Global warming spawned the age of reptiles

Researchers can examine the effects of environmental crises on organismal evolution by looking at major extinctions brought on by climate change that occurred in the distant past. The Permian-Triassic climatic crises, a sequence of climatic changes brought on by global warming that took place between the Middle Permian (265 million years ago) and Middle Triassic, serve as one prominent example (230 million years ago). At the conclusion of the Permian, these climatic changes led to two of the worst mass extinctions in the history of life, the first occurring at 261 million years ago and the second at 252 million years ago, the latter wiping out 86% of all animal species worldwide.

The end-Permian extinctions were significant not only because of their size but also because they signaled the beginning of a new period in Earth's history, one in which reptiles predominated as the main group of land-dwelling vertebrates. Synapsids, the forerunners of mammals, dominated the land vertebrate faunas during the Permian. In the Triassic Period (252–200 million years ago), following the Permian extinctions, reptiles underwent fast evolution, leading to an expansion in reptile variety. The development of numerous lost ecosystems as well as many existing ecosystems depended on this expansion. Most paleontologists thought that the reason for these quick rates of development and diversity was the extinction of rivals, which allowed reptiles to occupy new habitats and food sources that multiple synapsid taxa had previously dominated.

The rapid evolution and radiation of reptiles, however, actually began much earlier, before the end of the Permian, in connection with the steadily rising global temperatures through a long series of climatic changes that spanned almost 60 million years in the geological record, according to a new study published in Sciences Advances by researchers from the Department of Organismic and Evolutionary Biology at Harvard University and the Museum of Comparative Zoology with collaborators.

"We discovered a strong correlation between these times of fast reptile evolution and rising temperatures. While some groups evolved more quickly than others, nearly all reptiles were evolving more quickly than ever before "Tiago R. Simes, a postdoctoral fellow and the senior author, said.

Due to a lack of data, previous investigations on the effects of these changes frequently overlooked terrestrial vertebrates in favor of the responses of marine creatures.

Simes and senior author Professor Stephanie E. Pierce of Harvard University, along with colleagues Michael Caldwell of the University of Alberta in Canada and Dr. Christian Kammerer of the North Carolina Museum of Natural Sciences, examined early amniotes—the closest living relatives of all living mammals, reptiles, birds, and other vertebrates—at the beginning of their evolutionary history. The earliest reptile and mammal ancestors were currently separating from one another and embarking on their own unique evolutionary journeys.

According to Simes, "Reptiles represent an ideal and rare terrestrial system to study this question because they have a relatively good fossil record and have survived a series of climatic crises including those preceding the Permian-Triassic mass extinction, the largest extinction in the history of complex life."

Compared to our mammalian forebears, reptiles were quite uncommon throughout the Permian. However, throughout the Triassic, the number of species and morphological variety in reptiles experienced a huge expansion. The majority of the major living reptile groups (crocodiles, lizards, and turtles) as well as numerous groups that are now completely extinct first appeared as a result of this.

More than 1,000 fossil samples from 125 species of reptiles, synapsids, and their closest relatives that lived roughly 140 million years before and after the Permian-Triassic extinction were used to construct a dataset by the researchers. Then, they conducted an analysis of the data using cutting-edge analytical methods like Bayesian evolutionary analysis, which is also used to comprehend the evolution of viruses like SARS-COVID 19. This analysis allowed them to determine when these species first evolved and how quickly they were evolving. The new information was then integrated by the researchers with global temperature data from the geological record that dates back several million years in order to provide a broad perspective of the creatures' primary adaptive response to climatic shifts.

According to Pierce, "Our results show that periods of rapid climatic change and global warming are associated with exceptionally high rates of anatomical change in most groups of reptiles as they adapted to new environmental conditions. And this process started long before the Permian-Triassic extinction, since at least 270 million years ago, indicating that the diversification of reptile body plans was not triggered by the P-T extinction event, as previously thought, but in periods of rapid climate change and global warming."

Simes noted that one group of reptiles, the lepidosaurs, which produced the first lizards and tuataras, "veered in the opposite direction of most reptile groups and underwent a phase of very slow rates of change to their overall anatomy. Essentially, their body plans were constrained by natural selection, instead of going rogue and radically changing like most other reptiles at the time." According to the experts, this is a result of pre-adaptations made to their body size to better withstand extreme temperatures.

"According to Simes, body size has a significant impact on an organism's physiology. Smaller reptiles are better at exchanging heat with their surroundings. Being much smaller than other groups of reptiles and not all that unlike from their contemporary relatives, the early lizards and tuataras were better suited to withstand sudden changes in temperature. In order to quickly adjust to the new environmental conditions, the much larger progenitors of crocodiles, turtles, and dinosaurs had to modify their bodies since they could not expel heat as easily."

Simes, Pierce, and other researchers also charted how variations in body size occurred throughout the world at this time. They demonstrated that in the lethally hot conditions of this time, there was a limit body size at which reptiles could survive in tropical climates.

Large-sized reptiles essentially followed one of two paths to adapt to these climate changes, according to Pierce: either they moved closer to temperate regions or they migrated into aquatic environments, where they didn't have to worry about overheating because water can absorb heat and maintain its temperature much better than air.

Climate change may have played a significant role in the emergence and explosion of new reptile body plans during the most recent Permian and Triassic, according to Simes. "This strong association between rising temperatures in the geological past and a biological response by dramatically different groups of reptiles," he said.

For assistance with specimen access, the researchers are grateful to the Museum of Comparative Zoology (MCZ), Harvard University, the vertebrate paleontology team, and the curators of more than 50 natural history museums worldwide. The Deutsche Forschungsgemeinschaft Grant KA 4133/1-1, the National Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant #23458 and NSERC Accelerator Grant, the Faculty of Science, Chairs Research Allowance, University of Alberta, the Lemann Brazil Research Fund, and funds made available through Harvard University all contributed to the funding.

Harvard University, Department of Organismic and Evolutionary Biology