Birds are warm-blooded, while lizards are cold-blooded. Both of these groups are related to dinosaurs. For this reason, paleontologists have long wondered if these deniers have slow (cold-blooded) metabolisms like their lizard cousins or high metabolisms like their avian relatives. Scientists now know the answer: both.
An animal’s metabolism refers to the amount of energy its body uses to function normally. An animal with a high metabolism is often more active, but must compensate by consuming enough food and breathing enough oxygen to keep its metabolic engine running. As a bonus, this high metabolism generates heatthus keeping these animals warm.
The opposite metabolic strategy requires less energy. These animals therefore need less oxygen and food to evolve. However, this metabolism does not generate enough heat to keep them warm. They must therefore regulate their body temperature basking in the sun or hiding in the shade.
What about dinosaurs?
Today, only birds and mammals are warm-blooded, while others are cold-blooded, but what about millions of years ago? As part of a study, a team led by Jasmina Wiemann of CalTech became interested in fifty-five speciesincluding many dinosaurs, with the aim of tracing the evolution of metabolisms over time.
Previous studies have already addressed this question. However, this work had two major drawbacks. First, researchers tended to infer metabolism indirectly by analyzing eggshell thickness, tooth structure, or isotopes (variations of an element with a different number of neutrons) remaining after fossilization. These factors are often used to determine growth rate or body temperature which are indicators of metabolic rate. However, these measures remain indirect. On the other hand, the methods used to carry out this research are often invasivewith researchers forced to damage the fossils to get their data.
For this new study published in Naturethe researchers used a light scattering microscope to determine the chemical composition of bones. It is a non-invasive method. More specifically, they looked for waste products of the metabolism itself (such as broken down fat) which could refer to oxygen utilization in an animal’s body (a direct measure of metabolic rate).
Diverse and evolving metabolisms
In the Triassic, there is between 251.9 million and 201.3 million years ago, the dinosaurs were divided into two large groups with on one side the saurischians (dinosaurs with lizard hips) and on the other the ornithischians (dinosaurs with bird hips). The results of this work suggest that members of the first group, which notably includes the T-Rex and many other theropods, were warm-blooded creatures. Birds from this line have maintained a similar metabolism.
Ornithischians, which include ceratopsids (like Triceratops) or hadrosaurids (duck-billed dinosaurs) on the other hand, lost their fast metabolism over time and became cold-blooded animals. Ultimately, therefore, the dinosaurs had various metabolisms following an evolutionary pattern.
Dinosaurs aside, the study also reveals that warm-blooded metabolisms arose in two other distinct evolutionary lineages: in mammals and in a group of extinct marine reptiles known as plesiosaurs. Not only did these lines achieve higher metabolisms independently of each other, they all did so around the same time, during the Triassic period.
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