Fossil fish shows unexpected spine shape

CHICAGO, May 23 (UPI) -- A fossil discovery has scientists reconsidering whether a spine with multiple segments is a feature of land-dwelling animals only, U.S. researchers say.

The discovery of the identical anatomical feature in a 345-million-year-old eel-like fish suggests the complex anatomy arose separately from -- and perhaps before -- the first species to walk on land, the University of Chicago reported Wednesday.

Tarrasius problematicus lived in shallow bodies of water in what is now Scotland between 359 million and 318 million years ago. Like many fish, it was thought to have a vertebral column divided simply into body and tail segments.

A new description of Tarrasius by Chicago biologist Lauren Sallan, published in Proceedings of the Royal Society B, highlights a five-segment column much more similar to the spinal anatomy of land-dwelling animals called tetrapods, including humans.

The finding contradicts a common assumption paleontologists have used to determine from fossils whether an ancient species lived on land or in water, she said.

"It's the last trait to fall," Sallan said. "First, limbs were thought to show that a species was on land and walking, and now the vertebral morphology doesn't mean that they're on land either.

"So a lot of the things we associate with tetrapods actually arose first in fishes, and this is another example of that," she said.

The spine may have been useful in propelling the fish's body during fast swimming, she said.

"I think it must help with stiffening the body, because the tail is so flexible," Sallan said. "If you look at the general shape, it's more like a tadpole or an early tetrapod, so it might just function to hold the body steady because the tail is flapping."