Why Were So Many Prehistoric Animals So Big?

By: Karen Kirkpatrick & Sarah Gleim  | 
The Brachiosaurus is a sauropod that lived in North America during the Late Jurassic, about 154–150 million years ago. Its trademark long neck is thought to be one way it grew so massive. Aleks49/Shutterstock

There's ongoing debate about which dinosaur might be the largest creature to ever roam Earth. A 98-million-year-old skeleton of a long-necked titanosaur discovered in Neuquén Province of northwest Patagonia in 2012 could hold the record. In research published in 2021 in the journal Cretaceous Research, the authors said they believe it could be "one of the largest sauropods ever found" — even larger than a Patagotitan, like the one on display at the American Museum of Natural History that is 122 feet (37.1 meters) long.

But why did prehistoric animals like these get so big in the first place? Why mastodons, woolly mammoths (whose name means "huge") and many dinosaurs were so big is something of a mystery. There are several hypotheses about why these mammals grew so massive. Here are a few of them.


Better Oxygen

First, several studies hypothesize that environmental factors, such as higher oxygen content in the air, contribute to their expansion in North America. This is from a study published in 2019 from researchers from Rensselaer Polytechnic Institute and the University of Texas Austin. They used a new technique to analyze tiny amounts of gas trapped inside 215-million-year-old rocks from the Colorado Plateau and the Newark Basin.

"Our results show that over a period of around 3 million years, which is very rapid in geological terms, the oxygen levels in the atmosphere jumped from around 15 percent to around 19 percent," lead researcher, Rensselaer Polytechnic Institute professor Morgan Schaller said in a press statement. "For comparison, there is 21 percent oxygen in today's atmosphere. We really don't know what might have caused this increase, but we also see a drop in CO2 levels at that time." Around this same time the oxygen peaked, the first dinosaurs appear in the North American tropics and the massive sauropods soon followed.

Efficient Eaters

The existence of sauropods lead us to a second hypothesis about why these mammals grew so giant: efficient food uptake. The thought behind this idea is that because sauropods had such long necks, they must have been more efficient eaters than other large herbivores, meaning they could cover much larger feeding grounds and reach food that was inaccessible to other dinosaurs. So in theory, the massive sauropods must have been able to grow larger than other dinosaurs because they fed more efficiently.

 titanosaur Patagotitan
This scale diagram shows the giant Titanosaur Patagotitan mayorum compared to a human.

Cope's Rule

Then there's Cope's Rule, which is a hypothesis formulated by paleontologist Edward Cope that says animals in evolving lineages tend to get larger over time. And a study by the Department of Paleobiology at the National Museum of Natural History (NMNH) in 2012 found that Cope was right, well, some of the time. Using advanced statistical methods the team used dinosaur femur bones to detect animal size. They then used that femur data in their statistical model to look for two things: directional trends in size over time and whether there were any detectable upper limits for body size.

"What we did then was explore how constant a rule is this Cope's Rule trend within dinosaurs," Gene Hunt, curator in the Department of Paleobiology at the National Museum of Natural said in a press statement. What they found was some groups, or clades, of dinosaurs (including the long-necked sauropods) do grow larger over time, as Cope's Rule suggests. However, others, like theropods, which include the popular T.rex, did not.

Light Bones and Air Sacs

Finally, other evidence points to why prehistoric animals grew so massive, including those that took to the air. Studies indicate it could be because of their bones and lungs. Pterosaurs, for instance, conquered the skies long before the first dinosaurs. And pterosaurs started out small, but some species ballooned to unbelievable proportions. This could have been due in part to a highly effective flow-through respiratory system, which allowed them the ability to sustain flight.

Azhdarchids were one of the largest flying pterosaurs in the world. They had wingspans of 32.8 feet (10 meters) and weighed as much as 440 pounds (200 kilograms). Studies have shown that their bones were intricate structures that made them super strong and stable, but also super light so these reptiles could fly. And we mean fly — as far as 10,000 miles (16, 093 kilometers) possibly without resting or eating.

The Supersaurus is also thought have had extraordinarily light bones and a complex system of air sacs, which could have allowed it to grow huge without collapsing in on itself. They also had very efficient lungs, so their respiration and heat exchange could better support the larger size. The fact that they laid eggs and could reproduce relatively quickly may have given the bigger animals a reproductive advantage as well.

Of course, all of the animals we've mentioned were the top predators of their time, which made them a lot less susceptible to becoming another animals' dinner.

Azhdarchids like this one were one of the largest flying pterosaurs in the world.


Originally Published: May 6, 2015

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