Global warming spawned the age of reptiles

image: Artist’s reconstruction of adaptive reptile radiation in a terrestrial ecosystem during the hottest period in Earth’s history. The image depicts a massive, large-headed, carnivorous erythrosuchid (close relative to crocodiles and dinosaurs) and a tiny, hovering reptile about 240 million years ago. The erythrosuchid pursues the gliding reptile and propels itself using a fossilized skull of the extinct Dimetrodon (ancestor of the first mammals) in a hot and dry river valley.
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Credit: Image created by Henry Sharpe

Studying climate change-induced mass extinctions in the deep geological past allows researchers to explore the impact of environmental crises on the evolution of organisms. A prime example is the Permian-Triassic climate crises, a series of climatic changes caused by global warming that occurred between the Middle Permian (265 million years ago) and the Middle Triassic (230 million years ago). million years). These climatic changes caused two of the greatest mass extinctions in the history of life at the end of the Permian, the first at 261 million years ago and the other at 252 million years ago, the latter eliminating 86% of all animal species in the world.

The Late Permian extinctions are significant not only because of their scale, but also because they mark the beginning of a new era in the planet’s history when reptiles became the dominant group of vertebrate animals. living on earth. In the Permian, terrestrial vertebrate faunas were dominated by synapsids, ancestors of mammals. After the Permian, Triassic extinctions (252 to 200 million years ago), reptiles evolved at a rapid rate, creating an explosion of reptile diversity. This expansion has been key to building modern ecosystems and many extinct ecosystems. Most paleontologists believed that these rapid rates of evolution and diversification were due to the extinction of competitors allowing reptiles to take over new habitats and food resources that several groups of synapsids had dominated before their extinction.

However, in a new study in Scientists progress researchers from the Department of Organismal and Evolutionary Biology and the Museum of Comparative Zoology at Harvard University and collaborators reveal that the rapid evolution and radiation of reptiles began much earlier, before the end of the Permian, in relation to the constant increase in global temperatures for a long period. series of climatic changes that lasted nearly 60 million years in the geological record.

“We found that these periods of rapid evolution in reptiles were intimately linked to rising temperatures. Some groups changed very quickly and others less quickly, but almost all reptiles evolved much faster than they expected. have never done before,” said lead author, postdoctoral fellow Tiago R Simões.

Previous studies of the impacts of these changes have often overlooked terrestrial vertebrates due to limited data availability, focusing primarily on the response of marine animals

In this study, Simões and lead author Prof. Stephanie E. Pierce (both at Harvard) worked alongside collaborators Prof. Michael Caldwell (University of Alberta, Canada) and Dr. Christian Kammerer (North Carolina Museum of Natural Sciences) to examine early amniotes, which represent the precursors of all modern mammals, reptiles, birds, and their closest extinct relatives, at the initial stage of their evolution. At that time, the earliest reptile and mammalian ancestor groups separated from each other and evolved along their own evolutionary paths.

“Reptiles represent an ideal and rare Earth system to study this question because they have a relatively good fossil record and have survived a series of climatic crises, including those that led to the greatest extinction in the history of life. complex, the Permian-Triassic mass extinction,” said Simões.

Reptiles were relatively rare during the Permian compared to mammalian ancestors. However, things took a major turn during the Triassic when reptiles underwent a massive explosion in species number and morphological variety. This led to the appearance of most major living groups of reptiles (crocodiles, lizards, turtles) and several groups that are now entirely extinct.

The researchers created a dataset based on extensive first-hand data collection of more than 1,000 fossil specimens of 125 species of reptiles, synapsids and their closest relatives for approximately 140 million years before and after the Permian-Triassic extinction event. They then analyzed the data to detect when these species first appeared and how quickly they evolved using state-of-the-art analytical techniques such as Bayesian evolutionary analysis, which is also used to understand the evolution of viruses such as SARS-COVID 19. Researchers then combined the new dataset with global temperature data spanning millions of years in the geological record to provide broad insight into the major adaptive response of animals to climate change .

“Our results reveal that periods of rapid climate change and global warming are associated with exceptionally high rates of anatomical changes in most reptile groups as they adapted to new environmental conditions,” Pierce said, “ and this process began long before the Permian-Triassic extinction, for at least 270 million years, indicating that the diversification of reptile body plans was not triggered by the PT extinction event as previously thought. before, but actually started tens of millions of years before that.

“A lineage of reptiles, the lepidosaurs, which gave rise to the first lizards and tuataras, veered in the opposite direction of most reptile groups and underwent a phase of very slow change in their overall anatomy,” Simões said. , “essentially, their body plans were limited by natural selection, instead of going rogue and changing drastically like most other reptiles at the time. Researchers suggest this is due to pre-adaptations in their size body to better cope with high temperatures.

“The physiology of organisms really depends on their body size,” Simões said, “small-sized reptiles can better exchange heat with their surrounding environment. Early lizards and tuataras were much smaller than other reptile groups, not so different from their modern relatives, and so they were better adapted to cope with drastic changes in temperature.The much larger ancestors of crocodiles, turtles and dinosaurs could not lose heat so easily and had to quickly change their bodies to adapt to new environmental conditions.

Simões, Pierce and their collaborators also mapped changes in body size across geographic regions during this time period. They revealed that climatic pressures on body size were so high that there was a maximum body size for reptiles to survive in tropical regions during the deadly heat periods of that time.

“Large-bodied reptiles basically took two routes to cope with these climate changes,” Pierce said, “they either migrated closer to temperate regions or invaded the aquatic world where they didn’t have to worry. overheating because water can absorb heat and maintain its temperature much better than air.

“This strong association between rising temperatures in the geological past and a biological response of radically different reptile groups suggests that climate change has been a key factor in explaining the origin and explosion of new reptile body plans over the Late Permian and Triassic,” said Simões. .

The researchers would like to thank the Museum of Comparative Zoology (MCZ), Harvard University, vertebrate paleontology staff, and curators of more than 50 natural history collections worldwide for their assistance in accessing specimens. . Funding was provided by: Alexander Agassiz Postdoctoral Fellowship, MCZ; National Science and Engineering Research Council of Canada (NSERC) Postdoctoral Fellowship; Grant KA 4133/1-1 of the Deutsche Forschungsgemeinschaft; NSERC Discovery Grant #23458 and NSERC Accelerator Grant; Faculty of Science, Chairs Research Allocation, University of Alberta; Lemann Brazil Research Fund; Funds made available by Harvard University.

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Teresa H. Sadler