Why Were Dinosaurs So Big?

Anyone who recently saw Jurassic World Rebirth—or any of the previous six movies in the franchise—has likely wondered how dinosaurs got so big. “Oh boy, I don’t think there’s one answer to that question,” said Jordan Mallon, a paleobiologist at the Canadian Museum of Nature. “When we think of the biggest dinosaurs, we think about sauropods, and I think there are multiple reasons why they were able to get so big.”

How Did Dinosaurs Get So Big?

“That’s a very interesting and difficult question to answer. We’ve been tackling it in various ways,” said David Hone, a paleontologist at the Queen Mary University of London.

Scientists have found evidence that the long-necked and long-tailed sauropods had a comparatively slower metabolism relative to today’s large mammals, which indicates that they likely didn’t have to eat as much.¹ And when sauropods did eat, they likely swallowed most of their food whole: Sauropods’ teeth had very little wear, unlike those of other herbivorous species, such as the duck-billed dinosaurs, and even mammals.^2,3 “They could ‘vacuum up’ a lot of food without having to spend a lot of time chewing,” Mallon said. Some sauropod bones were also filled with air sacs, which could make even larger body sizes mechanically feasible.⁴ This adaptive anatomical feature is found in many birds, dinosaurs’ closest living relatives, and it helps reduce their body weight and promote flight.

But how do scientists learn about dinosaurs’ physiology and behavior, considering they can’t observe these extinct creatures in real life? “With great care and trepidation,” according to Mallon. “It's a challenge, but also lots of fun.”

For example, to predict the efficiency of sauropod digestion, some researchers tried to artificially reproduce this process in the laboratory.^5,6 However, most scientists looked for indirect lines of evidence, such as using markings on the thigh bone to figure out if a dinosaur was a “warm-blooded” endotherm, which would generate heat metabolically, or a “cold-blooded” ectotherm, which would rely on the environment as a source of heat.⁷ While there are likely differences between dinosaur types, Mallon expected that the largest ones would be borderline ectothermic. “At really large body sizes, your problem isn’t staying warm. Your problem is staying cool,” Mallon said. “When you’re that large, overheating becomes a major issue.”

Which Dinosaur Is the Biggest, and How Big Is It?

It would be remiss to talk about how dinosaurs became the largest animals that ever walked the Earth without discussing their sizes and the quest to determine the biggest one.

In 2014, the media dubbed the sauropod Patagotitan mayorum—which based on its thigh bones, researchers estimated to be around 40m long and 20m tall and weigh about 70,000kg—as the “biggest dinosaur ever.” However, scientists are still debating which dinosaur, among several contenders, is actually the largest and how big it is exactly. “Every kid’s book in the world wants to know this, but we really don't know,” Hone said.

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According to both Hone and Mallon, estimating the size of dinosaurs is a tall order. For one, most dinosaur skeletons are recovered incompletely. “Estimating the mass of things from an incomplete skeleton is hilariously problematic,” Hone said. And to make matters worse, for many species, researchers made size estimates based on a few bones from just a single dinosaur. “That one dinosaur could be the equivalent of a six-foot-ten human,” Hone said. “Sure, if you’ve only got one skeleton, the odds of that being the equivalent of a six-foot-ten human is very, very small indeed, but we also don’t know that we haven’t done that.”

To highlight this problem, Mallon and Hone recently performed a computational simulation to predict the body size distribution in a population of 140 million Tyrannosaurus rex, a well-studied, large, carnivorous dinosaur.⁸ Hone said that T. rex was the largest dinosaur that had enough data on which they could perform such analysis. Using their model, Mallon and Hone estimated that the absolute largest T. rex could have been 70 percent heavier than the largest known specimen. “That’s an impressive number—it’s nothing to sneeze at,” Mallon said.

Based on this analysis, Mallon and Hone predicted the largest T. rex to weigh close to 15,000kg, which meant that this carnivore could’ve been capable of eating a sauropod. This predation could then, in turn, prompt sauropods to grow even larger. “There’s almost like an arms race going on,” Mallon said. This suggests that evolutionary pressure may have contributed to sauropods’ gigantism. It also hints at the possibility that the largest sauropod could be even bigger than the ones whose fossils scientists partially unearthed in 2014.

Scientists will likely never know the exact size of the largest dinosaur or what they looked like, but that doesn’t bother Hone. He would rather find answers to the questions about which dinosaurs ate what and how that shaped both the evolution of dinosaurs and their environment. “Dinosaurs are so, so much larger than any other animal that ever walked the Earth—that's the interesting bit. Who cares if the T. rex was 60 centimeters longer than the Spinosaurus? It doesn't tell you anything,” said Hone.