Michael Reilly in Discovery News:
Slowly rolling across the ocean floor, a humble single-celled creature is poised to revolutionize our understanding of how complex life evolved on Earth.
A distant relative of microscopic amoebas, the grape-sized Gromia sphaerica was discovered once before, lying motionless at the bottom of the Arabian Sea. But when Mikhail Matz of the University of Texas at Austin and a group of researchers stumbled across a group of G. sphaerica off the coast of the Bahamas, the creatures were leaving trails behind them up to 50 centimeters (20 inches) long in the mud.
The trouble is, single-celled critters aren't supposed to be able to leave trails. The oldest fossils of animal trails, called 'trace fossils', date to around 580 million years ago, and paleontologists always figured they must have been made by multicellular animals with complex, symmetrical bodies.
G. sphaerica's traces are the spitting image of the old, Precambrian fossils; two small ridges line the outside of the trail, and one thin bump runs down the middle.
At up to three centimeters (1.2 inches) in diameter, they're also enormous compared to most of their microscopic cousins.
“If these guys were alive 600 million years ago, and their traces got fossilized, a paleontologist who had never seen this thing would not have a shade of doubt attributing this kind of trace to the activity of a big, multicellular, bilaterally symmetrical animal,” Matz said.