Warm-bloodedness may have first arisen in dinosaurs some 180 million years ago.
Dinosaurs were once thought to have been cold-blooded like their modern-day reptilian cousins. Recent findings suggest that some species of dinosaur had indeed evolved to be warm-blooded, but until now, paleontologists weren't sure when this trait arose.
According to a new paper in the journal Current Biology, warm-bloodedness may have evolved in the early Jurassic period, with dinosaurs gaining the ability to regulate their temperature in the same "unique" way that birds do.
Warm-bloodedness, also known as endothermy, is a characteristic of animals that can maintain a stable internal body temperature regardless of external environmental conditions.
Warm-blooded animals, such as mammals and birds, achieve internal temperature control through metabolic processes that generate heat. They consume more food than cold-blooded (ectothermic) animals, such as reptiles and amphibians, because maintaining a constant body temperature requires more energy.
The ability to maintain a stable temperature helps warm-blooded animals remain active even when temperatures are cool, giving them a significant advantage in some environments over ectotherms, which often must rely on external heat sources to regulate their body temperatures.
Birds generally maintain a higher body temperature than mammals, which may help with faster muscle reactions and higher metabolic rates, which are beneficial for flight. Additionally, mammals primarily use fur or hair to retain heat, while birds use feathers.
The researchers made this discovery after analyzing the spread of dinosaurs across varying climates during the Mesozoic Era between 230 and 66 million years ago. They found that two of the three main dinosaur groups at the time moved to colder environments during the Early Jurassic. This implies that theropods (a group including Tyrannosaurus rex and velociraptors, the ancestors of modern birds) and herbivores may have subsequently evolved to be warm-blooded to stay alive in the colder climes, while sauropods like brontosaurus stayed cold-blooded as they stuck to the warmer areas.
"We found out that dinosaurs were incredibly diverse not just in shape and size but also in the deep of their biology: one thing that doesn't leave obvious traces is their metabolism, and we hypothesize that some groups were warm-blooded, capable of warming up and being active independently from the outside environment (theropods and ornithischians), while some others (sauropods) were likely more dependent from the environmental heat to warm up and be active," study co-author Alfio Alessandro Chiarenza, a researcher at University College London, told Newsweek.
"Interestingly this diversity might have originated after an environmental crisis caused by volcanically induced global warming."
This change is thought to have occurred as a result of a period of intense volcanic activity known as the Jenkyns event, which occurred around 183 million years ago. This resulted in major climatic changes across the globe, significantly increasing the temperatures in certain regions.
"This research suggests a close connection between climate and how dinosaurs evolved. It sheds new light on how birds might have inherited a unique biological trait from dinosaur ancestors and the different ways dinosaurs adapted to complex and long-term environmental changes," study co-author Juan L. Cantalapiedra, a researcher at the Museo Nacional de Ciencias Naturales in Madrid, Spain, said in a statement.
The researchers note that these dinosaurs may have evolved traits like feathers or changed body sizes to help insulate their bodies in order to retain this self-generated heat, which allowed them to regulate their temperature like modern birds do.
"Previously we thought that the appearance of feathers in one lineage (the theropods, which include birds) was the only unambiguous trait related to independent thermoregulation, but it is likely that dinosaurs evolved different strategies (from feathers to size to adaptations in their skeleton) to heat up and cool down more efficiently, allowing them to be more active for longer times and in harsh environments," Chiarenza said.
"Since feathers were previously recognized to be around 160 million years old, we pre-date of a few tens of million years the likely origination of this trait."
Additionally, the sauropods that stayed were found not to have stuck around to feed on foliage that wasn't available in other places, but because of the temperatures.
"Sauropods, on the other hand, which stayed in warmer climates, grew to a gigantic size at around this time – another possible adaptation due to environmental pressure. Their smaller surface area to volume ratio would have meant these larger creatures would lose heat at a reduced rate, allowing them to stay active for longer," study co-author Sara Varela, a researcher at the Universidade de Vigo in Spain, said in the statement.
This discovery is hoped to lead to a deeper understanding of how climate affected the evolution of traits in dinosaurs and the earliest mammals.
"In the future, we plan to extend this kind of analysis to more species and better resolved evolutionary tree, with more complex methods, but also to extend this kind of analysis to the only other group of land vertebrates that evolved warm-bloodedness, ours, the mammals," Chiarenza said.
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