The dinosaurs looked ready to tear each other apart. In 1897, the famed nature artist Charles R. Knight depicted a pair of carnivorous dinosaurs called “Laelaps” in deadly combat. Claws and teeth bared, these scaly reptiles were captured in mid-tussle—a very dynamic, warm-blooded interpretation of animals previously thought to be little different than big, tail-dragging lizards. But Knight’s vision of Leaping Laelaps was hardly the last word on the subject. Through the following century, and even into the 21st, paleontologists would argue with almost dinosaurian ferocity over whether these stupendous animals were more like mammals and birds or could be considered “cold-blooded” like lizards. Even in our present age of seemingly supercharged dinosaurs, there remains a great deal to uncover.
Now, a recent Nature study has renewed interest in the longstanding puzzle. The study found that various dinosaurs had different metabolisms related to body temperature and activity level, with some being more bird-like and others closer to crocodiles. While many dinosaurs like Apatosaurus and Tyrannosaurus had very active metabolisms and were warm-blooded like birds, Yale University paleontologist Jasmina Wiemann and colleagues found, dinosaurs like Triceratops and Stegosaurus were different and even seemed more similar to animals thought of as cold-blooded. This unexpected difference has raised new questions about how paleontologists reconstruct the biology of long-extinct species, with implications for everything from how fast dinosaurs grew to how much they needed to eat.
“I think most of us, from paleontologists to the public, are most interested in the end result of the dinosaurs’ metabolic pathway. How active were they?” says University of Maryland paleontologist Thomas Holtz Jr., who was not involved in the new study. That question’s been in place before the word “dinosaur” even existed. How fast an animal can run, what sort of nutrition they need, what climates they can inhabit, and more are all informed by physiology, details held in the working organs of dinosaur bodies that have long since decayed away. It’s like trying to give an Ankylosaurus a check-up more than 66 million years too late.
Early on, based on little more than skeletal scraps, 19th century paleontologists thought that Iguanodon was essentially an enormous iguana lizard and Megalosaurus shared more in common with crocodiles than birds. It seemed logical that dinosaurs, as they were officially named in 1842, would be endothermic—or have body temperatures regulated by their environment and behavior within it, needing to bask in the sun to raise their metabolic rate. Scientists envisioned dinosaurs as slow, lumbering animals that seemed overall inferior to the mammals that would eventually supplant them.
But that conclusion was not set in stone. 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. Fossils of dinosaurs like Dryptosaurus and Hadrosaurus were not lizard-like, but belonged to animals that walked on two legs and did not share the squat, low-to-the-ground look of crocodiles. Much like a red convertible, some of these dinosaurs just looked fast—an impression that would take decades of research to back up with hard fossil evidence.
Further finds led some paleontologists to suspect that at least some dinosaurs must have been very active creatures and maintained elevated body temperatures. A small, carnivorous dinosaur called Ornitholestes, for example, was difficult to envision as being sluggish. This was a small, delicately-built predator with grasping hands, so named because it seemed like the dinosaur might have been a bird hunter. Nevertheless, paleontologists had classified dinosaurs as reptiles and modern reptiles are famously ectothermic, relying on their surroundings to warm up. For the big species, at least, the cold-blooded interpretation stuck around and was codified in visions of dinosaurs munching on soft water plants among murky Mesozoic swamps in everything from Disney’s Fantasia to World’s Fair dioramas. It wasn’t until the 1970s that paleontologists began to investigate and argue over a new idea—that dinosaurs, as a group, might have been warm-blooded and much more behaviorally interesting than previous generations had supposed. That view has largely taken hold, with dinosaurs seemingly having more in common with birds and mammals than modern lizards and crocodiles.
“In recent years, new research on dinosaurs has transformed their image,” says Wiemann. Paleontologists have been able to discern dinosaur colors, work out the timing of how quickly baby dinosaurs developed in their eggs, found dinosaurs that survived through the cold of Polar winters, and more. The rate of discovery is difficult to keep up with. Still, she notes, the actual physiological details of dinosaurs are difficult to examine.
Physiology is the study of how a living thing functions, ranging from how an organism regulates its body temperature to how quickly it grows. Such considerations are essential to understanding how dinosaurs lived. Consider a little compare and contrast involving a famed dinosaur, like the long-necked herbivore Brachiosaurus. A cold-blooded Brachiosaurus might rely on the temperature of their environment to regulate their body heat rather than generating that heat from their internal processes. That would mean that they require less energy to keep their body running, so they could get by on smaller meals or longer stretches between nibbling at conifers. A warm-blooded Brachiosaurus that grows fast and maintains a warm body, by contrast, would have to eat almost-constantly to feed their body, a hunger that would have significant consequences for the plant life in the area. In Wiemann’s study, Brachiosaurus and similar dinosaurs are categorized as endotherms that maintained high internal body temperatures. That means these dinosaurs had to consume incredible quantities of vegetation each day, no doubt influencing the evolution of horsetails, ginkgoes and other plants of the time to be resilient and grow back faster in the face of so many hungry herbivores. Discerning the details of dinosaur physiology can open up a range of scientific possibilities that bear on how we envision non-avian dinosaurs and how they interacted with the world around them.
Many different lines of evidence have become involved in this fossiliferous sleuthing. Many dinosaurs grew astonishingly quickly, and some of which—like Tyrannosaurus rex—went through teenage growth spurts not unlike our own. Much like trees, many dinosaurs went through phases of quick development and rest that left rings inside their bones. By counting those rings and examining how quickly the bone within each ring accumulated, paleontologists have been able to discern that many dinosaurs grew very fast after they hatched. That’s a point in favor of elevated body temperatures and a fast-running metabolism. The fact that tyrannosaurs, horned dinosaurs, armored dinosaurs and more lived in polar habitats affected by months of darkness and cold temperatures also hints at a warm-blooded lifestyle. Not to mention that some dinosaur poop, such as a T. rex plop found in Canada, indicate that digested food was voided quickly, matching a view that dinosaurs ate a great deal because their bodies moved that food through their digestive systems at a fast pace. The fact that many dinosaurs had fluff, fuzz and feathers, too, indicates that they evolved these insulating coats to help retain body heat, their protofeathers being more of a hinderance if they had to rely on the environment to regulate their temperature.
But it’s one thing to understand that dinosaurs likely had warm body temperatures associated with fast growth, where they lived, or their digestion and another to get a closer look at whether dinosaurs were warm- or cold-blooded. That’s a tricky task. Even in our modern world, life isn’t neatly divided between the cold- and warm-blooded. Some mammals like tenrecs, Holtz notes, change their physiology to become more ectothermic during part of the year. On the other side of the puzzle, some ectotherms can raise their body temperatures, such as brooding snakes squeezing their muscles to heat their body and then their eggs or like leatherback sea turtles that are so big they can hold on to heat generated by their muscles to stay warmer than the surrounding seawater. Being “warm-blooded” isn’t just about the body’s temperature, in other words, but how that temperature is regulated. Some animals, like you and I, generate our heat internally and maintain it fairly steadily, which is very different from a lizard that shares a similar body temperature after heating in the sun but will become cooler and more sluggish at night.
The new study by Wiemann and coauthors offers a new way to get at this conundrum by looking at the biological signatures of liver stress that are still preserved in dinosaur bones. The technique works for dinosaurs because dinosaur bones don’t entirely turn to stone, as is so often said, but often contain a significant amount of their original biological makeup that paleontologists can investigate for new clues invisible to the naked eye.
By comparing those biochemical clues in the dinosaur fossils to the same signatures in living animals whose physiology scientists can directly observe, Wiemann and colleagues were able to determine that dinosaurs actually inherited high metabolic rates from their ancestors. It seems that the last common ancestor of dinosaurs and their flying pterosaur cousins had a high metabolic rate, sometime before 247 million years ago. This finding hints that dinosaurs and pterosaurs had a warm-blooded ancestor, an evolutionary inheritance that later lineages tweaked over time. While early birds evolved even higher metabolic rates starting around 150 million years ago, for example, Wiemann and colleagues found that the large family of dinosaurs called ornithischians—the horned dinosaurs, armored dinosaurs, duckbilled dinosaurs, and their relatives—seem to have evolved slightly lower metabolic rates over time, looking more like ectotherms than the theropods and sauropods. As we might expect from dinosaurs coming in so many different shapes and sizes over millions of years, different species and even groups of dinosaurs had differing physiologies.
Being able to better perceive the metabolism and bodily processes of dinosaurs has big picture implications. “The high metabolic rates in theropods and pterosaurs,” Wiemann says, “allowed their evolutionary exploration of energy-consumptive lifestyles.” That is, inheriting a hot-running, active metabolism allowed these reptiles to evolve powered flight—opening evolutionary possibilities that would have otherwise been closed. Likewise, the high metabolic rates of dinosaurs and other prehistoric creatures might have helped them survive in a broader array of climates. This might explain why dinosaurs were able to so readily withstand the Triassic-Jurassic extinction around 201 million years ago in which extreme volcanic outpourings quickly altered the global climate, the chill of a volcanic winter causing crocodile relatives and other reptiles to go extinct while dinosaurs were apparently unscathed.
But paleontologists can’t rest easy just yet. Just as mammals differ in their metabolic details, so did dinosaurs. More than that, Wiemann notes, it’s unclear why dinosaurs like Allosaurus and Brachiosaurus had high metabolic rates while their relatives among the ornithischians—like Stegosaurus—may have evolved a lower metabolic rate and what consequences those details might have for the big picture of dinosaur evolution. Then again, another study published this year found a different pattern—with warm-running ornithischian dinosaurs and cooler-bodied sauropods. Based on where fossils of sauropod dinosaurs like Brachiosaurus are found, paleontologist Alfio Alessandro Chiarenza and colleagues proposed that sauropods preferred warm habitats and might have been more like ectotherms and reliant on local climates more than heat generated inside their bodies. But this is how paleontology works, with experts looking at different lines of evidence to test ideas. Any hypothesis about a particular dinosaur or group is likely to be tested and tested again as paleontologists dig in. Dinosaurs were likely active animals with amazing behaviors, but the mechanics underpinning those particulars will take time to emerge. “The nitty-gritty details of how dinosaurs achieve their activity levels is more of a molecular black box,” Holtz notes, and investigating what’s inside will keep paleontologists busy for decades to come.