Modern-day crocodiles and birds share a common archosaur ancestor that survived the Permian-Triassic extinction event roughly 250 million years ago, according to paleontologists. New research reveals that unlike long-held scientific assumptions that warm-bloodedness only evolved once toward avian lineages, ancient crocodile forebears also possessed a warm-blooded physiology until the Chicxulub asteroid impact roughly 66 million years ago, as detailed by Adelaide University emeritus professor of physiology Roger Seymour and an international team of physiologists, paleontologists, and geoscientists.
Evolutionary Shift From Warm-Blooded Killers to Cold-Blooded Ambush Predators
Scientists previously assumed that warm-blooded animals evolved exclusively from cold-blooded ones rather than reversing the process, according to Seymour writing in an essay for The Conversation. However, analyzing fossilized bones from 81 archosaur species spanning earliest known relatives revealed that crocodile ancestors maintained high metabolisms until the end of the Cretaceous period. Reverting to a cold-blooded metabolism provided distinct survival advantages.
According to Seymour and his coauthors, slowing down metabolic rates allowed crocodiles to hide underwater for extended periods and remain submerged while drowning prey. This physiological reversal likely protected the crocodile lineage from extinction when the toxic atmosphere followed the cataclysmic Chicxulub impact 66 million years ago.
The Tell-Tale Heart Anatomy Reveals Active Past
Living crocodiles possess a four-chambered heart, which served as the first anatomical clue that their lineage was originally warm-blooded, according to Seymour. Outside of mammals, birds are the only other living group with four-chambered hearts, which feature a functional connection to endothermy.
These segregated chambers separate low-pressure blood flow to the lungs from high-pressure flow to the rest of the body, supporting high energy needs. While birds maintain high metabolic rates for sustainable flight, crocodiles today are wait-and-see predators relying on brief bursts of anaerobic activity. Yet, their heart anatomy preserves the structural remnants of a much more active, high-energy lifestyle.
Fossil Bone Measurements and Blood Flow Rates
To test whether ancient archosaurs were truly warm-blooded, researchers examined nutrient artery blood-flow rates by measuring suggestive holes in fossilized leg bones. Past research established that these blood vessel passageways are significantly larger in warm-blooded species than in cold-blooded ones, according to the study.
Calculations showed that the extinct archosaurs averaged blood-flow rates closer to modern mammals and high-metabolism reptiles like Komodo dragons than to today’s cold-blooded crocodiles. While some paleontologists suggest large cold-blooded dinosaurs achieved higher blood flows through gigantothermy, microfracture analysis of the fossilized bones supports the conclusion that these archosaurs lived active, high-energy lives.
Jurassic Adaptations and Marine Migrations
Fossil teeth analysis from other prehistoric crocodile relatives provides additional insight into ancient metabolic adaptability. According to research from a team including scientists from Edinburgh reported by scienceaq.com, at least one prehistoric crocodile group known as metriorhynchids was warm-blooded, allowing them to raise their body temperature during a global cooling period around 150 million years ago.
Findings on fossil tooth enamel oxygen levels indicate that metriorhynchids used their metabolism to generate heat and thrive in open oceans. This warm-bloodedness enabled their transition into dolphin-like bodies with flippers and tail fins. In contrast, closely related teleosaurids were cold-blooded and relied on basking in the sun, which likely contributed to their die-off when sea temperatures dropped at the end of the Jurassic Period, according to Dr. Mark Young.
Did you know? Modern crocodiles possess four-chambered hearts—a physiological trait they share with birds and mammals, reflecting an ancient ancestry of high-energy, warm-blooded movement before reverting to cold-blooded ambush tactics.
Frequently Asked Questions
Were all ancient crocodile ancestors warm-blooded?
Research on fossilized bones indicates that the crocodile lineage was warm-blooded until approximately 66 million years ago, when they reverted to a cold-blooded metabolism following the Chicxulub asteroid impact, according to Roger Seymour and colleagues.
How do scientists know prehistoric crocodiles were warm-blooded?
Scientists measure the diameters of nutrient artery passageways in fossilized leg bones, which are significantly larger in warm-blooded animals. Additionally, researchers analyze oxygen levels in fossil tooth enamel to determine ancient body temperatures.
Reverting to a cold-blooded metabolism lowered their metabolic rate, enabling crocodiles to remain submerged longer to hide and drown prey, while also helping them survive the harsh environmental conditions triggered by the asteroid impact.
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