Schematic drawing of a subset of the animals that were investigated as part of the investigation. The resulting metabolic rates and thermophysiological strategies are color-coded, orange tones characterize high metabolic rates that match warm-blooded, and blue tones characterize low metabolic rates that match cold-blooded. From left to right: Plesiosaurus, Stegosaurus, Diplodocus, Allosaurus, Calypte (modern hummingbird). Credit: © J. Wiemann
Paleontologists have been debating for decades whether dinosaurs were warm-blooded, like modern mammals and birds, or cold-blooded, like modern reptiles. Knowing if the dinosaurs were warm or cold blood could give us clues as to how active they were and how their daily lives were, but previous methods for determining their hot or cold blood, how quickly their metabolisms could turn oxygen into energy. they were not conclusive. However, in a new article in the journal Nature, scientists are introducing a new method for studying the metabolic rates of dinosaurs, using clues in their bones that indicated how much the animals breathed in their last hour of life.
“This is really exciting for us as paleontologists: the question of whether dinosaurs were warm-blooded or cold-blooded is one of the oldest questions in paleontology. hot, “he says. Jasmina Wiemann, lead author of the paper and postdoctoral researcher at the California Institute of Technology (Caltech).
“The new proxy developed by Jasmina Wiemann allows us to directly infer metabolism in extinct organisms, something we only dreamed of just a few years ago. We also found different metabolic rates that characterize different groups, which was previously suggested based on other methods. “But never tested directly,” says Matteo Fabbri, a postdoctoral researcher at the Chicago Field Museum and one of the study’s authors.
People often talk about metabolism in terms of how easy it is for someone to keep fit, but deep down, “metabolism is the effectiveness with which we convert the oxygen we breathe into the chemical energy that feeds our body. “says Wiemann, who is affiliated with Yale University and the Los Angeles County Museum of Natural History.
Microscopic view of soft tissue extracted from the bones of one of the dinosaur specimens (Allosaurus) that were investigated for metabolic signals (metabolic bonds) in the fossilization products of the protein bone matrix. Fossilization introduces additional cross-links that, in combination with metabolic bonds, generate the characteristic brown color of the fossil extracellular matrix that maintains bone cells (dark, branching structures) and blood vessels (tube-shaped structure in the tube). center) instead. Credit: © J. Wiemann
Animals with a high metabolic rate are endothermic, or warm-blooded; Warm-blooded animals such as birds and mammals take in a lot of oxygen and need to burn a lot of calories to maintain body temperature and stay active. Cold-blooded, or ectothermic, animals such as reptiles breathe less and eat less. Their lifestyle is less energy-intensive than that of a warm-blooded animal, but it comes at a price: cold-blooded animals depend on the outside world to keep their bodies at the right temperature to function (like a lizard taking the ground). and are usually less active than warm-blooded creatures.
With warm-blooded birds and cold-blooded reptiles, dinosaurs were trapped in the middle of a debate. Birds are the only dinosaurs that survived mass extinction in the late Cretaceous, but dinosaurs (and, by extension, birds) are technically reptiles; outside of birds, their closest living relatives are crocodiles and alligators. Would that make the dinosaurs warm-blooded or cold-blooded?
“This is really exciting for us as paleontologists: the question of whether dinosaurs were warm-blooded or cold-blooded is one of the oldest questions in paleontology. hot “. —Jasmina Wiemann
Scientists have tried to obtain the metabolic rates of dinosaurs from chemical and osteohistological analyzes of their bones. “In the past, people have looked at dinosaur bones with isotope geochemistry that basically works like a paleothermometer,” says Wiemann. “It’s a very interesting approach. It was really revolutionary when it came out, and it continues to provide very exciting insights into the physiology of extinct animals. But we’ve come to realize that we still don’t really understand how fossilization processes change the isotopic signals we capture. so it is difficult to unambiguously compare fossil data with modern animals.
Another method to study metabolism is the growth rate. “If you look at a cross section of dinosaur bone tissue, you can see a series of lines, like tree rings, that correspond to years of growth,” says Fabbri. “You can count the growth lines and the space between them to see how fast the dinosaur grew. The limit depends on how you transform the growth rate estimates into metabolism: growing faster or slower can take longer. to see the stage of the animal’s life rather than its metabolism, such as how we grow faster when we are young and slower when we are older. ”
The new method proposed by Wiemann, Fabbri and colleagues does not look at the minerals present in the bone or the speed with which the dinosaur grew. Instead, they observe one of the most basic characteristics of metabolism: the use of oxygen. When animals breathe, by-products are formed that react with proteins, sugars, and lipids, leaving behind molecular “residues.” These wastes are extremely stable and insoluble in water, so they are preserved during the fossilization process. It leaves behind a record of the amount of oxygen a dinosaur breathed and therefore its metabolic rate.
“We are living in the sixth mass extinction, so it is important for us to understand how modern and extinct animals responded physiologically to previous climate change and environmental disturbances, so that the past can inform the conservation of biodiversity in the present and report on our future actions. ”—Jasmina Wiemann
Researchers looked for these fragments of molecular waste in dark-colored fossil femurs, because these dark colors indicate that much organic matter is conserved. They examined the fossils using Raman and Fourier transform infrared spectroscopy: “These methods work like laser microscopes, we can basically quantify the abundance of these molecular markers that tell us about the metabolic rate,” says Wiemann. “It’s a particularly attractive method for paleontologists, because it’s not destructive.”
The team analyzed the femurs of 55 different groups of animals, including dinosaurs, their flying cousins, pterosaurs, their closest marine relatives, plesiosaurs, and modern birds, mammals, and lizards. They compared the amount of respiratory-related molecular byproducts with the known metabolic rates of living animals and used these data to infer the metabolic rates of extinct animals.
The team found that dinosaur metabolic rates were generally high. There are two major groups of dinosaurs, the saurischians and the ornithischians: lizard hips and bird hips. Bird hip dinosaurs, such as Triceratops and Stegosaurus, had low metabolic rates comparable to those of modern cold-blooded animals. Lizard-hip dinosaurs, including theropods and sauropods, two-legged, more bird-like predatory dinosaurs such as Velociraptor and T. rex, and long-necked herbivorous giants such as Brachiosaurus, were warm-blooded or thin. and all hot blood. The researchers were surprised to find that some of these dinosaurs were not only warm-blooded, but had metabolic rates comparable to those of modern birds, much higher than those of mammals. These results complement previous independent observations that hinted at these trends, but could not provide direct evidence, due to the lack of a direct proxy to infer metabolism.
These findings, researchers say, may give us fundamentally new insights into what dinosaur life was like.
“Dinosaurs with lower metabolic rates would have been, to some extent, dependent on outside temperatures,” says Wiemann. “Lizards and turtles sit in the sun and sunbathe, 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 the weather may have been a selective factor in where some of these dinosaurs might live. “
On the other hand, he says, warm-blooded dinosaurs would have been more active and would have needed to eat a lot. “Giant warm-blooded sauropods were herbivores and it would take a lot of plant matter to feed this metabolic system. They had very efficient digestive systems and because they were so large, it was probably more of a problem for them to cool than to warm up.” Meanwhile, theropod dinosaurs, the group that contains birds, developed high metabolisms even before some of their members evolved in flight.
“Reconstructing the biology and physiology of extinct animals is one of the most difficult things to do in paleontology. This new study adds a key piece of the puzzle to understanding the evolution of physiology in deep time and complements previous indicators used by paleontology. “We can now infer body temperature through isotopes, growth strategies using osteohistology, and metabolic rates using chemical proxies,” says Fabbri.
In addition to giving us an idea of what dinosaurs were like, this study also helps us to better understand the world around us today. Dinosaurs, with the exception of birds, died en masse …