The largest mass extinction in Earth’s history marked the end of the Permian period, some 252 million years ago. Over 80 percent of all species around at the time disappeared forever from the fossil record. This changed the nature of life on Earth and ushered in a new era in which the reptiles evolved and proliferated. In the past, biologists have understood that this rapid radiation of reptiles was driven by the plethora of ecological niches made available by the extinction of Permian species, but new research has now proposed that climate change was also a significant factor behind the spectacular evolution and expansion of reptiles at this time.
“We found that these periods of rapid evolution of reptiles were intimately connected to increasing temperatures. Some groups changed really fast and some less fast, but nearly all reptiles were evolving much faster than they ever had before,” said lead author Tiago R Simões, postdoctoral fellow at the Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology, Harvard University.
For the investigation, Simões and senior author Professor Stephanie E. Pierce (also from Harvard) worked alongside collaborators Professor Michael Caldwell (University of Alberta, Canada) and Dr. Christian Kammerer (North Carolina Museum of Natural Sciences) to examine early amniotes. These animals represent the forerunners of all modern mammals, reptiles, birds, and their closest extinct relatives, at the initial phase of their evolution. At this point in time, the first groups of reptiles and mammal ancestors were splitting from each other and evolving along their own separate evolutionary paths.
“Reptiles represent an ideal and rare terrestrial system to study this question as they have a relatively good fossil record and survived a series of climatic crises including the ones leading up to the largest extinction in the history of complex life, the Permian-Triassic mass extinction,” said Simões.
During the Permian period, it was the synapsids that dominated the vertebrate fauna. These were not reptiles, but the ancestors of mammals, known as stem mammals or proto-mammals. Most of them perished during the end Permian extinction, their ecological niches being taken over by reptiles, including numerous dinosaur species, that evolved and radiated rapidly during the Triassic Period (252-200 million years ago).
The new study, published in the journal Science Advances, makes use of morphological data from more than 1,000 fossil specimens from 125 species of reptiles, synapsids, and their closest relatives during approximately 140 million years before and after the Permian-Triassic extinction. This data set is 40 percent larger than the previous largest data set and most of the measurements were made by direct observations and micro-CT scans.
The researchers analyzed this data to identify when these fossil species first originated and how fast they were evolving, 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. They then combined the morphological data with information on body size and with global temperature data that spanned millions of years in the geological record, to provide a broad overview of the animals’ major adaptive responses towards climatic shifts.
The experts found that the appearance of new species, increases in the rates of morphological change and alterations in geographical distributions corresponded to periods of rapidly changing climatic conditions, including global warming and cooling,
“Our results reveal that periods of fast climatic shifts 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 fact started tens of million years before that,” said Professor Pierce.
“One reptile lineage, 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 rates of change to their overall anatomy,” said Simões, “essentially, their body plans were constrained by natural selection, instead of going rogue and radically changing like most other reptiles at the time.” The researchers suggest this is due to pre-adaptations on their body size to cope better with high temperatures.
“The physiology of organisms is really dependent on their body size. Small-bodied reptiles can better exchange heat with their surrounding environment. The first lizards and tuataras were much smaller than other groups of reptiles, not that different from their modern relatives, and so they were better adapted to cope with drastic temperature changes. The much larger ancestors of crocodiles, turtles, and dinosaurs could not lose heat as easily and had to quickly change their bodies in order to adapt to the new environmental conditions.”
According to Professor Pierce, large-sized reptiles basically took two routes to deal with these climate shifts – they either migrated closer to temperate regions or invaded the aquatic world where they didn’t need to worry about 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 by dramatically different groups of reptiles suggests climate change was a key factor in explaining the origin and the explosion of new reptile body plans during the latest Permian and Triassic,” said Simões.
“We show that the origin and phenotypic radiation of reptiles was not solely driven by ecological opportunity following the end-Permian extinction, as previously thought, but was also the result of multiple adaptive responses to climatic shifts spanning 57 million years,” concluded the study authors.