Revisiting the question of reptile evolution

Analysis of a vast database of reptilian information contradicts the theory that major transitions in evolution always happened in large quick bursts triggered by environmental shifts, according to palaeontologists in the US.

In a paper in Nature Communications, they suggest the evolution of extinct lineages of reptiles from more than 250 million years ago took place through many small bursts of morphological changes over 50 million years rather than during a single major event.

They also note that the early evolution of most lizard lineages was a continuously slower and more incremental process than previously understood.

“It wasn’t a sudden jump that kind of established the wide diversity that we see today in reptiles,” says lead author Tiago R Simões, from Harvard University. “There was an initial jump, but relatively small, and then a sustained increase over time of those rates [of evolution] and different diversity values.”

Evidence of this has been seen in other types of animals, the researchers say, but this is the first time it’s been found in reptiles.

The findings run contrary to the evolutionary theory of adaptive radiation that Harvard palaeontologist George G Simpson popularised in the 1940s, which sought to explain the origins of the planet’s biological diversity.

That theory is being regularly re-examined as advances in technology make it possible to precisely measure rapid rates of evolution in the fossil record.

For his work with colleagues from Harvard and the University of Alberta, Canada, Simões spent five years compiling a dataset of living and extinct major reptile groups, including marine reptiles, turtles, lizards and the ancestors of dinosaurs and crocodiles.

He visited more than 20 countries and many of the world’s leading natural history museums to take CT scans and photos of nearly 1000 reptilian fossils, some hundreds of millions of years old.

Using DNA information from modern species and hundreds of anatomical features from both modern and fossil species for statistical analysis, the researchers detected that periods of fast anatomical change during the origin of reptile groups often predated when those groups diversified into hundreds or thousands of species.

The study revealed that rates of evolution and morphological variety in reptiles prior to the Permian-Triassic Mass Extinction – the biggest mass extinction of all time – were equally high, or even higher, than after the event.

That finding, the researchers say, confirms that fast rates of anatomical change don’t need to coincide with genetic diversity or an abundance of species and further rebuts adaptive radiation as the only explanation for the origin of new animal groups and body plans.

They note that it took reptiles almost 10 million years to recover to previous levels of anatomical diversity.

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