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Mammals' unique arms started evolving before the dinosaurs existed

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Bats fly, whales swim, gibbons swing from tree to tree, horses gallop, and humans swipe on their phones—the different habitats and lifestyles of mammals rely on our unique forelimbs. No other group of vertebrate animals has evolved so many different kinds of arms: in contrast, all birds have wings, and pretty much all lizards walk on all fours. Our forelimbs are a big part of what makes mammals special, and in a new study in the Proceedings of the National Academy of Sciences, scientists have discovered that our early relatives started evolving diverse forelimbs 270 million years ago—a good 30 million years before the earliest dinosaurs existed.

"Aside from fur, diverse forelimb shape is one of the most iconic characteristics of mammals," says the paper's lead author Jacqueline Lungmus, a research assistant at Chicago's Field Museum and a doctoral candidate at the University of Chicago. "We were trying to understand where that comes from, if it's a recent trait or if this has been something special about the group of animals that we belong to from the beginning."

To determine the origins of mammals' arms today, Lungmus and her co-author, Field Museum curator Ken Angielczyk, examined the fossils of mammals' ancient relatives. About 312 million years ago, land-dwelling vertebrates split into two groups—the sauropsids, which went on to include dinosaurs, birds, crocodiles, and lizards, and the synapsids, the group that mammals are part of. A key difference between sauropsids and synapsids is the pattern of openings in the skull where jaw muscles attach. While the earliest synapsids, called pelycosaurs, were more closely related to humans than to dinosaurs, they looked like hulking reptiles. Angielczyk notes, "If you saw a pelycosaur walking down the street, you wouldn't think it looked like a mammal—you'd say, 'That's a weird-looking crocodile.'"