Monitoring lizard dispersal and evolution
Snakes, iguanas and the lizards are squamate reptiles, sharing an ancestry back into the Oligocene period and beyond. Many of the lizards originated in Laurasia but the varanids or monitors have a disputed origin.
Africa, Arabia, Australia and Asia hold the present-day species, with the most ancient Varanus spp. coming from the Late Eocene of Africa.
This means the Asian monitors or this African fossil species could have started off the line in the Cenozoic or a Gondwanan relative could have been their ancestor at the Jurassic/Cretaceous interface. (note Gondwana at the bottom left of the maps)
Credit: © Shutterstock
Reference Above: As the continents spread out, the monitors were developing in Eurasia. With the opening of the Atlantic, they were excluded from North America, but managed to cross later into Africa and then Australia. The Australian plate moves upward (with the small green area of India) after the sequence shown till it collides with Asia.
The regular modus operandi in genetic mapping now is to check DNA and RNA of representative species (in this case, 54, in all). A whole genome is not necessary to give all the clues needed about relationships between a Class such as the Reptilia or even the Order Squamata here.
Nicolas Vidal et al of the Departement Systematique et Evolution, Museum National d'Histoire Naturelle, in Paris, used probabilistic approaches which they call ML (maximum likelihood) and built up a perfect phylogeny of the whole group.
Identical trees were formed by two methods. With a split between New and Old World types in the early Cretaceous, 100 million years ago, the varanids simply become an African niloticus group and two other groups based on an Indo-Asian and an Indo-Australian biogeography.
Most species-rich groups have developed recently, but one or two date back to before the formation of the Atlantic, such as the Helodermatids (gila monsters) These groups have very few species in them.
Gila monster image; Credit: © Shutterstock
The gila monster above (Heloderma suspectum), here in Arizona, and its Mexican relative, Heloderma horridum are examples of a lizard family that developed early, with these two modern (Miocene) species, but many fossil relatives. The family date as far back as the early Cretaceous dinosaurs.
The monitors seem to have originated in Asia and may have used Iran as a route - stop on the way to Africa. As the intriguing Madagascar fauna lacks varanids, some support is available for later colonisation of the African continent, between 49 and 33 mya.
The spiny-tailed monitor (Varanus acanthurus) © Freek Vonk
This only leaves Australia to be considered. Between 39 and 26 mya, the late Eocene saw blindsnakes, pythons, agamids and geckos reach the continent, and it seems the monitors travelled there within the same time-limits, before its collision with the Asian plate.
The large Komodo dragon would have developed since then, along with the other larger monitors in both Australia and Asia, during the Miocene and Pliocene. The detailed paper with all 54 species tagged on a family tree can be found in Biology Letters today.
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