Mammals boast an unprecedented diversity of forelimbs, allowing mammalian species to adopt a variety of lifestyles and adapt to a wide range of habitats.

According to a new study, the earliest mammalian predecessors began evolving unique forelimbs 270 million years ago, 30 million years before the first dinosaurs arrived.

"Aside from fur, diverse forelimb shape is one of the most iconic characteristics of mammals," Jacqueline Lungmus, a doctoral candidate at the University of Chicago and a research assistant at the Field Museum, said in a news release. "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."

When Lungmus and Field Museum curator Ken Angielczyk surveyed the arms of mammals' ancient relatives, they found forelimb diversity began to dramatically expand around 270 million years ago, around the time therapsids emerged.

Some 320 million years ago, land-dwelling vertebrates were comprised of two groups, sauropsids and synapsids. From sauropsids came dinosaurs, birds, crocodiles and lizards. Mammals eventually emerged from synapsids.

Today, scientists know synapsids are more closely related to humans than tyrannosaurs, but 300 million years ago, it would have been hard to tell the difference between sauropsids and synapsids.

"If you saw a pelycosaur walking down the street," Angielczyk said, "you wouldn't think it looked like a mammal -- you'd say, 'That's a weird-looking crocodile.'"

But with the emergence of therapsids, synapsids began to modernize and diversify. To determine whether this expansion was linked with forelimb evolution, Lungmus and Angielczyk analyzed the arm bones of 73 kinds of pelycosaurs and therapsids.

"This is the first study to quantify forelimb shape across a big sample of these animals," said Lungmus.

The survey -- detailed this week in the journal PNAS -- revealed greater variation among therapsid forelimbs.

"The therapsids are the first synapsids to increase the variability of their forelimbs -- this study dramatically pushes that trait back in time," said Lungmus.

Previously, scientists have only successfully traced mammalian forelimb evolution to 160 million years ago. The work of Lungmus and Angielczyk pushes the evolutionary origin of the mammalian forelimb back another 100 million years.

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