Why were dinosaurs so big?

“Mysteries of the Dinosaurs” reveals the secret side of the “terrible lizards” and all the questions that keep paleontologists up at night.

YOU NEVER KNOW how small you are until you’re next to a big old dinosaur. Find the right lighting in the museum hall and you can literally stand in the shadow of the skeletons of Apatosaurus, Patagotitan, Brachiosaurus, and other reptiles that have become much larger than any other terrestrial creature in the last 66 million years. But even after nearly two centuries of study, we have only the vaguest ideas why some dinosaurs were larger than any land mammal to date.

While a number of dinosaurs fall into the super-large category –T-Rex weighed more than a mature male African elephant—the sauropods held the all-time title. They had small heads with simple teeth, impressively long necks, stout bodies and tapering tails. So many species of sauropods reached more than 100 feet in length that paleontologists still aren’t sure which stretched the farthest. While the largest land mammals, such as the hornless rhinoceros Paraceratherium and the largest fossil elephants, reaching about 18 tons, sauropods evolved to be at least 36 times more massive during their evolutionary history—a continuing revenge of huge herbivores throughout the Jurassic and Cretaceous.

The staggering weight of these creatures has often left us wondering why they are so much larger than any earthly creature before or since. But in the field of paleontology, “why” questions are extremely difficult to answer. Inquiries beginning with “why” are questions of history, and in this case, history has played out dozens of times on multiple continents over more than 130 million years. Although we see the end effect, we cannot understand exactly the causes.

However, dinosaurs have a habit of digging their claws into our imaginations, so researchers have continued, uncovering several clues over the past two decades about the abundance of superior sauropods. While higher oxygen levels have been linked to larger body sizes in several ancient insects, the atmosphere in the heyday of the dinosaurs was about the same as it is today. Moreover, Earth’s gravitational pull was just as strong during the Mesozoic Era as it is in the modern era. So we know that the impressive sizes of Argentinosaurus and other higher sauropods was not a matter of an abiotic factor such as increased oxygen in the atmosphere or lower gravity. Our explanation is elsewhere.

These facts only show us what allowed sauropods to grow large. Dinosaurs weren’t supposed to drift in that direction.

Paleontologists get closer to the truth by looking at the dinosaurs themselves. For example, experts have identified a set of characteristics that distinguish sauropods from the mastodons and giant rhinoceroses of the Cenozoic. Eggs have a lot to do with it.

The largest mammals of all time were placental, giving birth to their offspring from within so that they could emerge more developed. This reproductive strategy comes with some limitations. To reach even larger adult sizes, females of each species would have to carry their babies in the womb longer. African elephants, for example, are already pregnant for about two years – during which many things can go wrong. But sauropods, like all non-avian dinosaurs, laid several eggs at a time, bypassing the reproductive limitations of live birth and flooding their ecosystems with tons of babies that had the potential to grow huge (even if most became snacks for carnivores in time). Different reproductive strategies gave dinosaurs some advantages over mammals.

Camarasaurus and other sauropods also received some help from their anatomical features. Sauropods have complex air sac systems in their airways that create air pockets in and around their bones. These nifty features kept their skeletons light without sacrificing strength, and also made the extraction of oxygen from the air and the release of excess body heat more efficient. The distinctive dinosaurs were also able to have long necks because they didn’t have heavy heads full of massive, grinding teeth like the large herbivorous mammals of the past 66 million years. Instead, sauropods had small, light legs full of spoon- or pencil-shaped teeth that were mostly capable of cutting up vegetation to be broken down and fermented through their gastrointestinal tract. In other words, their guts did the work, not their teeth. Studies of ginkgo, horsetail, and other common Mesozoic plants show that ancient vegetation was more calorie-dense than previously thought, so an abundance of green food likely fueled the reptilian giants’ unprecedented growth.

But these facts only show us what allowed sauropods to grow large. Dinosaurs weren’t supposed to drift in that direction. In fact, some were relatively small: the island-dwelling species Madjarosaurus it was the size of a large cow. Sauropods could have thrived at smaller sizes, but instead continued to spin off giant lineages. We know something about what made large-sized life possible, but what we don’t yet know is what evolutionary pressures caused sauropods to develop huge bodies.

Predators definitely played their part. All sauropods are born small—even the largest species hatch from football-sized eggs. They were vulnerable to a variety of predators from the Jurassic and Cretaceous, but rapid growth was one way to fend off those hungry jaws. Hunting megafauna could be dangerous and even deadly, as we see with lions, wolves, and even humans today, and so sauropods may have increased in size to be less attractive to humans as Allosaurus and T. rex.

But if carnivorous appetites were the main driver of sauropod size, we would have seen a more even and prolonged “arms race” between dinosaurs over time, leading to progressively larger predators and prey. The fossil record instead shows that sauropods increased at different times and in different places, possibly for a number of reasons ranging from local food to what the mating sauropods found sexy in each other. The repeated evolution of the giant dinosaurs suggests that there were many paths to the impressive growth of sauropods, not just one. Biology was as complex then as it is now, and we’ll never know the whole story without experiencing 100-foot-long reptiles for ourselves.

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