Two skull fossils unearthed in Germany provide the first direct evidence that lepidosaurs – reptiles closely related to lizards, snakes and tuatara – were alive 240 million years ago during the Middle Triassic period. The fossils predate all other lepidosaur records by 12 million years.
“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,” said Dr Marc Jones from the University College London, UK, who is a lead author of a paper reporting the discovery in the journal BMC Evolutionary Biology.
According to the study, the fossils are most closely related to the tuatara, a lizard-like reptile. The small teeth and lightly built jaws suggest that the extinct animal preyed on small insects.
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. Tuatara feed on beetles, spiders, crickets and small lizards, also enjoying the occasional sea bird.
Today, there are over 9,000 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.
Dr Jones with colleagues used a dating technique known as a ‘molecular clock’ to establish the age of the fossil remains. 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.
Molecular clocks have been used by biologists to answer questions as important as when the first modern humans emerged, and when humans and chimpanzees shared a common ancestor. 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. Previous estimates have varied over a range of 64 million years and the scientists are keen to help narrow this down.
“Some previous estimates based on molecular data suggested that lizards first evolved 290 million years ago. To a paleontologist this seems way too old and our revised molecular analysis agrees with the fossils,” said study co-author Dr Cajsa Lisa Anderson from the University of Gothenburg.
Revised molecular dating in light of this new fossil find now suggests lizards began to diversify into most of the modern groups we recognize today, such as geckos and skinks, less than 150 million years ago in the Cretaceous period, following continental fragmentation.
Bibliographic information: Marc Jones et al. 2013. Integration of molecules and new fossils supports a Triassic origin for Lepidosauria (lizards, snakes, and tuatara). BMC Evolutionary Biology 13: 208; doi: 10.1186/1471-2148-13-208