from the Miocene through the Pleistocene which are described based only on isolated vertebrae. The best-known fossil relatives of Varanus are two species of the genus Saniwa, which are morphologically quite similar to the extant monitors. Saniwa ensidens comes from the Paleocene through Oligocene of North America and S. feisti is described from good material from the Eocene of Europe. Both are known from relatively complete skeletons. Both appear to have been active, terrestrial predators.
The earliest members of the family were unearthed as part of the Polish expeditions to Mongolia, and come from Cretaceous sediments there. Cherminotus, Saniwides, and Telmasaurus are all known from good skull material, but postcrania are limited. Telmasaurus appears to be most closely related to Saniwa, and it may have been ancestral to forms which crossed the Bering land bridge into North America. Saniwa then arose in the New World and continued to migrate eastward (fossil varanid vertebrae are known from Ellesmere Island) into Europe during a period of tropicality during the Eocene. Although little fossil evidence exists, it is postulated that the genus Varanus arose separately from the Cretaceous stock and migrated westward through Asia to colonize Africa during the Early Tertiary.
Further afield, the teeth of all varanids show a labyrinthine infolding of the dentine which is visible as longitudinal striations on the surface. This feature is characteristic of the more inclusive group Varanoidea and unites goannas with those aquatic leviathans, the mosasaurs. When watching a Komodo dragon eat, the relationship is not hard to imagine. But things were even worse in Australia during the Pleistocene, when the gigantic Megalania stalked the land. Megalania was more than twice the size of modern Komodo dragons and could bring down the giant marsupial Diprotodon. There's an excellent diorama of just such a battle in the Australian Museum in Sydney. After seeing it, a person is glad that varanids haven't inhabited the Western Interior in millions of years!