Equatorial crocodile refuges stayed livable at 10% less sunlight after the asteroid, not 15%

Crocodiles and their relatives spent most of the last 245 million years following warm climates rather than adapting to new ones, and that habit may have left them in the right places when the dinosaur-killing asteroid hit, a UCL-led study in Science Advances reports. In the team's models, warm regions near the equator stayed livable for crocodile relatives if sunlight dropped by 5% or 10% after the impact, but not if it dropped by 15%.

The researchers note that the 15% scenario is now considered unlikely, so in most plausible versions of the aftermath the crocodiles' tropical strongholds held.

7,000 fossils matched to ancient climate maps

The team, led by Alfio Alessandro Chiarenza of UCL Earth Sciences, drew on over 7,000 fossils and other records for more than 400 species, living and extinct. Using fossil locations, family trees and the ranges of today's crocodiles, they estimated each species' preferred temperature and rainfall.

They then laid those preferences over computer reconstructions of Earth's climate and geography through time. The pattern was consistent: when the planet warmed, the group spread toward the poles, and when it cooled, they pulled back toward the tropics.

About 50 million years ago, in the early Eocene, global temperatures were roughly 10°C above today's and the poles had no ice. Crocodile relatives then ranged into the far north of North America and Europe, and into Antarctica.

Testing the asteroid winter

For the Chicxulub impact 66 million years ago, the team modeled conditions over the following decades to centuries. Dust and ash dimming the sun was the scenario that mattered. Higher carbon dioxide from volcanic eruptions, another candidate, made no difference to where crocodiles could live, according to the study.

Earlier work by members of the same team found that suitable climate for non-avian dinosaurs shrank far more sharply after the impact. "The asteroid did not make the whole planet equally hostile to every group," Chiarenza said.

The authors do not claim climate is the full answer. Small body size, a semi-aquatic life, broad diets and the low energy needs of cold-blooded animals probably helped too, they say.

Two lineages broke the rule

The pattern had exceptions. Around 94 million years ago, during the Cretaceous Thermal Maximum, when temperatures reached up to about 14°C above today, one branch adapted to heat beyond what the lineage tolerated at other times. Around 34 million years ago, a now extinct group of Australian crocodiles adapted to desert-like conditions as the continent dried.

Philip Mannion, a co-author at UCL, said the lesson for living crocodiles is that they will have to shift their ranges as temperatures rise, while roads, farms and cities shrink the space available to them.