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 endured through the sulfurous, inhospitable years of the Great Dying that followed the Permian-Triassic extinction about 250 million years ago. Scientists had long assumed that the critical moment that marked the evolutionary split of archosaurs on their separate journeys into birds and crocodiles was the moment when the bird-like ancestors of birds first became warm-blooded, but it turns out they were wrong. The ancient ancestors of crocodiles also evolved through a phase of warm-blooded species before becoming cold-blooded aquatic killers again. A new analysis of fossilized bones from 81 archosaur species, spanning the earliest known relatives of both lineages, has found that the crocodile’s ancient ancestors were probably warm-blooded until just 66 million years ago, when the Chicxulub asteroid wiped out most of the dinosaurs. “Scientists have assumed that warm-blooded animals evolved from cold-blooded ones, and not the other way around,” as Roger Seymour, professor emeritus of physiology at the University of Adelaide in Australia, explained in an essay for The Conversation. “As warm-blooded creatures, we have a habit of thinking that warm-blooded animals are physiologically superior to cold-blooded ones,” said Seymour, the first author of the new study. Returning to a cold-blooded physiology had real benefits for crocodiles, Seymour and his colleagues maintain. The process slowed down the metabolism of these creatures, allowing them to hide in the water longer to ambush their prey and “stay submerged while they drown them.” The telltale heart “Our first clue that the crocodile lineage was originally warm-blooded was that living crocodiles have four-chambered hearts,” Seymour said of previous findings that led to this new study. “The only other groups with such hearts are birds and warm-blooded mammals, and there is a functional connection to warm blood.” 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 secreted chambers help separate the low-pressure blood flow needed to prevent fluid from entering the lungs from the higher pressures needed to pump blood to the rest of the body, meeting the higher energy needs of those species. “Birds don’t get tired and fall from the sky,” Seymour noted. “They have high metabolic rates, producing energy quickly enough to warm up and stabilize their body temperature and perform strenuous and sustainable flight exercise.” Crocodiles, of course, are comparatively more lethargic; They are wait-and-watch predators whose deaths come in bursts of surprising anaerobic activity, striking and crushing their prey. And yet, the crocodiles’ hearts reveal a different era, the remains of a much more active lifestyle. Cold-blooded To assess whether the archosaur ancestors of crocodiles 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. Previous research had shown that these holes (corridors filled with blood vessels) were significantly larger in warm-blooded animals than in cold-blooded species. The team used careful measurements of the diameters of these holes to calculate the blood flow rates of the nutrient arteries for both their specimens and several species living today. The average blood flow rate of extinct archosaurs was closer to both high-metabolism reptiles, such as Komodo dragons, and mammals alive today, than the crocodile or its cold-blooded brethren. Seymour and his co-authors acknowledge that there is some uncertainty in these calculations. Some paleontologists have argued that many cold-blooded dinosaurs from this period may have had greater blood flow simply by virtue of their enormous size, which offered its own form of insulation and temperature regulation, called “gigantothermy.” 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 created after the cataclysmic Chicxulub impact, slowing things down (and, frankly, maybe breathing less) might have been what helped these species survive. “The reversion to a cold-blooded metabolism may have saved the crocodile lineage from extinction 66 million years ago,” Seymour said.
