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The History of Monitor Lizards Print E-mail
Extract from A Little Book of Monitor Lizards © D. Bennett 1995. Viper Press, UK

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 (probably many thousands of) species. Some of them appeared to have died out 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 behaved 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 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.


 
 

 

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