Following the Warmth: Why Crocodiles Outlived the Dinosaurs
AWhen the Chicxulub asteroid struck the Earth around 66 million years ago, the age of the dinosaurs came to an abrupt end. Yet crocodiles, which had shared the planet with dinosaurs for tens of millions of years, came through the catastrophe and are still with us today. Crocodiles are, after all, reptiles that rely heavily on warmth from their surroundings, which might seem to make them especially vulnerable to sudden cooling. Why one group perished while the other endured has long been a puzzle for palaeontologists. A new study from University College London, published in the journal Science Advances, suggests that a large part of the answer lies in a simple habit: for most of their history, crocodiles and their relatives have followed warm climates rather than adapting to new ones.
BThe research team, led by Alfio Alessandro Chiarenza, examined more than 7,000 fossils and records belonging to over 400 living and extinct species. Drawing on fossils, evolutionary family trees and the present-day distribution of crocodiles, they reconstructed the temperatures and rainfall levels that each species preferred. These preferences were then combined with computer models of climate and geography stretching back 245 million years. This approach allowed the scientists to trace not only where crocodile relatives lived, but also why their ranges shifted over time. Such a large dataset made it possible to look for patterns across the entire history of the group rather than in isolated species.
CThe picture that emerged was one of remarkable consistency, a pattern that biologists describe as climatic niche conservatism. Across roughly 250 million years, the group tended to keep the same climatic preferences, so that when the planet changed, they moved rather than changing themselves. During warm periods they spread towards the poles: in the early Eocene, about 50 million years ago, when global temperatures were around 10°C higher than today and there was no permanent ice at the poles, crocodile relatives lived at high latitudes in North America, Europe and even Antarctica. When the Earth cooled again, the area of suitable habitat shrank and they retreated towards the tropics.
DThis conservatism is all the more striking given how varied the group once was. The wider lineage, known as pseudosuchians, included heavily armoured plant-eaters, predators that hunted on land and animals weighing more than a tonne. Sarcosuchus, sometimes called the "super croc", could grow to about 12 metres long, while Postosuchus was a land hunter that could walk on either two legs or four. Some looked so unlike modern crocodiles that a casual observer might not have recognised them as relatives at all. Despite this variety of body forms and lifestyles, the group's relationship with temperature changed surprisingly little.
EThere were, however, two notable exceptions. Around 94 million years ago, during a period known as the Cretaceous Thermal Maximum, temperatures rose as high as 14°C above modern levels, and one branch of the group evolved a greater tolerance of heat. Much later, about 34 million years ago, an extinct group in Australia adapted to increasingly dry, desert-like conditions as a long period of cooling reduced rainfall. These cases show that adaptation was possible, but they appear to have been the exception rather than the rule.
FThe habit of tracking warmth may explain how crocodiles survived the asteroid impact. The team simulated the aftermath of the collision, in which dust and ash would have blocked sunlight while volcanic activity raised levels of carbon dioxide. If sunlight was reduced by 5 or 10 per cent, regions near the equator remained suitable for crocodile relatives. Only a 15 per cent reduction made those regions unsuitable as well, and the researchers point out that such an extreme scenario is now considered unlikely. Higher carbon dioxide levels, meanwhile, made no difference to the availability of suitable climates. Earlier modelling by members of the same team had found that the habitats of non-avian dinosaurs shrank far more severely after the impact. In effect, the tropics may have acted as a warm refuge in which crocodiles could wait out the crisis.
GThe researchers are careful not to claim that temperature was the whole story. Other traits probably helped as well: many crocodile relatives were small, could live partly in water, ate a wide range of foods and, as cold-blooded animals, needed relatively little energy. It would be a mistake, therefore, to attribute their survival to a single advantage. Rather, a strategy of following familiar conditions, combined with a flexible way of life, seems to have served them well when conditions changed suddenly.
HThe findings also carry a warning for the present. For millions of years, moving to a suitable climate has been the main way in which crocodiles have coped with climate change. Today, however, roads, farmland and cities increasingly stand in the way of such movement. Where a river is dammed or a wetland drained, there may simply be nowhere for an animal to go. As Professor Philip Mannion, one of the study's authors, has pointed out, living species will need room to move as the planet warms, and the loss of their habitats is making this more difficult. The strategy that carried crocodiles through one of the greatest disasters in the Earth's history may not be enough to protect them in a world reshaped by humans.