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Goannas, or monitors as they are known elsewhere in the world, are a very distinctive group of lizards. Goannas evolved in the northern hemisphere in the Upper Cretaceous Period about 90 million years ago.

They range in size from the diminutive Australian Short-tailed Pygmy Monitor Varanus brevicauda (total length of 25 cm), to the largest lizard living today, the 3 metre-long Komodo Dragon of Indonesia, Varanus komodoensis.

There are 69 living species of goanna and most of them are carnivores. Goannas find their food by searching widely across the landscape, catching animals by stalking or digging them out of shelters and nests. They are aided in their search for food by their long forked tongues which they flick in and out, picking up chemicals in the air and on the ground. Goannas then "read" these chemicals with a special organ in the roof of the mouth.

Goannas evolved in the northern hemisphere in the Upper Cretaceous Period about 90 million years ago. However, in the Miocene Epoch, about 15 million years ago, goannas moved south into Africa and Australia. In Africa, only a few species evolved, but in Australia, goannas evolved into roughly 28 species of which at least one is extinct (Varanus priscus, a giant species which grew to more than 6 m in length). Goannas may have evolved in such profusion in Australia because, when they arrived, there were relatively few medium-sized carnivores competing for the same resources. Africa, however, had a large number of medium-sized carnivores, especially mammals in the dog and weasel families.

Distinctive features

One of the most distinctive features of the evolution of goannas in Australia is that their size range is enormous but their body shape has changed very little over time. The scarcity of competitors provided goannas with an evolutionary opportunity: to stick with one way of making a living and apply it over a range of sizes rather than changing the way of making a living within the same size range.

Words to know:

  • ENDEMIC - native to one area or continent only.
  • UPPER CRETACEOUS PERIOD - approximately 97 - 65 million years ago.
  • MIOCENE EPOCH - approximately 23 - 5 million years ago.
  • RADIATION - the evolution of a species into new and different species as a result of adaptations to the environment.