Around 38 million years ago, in a lush forest in what's now South Dakota, the ground shook with the thundering footsteps of a herd of giants.

They were massive, standing about 2.5 meters  tall, nearly 5 meters long, and  weighing close to 3 metric tons.

And while they were nearly elephant-sized, they were actually more closely related to horses.

Megacerops coloradensis was just one of many  humongous members of a group called brontotheres, aka thunder beasts, that lived around this time.

But in the early Eocene epoch, their ancestors were only about the size of a Border Collie.

Yet somehow, in the span of a mere 20 million years, species weighing over 1 metric ton would become a common sight.

In fact, brontotheres experienced one of the  most extreme size increases  ever seen among mammals.

And the journey the thunder beasts took to reach such mega proportions from such humble beginnings forces us to ask an important question, one that paleontologists have been  asking for more than a century… From an evolutionary perspective, is bigger always better?

Everything big started out much smaller.

This may sound simple and obvious, but the actual reason that size increases happen in animal lineages has been tricky for scientists to explain.

In the 19th century, American paleontologist Edward Drinker Cope of ‘Bone Wars’ fame wrote about the seemingly widespread trend  in the fossil record of species  getting bigger over geologic time.

This idea became known as ‘Cope’s rule.’ It basically argues that there’s a constant, gradual push toward bigger and bigger sizes as lineages evolve over time, with successive species generally increasing their body sizes compared to the ones that came before.

And the idea was that this trend is driven by a simple principle: when it comes to survival and reproduction, being bigger generally comes  with a bunch of selective  advantages over being smaller.

See, scientists like Cope assumed that being bigger meant better defense against predators.

Or, if the species itself is a predator, then it can take down larger prey more easily.

Many species also fight for mates and territory,  and you can generally bet on larger  individuals winning those fights.

Plus, bigger bodies generally have bigger brains, possibly opening up the door for greater intelligence to evolve, which can offer survival advantages, too.

So why don’t we live in a world filled with giant animals today?

Well, for most of our species’ history, that’s exactly the world we did live in.

Megafauna were everywhere.

From giant sloths in the Americas, to giant deer in Eurasia, to giant marsupials in Australia.

And their absence today is not entirely natural - in many cases, it’s likely that we had a direct hand in their extinction.

But on the flip side, the largest animal ever  known to have existed is actually  around today - the blue whale.

The most gigantic animal of all time is a modern one, not a prehistoric one.

So does that mean that Cope's rule is actually true?

And that it neatly explains why so many animal  lineages - including brontotheres  - reached such epic proportions?

Well, not exactly.

See, the so-called ‘rule’ has been criticized for pretty much as long as it’s been around.

It can't be applied to all lineages in the fossil record - not everything ends up getting much larger than its ancestors.

And these kinds of evolutionary trends unfold over tens of millions of years, and alongside many ecological variables, so trying to identify  such large-scale patterns with any  degree of confidence is really hard.

You need a lot of fossil data and, ideally, a lot of computing power to find trends in all that data - something that wasn't possible until relatively recently.

Plus, even for groups that do seem to fit the trend, on deeper investigation, it rarely seems to be as simple as a constant, linear push towards ever-larger sizes.

Take, for instance, pterosaurs - the biggest things that ever flew.

By the end of their reign, the wingspan of some pterosaurs exceeded 10 meters.

But for their first 70 million years, their wingspans stayed below 1.6 meters.

The upward trend in size only kicked in about  halfway through their history, at  the end of the Jurassic period.

So it isn't as simple as bigger always equals better.

But then how do we explain the thunder beasts rapidly increasing in size by orders of magnitude, pretty much as soon as ecosystems could support such large mammals?

After all, if you line up the members of their group in chronological order, they seem like a perfect example of Cope’s rule.

They started off as small, dog-sized species like Eotitanops, that roamed North America and Asia in the early Eocene.

By the mid-Eocene, more hefty species had evolved, like the cow-sized Palaeosyops.

And by the end of the Eocene, less than 20  million years after they  first appeared, brontotheres had truly earned their nickname of thunder beasts - a name inspired by Lakota cultural legends of giant beings that caused violent storms that shook the earth.

Because those later brontotheres  were rhinoceros-to-elephant-sized  mammals that had no equals on land at the time.

And these giants emerged in multiple places, too.

Asia had the chunky Rhinotitan, as well as Embolotherium, whose name means ‘battering ram beast’, for example.

That's a pretty metal name.

And North America had, among others, perhaps the most famous of the massive brontotheres: Megacerops, which has also gone by a few  other names in its history, including  Titanotherium, Brontops, and Brontotherium.

Cope personally spent time studying fossils from across this timeline, even naming a handful of brontothere species himself, but did they actually obey his rule or not?

Well, to figure it out, in 2023 a team of researchers published a new analysis of 276 fossils from 57 brontothere species, more  than half of which grew to over a  ton as the Eocene epoch unfolded.

They took into account several other ecological variables throughout that time period, too.

Like the presence of other herbivores that might have competed with them and carnivores that might have preyed on them.

And they analyzed all that data using computer models to try and test three evolutionary pathways that brontotheres could have taken on their journey toward gigantism.

These pathways included Cope’s rule, that gradual, sustained increase in size over time.

And they also included two others that argued that body sizes grew in successive phases, or just diversified without any overall direction.

And they found that it was actually the third scenario that best fit the data.

Because brontotheres weren’t simply getting bigger with time in a selective one-way push that was consistent with Cope's rule.

Instead, they showed a completely different overall trend… A trend in which brontothere lineages continued to produce both larger and smaller new species as they diversified, with no particular preference in either direction.

The reason why there still seemed to be an overall net-increase in body size over millions of years was due to a process called species sorting.

While smaller brontothere species were still evolving, the ecological conditions of the Eocene acted as a sort of filter that led to them generally doing much worse than the larger ones over the long term.

And the researchers think this was primarily because of competition.

See, mammals that survived the extinction event at the end of the Mesozoic that had de-throned the dinosaurs were all pretty small.

So competition in smaller niches was pretty intense as ecosystems recovered and mammals took center stage for the first time.

And while brontotheres were producing species with a range of sizes, larger lineages generally found themselves occupying niches with less competition.

So there was a benefit to being one of the  first mammal groups to  produce big and bulky species.

This meant that smaller lineages in crowded niches were more prone to going extinct than larger ones, resulting in the overall pattern  of body sizes increasing that  we see in the fossil record.

Instead of the brontothere evolutionary tree growing only in one linear direction, it was more like the tree branched off in a range of directions, with the ecosystem pruning off the smaller branches, giving the illusion of a simple trend toward becoming huge.

So, for the brontotheres, Cope’s rule turned out to be wrong.

And by the end of the Eocene, around 34 million years ago, brontotheres of all sizes were extinct, probably as a result of environmental changes.

So it looks like bigger isn’t always better.

Like so many evolutionary traits that we’re tempted to point to as signs of ‘progress’ or ‘improvement,’ whether they’re good or not all depends on the ecological context…and if deep time has taught us anything, it’s that change is the only constant.