Was dinosaur blood hot or cold? A long-standing question may finally have an answer

This illustration depicts, from left, plesiosaurus, stegosaurus, diplodocus, allosaurus and calypte (modern hummingbird), with red hues indicating warm-bloodedness and blue hues for cold-bloodedness.
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This illustration depicts, from left to right, Plesiosaurus, Stegosaurus, Diplodocus, Allosaurus, and Calyptus (modern hummingbird), with red tints indicating warm blood and blue tints indicating cold blood. (J. Wiemann)

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ATLANTA — Fierce predators like the T. rex and towering, telescopic-necked dinosaurs like the brachiosaurus were warm-blooded creatures much like birds and mammals, according to a groundbreaking new study.

Whether the blood flowing through giant dinosaur frames was hot or cold, like that of reptiles, is a long-standing question that has vexed paleontologists. Knowing that this foundational information could shed significant light on the lives of prehistoric creatures.

Warm-blooded animals have a high metabolic rate – they take in a lot of oxygen and need a lot of calories to maintain their body temperature, while cold-blooded animals breathe and eat less.

“It’s really exciting for us as paleontologists – whether dinosaurs were warm or cold blooded is one of the oldest questions in paleontology, and now we think we have a consensus that most dinosaurs were warm-blooded,” said the study’s lead author, Jasmina Wiemann, a postdoctoral researcher at the California Institute of Technology, in a press release.

Previous recent attempts to answer this question have suggested that dinosaurs were warm-blooded, but these findings, which involved analyzing growth rings or chemical isotopic signals in bones, were ambiguous because fossilization can alter these markers. . In addition, these analytical techniques damage fossils, making it more difficult to build a large data set.

Wiemann and his colleagues, however, came up with a new – and in their view, more definitive – method for assessing a dinosaur’s metabolism.

Final answer ?

The researchers looked at the waste products that form when oxygen is inhaled into the body and reacts with proteins, sugars and lipids. The abundance of these waste molecules, which appear as dark-colored specks in fossils, changes with the amount of oxygen it takes in and indicates whether an animal is warm or cold-blooded.

The molecules are also extremely stable and do not dissolve in water, meaning they are preserved during the fossilization process.

Wiemann and his team analyzed a femur – thigh bone – from 55 different creatures, including 30 extinct animals and 25 modern animals. Among the samples were bones belonging to dinosaurs, giant flying reptiles called pterosaurs, marine reptiles like plesiosaurs, and modern birds, mammals and lizards.

The scientists used an approach called infrared spectroscopy, which targets interactions between molecules and light. This technique allowed them to quantify the number of waste molecules in the fossils. The team then compared these results with the known metabolic rates of modern animals and used this data to infer the metabolic rates of the extinct creatures.

what they found

Previous generations of paleontologists had grouped dinosaurs with reptiles, leading to the hypothesis of a reptilian appearance and lifestyle. Today, most paleontologists agree that dinosaurs were much more birdlike after the discovery of feathered fossils in the 1990s led to the understanding that modern birds descended directly from dinosaurs.

The study, published Wednesday in the journal Naturefound that the metabolic rates of dinosaurs were generally high and in many cases higher than those of modern mammals – which typically have a body temperature of around 98.6 degrees Fahrenheit – and more like birds, which have an average body temperature about 107.6 degrees Fahrenheit.

“With our new evidence for an avian-level metabolism ancestral to all dinosaurs and pterosaurs, all warm-blooded dinosaurs likely had elevated body temperatures, comparable to those of modern birds,” Wiemann said via email.

However, there were notable exceptions. Dinosaurs classified as ornithischians — an order characterized by lizard-like hips that include instantly recognizable creatures such as triceratops and stegosaurus — evolved to have low metabolic rates comparable to modern cold-blooded animals.

“Lizards and tortoises sit in the sun and bask, and we may need to consider similar “behavioral” thermoregulation in ornithischians with exceptionally low metabolic rates. Cold-blooded dinosaurs may also have had to migrate to warmer climates during the cold season, and climate may have been a selective factor for where some of these dinosaurs might live,” Wieman said.

Having a high metabolic rate has been proposed as one of the reasons birds survived the mass extinction that wiped out the dinosaurs 66 million years ago. However, Wiemann said this study indicated that this was not true: many dinosaurs with exceptional bird-like metabolic abilities became extinct.

The research will “radically change” the way the biology and behavior of many extinct animals are interpreted, said Jingmai O’Connor, associate curator of fossil reptiles at the Field Museum in Chicago. She did not participate in the study.

“I consider these results to be fairly definitive. Wiemann’s methods are meticulous and have been thoroughly tested,” she said.

“Some dinosaurs were warm-blooded, that was the ancestral state, but others later evolved to be ectothermic (cold-blooded). The next question to ask is why and what does that mean on their behavior, ecology and evolution?

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