PHILADELPHIA, April 2 (UPI) -- U.S. scientists said Friday they found a tiny fossil at a rock outcropping in Pennsylvania that could be the earliest example of an arm bone.
The humerus, or upper-arm bone, is estimated to be 360 million to 370 million years old, the scientists reported in the journal Science. Its existence on a primitive fish indicates limbs might have evolved first in water animals and not on land.
The earliest limbed animals could have been fish navigating shallow rivers. The development could have begun with fins on fish and led to the emergence of tetrapods, or four-legged animals that live primarily on land.
"It's like a Rosetta stone, it helps us translate between these two different forms: the fish fin and the tetrapod limb," said Ted Daeschler, of the Academy of Natural Sciences in Philadelphia.
The scientists think the fossilized animal probably was a predator that used its limbs to hold itself steady in the current of a shallow stream to ambush prey.
