During the late Oligocene to early Miocene, the group had dispersed to Australia, New Guinea, and the Solomon Islands three separate times. The genus Varanus first emerged in Laurasia. Like snakes, monitor lizards have forked tongues, which they use to sense odors. Snakes were believed to be more closely related to monitor lizards than any other type of extant reptile however, snakes have been more recently proposed to be the sister group of the clade of iguanians and anguimorphs. Varanids last shared a common ancestor with their closest living relatives, earless "monitors", during the Late Cretaceous.ĭuring the Late Cretaceous Period, monitor lizards or their close relatives are believed to have evolved into amphibious and then fully marine forms, the mosasaurs, some of which reached lengths of 12 m (39 ft) or more.
![argus monitor lizard california argus monitor lizard california](https://www.naturepl.com/cache/pcache2/01496996.jpg)
Monitor lizards probably expanded their geographic range into Africa between 49 and 33 million years ago, possibly via Iran, and to Australia and the Indonesian archipelago between 39 and 26 million years ago. The family Varanidae probably originated in Asia at least 65 million years ago, although some estimates are as early as the late Mesozoic (112 million years ago). The giant extinct megalania ( Varanus priscus) Some monitor lizards, including the Komodo dragon, are capable of parthenogenesis. Monitor lizards are oviparous, laying from seven to 37 eggs, which they often cover with soil or protect in a hollow tree stump. The venom of monitor lizards is diverse and complex, as a result of the diverse ecological niches monitor lizards occupy. This allows monitor lizards to create mammalian-equivalent pressure differentials between the pulmonary and systemic circuits, which in turn ensure that oxygenated blood is quickly distributed to the body without also flooding the lungs with high-pressure blood.Īnatomical and molecular studies indicate that all varanids (and possibly all lizards) are partially venomous. Whereas most reptiles are considered to have three-chambered hearts, the hearts of monitor lizards – as with those of boas and pythons – have a well developed ventricular septum that completely separates the pulmonary and systemic sides of the circulatory system during systole. Monitor lizards have a high aerobic scope that is afforded, in part, by their heart anatomy.
![argus monitor lizard california argus monitor lizard california](https://www.researchgate.net/profile/Robert-Mendyk/publication/287469845/figure/fig2/AS:404969296547840@1473563612346/Examples-of-elevated-surface-basking-temperatures-selected-and-utilized-by-different_Q320.jpg)
The general consensus is that monitor lizards have the highest standard metabolic rates of all extant reptiles. The active nature of monitor lizards has led to numerous studies on the metabolic capacities of these lizards. Monitor lizards maintain large territories and employ active-pursuit hunting techniques that are reminiscent of similar-sized mammals. Finer morphological features such as the shape of the skull and limbs do vary, though, and are strongly related to the ecology of each species. The genus Varanus is considered unique among animals in that its members are relatively morphologically conservative, yet show a very large size range.