Long ago, before the dinosaurs, there was a time when proto-mammals flourished. These early ancestors of ours were some of the most ecologically significant animals on the planet, filling landscapes to the brim with a variety of herbivores, carnivores and omnivores during what paleontologists know as the Permian period.
The mass extinction changed all that.
About 252 million years ago, intense volcanic activity caused Earth’s third mass extinction. An estimated 70 percent of known species are rapidly dying out, including most protomammals. Reptiles fared better during the aftermath, the early relatives of crocodiles, dinosaurs and other scaly creatures diversifying into a spectacular array of shapes and sizes.
Paleontologists are now beginning to understand how the age of reptiles began.
Surviving the mass extinction
No species can prepare for mass extinction. Events are often so fast that it is extremely difficult to adapt.
But over long evolutionary periods, organisms can develop certain traits that give them an advantage when mass extinction pressures mount. Reptilians had such an advantage.
The ways in which reptiles adapted to the greenhouse world created by the greenhouse gases spewed out by volcanoes set them up for ecological success during the Triassic, Jurassic, and Cretaceous periods.
“Reptiles underwent an evolutionary explosion of new species and body plans during the Triassic,” says paleontologist Thiago Simoes of Harvard University.
New findings reported in Scientific progress, does not come from a single fossil discovery. Instead, Simões and colleagues compared details of reptile evolution—such as body size and how fast early reptiles evolved—with data on global temperature in the prehistoric past.
Paleontologists had previously thought that reptiles predated ancient mammalian relatives since the Late Permian Catastrophe—seen as the largest mass extinction in Earth’s history — cleared the field and offered plenty of ecological free space for surviving species.
Read more about mass extinction events on Earth:
It is assumed that reptiles simply filled this space more quickly during the early part of the Triassic period. But this is a pattern rather than a reason for the change.
More likely, Simões and co-authors argue, is that increased temperatures during the earliest part of the Triassic forced reptiles to adapt in ways that opened up more evolutionary possibilities.
Temperatures soared during the Early Triassic. The average sea surface temperature at the end of the Permian was about 80 degrees Fahrenheit just before the mass extinction.
During the first 10 million years of the Triassic, average sea surface temperatures reached over 90 degrees Fahrenheit, indicating higher temperatures in the terrestrial environment as well. And even though these changes took place over millions of years, they still put pressure on the Permian survivors.
Organisms had to move, adapt or become extinct.
How ancient reptiles have changed
At least three possibilities were open to ancient reptiles, Simões and co-authors suggest.
Some reptiles, such as long-necked forms called protorosaurs, became smaller as temperatures rose. The smaller size makes it easier to shed excess body heat. Water may have been another refuge.
Before the mass extinction there was none aquatic reptiles living in the seas or have spent their entire lives in the water. But the Triassic saw an abundance of diverse groups—such as the ancestors of the shark-like ichthyosaurs and four-bladed plesiosaurs, as well as one-offs like the turtle-like placodonts—that became increasingly adapted to life in the water.
These were some of the first reptiles to really grow large, as life in the water allowed these creatures to reach sizes not yet possible on land.
Finally, paleontologists suggest, the reptiles may have migrated from the hottest, tropical habitats to more temperate habitats.
“The Triassic is often considered an age of freaks because of the very wild morphologies that reptiles [experienced],” says Yale University paleontologist Dalton Meyer, who was not involved in the new study.
Some reptiles, such as the wider family that includes dinosaurs and crocodiles, have evolved rapidly with changing temperatures. Other reptiles, such as the ancestors of the lizards, became smaller and did not assume such a wide variety of forms.
There were more nuances to why organisms survived and thrived, Meyer says, than previously appreciated.
Reptiles Prepared Before Climate Change
There was also an unexpected twist in the story.
Tens of millions of years before the end of the Permian, reptiles adapted to cope with a different, less intense climate change. This period of time is welcome the earliest turtlesthe ancestors of lizards and snakes, etc.
Although these forms were often small and seemed less significant than the Permian proto-mammals, they still came in a diverse range of forms and ecologies that gave reptiles as a whole a better chance to recover than -further disaster.
“Reptiles have already evolved new body plans or key new anatomical traits that allowed them to rapidly diversify into a wide range of ecosystems after the end of the Permian extinction that were previously dominated by their immediate land competitors,” says Simoes.
Previous environmental changes had caused the reptiles to cope in new ways, making it easier for them to do it again.
Reptilians didn’t just walk into an almost empty world and take it over. They evolved to cope with intense conditions after the extinction, the pressures of climate change spawned a variety of new forms that inhabited a wider swath of the planet than ever before.
When temperatures started to drop back to their pre-extinction levels, then there was a whole host of different reptiles that could breed even more. By the middle of the Triassic, the age of reptiles was in full swing.