November 28, 2022.

>Whatcheeria was a six-foot-long lake-dwelling creature with a salamander-like body and a long, narrow head; its fossils were discovered in a limestone quarry near the town of What Cheer, Iowa.

>There are around 350 Whatcheeria specimens, ranging from single bones to complete skeletons, that have been unearthed, and every last one of them resides in the Field Museum’s collections.

>In a new study in Communications Biology, these specimens helped reveal how Whatcheeria grew big enough to menace its fishy prey: instead of growing “slow and steady” the way that many modern reptiles and amphibians do, it grew rapidly in its youth.

>Whatcheeria was a top predator. Bony grooves in its skull for sensory organs shared by fish and aquatic amphibians reveal that it lived underwater, and its sturdy leg bones could have helped it hunker down in one spot and wait for prey to swim by.

>While Whatcheeria looks like a giant salamander, it isn’t one-- it’s a “stem tetrapod,” an early four-legged critter that’s part of the lineage that eventually evolved into the four-limbed animals alive today.

> 

>“Whatcheeria is more closely related to living tetrapods like amphibians and reptiles and mammals than it is to anything else, but it falls outside of those modern groups,” says Ken Angielczyk, a curator at the Field Museum and co-author of the study. “That means that it can help us learn about how tetrapods, including us, evolved.”

>To see how Whatcheeria grew, [Ben Otoo, co-author; PhD student at the University of Chicago] and Angielczyk offered up thigh bones from nine Whatcheeria individuals ranging from juvenile to adult.

>[Lead author Megan Whitney] and her advisor, Harvard University’s Stephanie Pierce, took thin slices of bone and examined them under a microscope. When an animal is growing, it creates new layers of bone every growing season, says Otoo.

>In addition to helping give us a better sense of the evolutionary pressures on early tetrapods, the researchers say the findings are a reminder that evolution isn’t a neat stepwise process: it’s a series of experiments.

Communications Biology, 2022. DOI 10.1038/s42003-022-04079-0

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So growing up = evolution now.

Disappointed when I found out "tetrapod" didn't mean "three legged"