Bone is 'missing fish-animal link'

SCIENTISTS believe they have found the long-sought missing link between fish and land animals in the shape of the oldest known arm bone.

The 365-million-year-old humerus belonged to a creature that was just learning to prop up its body.

It is believed to bridge the gap between the fins of fish and the limbs of amphibians.

Precisely how land-living animals evolved from fish has been a scientific mystery. An important part of the transition is the change from fins to arms and legs.

The fossil, found in an unlikely site beside a busy US highway, is the earliest arm bone known to belong to any limbed animal.

While possessing features seen in primitive fish fins, it also has the characteristics of a true limb bone.

Researcher Dr Michael Coates, from the University of Chicago, said: "This bone is a lot more robust than a humerus from any of the ancient species.

"Relative to other tetrapods (four-limbed animals) this is almost over-engineered. There's a massive space for the attachment of substantial muscle going across to the chest."

The action of these muscles would have produced a motion similar to a "push-up or a bench-press," indicating that the bone served to prop up the body.

"This function represents the intermediate condition between primitive steering and braking functions in fins and the derived aquatic or terrestrial walking gait," the researchers wrote in the journal Science.

Dr Coates said it was clear that the ability to prop the body dated back further than was previously thought. "This means that many of the features that we thought evolved to enable life on land originally evolved in fish living in aquatic ecosystems," he said.

The specimen was unearthed in 1993 from a spot beside the Clinton County Road in north-central Pennsylvania.

Layered rocks were deposited by ancient stream systems. Trapped within these rocks is fossil evidence of an ecosystem teeming with plant and animal life.