Fossils provide us with a tantalising glimpse of a world that we will never know. They have the unerring ability to create more questions than they answer. The chances of an animal or plant being preserved as a fossil is extremely slight. It must be covered with a protective layer as soon as it dies and many millions of years later it must somehow get back onto, or close to, the surface of the rock. Then somebody has to find it. The vast majority of known fossils come from marine organisms, only a tiny proportion are of terrestrial vertebrates and the monitor lizards are poorly represented there. All the monitor fossils I have seen have been unexceptional. Our knowledge of this family before the dawn of civilisation comes from fossilised remnants which are sometimes nothing more than a single vertebrae or fragment of jaw. Often it is very difficult to tell what sort of animal a scrap of bone belonged to over 80 million years ago, with the result that whilst some authors will consider a fossil bone to be that of a monitor lizard the next may claim that it is in fact a piece of prehistoric tortoise. Fossils records of monitor lizards from Africa and Australia are very rare, probably reflecting the unsuitable conditions that existed for fossilisation rather than the scarcity of the animals. Thus this little "history" of the monitor lizards must be taken with a large pinch of salt. Except where indicated the following account follows Estes (1983).
We do know that by 300 million years ago at least three major groups of reptiles had established themselves on Earth. The synapsids included the lizard-like pelycosaurs, some of which closely resembled the monitor lizards of today (e.g. Varanosaurus from what were then the swamps of Texas) and the "theraspids" which may have survived to the present in the form of modern mammals. The anapsids include the living turtles and tortoises and other orders, all of which had died out by 250 million years ago. The diapsids gave rise to dinosaurs and other ruling reptiles as well as birds, crocodiles, tuatara, snakes and lizards. True monitor lizard-like animals (varanoids) appeared in the Late Jurassic era, about 180 million years ago. Aigialosaurs were small aquatic lizards that were probably closely related to the monitors. They gave rise to mosasaurs, a diverse group of water lizards growing up to 50m long that roamed the seas for over 100 million years before dying out altogether (Cox et al 1988; Zug 1994; Steel 1996). The large mososaurs are the beiggest lizards known to have ever existed. As they disappeared monitor lizards first appeared on the land.
According to the available evidence monitor lizards and their close relatives the heloderms (Gila lizards) and lanthonotids (earless monitors) probably originated in northern Asia at least 90 million years ago (Pregill et al 1986). At this time the reign of the dinosaurs was coming to an end and flowers had begun to cover the Earth. The oldest monitor lizards known are from Mongolia: Telmasaurus grangeri, Saniwides mongoliensis and Estesia mongoliensis. All of them must have been quite similar to modern monitor lizards in appearance, but the latter possessed grooved teeth which probably transmitted venom in the same manner as modern-day Gila monsters (Pregill et al 1986, Norell et al 1992). The exact relationship between these lizards and the modern heloderms and varanids is not clear.
Early fossils tentatively identified as monitor lizards have also been found in Alberta and Wyoming in North America. Most authorities agree that this part of America was still attached to Asia when monitor lizards appeared. Paleosanawina canadensis lived at least 70 million years ago and probably reached a total length of about 240cm (Gilmore 1928). These lizards had long backward pointing, serrated teeth that show grooves similar to those of the Mongolian Estesia. Although they too must have been very similar to the present day monitor lizards, their inclusion in the family Varanidae has been questioned.
The oldest fossils definitely identified as belonging to the monitor lizard family belong to the once widespread genus Saniwa which appeared at least 55 million years ago. Described species include Saniwa ensidens, S.grandis, and S.crassa from Wyoming, S.paucidens from Wyoming and Utah. S.brooksi from California, S.orsmaelensis from Belgium and unidentified species from France, New Mexico, Wyoming and Nebraska. Apart from differences in size there is little to distinguish these fossils from each other, nor from living monitor lizards. Saniwa may not have survived for long in Europe but they persisted in North America until at least 15 million years ago.
The living genus, Varanus, does not appear in the fossil record until about 25 million years ago. The oldest fossils come from Khazakstan are too fragmented to be assigned to species (Reshetov et al. 1978, Lungu et al. 1989). About 20 million years ago Varanus rusingensis inhabited Rusinga Island (now in Lake Victoria) and other areas of Kenya (Clos 1995). This monitor was very similar to the living African monitors V.niloticus, V.exanthematicus and V.albigularis. Like the living species, it probably fed largely on molluscs and reached a length of at least 2m. It may have lead a semi-aquatic existence in forests, much like V.niloticus ornatus.
The oldest European monitor is Varanus hofmanni, about 10 million years younger, which is known from France, Spain and Germany. At the same time the closely related Iberovaranus catalonicus lived in Spain and Portugal and Varanus pronini lived in Khazakstan (Zerova & Ckhikvadze 1986). V. marathonensis appeared at least 5 million years ago and is known from Greece, Hungary and Turkey. An unconfirmed record of this species from Italy suggests that the monitor lizards may have survived in Europe until less than a million years ago. Orlov & Tuniev (1986) suggest that V.marathonensis was very closely related to the living V. griseus and to V. darevskii, which lived in Tadjikistan about 5 million years ago (Levshakova 1986). V.semjonovi is known from Ukraine and another species, V.lungui has been described from Moldavia (Zerova & Ckhikvadze 1986, Lungu et al 1989). At this time a very large monitor lizard approaching 3m in length, V.silvalensis, lived in India. Other extinct fossil species include V.hooijeri, a close relative of the present day V.olivaceus, which lived on Flores less than 5 million years ago and possibly V.bolkayi, known to have inhabited Java and Timor about 2 million years ago (these fossils may represent the living species V.salvator (Auffenberg 1981)).
Unfortunately virtually nothing is known of the monitor lizards' history in Australia. The earliest fossils known come from South Australia and are around 10 million years old (Estes 1984). Fossil vertebrae of a species similar to V.giganteus from New South Wales are less than 2 million years old. On immunological evidence Baverstock et al (1994) suggest that monitor lizards reached Australia from south-east Asia less than 20 million years ago. When monitor lizards reached Australia, something very strange happened to them. Throughout the world fossil monitors appear as large or medium sized lizards, but few, if any, ever exceeded 300cm in length. In Australia both gigantic and dwarf monitor lizards evolved. Megalania (or Varanus) prisca was the largest land-dwelling lizard that has ever lived. Adults may have weighed over 600kg and measured more than 7m in length. They appear to have been widespread in Australia (remains have been found in New South Wales, Queensland and South Australia). This immane goanna is not a long dead and buried species. They may have survived until less than 25,000 years ago and are believed to have preyed upon the giant ancestors of kangaroos and wombats. Giant goannas may also have preyed on early human settlers, who must have regarded its extinction with great relief, even if they did not play a direct role in its demise themselves (Owen 1860, 1880, Anderson 1931, Hecht 1975, Rich 1985, Molnar 1990). The artist's impression of the giant goanna given here does not take into account the fact that this enormous monitor lizard may have had a bony crest on top of its head. Other ancient Australian goannas include an unidentified species that lived in South Australia 5 million years ago and had very large blunted teeth (Archer & Wade 1976) and the fossil V.emeritus from Queensland, which may represent another extinct species. Recent fossils of the living lace and sand goannas have been recovered from cave deposits in Victoria (Wells et al 1984).
Whilst some of the Australian monitor lizards became massive the more successful ones had adopted an opposite strategy. They shrank and diversified to form a unique group of diminutive varanids that spread throughout Australia and then began to move northwards (Storr 1980). To date they have not got very far; to the south of New Guinea and a few islands in the Timor Sea. Nevertheless, they are a very young group of lizards and already account for two-thirds of the living species of the Varanidae in Australia, and a third of the family world-wide. The larger monitor lizards have also persisted in Australia with at least 9 species living there today.
Today at least 46 species of monitor lizards are known to exist in Africa, Asia and Australasia. Baverstock et al (1994) suggest that all living species have evolved from a common ancestor within the last 45 million years. The confusion over the extinct varanids is unlikely ever to be resolved fully, but ample opportunities remain to study the surviving species. It is hoped that by examining many different characteristics of the living monitors it will be possible to gain some idea of how they are related to each other and eventually, it is hoped, their dispersal routes and the chronological order of species evolution will become apparent.