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Oldest lizard-like fossil hints at scaly origins

Artwork showing what a lepidosaur would have looked like, and a fossilised jaw discovered in Vellberg, Germany.
Image: Marc Jones.

Artwork showing what a lepidosaur would have looked like, and a fossilised jaw discovered in Vellberg, Germany.
Image: Marc Jones.

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Thursday, 26 September 2013

An international team of scientists including the University of Adelaide's Dr Marc Jones has reported that the fossilised remains of a reptile closely related to lizards unearthed in Germany are the oldest 'lepidosaur' fossils yet to be discovered.

Two new fossil jaws discovered in Vellberg, Germany provide the first direct evidence that the ancestors of lizards, snakes and tuatara (known collectively as lepidosaurs), were alive during the Middle Triassic period - around 240 million years ago and 12 million years before all other lepidosaur records.

Publishing their findings in BMC Evolutionary Biology, the international team of scientists behind dating the fossil jaws provide evidence that lepidosaurs first appeared after the end-Permian mass extinction event - a period when fauna began to recover and thrive in the more humid climate.

Dr Jones, new ARC Discovery Early Career Research Award Fellow in the University's School of Earth and Environmental Sciences, is lead author and conducted the research while at University College London (UCL).

"The Middle Triassic represents a time when the world has recovered from the Permian mass extinction but is not yet dominated by dinosaurs. This is also when familiar groups, such as frogs and lizards, may have first appeared."

The small teeth and lightly built jaws, suggest that the extinct animal preyed on small insects. The new fossils are most closely related to the tuatara, a lizard-like reptile.

Tuatara can be found on 35 islands lying off the coast of New Zealand and were recently reintroduced to the mainland. However, they are the sole survivors of a group that was once as globally widespread as lizards are today.

Today, there are more than 9000 species of lizards, snakes and tuatara. Knowing when the common ancestor of this grouping first appeared is crucial for understanding the ecological context in which it first evolved as well as its subsequent diversification.

To establish a date, biologists use a molecular dating technique known as a "molecular clock". This method compares the amount of genetic divergence between living animals, caused by changes in their DNA sequences that have accumulated since they split from a common ancestor. These mutations occur fairly regularly, ticking along at a clock-like rate. However, for the clock to convert genetic differences into geological time, it has to be calibrated using one or more fossils of known relationship and time.

The new fossil jaws can improve molecular dating estimates of when reptiles began to diversify into snakes, lizard and tuatara, and when the first modern lizards inhabited the earth.

Revised molecular dating in light of this new fossil find now suggests lizards began to diversify into most of the modern groups we recognise today, such as geckos and skinks, less than 150 million years ago in the Cretaceous period, following continental fragmentation.

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