Perhaps surprisingly, the closest living relatives of crocodiles and crocodiles are birds. Both are archosaurids, a group of species that arose about 250 million years ago near the beginning of the Triassic.
The group soon split into two lineages: one gave rise to crocodiles (Fake sochia), while the other led to dinosaurs and then birds (Avmitatarsalia). This means that there is a combined 500 million years of evolutionary change between modern birds and crocodiles.
The eons have erased almost all evidence of common descent. Crocodiles are cold-blooded (physiologists call them “ectothermic”). Their body temperature is determined by the environment, and they have low metabolic rates (the speed at which their bodies convert food into energy). They appear sluggish, except for short periods of vigorous activity while drowning prey or fighting among themselves.
Birds are warm-blooded (or warm-blooded). They have high metabolic rates, producing energy quickly enough to warm and stabilize their body temperatures and undertake sustained, strenuous flight exercise. Birds do not get tired and fall from the sky.
Even if crocodiles somehow had wings, the metabolic difference in strength alone meant they could never fly.
How did birds and crocodiles become so different? The traditional assumption has been that their common ancestor was more like a crocodile than a bird, but our new research shows that the opposite is likely true.
Traditional image
Scientists have assumed that warm-blooded animals evolved from cold-blooded animals, and not the other way around. As warm-blooded creatures, we have become accustomed to believing that warm-blooded animals are physiologically superior to cold-blooded animals.
By this logic, if a group of animals evolved to become warm-blooded, they would never be able to return to a cold-blooded state without quickly becoming extinct.
From this perspective, it is logical to believe that the crocodile lineage was cold-blooded from its origins. Scientists thought that warm blood only appeared in the bird lineage of dinosaurs.
New photo
Our first clue that the crocodile lineage was originally warm-blooded is that living crocodiles have four-chambered hearts. The only other groups that have such hearts are birds and warm-blooded mammals, and there is a functional connection with warm-blooded ones.
High metabolic rates in warm-blooded animals require high rates of blood flow and high blood pressure. Four-chambered hearts completely separate low-pressure blood, which passes through the pulmonary arteries to the lungs, from high-pressure blood, which travels through systemic arteries to the rest of the body.
Decompression is vital to prevent high-pressure blood from reaching the lungs. If this happens, it can cause a deadly buildup of fluid that prevents oxygen uptake. So modern crocodiles have a warm-blooded heart but produce the low blood flow rates and pressures we expect for their cold-blooded nature. This suggests that their hearts had once evolved to live a warm-blooded life.
Because warm blood is associated with high rates of blood flow, we looked for fossil evidence of large blood vessels to accommodate the high flow. We had previously discovered that the holes in leg bones that pass blood vessels through the bones of living mammals were relatively larger than those in living cold-blooded reptiles.
We checked this against these holes in ten species of dinosaur, and confirmed that they were also warm-blooded. But we had no measurements from older archosaurs, nor any from ancient members of the crocodile lineage.
Our new study of foramen size in fossil leg bones focused on the closest relatives of archosaurs from both lineages that we could find. Starting in 2018, we collected data from 81 species, which showed that both crocodilian lineages, dinosaurs, and birds had high metabolic rates until relatively recently — perhaps about 66 million years ago, around the time most dinosaurs became extinct — when crocodiles abandoned them in favor of a cold-blooded metabolism.
These results are consistent with many other pieces of evidence regarding the evolution of metabolism in crocodilian lineages, dinosaurs, birds, and also among mammals.
From active land dwellers to aquatic predators
The new look at the history of the crocodile lineage begins in the Triassic period, when they were active, warm-blooded land-dwelling animals, in many cases running on two legs. It diversified into hundreds of species during the Jurassic and Cretaceous periods, and lived on land, in salt and fresh water, and everything in between. However, they appear to have remained warm-blooded until recently.
Why did their metabolism slow down? We think it may have been related to their current behavior as predators in the water. The lower metabolic rates that come with being cold-blooded allow them to hold their breath longer to remain hidden while waiting for predators and while drowning them.
There are no warm-blooded water predators alive today. Warm-blooded animals simply cannot hold their breath long enough.
A return to a cold-blooded metabolism may have saved the crocodile lineage from extinction 66 million years ago when an asteroid ended the lines of most dinosaurs, except birds, of course.
