Despite an incredible number of visible differences, modern-day crocodiles and birds share a common ancestor dating back to the Early Triassic. Paleontologists call them archosaurs: a hardy clade of reptiles that toughed it out through the sulfurous, inhospitable years of the Great Dying that followed the PermianâTriassic extinction event roughly 250 million years ago.

Scientists had long assumed that the critical juncture marking archosaursâ evolutionary split on their separate journeys toward birds and crocs was the moment that birdsâ avian-like ancestors first became warm-bloodedâbut it turns out they were wrong. Crocodilesâ ancient forebears also evolved through a phase of warm-blooded species before reversing tack back into aquatic, cold-blooded killers. A new analysis of fossilized bones from 81 archosaur species, spanning the earliest known relatives along both lineages, has found that the crocodileâs ancient ancestors were likely warm-blooded until as recently as 66 million years ago, when the Chicxulub asteroid killed off most dinosaurs.

âScientists have assumed that warm-blooded animals evolved from cold-blooded ones, not the reverse,â as Roger Seymour, an emeritus professor of physiology with Adelaide University in Australia, explained in an essay for The Conversation.

âAs warm-blooded creatures ourselves, we have had a habit of thinking warm-blooded animals are physiologically superior to cold-blooded ones,â Seymour, the new studyâs first author, noted.

Reverting back to a cold-blooded physiology had real benefits for crocodiles, Seymour and his colleagues argue. The process slowed these creaturesâ metabolism, allowing them to hide in water longer to ambush prey and âremain submerged while drowning prey.â

The tell-tale heart

âOur first clue that the crocodile lineage was originally warm-blooded was that living crocodiles have four-chambered hearts,â Seymour said of prior findings that led to this new study. âThe only other groups with such hearts are warm-blooded birds and mammals, and there is a functional connection with warm-bloodedness.â

As an organic innovation of evolution, the four-chambered heart accomplishes several things that make the high metabolism of a warm-blooded animal possible. For starters, these additional segregated chambers help separate the low-pressure blood flow needed to prevent fluid entering into the lungs from the higher pressures needed to pump blood throughout the rest of the body, meeting those speciesâ higher energy needs.

âBirds donât fatigue and fall out of the sky,â Seymour noted. âThey have high metabolic rates, producing energy quickly enough to warm and stabilise their body temperatures and to undertake sustainable, strenuous exercise of flight.â

Crocodiles, of course, are comparatively more lethargic; theyâre wait-and-see predators whose kills come in bursts of surprising anaerobic activity, thrashing and crushing their prey. And, yet, the crocodilesâ heart betrays a different epoch, the remnants of a much more active lifestyle.

In cold blood

To assess whether or not crocodilesâ archosaur ancestors were truly warm-blooded, Seymour and an international team of physiologists, paleontologists, and geoscientists focused on highly suggestive holes in the fossilized leg bones of these species. Past research had shown that these holesâpassageways threaded with blood vesselsâwere significantly larger in warm-blooded animals than cold-blooded species.

The team used careful measurements of the diameters of these holes to calculate nutrient artery blood-flow rates for both their specimens and various species living today. The extinct archosaursâ average blood-flow rate was closer to both high-metabolism reptiles, like Komodo dragons, and mammals alive today than either the crocodile or its cold-blooded brethren.

Seymour and his coauthors acknowledge there is some uncertainty to these calculations. Some paleontologists have argued that many cold-blooded dinosaurs from this period may have had higher blood flows simply by virtue of their enormous size, which offered its own form of insulation and temperature regulation, dubbed âgigantothermy.â

But a study of microfractures within these fossilized bones, they argued, suggests that these archosaurs lived more active, high-energy lives. But, in the toxic atmosphere kicked up after the cataclysmic Chicxulub impact, slowing things down (and frankly, maybe breathing less) might just have been what helped these species survive.

âReversion to a cold-blooded metabolism may have saved the crocodile lineage from extinction 66 million years ago,â Seymour opined.