Scientists have been debating for decades whether dinosaurs were warm- or cold-blooded. Each side has conflicting views and evidence to support its claims.
A new study may have provided an answer to the debate. It suggests that the ancestors of dinosaurs may have been warm-blooded, although not all stayed that way.
The researchers used a new method for determining whether a dinosaur is warm- or cold-blooded by studying the bones that indicate how much each animal breathed in their last hour of life.
Studying Dinosaurs' Metabolism
Metabolism is usually associated with how easy it is for a person to stay in shape. Study lead author Jasmina Wiemann from the California Institute of Technology said that at its core, metabolism is how an organism effectively converts oxygen into chemical energy or fuels for the body.
According to Science Daily, animals with high metabolic rates are endothermic or warm-blooded. For example, birds and mammals take in a lot of oxygen and use lots of calories to keep their bodies warm.
On the other hand, exothermic or cold-blooded animals like reptiles breathe less and eat less with their lifestyle, which is less energetic than warm-blooded animals. That also means they rely on the outside world to keep their bodies warm.
Since birds are warm-blooded and reptiles are cold-blooded, that leaves dinosaurs in between. Today, birds are the only surviving dinosaurs after the mass extinction some 66 million years ago that ended the Cretaceous period. Technically, dinosaurs are reptiles, with their closest living relative the crocodiles and alligators.
Scientists tried to study the metabolic rates of dinosaurs from the chemical and osteohistological analyses of dinosaur bones. Researchers examined the minerals in a fossil and identified the temperatures at which they were formed. Wiemann said that it provided them with exciting insights into the physiology of extinct animals.
Another method considered the growth rate of dinosaurs' bone tissue. The series of lines correspond to the years of growth and the space between them. The faster or slower the dinosaur grows could have something to do with its metabolism.
Warm-Blooded and Cold-Blooded Dinosaurs Identified
The team examined dinosaur bones for advanced lipoxidation end-products (ALEs), compounds found during metabolic stress in living animals. They do not dissolve in water and are extremely stable, which means they are likely preserved in the fossils.
Birds, reptiles, and mammals, have different levels of ALEs in thigh bones, so the team used it to identify whether dinosaurs were endothermic or ectothermic. The method was non-destructive to preserve fossils for future use.
They found that mammals, pterosaurs, and plesiosaurs were endothermic. That means theropods, such as T-rex (Tyrannosaurus Rex), deinonychus, sauropods, and allosaurus are likely warm-blooded. However, other dinosaurs, like Triceratops, stegosaurus, and hadrosaurs were cold-blooded.
The team interprets their findings as evidence that ancestors of mammals and birds were endothermic, but some of their descendants went on to lose their ability to warm themselves and became ectothermic.
Scientists were amazed by the findings but noted that it would be better to see more examples from each to confirm the results. They pointed out that the extinct monitor lizard has become warm-blooded in their study when no living lizards today are endothermic.
Researchers behind the study are now expanding their current data set to include more species, including marine megafauna and woolly mammoth.
They discussed their findings in full in the study titled "Fossil Biomolecules Reveal an Avian Metabolism in the Ancestral Dinosaur," published in the journal Nature.
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