Photo by MARK GARLICK/SCIENCE PHOTO LIBRARY/Getty Images
Dinosaurs are adored for their size, their ferocity and their strangeness; nothing sparks the imagination more than daydreaming of strange, ancient creatures traversing lush ancient landscapes millions of years ago. Some of the largest—such as Patagotitan—stretched more than 100 feet in length and weighed over 70 tons. The jaws of the fearsome Tyrannosaurus could crush bone. The armored Stegosaurus sported a flashy array of plates and three-foot-long spikes at the end of its tail. But, despite the striking appearance of our favorites, not all dinosaurs were fierce, giant, highly decorated, or even all that strange.
What made the Mesozoic world go ’round weren’t the flashiest dinosaurs, necessarily. Credit goes to the average dinosaurs, the seemingly humdrum herbivores that were critical to ancient ecosystems who lived from 66 to 251 million years ago. These sorts of dinosaurs often get nicknames like “the cows of the Cretaceous,” but that moniker belies their story. Average dinosaurs—both in terms of size and appearance—were core parts of ancient dinosaur communities. Carnivores have to eat, after all. By getting a clearer picture of what the average dinosaur was like, the better paleontologists can understand the nature of the ancient ecosystems where these dinosaurs roamed.
The very first dinosaurs, so far as science has been able to tell, evolved about 243 million years ago. Even compared to other animals in the same Triassic environment, they were not very big. One of the candidates for earliest dinosaur, Nyasasaurus, was about the size of a German shepherd and was a lanky omnivore that likely fed on leaves and beetles. “When exploring patterns of body size evolution, we found that the ancestral body size of all dinosaurs was actually quite small, ranging between 14 and 24 kilograms,” says University of New England paleontologist Nicolas Campione.
But dinosaurs did not stay pipsqueaks for long. Not long after their appearance, dinosaurs began to split into different groups. There were the sauropodmorphs, or the ancestors and relatives of dinosaurs like the long-necked, long-tailed Apatosaurus. So, too, roamed the theropods, which were related to carnivores like the bipedal, sharp-clawed Allosaurus, and the early ornithischians that set the stage for the evolution of horned, armored and duckbilled dinosaurs. By about 225 million years ago, in the Late Triassic, some even got pretty big. “By the Late Triassic and definitely by the Early Jurassic,” Campione says, “ornithischians, sauropods and non-avian theropods would explore most of the body size ranges they would have for the rest of the Mesozoic.” The smallest dinosaurs were about the size of a pigeon, while the largest stretched longer than a blue whale.
Through the Jurassic and Cretaceous, between 66 and 200 million years ago, dinosaurs of all shapes and sizes continued to evolve
. The range of dinosaur sizes from tiny to titanic speaks to their evolutionary success and ecological importance. “Body size is a known predictor of ecology and likely reflects the range of ecological roles that non-avian dinosaurs played during the Mesozoic,” Campione says. Dinosaurs so filled their ancient ecosystems that they suppressed the evolution of mammals. “Only after the extinction of non-avian dinosaurs could mammals finally explore other ecologies,” Campione says.
Out of all these impressive dinosaur species, however, what was average-sized for a dinosaur? Drawing from a data set of 584 dinosaurs, Campione notes, most non-avian dinosaurs had a mass of about 7,700 pounds. That’s a pretty big animal, somewhere between modern-day rhinos and elephants. But that wasn’t uncommon in the Mesozoic. Most non-avian dinosaurs were about this size, Campione notes, exemplified by duckbilled dinosaurs that were common through the Late Cretaceous.
A duckbilled hadrosaur like Edmontosaurus is a perfectly average-sized beast; most dinosaurs were about this size, with the absolute biggest and smallest being relatively rare. And that’s significant for outlining how different the Mesozoic was from our present time.
We live in a megafaunal lull, when large animals are scarce. To know that the average dinosaur was roughly elephant-sized indicates that plant life must have been incredibly productive to support such creatures, and perhaps that dinosaurs were better protected from predators when they reached a certain size. Studies of dinosaur growth have indicated that hadrosaurs rapidly packed on the pounds as a defense against carnivores, so an adult hadrosaur represents the size threshold when potential lunch became too much of a bother.
But average has another, less quantitative meaning, too. Many famous dinosaurs were festooned with horns, had impressive teeth or otherwise stand out because they look odd. Dinosaurs likely evolved these traits to impress each other, and so we, in turn, are impressed. With this in mind, what dinosaurs were just plain boring? Ornithopods
This group of herbivorous dinosaurs includes some favorites such as the crested hadrosaur Parasaurolophus, but also small, beaked plant-eaters like Dryosaurus. What unites these dinosaurs is that they all have three-toed feet, were capable of walking on two legs, and had both beaks and teeth to help them process lots of plant food, with the smallest being about five feet long and the largest stretching to 50 feet long. Often, they’re cast as the prey for the charismatic, toothy predators of their times. “These dinosaurs definitely get made fun of for being boring or, in the case of Dryosaurus, being dry,” says Central Michigan University paleontologist Karen Poole. Most of the time, these dinosaurs are shunted off to the side in museum halls while the more ornate and scary dinosaurs take center stage.
As plain as many of them might seem, though, ornithopods were nonetheless important. “If we were to make an analogy to modern herbivore groups,” Poole says, “ornithopods would be akin to horses and donkeys.” And much like horses and donkeys, part of what makes some ornithopods special is that there are a lot of them.
While rarity often gains a great deal of attention in news reports, paleontologists can learn relatively little from just a single specimen. To understand growth, variation, pathology and many other aspects of paleobiology, researchers need a larger sample size. The fossil record only contains a fraction of all the creatures that have ever lived, so having a large collection of the spike-thumbed, bipedal dinosaur Iguanodon, for example, can let scientists examine questions that can’t be answered from singletons—species only known from a single specimen. By studying large samples of the ornithopod Maiasaura—another common dinosaur and a relative of Edmonotsaurus—experts have been able to determine that these dinosaurs suffered terrible mortality rates during their first year of life. It was hard being a baby dinosaur, these fossils indicate, and getting to adulthood required navigating a very harsh first year.
In fact, Poole notes, ornithopods offer a tantalizing puzzle to paleontologists. Where other dinosaur groups are delineated on the basis of flashy features like horns and crests, ornithopods are sometimes grouped together by their lack of telltale ornaments. But as paleontologists have started looking more closely at the anatomy of these animals, Poole notes, “we’re finding that ornithopods may not all be one group.” Some might be more closely related to horned dinosaurs, for example, while others are closer to armored dinosaurs, hidden connections popping up through ongoing study. By sorting through these connections, paleontologists can sort out how many times certain features evolved—like beaks suited to nipping plants—and update who’s related to whom in the dinosaur family tree.
If paleontologists only focused on the most superlative and strange species, the resulting image of the Mesozoic world would be distorted. The focus would be on the rare, the terrifying and the anatomical outliers. But the less hyped, more common species are where learning happens. These were the dinosaurs that altered ecosystems depending on what plants they ate and even where they walked, trampling some areas and letting others grow. They often were the food our favorite carnivores relied on. And these dinosaurs were often so abundant that they’re more useful for paleontologists who want to know how dinosaurs varied, how they grew and other basic facts about what dinosaurs were like.
Just as a modern forest can’t be understood without the deer, squirrels and other common animals, understanding the Mesozoic requires an appreciation of the most average dinosaurs.
Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.