The dinosaurs appeared ready to rip each other apart. Charles R. Knight, a famous nature artist, depicted two carnivorous dinosaurs in deadly combat in 1897. These scaly reptiles, with their teeth and claws bared, were captured mid-tussle. This was a dynamic interpretation of animals that had been thought to be no different from big tail-dragging lizards. Knight's Leaping Laelaps vision was not the final word on the matter. The debate over whether these magnificent animals could be classified as "cold-blooded" lizards or mammals continued on into the 21st century. There is still much to discover, even in this age of supercharged dinosaurs.
A Nature study recently reopened the debate on this long-standing mystery. Study results showed that different dinosaur metabolisms were related to body temperature, activity level and body temperature. Some dinosaurs were more bird-like while others were closer to crocodiles. Apatosaurus, Tyrannosaurus, and many other dinosaurs had active metabolisms, making them warm-blooded. However, Yale University paleontologist Jasmina Wiemann, and her colleagues discovered that dinosaurs such as Triceratops, Stegosaurus, and Triceratops were more like animals considered cold-blooded. This surprising difference raises new questions about how paleontologists reconstruct biology of long-extinct species. It has implications for everything, from how fast dinosaurs grew up to how much food they needed.
"I believe most people, paleontologists and the general public, are most interested the final result of the metabolic pathway of dinosaurs." Thomas Holtz Jr., University of Maryland paleontologist, said that how active were they. He was not part of the new study. This question has been around since before the term "dinosaur” was even invented. The physiology of dinosaur bodies, which contains details such as how fast an animal can run and what kind of nutrition they need, can help us understand these things. It's like giving Ankylosaurus a checkup after it has been 66 million years.
Based on very little other than skeletal material, 19th-century paleontologists believed that Iguanodon was an essentially a huge iguana Lizard and Megalosaurus had more similarities with crocodiles then birds. As dinosaurs were named in 1842, it seemed natural that they would be endothermic. This means their body temperature is controlled by the environment and their behavior within it. They need to soak in the sun to increase their metabolic rate. Scientists saw dinosaurs as slower, less agile animals than the mammals that would eventually replace them.
However, this conclusion was not final. It was difficult to look at the bones of dinosaurs like Dryptosaurus--eventually decided to be the proper name for the "Laelaps" Knight depicted--and perceive them as belonging to ectothermic animals that spend much of their days resting. The fossils of dinosaurs such as Dryptosaurus or Hadrosaurus are not lizard-like. They belonged to animals who walked on two feet and didn't share the low, low-tothe-ground appearance of crocodiles. Some dinosaurs looked fast, much like a red convertible. Hard fossil evidence would be difficult to confirm this impression.
Some paleontologists have concluded that some dinosaurs were active and had high body temperatures. For example, Ornitholestes, a small carnivorous dinosaur, was hard to imagine as slow. This predator was small and delicately built. It had grasping hands. The name came from the possibility that it might have been an animal hunter. Paleontologists classified dinosaurs as reptiles. Modern reptiles, however, are ectothermic and rely on their environment to heat up. The cold-blooded theory was accepted for the large species. It was codified by visions of giant dinosaurs eating soft water plants in murky Mesozoic swamps. In the 1970s paleontologists started to debate and investigate a new idea: that dinosaurs as a whole might be warm-blooded and more behaviorally intriguing than previously thought. This view is now largely accepted, with dinosaurs having more similarities to birds and mammals than modern-day lizards or crocodiles. Wiemann says that recent research has changed the perception of dinosaurs. Paleontologists were able to identify dinosaur colors and determine when baby dinosaurs develop in their eggs. They also discovered dinosaurs that have survived the harsh cold of Polar winters. It is hard to keep up with the pace of discovery. She notes that it is difficult to determine the physiological details of dinosaurs.
This fossiliferous sleuthing has involved many different types of evidence. Many dinosaurs grew at an astonishing rate, with some species (like Tyrannosaurus Rex) going through teenage growth spurts that were similar to ours. Many dinosaurs experienced phases of rapid development and rest, much like trees. Paleontologists were able to determine that many dinosaurs grew quickly after hatching by counting the rings and looking at how fast each bone accumulated. This is a strong argument in favor of high body temperatures and a quick-running metabolism. It is possible to see evidence of a warm-blooded lifestyle in the fact that tyrannosaurs and horned dinosaurs lived in polar habitats subjected to cold temperatures and months of darkness. A few dinosaur poop samples, like a T. Rex plop from Canada, show that dinosaurs digested food quickly. This is consistent with the view that dinosaurs ate a lot because their bodies moved food through their digestive system at a rapid pace. Many dinosaurs also had fluff, fur, and feathers. This suggests that their insulating layers were designed to retain heat. Their protofeathers would have been more detrimental if they had had to rely on the surrounding environment to regulate their temperature.
It's one thing to know that dinosaurs had high body temperatures due to fast growth, their diet, and other things to be able to determine if they were cold-blooded or warm-blooded. It's not an easy task. Life isn't always easy for the cold-blooded to distinguish themselves in modern times. Holtz points out that some mammals, such as tenrecs change their physiology to be more ectothermic in certain seasons. Some ectotherms can also raise their body temperature, like brooding serpents who squeeze their muscles to heat themselves and then their eggs, or leatherback sea turtles, which can store heat from their muscles in order to keep them warmer than the seawater. Warm-blooded is more than just the temperature of your body. It also refers to how it is controlled. Some animals, such as you and me, produce heat internally and keep it steady. This is quite different from a Lizard, who will have a similar body temperature but become slower and cooler at night.
Wiemann and his coauthors have a new approach to this problem. They look at the biological signatures that liver stress has left behind in dinosaur bones. This technique is applicable to dinosaurs, since dinosaur bones don’t completely turn to stone as so often claimed. However, they often retain a substantial amount of their original biological makeup, which paleontologists can use to search for clues that are not visible to the naked eye.
Wiemann and his colleagues were able, through comparing biochemical clues found in dinosaur fossils with those in living animals whose physiological science scientists can directly observe them, to establish that dinosaurs inherited high metabolic rates. The last common ancestor of both dinosaurs and flying pterosaur relatives had a high metabolic speed, which was around 247 million years ago. This suggests that dinosaurs, pterosaurs and their flying pterosaur cousins had a warm-blooded ancestral. Later lineages have modified this evolutionary inheritance. Wiemann and his colleagues discovered that early birds had higher metabolic rates than modern ones, starting at 150 million years old. The large dinosaur family called ornithischians, which includes the horned, armored, and duckbilled dinosaurs as well as their relatives, seem to have developed slightly lower metabolic rates over the course of time. They look more like ectotherms rather than the theropods. Different species, and even different groups of dinosaurs, had different physiologies. This is what we would expect given the many shapes and sizes of dinosaurs over millions of years.
It is important to be able to see the metabolic and bodily processes of dinosaurs. Wiemann states that the high metabolic rates of theropods, pterosaurs allowed them to explore energy-consumptive lifestyles. This is because these reptiles were able to develop powered flight, opening up evolutionary possibilities that might not have been possible otherwise. The high metabolic rates of dinosaurs, and other prehistoric creatures, may have allowed them to survive in more diverse climates. This could explain why dinosaurs survived the Triassic–Jurassic extinction that occurred around 201 million years ago. Extreme volcanic eruptions altered the climate quickly, causing the chill of a volcanic freeze to cause crocodile relatives to die and other reptiles.
Paleontologists aren't content to rest until now. Dinosaurs evolved in a similar way that mammals have different metabolic characteristics. Wiemann points out that it is not clear why Allosaurus and Brachiosaurus had high metabolic speeds while their relatives, like Stegosaurus, had a lower metabolism. This could have implications for how the evolution of dinosaurs. Another study, published in the same year, found a completely different pattern: warmer-running sauropods and ornithischian dinosaurs. Alfio Alessandro Chiarenza, paleontologist and colleague suggested that sauropods prefer warm habitats. They might also have relied more on the local climates than heat generated within their bodies. This is how paleontology works. Experts look at various evidence to verify their ideas. As paleontologists dig deeper, any hypothesis regarding a dinosaur group or particular dinosaur is likely to be tested. While dinosaurs were probably active animals that displayed amazing behaviors, the mechanics behind those behaviors will need to be revealed over time. Holtz says that the molecular details of how dinosaurs get their activity levels are more of a black box. This will keep paleontologists busy for many decades.