A preference for warm climates may have helped crocodiles and their ancient relatives survive the asteroid impact that caused the extinction of most dinosaurs 66 million years ago. A new study suggests that tropical and subtropical regions near the equator may have remained suitable refuges for crocodilian populations, even as habitats for many dinosaurs deteriorated.
Scientists led by UCL (University College London) found that crocodiles and their relatives spent much of their roughly 250 million-year evolutionary history tracking temperate climates rather than adapting to dramatic temperature changes. As Earth warmed, they expanded into new regions, at times reaching Antarctica. When temperatures fell, their range contracted toward the tropics.
The study analyzed more than 7,000 fossils and records from more than 400 living and extinct species. Researchers combined those data with computer models that reconstructed Earth’s climate and geography over 245 million years.
Why crocodiles survived the dinosaur extinction
The researchers modeled environmental conditions after the Chicxulub asteroid impact 66 million years ago. The simulations showed that regions near the equator may have remained warm enough to support crocodilian relatives, allowing surviving populations to persist and eventually recover.
Previous modeling by members of the same research team found that habitat suitable for non-avian dinosaurs shrank much more severely after the impact. In the new study, tropical and subtropical regions remained potential refuges for crocodilians in most scenarios. Only the most extreme climate simulations eliminated those areas.
“The asteroid did not make the entire planet equally hostile to any group,” said lead author Dr Alfio Alessandro Chiarenza of UCL Earth Sciences.
“Crocodilians have long favored warmer environments and may have placed their survivors in areas where suitable climate is best, such as near the equator. These refugia may have protected populations that then recovered and spread outward.”
The finding offers a possible explanation for how a group of animals that emerged around the same time as dinosaurs survived two of Earth’s five major mass extinctions.
Ancient crocodilians were far more diverse
Although modern crocodiles and their relatives share many recognizable features, their evolutionary history includes a much broader range of body shapes, sizes and lifestyles.
The wider crocodilian lineage, known as pseudosuchians, once included heavily armored plant eaters, fully terrestrial predators and giant animals weighing more than a ton.
One notable example was Sarcosuchus, a gigantic predator often called a “super crocodile” that could reach up to 12 meters in length. Another prehistoric relative, Postosuchus, was a land-dwelling hunter that could move on two or four legs.
“Although crocodiles are often referred to as living fossils, their ancestors and relatives have undergone significant changes throughout their history, ranging from giant crocodiles the size of a bus to those that can swallow people,” Chiarenza said.
“Our study shows that despite this diversity, one thing that was surprisingly consistent was the relationship with climate.”
What fossils reveal about 245 million years of climate history
To investigate that relationship, the scientists reconstructed the temperature and precipitation preferences of 400 crocodilian species and their ancient relatives. They used fossil evidence, evolutionary family trees and detailed maps showing where living crocodile species occur today.
The team compared those preferences with reconstructions of ancient geography and climate to determine where suitable habitat existed at different points in Earth’s history.
A clear pattern emerged: crocodilian relatives generally followed warmer environments as the planet’s climate changed. Their ranges expanded during hotter periods and contracted when temperatures cooled.
About 50 million years ago, during the particularly warm early Eocene, global temperatures were approximately 10 degrees Celsius higher than today, and the polar regions had no permanent ice. Crocodilian relatives lived at high latitudes in North America, Europe and even Antarctica.
Over the following several million years, global cooling gradually reduced the area in which they could thrive. Their geographic range contracted toward warmer tropical regions.
Two important exceptions
Although the preference for warm environments was remarkably consistent, the researchers identified two important exceptions.
During the Cretaceous Maximum, about 94 million years ago, global temperatures rose 14 degrees Celsius above modern levels. One lineage in the crocodilian family tree evolved the ability to tolerate hotter conditions than other lineages could withstand.
A second adaptation occurred in Australia about 34 million years ago. As prolonged cooling reduced precipitation and dried the continent, extinct crocodilian groups adapted to increasingly arid, desert-like environments.
These exceptions indicate that some crocodilian relatives could evolve to cope with different climates, even though most continued to track the warmer conditions they preferred.
What happened after the Chicxulub impact?
The researchers used climate simulations to examine the decades and centuries immediately following the Chicxulub asteroid impact, which caused the extinction of most dinosaurs.
They considered several possible environmental effects, including sunlight blocked by dust and ash and increased carbon dioxide levels associated with volcanic eruptions.
When the simulations reduced sunlight by 5% or 10%, warm regions near the equator remained suitable for crocodilian relatives. These areas may have served as important refuges during the environmental crisis.
In a more severe scenario involving a 15% reduction in sunlight, however, those regions also became unsuitable. The researchers noted that such an extreme decline in sunlight is now considered unlikely.
Other simulated changes, including an increase in atmospheric carbon dioxide, did not alter the availability of climates suitable for crocodilians.
Taken together, the results suggest that crocodiles may have benefited from living in areas where temperatures remained within their preferred range, even as conditions became far less favorable for many dinosaurs.
A survival strategy that could become a weakness
The researchers emphasized that favorable temperatures were probably only one reason crocodiles survived the mass extinction.
Other characteristics may also have helped, including relatively small body size, the ability to live partly underwater, a flexible diet and moderate energy requirements. As cold-blooded animals, crocodiles require less food to sustain themselves than animals that generate their own body heat.
The findings also raise questions about how crocodiles will respond to modern climate change.
For millions of years, moving to areas with suitable temperatures appears to have been one of their main responses to a changing planet. Today, however, the expansion of cities, farmland and roads increasingly limits where wild animals can move.
“These findings have implications for the species today,” said Professor Philip Mannion of UCL Geosciences. “If crocodiles have coped with climate change in the past primarily by moving to suitable habitats, then as the world warms, species now need room to move, but this is becoming more difficult as suitable areas for roads, farms and cities disappear.”
The research was funded by the Royal Society, the Leverhulme Trust and the National Environmental Research Council (NERC).
Source: www.sciencedaily.com
