A fangy, 6-foot-long (1.8 meters) carnivore that haunted the lakes of what is now the American Midwest would have been a top predator in its freshwater ecosystem — a "T. rex of its time," according to scientists who studied the creature. And it grew up fast, new research finds.

The predator, an early four-legged vertebrate known as a tetrapod, lived around 328 million years ago during the early Carboniferous period. Its name is Whatcheeria deltae, after the town of What Cheer, Iowa, where many of its fossils are found. It lived at a time when the region was lushly vegetated and dotted with sinkholes that had turned into lakes. W. deltae would have lurked in these lakes, growing to 6.5 feet (2 meters) long and looking something like toothy, enormous salamanders.

"It would have made Whatcheeria the biggest thing in the lake: Go wherever you want, eat whoever you want," said Ben Otoo, a doctoral candidate at Chicago's Field Museum and the University of Chicago and one of the authors of a new study describing W. deltae published Monday (Nov. 28) in the journal Communications Biology.

Otoo and was studying the Field Museum's collection of 375 W. deltae specimens — some bone fragments and some nearly complete skeletons — when they noticed that the limb bones of the animal came in different sizes. The size differences weren't based on when or where the fossils were found, so Otoo realized that they were looking at bones from animals of different ages. Smaller limb bones measured about 4 inches (10 centimeters) long, while the largest were 2.6 feet (0.8 m).

That meant that Otoo and their colleagues had an opportunity to study how W. deltae grew. Early tetrapods like whatcheeriads were related to modern reptiles, amphibians and mammals but was in a different evolutionary lineage than the ancestor of those three groups. Modern-day birds and mammals tend to grow quickly in their youth and then stop growing, while reptiles tend to grow quickly at first and then continue growing, but more slowly. Meanwhile, some amphibians grow at a slow and consistent rate throughout their lives. Little was known about how early tetrapods may have grown.

By looking at growth rings in the bones, the team found that W. deltae got big fast and then slowed to a more leisurely but steady growth rate. All of the nine specimens they studied were older juveniles and young adults, Otoo said, so it seems that the animals bulked up to around 3.3 feet (1 meter) in length as they neared sexual maturity and then grew larger more slowly in later adulthood.

"You have this animal that is racing to get to reproductive age to get to at least a decent size really quickly, because the best way to get yourself out of a predator's range of prey items is to get bigger," Otoo told Live Science.

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It was surprising to see this pattern in such an early tetrapod, Otoo added, because scientists expected rapid early growth to be linked to a terrestrial lifestyle and restricted to mammals, birds and reptiles with higher metabolisms than those of early tetrapods.

"To find [rapid growth] in as old an animal as Whatcheeria and as primitive as Whatcheeria was really unexpected," Otoo said.

Other types of tetrapods from this era grew more slowly and steadily, Otoo said, so it's clear that these early four-legged animals were trying a number of different evolutionary pathways to success.

"Early tetrapods, even those far away from the origin of the modern living lineages — they basically had a lot more going on than we thought," Otoo said.