How did the beings of primeval times get up to temperature? It is still unclear to what extent the dinosaurs were warm like birds and mammals or cold blood like today’s reptiles. Now researchers present a new method that may shed more light on this question. It provides information on the metabolism of the animals based on fossil traces of their oxygen consumption. The results confirm that most dinosaurs were warm and some even had a particularly high metabolic rate. Some of them were obviously also cold-blooded. The information in turn allows conclusions to be drawn about the way of life, say the paleontologists.
Lizard and co lie down in the sun or have to deal with the ambient values as physical operating temperature. Birds and mammals, on the other hand, do not need this – they generate optimal body heat themselves thanks to their high metabolic rate. These two concepts divide modern animals into cold-blooded and warm-blooded beings. Both systems have advantages and disadvantages: cold-blooded animals save energy because the body heat comes from outside for free. However, cool conditions literally chill them, and even in warm conditions they are not as agile as warm-blooded animals. However, mammals and birds have to be able to afford their own “heating”: they need a comparatively large amount of food to fire up their metabolism.
But to which category did the dinosaurs belong? For a long time it was assumed that they were as variegated as today’s reptiles. But in the meantime, various studies have already provided clear indications that at least some representatives were equally warm. But the detection methods used so far leave questions unanswered, say the researchers working with Jasmina Wiemann at the California Institute of Technology in Pasadena. “Until now, certain isotopes in dinosaur bones have been studied, which basically work like a paleothermometer,” says Wiemann. “That was revolutionary and the process continues to provide very exciting insights. But it remains questionable how fossilization processes have altered the isotope signals, making it difficult to unequivocally compare data from fossils with those of modern animals,” explains Wiemann.
Thermoregulation in the level of oxygen consumption
Another method that has provided clues to thermoregulation was the detection of growth rates in dinosaur bones. For example, research has shown that dinosaurs grew rapidly, suggesting high metabolic rates typical of warm-blooded animals. However, the method developed by Wiemann and her colleagues now allows direct conclusions to be drawn about the metabolism of the extinct animals. Because it is based on the detection of fossil traces of oxygen consumption. As the researchers explain, these are molecular substances that are formed from proteins, sugars and lipids in connection with cellular respiration. These reaction products are extremely stable and insoluble in water, allowing them to be preserved during the fossilization process. The levels of these substances thus provide indications of a dinosaur’s oxygen consumption and thus its metabolic rates.
According to the researchers, dark-colored fossil femurs are particularly rich in these traces, because this coloration indicates that a lot of organic material has been preserved. The scientists explain that the substances can be detected using special microscopy methods. “They are similar to laser microscopy and allow us to non-destructively quantify the abundance of these molecular markers, which tell us something about metabolic rates,” says Wiemann.
To explore the potential of the method, the team analyzed the femurs of 55 different groups of animals. Among them were different representatives of the dinosaurs and some of their relatives as well as modern birds, mammals and reptiles. The researchers explain that the comparisons with the results in these creatures with known metabolic rates allowed conclusions to be drawn about the characteristics of the extinct animals. As they report, their results now decisively round out the evidence from previous studies: “The question of whether dinosaurs were warm-blooded or cold-blooded is one of the oldest questions in paleontology, and we now believe that we have reached a consensus that the Most dinosaurs were warm-blooded,” says Wiemann.
Hot-blooded T. rex and cool stegosaurus
Interesting features emerge in the details. Apparently, there were also cold-blooded dinosaurs: According to the results, representatives from the group to which Triceratops and Stegosaurus belonged had low metabolic rates that are comparable to those of modern reptiles. But the large groups of theropods and sauropods — the two-legged, more bird-like predatory dinosaurs like Velociraptor and T. rex and the giant, long-necked herbivores like Brachiosaurus — were warm-blooded, the analyzes confirmed. What was surprising was that some of these dinosaurs could even be described as “hot-blooded”. The researchers report that metabolic rates were found that were comparable to those of modern birds and far above those of mammals.
“Our work adds a fundamental piece of the puzzle for understanding the evolution of physiology and complements previous methods: we can now draw conclusions about body temperature based on isotopes, on growth strategies based on osteohistology, and now also on metabolic rates based on chemical traces,” sums up co-author Matteo Fabbri, a postdoctoral fellow at the Field Museum in Chicago.
According to the researchers, their and future findings can shed new light on the way of life of extinct animals: “For dinosaurs with exceptionally low metabolic rates, we have to consider similar ‘behavioural’ thermoregulations as in some modern-day reptiles,” says Wiemann. “They may also have had to move to warmer climes during the cold season, and climate may have dictated where some of these species could live.” The warm-blooded dinosaurs were arguably more flexible. “Some scientists have also suggested that the ancestors of today’s birds, unlike non-avian dinosaurs, survived the mass extinction event 65 million years ago because they have high metabolic rates. However, our results show that this is not the case. Because many dinosaurs with bird-like high metabolic rates are also extinct,” emphasizes Wiemann.
Source: Field Museum, Article: Nature, doi: 10.1038/s41586-022-04770-6