Birds keep warm with heat generated by some of the hardest-working metabolic processes on the planet, while lizards rely on the sun to keep them warm. Both groups are connected to each other dinosaurand for this reason paleontologists have long wondered if dinosaurs had so-called cold-blooded animals metabolism like their lizard cousins, or warm-blooded metabolites like their bird relatives. Now scientists know the answer: It’s both.
An animal’s metabolism refers to how much energy its body uses to carry out normal functions. A higher metabolism — which requires more energy to maintain — means an animal can be more active, but the animal must be consuming enough food and breathing enough oxygen to keep its metabolic engine running. As an added bonus, a high metabolism generates heat that keeps animals warm, hence the term warm-blooded or endothermic. The opposite metabolic strategy requires less energy and is known as cold-blooded or ectothermic. Cold-blooded animals require less oxygen and food than endothermic creatures, but must regulate their body temperature through behavior. Instead of generating their own heat, they maintain their internal temperature by basking in the sun or hiding in the shade.
“Birds inherited their exceptionally high metabolic rates from their dinosaur ancestors, which is pretty cool,” Jasmina Wiemann, currently a postdoctoral researcher at CalTech and lead author of a new study on dinosaur metabolism, told Live Science. In an analysis of 55 living and extinct species (including many dinosaurs), Wiemann and co-authors found that warm-blooded animals, which are currently found only in mammals and birds, were fairly widespread among dinosaurs, but not all dinosaurs were warm-blooded.
By analyzing species from different dinosaur groups, the team tracked the evolution of warm-blooded and cold-blooded metabolisms over time. They found that dinosaurs that descended from an ancestor were likely warm-blooded, but not all dinosaurs stayed that way. In the Triassic Period, 251.9 million to 201.3 million years ago, dinosaurs split into two major groups: the Saurischians (“lizard-hipped” dinosaurs) and the Ornithischians (“bird-hipped” dinosaurs). Evidence suggests that the saurians, including carnivorous theropods, like Tyrannosaurus and Allosaurus were warm-blooded creatures like their ancestors, among many others. Birds are descended from this lineage and have retained a warm-blooded metabolism.
The Ornithischians, to which belong triceratops and duckbill Hadrosauruslost their fast metabolism over time and became cold-blooded.
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These data corroborate findings from previous research. This is what previous studies have found, for example Stegosaurusan ornithic genus of armored herbivores, had one exceptionally low growth rate — a hallmark of a slow, cold-blooded metabolism. Other to learn found that hadrosaurs, a group of duckbill herbivores, appeared to have body temperatures far too different for the animals to be warm-blooded. Other studies have pointed to warm-blooded animals, such as noting that some dinosaur species lived in the Arctic all year round (opens in new tab). This is the first study to show that dinosaurs had different metabolisms and followed an evolutionary pattern. “It’s very nice to get to the bottom of this and realize that these are real patterns, not just artifacts,” Wiemann said.
According to Wiemann, studies examining dinosaur metabolism had two major drawbacks. For one, they tended to infer metabolism indirectly by analyzing eggshell thickness, tooth structure, or isotopes—variations of an element with different numbers of neutrons – which remain after fossilization. These are often used to determine growth rate or body temperature, which are proxies for metabolic rate. These proxies can provide clues about an animal’s metabolism, but do not measure metabolism directly. Second, the methods used to conduct this research are often destructive, requiring researchers to damage fossils in order to tease out their mysteries.
Instead of grinding priceless fossils into dust, Wiemann and her colleagues used a light scattering microscope to determine the chemical composition of dinosaur bones for the new study. In particular, they looked for waste products from metabolism itself (such as broken down fats) that might indicate oxygen consumption in an animal’s body – a direct measure of metabolic rate.
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While this study supports findings from some previous work on dinosaur metabolism, Wiemann’s non-destructive sampling method could provide scientists with an unprecedented opportunity to study metabolic evolution in other extinct lineages, not just dinosaurs.
With this non-destructive method, paleontologists can delve into museum collections, “take a bone off the shelf and analyze it without much preparation,” Wiemann said. “Because of that, for the first time we were able to create one of these really big datasets that then actually connects the dots.”
Deducing the patterns of metabolic development in dinosaurs has also raised questions about the metabolism of living animals.
For example, birds are the only group of dinosaurs to have survived the mass extinction event at the end of the Cretaceous (roughly 145 million to 66 million years ago), so it might seem as if their highly active metabolism gave them an advantage. However, many other dinosaurs that appeared to have energy-hungry metabolic rates were not so lucky. Whether metabolism played a major role in survival at the time is a question Wiemann hopes to be able to answer soon.
The study also found that warm-blooded animal metabolism occurred in three different evolutionary lineages: in dinosaurs, in mammals, and in a group of extinct marine reptiles known as plesiosaurs. Not only did these lineages achieve higher metabolisms independently, they all did so at about the same time during the Triassic period. “I find it quite fascinating to realize that it all happened more or less at the same time,” Wiemann said.
Wiemann noted that future studies using the team’s research method could expand scientists’ knowledge of metabolic evolution. “They could eventually tell us what role mass extinctions and evolutionary bottlenecks actually play in giving different groups of animals the opportunity to expand and explore their metabolic capacities,” Wiemann said. “I think there’s something very exciting out there in the future.”
The results were published in the journal on May 25 Nature.
Originally published on Live Science.
https://www.livescience.com/dinosaur-metabolism-warm-cold-blooded T. rex and its close relatives were warm-blooded like modern birds