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A new study refutes the 75-year belief in reptile evolution

A timeline on the evolution of reptiles

Animals sampled in the analysis. Credit: Tiago R. Simões

Challenging a 75-year notion of how and when reptiles evolved over the past 300 million years and more requires a lot of shooting work, lots of CT scans, and most importantly, thousands of miles of travel. Just check the stamps on Tiago R. Simões̵

7; passport.

Simões is Alexander Agassiz Postdoctoral Fellow in the laboratory of Harvard paleontologist Stephanie Pierce. From 2013 to 2018, he traveled to more than 20 countries and more than 50 different museums to perform CT scans and photos of nearly 1,000 reptile fossils, a few hundred million years. It amounted to about 400 days of active harvesting, helping to form what is believed to be the largest timeline available on the evolution of major living and extinct reptile groups.

Now, a statistical analysis of that vast database is helping scientists better understand the evolution of these cold-blooded vertebrates by contradicting a widely accepted theory that major transitions in evolution always occurred in large, rapid (geologically speaking) explosions. , triggered by major environmental changes. The results are described in a recently published paper in Nature Communications.

In it, the researchers show that the evolution of extinct reptile lineages from over 250 million years ago took place through many small bursts of morphological changes, such as the development of armored body planes or wings for gliding, for a period of 50 million years instead of during a single major evolutionary event, as previously thought. They also show that the early evolution of most lizard lineages was a continually slower and more incremental process than previously thought.

“It wasn’t a sudden leap that established the wide diversity we see in reptiles today,” said Simões. “There was an initial, but relatively small, jump, and then a sustained increase in those rates over time [of evolution] and different values ​​of diversity “.

Evidence of this has been seen in other types of animals, but this is the first time that it has been seen in reptiles, one of the most diverse animals on the planet, with more than 10,000 different species and a dizzying variety of abilities and traits. Consider how some lizard species can freeze overnight and then thaw the next morning, or how turtles develop protective armor.

The findings run counter to the evolutionary theory of adaptive radiation that Harvard paleontologist George G. Simpson popularized in the 1940s, which sought to explain the origins of the planet’s biological diversity. Adaptive radiation has been the focus of intense investigation for decades, but only in recent years have the technology, methods and data existed to accurately measure the rapid rates of evolution in the fossil record in terms of different animal species, morphologies and at the molecular level. using DNA.

Researchers in this study also included Pierce, associate professor of organismic and evolutionary biology Thomas D. Cabot and curator of vertebrate paleontology in the Museum of Comparative Zoology; Oksana Vernygora, a graduate student at the University of Alberta in Canada; and Professor Michael Wayne Caldwell of Alberta.

Simões visited nearly every major natural history museum in the world to collect data for the study, including the national natural history museums in London, Paris, Berlin, Ottawa, Beijing, and Tokyo. In the United States, he visited the Smithsonian National Museum of Natural History, the Carnegie Museum of Natural History, and Harvard’s Museum of Comparative Zoology.

Scientists believe that by understanding how animals evolve over longer periods of time, they can draw a range of lessons about ecology and how organisms are affected by environmental changes. Using the database, researchers can determine when major reptile lineages or morphologies originated, see how those changes impacted reptile DNA, and learn important lessons about how species were affected by historical events.

Reptiles, for example, have survived three major mass extinction events. The largest was the Permian-Triassic mass extinction about 250 million years ago, which killed about 90% of the planet’s species, earning it the nickname Great Dying. It is believed to have been caused by an accumulation of natural greenhouse gases.

The timeline created by the researchers found that the speeds at which the reptiles were evolving and the anatomical differences between them before the Great Dying were almost as high as after the event. However, it wasn’t until long after the Great Dying that reptiles became dominant in many ecosystems and extremely diverse in terms of the number of different species.

This finding consolidated the fact that rapid rates of anatomical change do not necessarily have to coincide with genetic diversity or an abundance of species (called taxonomic diversity) and further refuted adaptive radiation as the only explanation for the origin of new animal groups and body plans. The researchers also note that it took the reptiles nearly 10 million years to recover to previous levels of anatomical diversity.

“This kind of information tells you about the broad scheme of things and on a global scale what impact, over the course of life history, sudden environmental changes can have,” said Simões.

Further evidence contradicting adaptive radiation included similar but startling findings about the origins of snakes, which attained major aspects of their lean, elongated body planes early in their evolution some 170 million years ago (but did not completely lose limbs for others. 105 million years). They also underwent rapid changes in their skulls around 170-165 million years ago which resulted in mouths so powerful and flexible that today they can swallow prey whole many times their size. But while snakes experienced the fastest rates of anatomical change in the history of reptile evolution, these changes did not coincide with increases in taxonomic diversity or high rates of molecular evolution as predicted by adaptive radiation, the researchers said.

Scientists have not been able to pinpoint why this discrepancy occurs and have suggested that more research is needed. Specifically, they want to understand how body planes evolve and how changes in DNA relate.

“We can see better now what are the big changes in the history of life and especially in the life history of reptiles on Earth,” said Simões. “We will continue to dig.”

Did adaptive radiation shape the evolution of reptiles?

More information:
Tiago R. Simões et al, Megaevolutionary dynamics and the timing of evolutionary innovation in reptiles, Nature Communications (2020). DOI: 10.1038 / s41467-020-17190-9

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New Study Refutes 75-Year Belief in Reptile Evolution (2020, Oct.7)
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