Why Were Dinosaurs So Big?

The Facts and Theories Behind Dinosaur Gigantism

HK TST Science Museum Bones exhibit 02 恐龍 dinosaur
Amongesle/Wikimedia Commons/CC-BY-SA-3.0

One of the things that makes dinosaurs so appealing is their sheer size: plant eaters like Diplodocus and Brachiosaurus weighed in the neighborhood of 25 to 50 tons, and a well-toned Tyrannosaurus Rex tipped the scales as much as 10 tons. From the fossil evidence, it's clear that--species by species, individual by individual--dinosaurs were more massive than any other group of animals that ever lived (with the logical exception of certain genera of prehistoric sharks, prehistoric whales and marine reptiles like ichthyosaurs and pliosaurs, the extreme bulk of which were supported by the natural buoyancy of water).

However, what's fun for dinosaur enthusiasts is often what causes paleontologists and evolutionary biologists to tear their hair out. The giant size of dinosaurs demands an explanation, and one that's compatible with other dinosaur theories--for example, it's impossible to discuss dinosaur gigantism without paying close attention to the whole cold-blooded/warm-blooded metabolism debate.

So what's the current state of thinking about plus-sized dinosaurs? Here are a few more-or-less interrelated theories.

Theory #1: Dinosaur Size Was Fueled by Vegetation

During the Mesozoic Era--which stretched from the beginning of the Triassic period, 250 million years ago, to the extinction of the dinosaurs at the end of the Cretaceous period, 65 million years ago--atmospheric levels of carbon dioxide were much higher than they are today. If you've been following the global warming debate, you'll know that increased carbon dioxide is directly correlated with increased temperature--meaning the global climate was much warmer millions of years ago than it is today.

This combination of high levels of carbon dioxide (which plants recycle as food via the process of photosynthesis) and high temperatures (an average of 90 or 100 degrees Fahrenheit, even near the poles) meant that the prehistoric world was matted with all kinds of vegetation--plants, trees, mosses, etc.

Like kids at an all-day dessert buffet, sauropods may have evolved to giant sizes simply because there was a surplus of nourishment at hand. This would also explain why certain tyrannosaurs and large theropods were so big; a 50-pound carnivore wouldn't have stood much of a chance against a 50-ton plant eater.

Theory #2: Hugeness in Dinosaurs Was a Form of Self-defense

If Theory #1 strikes you as a bit simplistic, your instincts are correct: the mere availability of huge amounts of vegetation doesn't necessarily entail the evolution of giant animals that can swallow it down to the last shoot. (After all, the earth was shoulder-deep in microorganisms for hundreds of millions of years before the appearance of multicellular life, and we don't have any evidence of one-ton bacteria.) Evolution tends to work along multiple paths, and the fact is that the drawbacks of dinosaur gigantism (such as the slow speed of individuals and the need for limited population size) could easily have outweighed its benefits in terms of food-gathering.

That said, some paleontologists do believe that gigantism conferred an evolutionary advantage on the dinosaurs that possessed it: for example, a jumbo-sized hadrosaur like Shantungosaurus would have been virtually immune to predation when fully grown, even if the tyrannosaurs of its ecosystem hunted in packs.

(This theory also lends some indirect credence to the idea that Tyrannosaurus Rex scavenged its food--say, by happening across the carcass of an Ankylosaurus that died of disease or old age--rather than actively hunting it down.) But once again, we have to be careful: of course giant dinosaurs benefited from their size, because otherwise they wouldn't have been gigantic in the first place, a classic example of an evolutionary tautology.

Theory #3: Dinosaur Gigantism Was a Byproduct of Cold-bloodedness

This is where things get a bit sticky. Many paleontologists who study giant plant-eating dinosaurs like hadrosaurs and sauropods believe that these behemoths were cold-blooded, for two compelling reasons: first, based on our current physiological models, a warm-blooded Mamenchisaurus would have cooked itself from the inside out, like a potato, and promptly expired; and second, no land-dwelling, warm-blooded mammals living today even approach the size of the largest herbivorous dinosaurs (elephants weigh a few tons, max, and the largest terrestrial mammal in the history of life on earth, Indricotherium, only topped out at about 20 tons).

Here's where the advantages of gigantism come in. If a sauropod evolved to large-enough sizes, scientists believe, it would have achieved "homeothermy"--that is, the ability to maintain its interior temperature despite the prevailing environmental conditions. This is because a house-sized, homeothermic Argentinosaurus would warm up slowly (in the sun, during the day) and cool down equally slowly (at night), giving it a fairly constant average body temperature--whereas a smaller reptile would be at the mercy of ambient temperatures on an hour-by-hour basis.

The problem is, these speculations about cold-blooded herbivorous dinosaurs run counter to the current vogue for warm-blooded carnivorous dinosaurs. Although it's not impossible that a warm-blooded Tyrannosaurus Rex could have coexisted alongside a cold-blooded Titanosaurus, evolutionary biologists would be much happier if all dinosaurs, which after all evolved from the same common ancestors, possessed uniform metabolisms--even if these were "intermediate" metabolisms that don't correspond to anything seen in modern animals.

Dinosaur Size: What's the Verdict?

If the above theories leave you as confused as you were before reading this article, you're not alone. The fact is that evolution toyed with the existence of giant-sized terrestrial animals, over a time span of 100 million years, exactly once, during the Mesozoic Era. Before and after the dinosaurs, most terrestrial creatures were reasonably sized, with the odd exceptions (like the above-mentioned Indricotherium) that proved the rule. Most likely, some combination of theories #1, #2 and #3, along with a possible fourth theory that we have yet to formulate, explains the huge size of dinosaurs; in exactly what proportion, and in what order, will have to await future research.