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  Section: Monitor Lizards » The History Of Monitor Lizards
 
 
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The History Of Monitor Lizards

 
     
 
As the monitors spread across the Earth experiencing different habitats and climates they diversified. Over many millions of years this process has resulted in the emergence of at least seventy or eighty (probahly many thousands of) species. Some of them appeared to have died Ollt quickly, whilst other, apparently ancient, species have survived until the present. Many monitor lizards appear to have evolved comparatively recently. It would be nice to know where the monitor lizards first came from, what the early species looked like, how they hehaved and why they died out.

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 indicate 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 modem 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 10m long that roamed the seas for over 100 million years before dying out altogether (Cox et al 1998; Zug 1994). As they disappeared monitor lizards first appeared on the land.

According to the available evidence monitor lizards and their close relatives the heloderrns (Gila lizards) and lanthonotids (earless monitors) probably originated in northern Asia at least 9O 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 mongoliel1sis and Estesia mongoliensis. All of them must have been quite similar to modem monitor lizards in appearance, but the latter possessed grooved teeth which probably transmitted venom in the same manner as modem-day Gila monsters (Pregill et al 1986, Norell el al 1992). The exact relationship between these lizards and the modern heloderms and varanids is not clear.

Early fossils tematively 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 Live at least 70 million years ago and probably reached a total length of about 240cm (Gilmore 1928). These Lizards had long backward pointing, serrate 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 defmitely identified as belonging to the monitor lizard family belong to the once widespread genus Saniwa which appeare 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.orsmaelellsis 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 survive for long in Europe but they persiste in North America umil 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 Kenya and Khazakstan and are too fragmented to be assigne to species. 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 live 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 survive 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 live in Tadjikistan about 5 million years ago (Levshakova 1986). V.semjonovi is known from Ukraine and another species, V.lungui has been dcscribe from Moldavia (Zerova & Ckhikvadze 1986, Lungu et al 1989). At this time a very large monitor lizard approaching 3m in length, V.silvalensis, Iive in India. Other extinct fossil species include V.hooijeri. a close relative of the present day V.olivaceus, which live 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 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).
Megalania prisca
Above: Megalania prisca (Stuart Dilks after
Anderson 1930) Below: The giant goanna
meets its match (lain Curran).
giant goanna meets its match (lain Curran). 

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 playa 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 nonhwards (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 opponunities 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 evenrually, it is hoped, their di spersal routes and the chronological order of species evolution will become apparent.

Monitor lizard taxonomy is a lively and controversial subject. For practical purposes, it must be possible to distinguish between species on the basis of external morphological characteristics, but these criteria can be misleading. Species which inhabit many different habitats over a huge range often show great variation in colour, pattern, scalation, shape and size, and very small populations often show similar variation between individuals, even amongst groups of siblings. Just as it is not always possible to tell which monitors are closely related to each other merely by looking for similarities in their external appearance, nor is appearance always indicative of the animals' lifestyles, because species can develop independently of each other and in very different habitats, yet appear remarkably similar. A good example is the belief, held for many years, that the yellow monitor was a close relative of Bosc's monitor, because the species looked so much alike. Unfonunately for the marshdwelling yellow monitor, it was also presumed that they shared similar habits, with the result that captive specimens were kept in quite unsuitable conditions and were unable to tlourish (Rotter 1963, Visser 1985). It is now known that Bosc's monitor is a close relative of the Nile monitor, to which it bears little obvious resemblance, and that the yellow monitor is more closely aligned to the Bengal monitors and some other South East Asian species (Bohime 1988).

Over the last twenty years the accepted relationships between species of monitor lizard have been largely revised using a variety of methods, all of which have advantages and disadvantages beyond the scope of this book. In recent years a number of studies have examined less obvious characteristics of the monitor lizards in order to clarify their relationships with each other. They include studies of protein and DNA, examination of morphological characteristics such as lung and hemipenal morphology and shared behaviow'al traits. There are some differences of opinion, but in general these studies differ only slightly from each other in their conclusions. Unfortunately some species have not been available for analyses and sample sizes for many others are very small. Much more work must be done before we have complete and certain knowledge of these relationships. The following is a summary of the concurrent (and not so concurrent) points. Full di scussions can be found in Mertens 1942, 1963), King & King (1975), Holmes et al (1975), Becker (1991)
Bohme (1988, 1991), King et al (1991), Sprackland (1991 a), Baverstock et al (1994) and Card & Kluge (1995).
  1. The African monitor lizards, V.albigularis, V.exanthematicus and Vniloticus and V.yemenensis are currently classified as belonging to the subgenus Polydaedalus. Some studies suggest that V.griseus (at present in its own subgenus, Psammosaurus) also belongs to this group. On this basis the expansion of V.griseus into Asia could be interpreted as being relatively recent. Other studies suggest that V.griseus has no close living relatives, and should remain in its own subgenus.

  2. The Asian monitor lizards V.bengalensis / V.nebulosus, V.flavescens, V.dumerilii and probably V.rudicollis form the subgenus Empagusia.

  3. V.komodoensis appears to have its closest living relatives amongst the large Australian monitors, particularly V.varius. According to recent studies these two lizards together with V.giganteus, V.rosenbergi, V.flavirufus, V.gouldii and V.spenceri form a distinct lineage, the subgenus Varanus.

  4. V.salvator appears to have no close living relatives (except possibly for V.rudicollis). However, "V.salvator" may be a conglomeration of several species.

  5. V.salvadorii is the sole member of the subgenus Papusaurus (Mertens 1962).

  6. The dwarf monitor lizards form a distinct lineage, (subgenus Odatria) which probably includes V.mitchelli (previously considered a Varanus-type animal). All Odatria are restricted to Australia with the exceptions of V.timorensis and the V.scalaris type animals of New Guinea. One study suggests that V.eremius does not belong to this subgenus and forms its own unique lineage. The same study (Baverstock et al 1994) indicates that Odatria monitors can be divided into two groups, one containing the spiny-tailed monitors (V.acanthurus , V.brevicauda etc.) and V.gilleni, the other containing the long-tailed rock goannas (V.glauerti, V.glebopalma, V.pilbarensis), together with V.trristis, V.scalaris and V.mitchelli. The fact that dwarf monitor are found nowhere else is strong evidence that this group evolved in Australia.

  7. V.indicus, the V.prasinus group, V.jobiensis and V.doreanus form the subgenus Euprepiosaurus. The centre of distribution of Euprepiosaurus is New Guinea, where all species are found. Only the highly aquatic V.indicus has extended its range over a much wider area; over much of the north coast of Australia and many of the islands of the South Pacific (due in part to mankind's' intervention).
giant goanna meets its match (lain Curran). 
At present monitor lizards reach their highest diversity in Australia, where half the living species occur, including virtually all of the dwarf forms and many of the larger species. Nobody really understands why the monitors are so successful in Australia. The absence of large carnivorous mammals which would act both as predators and as competition for food resources is undoubtedly a major factor in their present distribution. Although in the past plenty of giant carnivorous marsupials have inhabited the continent the only large predators to have survived to the present in many areas of Australia are the monitor lizards. Perhaps their more economical physiologies and greater water-conserving abilities allowed them to survive when the mammals perished. Unfortunately the past climate of Australia and the age of its deserts is not a subject of universal agreement (Pianka 1986). It has been suggested that a massive decrease in rainfall leading to a "dry age" occurred in Australia within the last two million years, leading to desertification and the demise of many animal species. However some of the Australian deserts which are home to large numbers of monitor lizards species may be very much older than this. I would like to think that the ability to stand and walk bipedally may have contributed to their success in Australia. Some Australian goannas use bipedalism to a much greater extent than other monitor lizards. Perhaps only the arrival of mankind, somewhat steadier on its hindlegs, prevented them from perfecting the technique.

Bioblography

Attribution / Courtesy: Daniel Bennett. 1995. A Little Book of Monitor Lizards. Viper Press U.K.
 
     
 
 
     



     
 
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