Wednesday, October 6, 2010

Egyptian Monitor Lizard Fossils Support an African Origin for Varanus

Monitor Lizards (family Varanidae) are thought to have evolved in the late Mesozoic of Laurasia and dispersed to Africa. However, the place of origin for genus Varanus  has been open to speculation. The greatest diversity of monitor lizards today is found in Australia. But, previous fossil and molecular data suggested that the genus Varanus originated in Asia and later dispersed into Africa and Australia, based upon Miocene fossils present on both continents. When the Asian origin was suggested the oldest African Varanus fossils dated only to the Miocene. Now, Robert B. Holmes and colleagues have described the oldest known fossils of  the genus Varanus from the upper Eocene of the Birket Qarun Formation of Egypt's Fayum Province. The fossil beds have produced a variety of vertebrate material over the last century, including primates, crocodilians, turtles, snakes, and fish. Except for a single varanid vertebrae, remains of lizards have not been previously discovered. The documentation of the early appearance of Varanus in Africa is consistent with an African origin of the genus, as previously suggested by Molnar and Pianka (2004) on the basis of two molecular phylogenetic analysis. One study placed the African species V. albigularis and V. niloticus as successive sister groups to the Asian and Australian species, and another recovered three African Varanus species (V. griseus, V. exanthematicus and V. niloticus) as a monophyletic clade that is the sister group to all other known Varanus. An Eocene to early Oligocene connection between Africa and Australia is implied with the African origin of Varanus. A Gondwanan connection between the land masses is  too old to explain the distribution of a single genus. But a hypothesized pre-Miocene Tertiary connection between Africa and Asia has already been proposed based on the palaeogeographic distributions of fossil mammals and freshwater fishes. An island chain may have allowed varanid lizards as well as the other organisms to move from Africa, to Asia to Australia.
The African Monitor Lizard, Varanus exanthematicus. JC
Holmes, R. B., A. M. Murray, Y. S. Attia, E. L. Simons, P. Chatrath. 2010. Oldest known Varanus (Squamata: Varanidae) from the Upper Eocene and Lower Oligocene of Egypt: support for an African origin of the genus . Palaeontology, 53:1099-1110
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