Were you to have wandered near what we now call What Cheer, Iowa, 340 million years ago, you could’ve been predated on by a giant two-meter, salamander-like macropredator. The remains of Whatcheeria, as the monstrous creature was eventually named, are now revealing new insights into their development, showing that they experienced growth spurts early in life that enabled them to become so enormous. This is a trait that was previously thought to have evolved much later.

“If you saw Whatcheeria in life, it would probably look like a big crocodile-shaped salamander, with a narrow head and lots of teeth,” said study co-author Ben Otoo in a statement, a PhD student at the University of Chicago and the Field Museum. “If it really curled up, probably to an uncomfortable extent, it could fit in your bathtub, but neither you nor it would want it to be there.”

Whatcheeria is an interesting specimen because it’s an early relative of tetrapods, the four-legged animals that wormed their way out of the water so we could seize the land and suffer as rent-paying, air-breathing animals. This means that unraveling the mystery of Whatcheeria can tell us more about how we came to exist, which is always fun.

Fortunately, the Field Museum in Chicago is blessed with a rich collection of Whatcheeria specimens ranging in size from little to large, stretching as much as 1.98 meters (6.5 feet). The diversity of remains gave researchers an opportunity to study their development, for which they sawed up Whatcheeria thigh bones so they could look for growth rings to establish how old the macropredators were at different sizes.

Their results showed that Whatcheeria experienced a growth spurt early on in life – which was surprising, as it had previously been expected that they grew slow and steady throughout their lives, like its close amphibious relatives. Instead, it seems they grew rapidly before leveling off, a fact that “breaks all of the rules that we thought of for how growth is evolving in these early tetrapods,” according to lead author Megan Whitney, a professor at Loyola University in Chicago.

Bulking up so early on in life isn’t easy, requiring a lot of food fast, but doing so may have helped Whatcheeria survive in what would’ve been a treacherous environment.

“If you’re going to be a top predator, a very large animal, it can be a competitive advantage to get big quickly as it makes it easier to hunt other animals, and harder for other predators to hunt you,” said co-author Stephanie Pierce of Harvard University. “It can also be a beneficial survival strategy when living in unpredictable environments, such as the lake system Whatcheeria inhabited, which went through seasonal dying periods.”

The discovery adds to a growing body of evidence that shows Whatcheeria was a puzzling predator for its time and demonstrates that evolution can sometimes work in mysterious ways.

“Evolution is about trying out different lifestyles and combinations of features,” said study co-author Ken Angielczyk, a curator at the Field Museum. “And so, you get an animal like Whatcheeria that’s an early tetrapod, but it's also a pretty fast-growing one. It's a really big one for its time. It has this weird skeleton that's potentially letting it do some things that some of its contemporaries weren't.”

“It’s an experiment in how to be a big predator, and it shows how diverse life on Earth was and still is.”

The study was published in communications biology.