While once thought to be cold blooded, after years  of debate it is now accepted that the non-avian  

dinosaurs were warmed blooded like modern birds.  As birds are dinosaurs themselves, the closest  

living relatives of the dinosaurs are the  crocodilians, who are cold blooded like most other  

reptiles. Therefore, it was previously assumed the  dinosaurs’ warm-blooded metabolism evolved after  

their last common ancestor with crocodilians. But  while reasonable given the evidence available at  

the time, additional evidence was since shown that  assumption to be wrong. The first paper to provide  

evidence for this, titled “Evidence for  Endothermic Ancestors of Crocodiles at  

the Stem of Archosaur Evolution'' was  published in 2004. It hypothesized that,  

like the dinosaurs, the ancestors of crocodilians  were also warm blooded, only later reverting back  

to a cold-blooded metabolism. Since then,  more research has added to the original body  

of evidence, confirming the hypothesis. This  video will examine this evidence, from fossils  

to evidence within living crocodilians. Part 2  will then explore how crocodilian thermoregulation  

changed over time and why crocodilians reverted  back to a lower, cold-blooded metabolism.  

Before going any further, it is important to  establish what the terms “warm blooded” and  

“cold blooded” actually mean. “Cold blooded”  is the popular term for an ectothermic  

metabolism. The body temperature of ectothermic  animals like crocodilians is derived from the  

external environment. While this means their  internal body temperature will vary wildly,  

ectotherms can modify it through behavior, such as  sitting in the shade to cool off or basking in the  

sun to warm up. On the other end of the spectrum  are the warm blooded, or endothermic species,  

who generate their own body heat. Mammals and  birds achieve this by burning vast amounts of  

energy at the cellular level. This keeps their  body temperature consistently at a narrow range,  

which allows their internal biochemical reactions  to always happen close to their optimal rate.  

The result is that endothermic animals are able  to be much more active than ectothermic ones.  

It should be noted that this is an  oversimplification. There are animals who have  

an intermediate metabolism called mesothermy, with  examples including monotremes, the naked mole-rat,  

and the leatherback sea turtle. Some of these  mesotherms modify their body temperatures in  

ways other than burning energy. Additionally, some  ectotherms have a higher metabolism than the norm,  

with monitor lizards being one of the most  extreme modern examples. Finally, there is no  

fine line between ectothermy, mesothermy, and  endothermy. To avoid overcomplicating things,  

for the purpose of this video mesothermy  will be considered a form of endothermy.  

So, what traits are present in living  crocodilians that suggest an endothermic ancestry?  

While most reptiles have a three chambered  heart, crocodilians instead possess a four  

chambered heart, like birds and mammals. A four  chambered heart allows for the separation of  

oxygenated and deoxygenated blood, as well  as systemic and pulmonary blood pressure.  

This allows for a high systemic blood pressure  while keeping the pulmonary blood pressure low.  

A high systemic blood pressure is required  for the high blood flow needed in endotherms,  

though a high pulmonary blood pressure would be  dangerous for both endotherms and ectotherms.  

Therefore, without a four chambered heart,  it would be impossible to safely maintain  

a blood flow high enough to meet the high  oxygen requirements of endothermic cells.  

While clearly useful for endotherms,  the advantages of a four chambered heart  

for the cold-blooded crocodilians are less  apparent. That said, crocodilian hearts do differ  

from those of birds. Ectothermic reptiles can  mix their oxygenated and deoxygenated blood to,  

among other things, reduce oxygen consumption.  Despite their four chambered hearts, crocodilians  

can also mix their blood, but the site of this is  not the heart like in other reptiles, but from a  

complex set of outflow vessels. This suggests  the modern crocodilian heart is the result of  

them having to redevelop the ability to mix their  blood after evolving their four chambered hearts.  

Crocodilian limbs are also unusual. In general,  ectotherms hold their legs in a sprawling posture  

while endotherms have their limbs placed directly  beneath their bodies. An upright posture allows  

for the ability to both run and breathe at the  same time, which is necessary for long periods  

of activity. As for crocodilians, though they  sometimes keep their legs in a sprawling position,  

like other reptiles, they can also shift their  legs into a semi- erect stance, called the “high  

walk”, when they need to walk quickly. Some  species of crocodiles take this even further,  

being capable of galloping. While their semi-erect  posture would still otherwise interfere with  

breathing when running, their diaphragmaticus  muscle allows for crocodilians to overcome this.  

However, because of their low metabolism,  crocodilians still lack the endurance to be  

able to run like this for very long. Therefore,  it is thought this behavior is an evolutionary  

relic retained from more active ancestors.  Additionally, the crocodilian respiratory system  

exhibits unidirectional airflow like in birds,  which is much more efficient than the respiratory  

system of other reptiles and mammals. While  not on par with that of their avian relations,  

such a respiratory system is overbuilt for  the needs of an ectotherm. Crocodilians in  

particular hunt primarily by ambushing prey  from the shore and, as previously stated,  

are incapable of the long periods of activity than  would select for such lungs. Therefore, like their  

four chambered hearts and semi-erect limbs, their  efficient respiratory system seems to have been  

inherited from more active, endothermic ancestors.  It is apparent that crocodilians are very atypical  

for cold blooded reptiles. But does that prove  that they were actually once endothermic?  

Proving this required careful study of the  fossils of their prehistoric ancestors.  

Those reptiles with a more recent common ancestor  with crocodilians than with dinosaurs are called  

pseudosuchians. During the Triassic Period,  which lasted from 250 million years ago to 200  

million years ago, Pseudosuchia was very  diverse. It included large carnivores,  

armored herbivories, reptiles that  converged upon a crocodilian-like body plan  

independently of the actual crocodilians, and  creatures that would have been best described  

as dinosaur mimics if they hadn’t evolved  before the dinosaurs in question. Within  

Pseudosuchia is the clade Crocodylomorpha, which  was the only branch of Pseudosuchia to survive  

a mass extinction at the end of the Triassic  period, and is the branch crocodilians belong to.  

So, how do we know what the metabolism was  of these long dead basal crocodylomorphs  

and the other pseudosuchians? The easiest  indicator is their limbs. Whereas crocodilians  

can shift to a semi-erect posture, those of most  other pseudosuchians, including many prehistoric  

crocodylomorphs, were held directly beneath their  bodies. This remained true in Crocodylomorpha  

until the ancestors of the crocodilians  transitioned to life in the water. This is exactly  

what was predicted to be the case if crocodilians  were the descendants of endotherms. However,  

as will be elaborated on in Part 2, the limbs  of some crocodylomorphs who were ectothermic,  

if not to the same degree as most other  reptiles, were also fully erect. Therefore,  

though their erect limbs at the very least  suggest endothermic ancestry, limb posture alone  

is insufficient to determine a prehistoric  animal’s basic metabolism. Perhaps the most  

compelling evidence is the confirmation of  prehistoric pseudosuchians doing something  

ectotherms are incapable of. First, some  pseudosuchians simply couldn’t survive as  

ectotherms. Pushing blood vertically  requires a high systemic blood pressure.  

Which in turn requires a four chambered heart and  a high cardiac output. Given the limits to how  

active an ectothermic heart can be, this in  turn limits the height of the highest point  

of the body from their heart. While the bodies  of today’s crocodilians conform to this limit,  

some pseudosuchians from the Triassic period, such  as Ornithosuchus and especially the sail-backed  

Ctenosauriscus, exceeded it. Therefore, to  actually pump blood throughout their bodies,  

some of these Triassic pseudosuchians would have  to have been somewhat endothermic. Of course,  

it could be argued that these species evolved a  high metabolism only after they split off from the  

ancestors of the crocodilians. Fortunately,  there is more evidence of pseudosuchian  

thermoregulation. Fossilized bones can also  show what a prehistoric animals growth rate was.  

Endotherms are able to grow swiftly because their  constantly high internal temperatures ensure the  

chemical reactions required for growth happen  as quickly as possible. Meanwhile, ectotherms,  

even those in warm climates, take much longer to  grow. Examination of the bones of pseudosuchians  

to determine their growth rates found they  were well above what ectotherms are capable of.  

This included pseudosuchians very closely related  to the crocodylomorphs, such as Postosuchus  

and Batrachotomus. Indeed, Batrachotomus was  able to reach its full 6-meter-long size in just  

three years. For comparison, Nile crocodile  males, who are smaller than Batrachotomus,  

take at least twelve years to reach sexual  maturity and even longer to reach their full size.  

Bone growth rate can also be used to measure an  animal’s resting metabolic rate. One study did  

this by using the osteocyte shape, density, and  area as well as the vascular density of the bone  

as proxies. After examining the bones of various  pseudosuchians, dinosaurs, and their distant  

relations, it was found that a high metabolism was  almost universal among Triassic pseudosuchians.  

Overall, the evidence is conclusive that  crocodilians are the descendants of endotherms  

who later reverted to ectothermy. This evidence  includes how their extinct ancestors and relatives  

usually possessed a fully erect stance, an  internal bone structure like those of endotherms,  

growth rates impossible for ectotherms to achieve,  indications of a high resting metabolic rate,  

and in some cases bodies incapable of even  being supported by an ectothermic metabolism.  

Modern crocodilians also retain endothermic four  chambered hearts modified to suit an ectothermic  

metabolism, an overbuilt, a respiratory  system with bird-like unidirectional airflow,  

and a semi-erect stance. While the ancestors  of most of today’s reptiles were no more  

active than their modern descendants, the  ancestors of the crocodilians were once  

more like their famous dinosaurian cousins.  However, this brings up more questions:  

when did the ancestors of crocodilians  revert to an ectothermic metabolism and why  

did they do it? These two questions will be  addressed in Part 2. I hope to see you then.