Cope’s Law: Could dinosaurs have been bigger?

You may have heard of island gigantism, an interesting phenomenon in which small animals that migrate to islands tend to grow significantly larger. However, Cope’s Rule, which was proposed by an American paleontologist Edward Cope in the late 19th century, takes the “bigger is better” perspective a few steps further.

Cope postulated that increases in body size is the trend for population lineages over time. He argued that larger organisms tend to enjoy certain fitness benefits, such as better resistance against predators, short-term famine situations, and rapid climactic changes, as well as increased ability to capture prey, reproduce, and eliminate competition. Sounds like a wicked power-up, but fitness benefits always come with drawbacks. Larger organisms require more food and water, and their longer generation times translate to a longer period of dependence on their mothers.

While Cope’s ideas seem to hold true specifically in mammals, there are plenty of exceptions to this rule. A research team led by Gene Hunt, curator in the Department of Paleobiology at the National Museum of Natural History (NMNH) in Washington, D.C., wanted to see if Cope’s Rule applied to dinosaurs – and it did for some species.

As part of their statistical model, Hunt and colleagues Richard FitzJohn of the University of British Columbia and Matthew Carrano of the NMNH examined data from dinosaur femurs as an accurate representation of its size. They then looked for temporal correlations of species size, as well as whether there were any upper limits for body size. Interestingly, when the researchers examined the dinosaur family tree, they found that some clades of dinosaurs, such as ceratopsids and hadrosaurs, indeed abided by Cope’s Rule: these clades showed greater increases in size than decreases over time.

Although birds are the modern-day descendants of theropod dinosaurs, the research team decided to exclude them from the study. This was because the evolutionary pressure exerted on birds caused them to become lighter and smaller to enable better flight.

With regards to the limits on size, the researchers obtained mixed results. Sauropods and ornithopods, for one, did not appear to have limiters on how large they could evolve. Sauropods, a gigantic herbivore weighing at least 16 metric tons and more commonly known as the Brontosaurus, were among the largest animals to have ever walked on land.

On the other hand, the size of theropods, which include the infamous Tyrannosaurus rex, were found to have plateaued. According to Hunt, this is not particularly surprising, as there are physical limits to how massive a bipedal animal can get while still being able to move. Makes a lot of sense, seeing how most of the weight champions in the modern age are either quadrupedal as elephants and giraffes, aquatic as blue whales and saltwater crocodiles, or both.

Hunt concludes by saying that, when it comes to evolution, “bigger is better” is not necessarily a hard and fast rule (maybe you could make an argument for larger serving sizes at fast-food restaurants, but that depends on how badly you want to keep your figure). While it may be true that bigger animals are less likely to become prey, even the biggest animals start out small. In addition, it takes a surprisingly long time, usually many millions of years, for the animal to become gigantic enough to avoid becoming lunch. Hence, it is not well-understood why Cope’s Rule works.

Hunt, FitzJohn, and Carrano presented the results of their study at the annual meeting of The Geological Society of America on Nov. 4.