A team of geologists from the University of California have discovered a rice-like organism with a front and back, two symmetrical sides, and openings at either end connected by a gut.
Organisms such as sponges and algal mats are known as the earliest multicellular creatures, but they lack basic features of most animals, such as a mouth or gut. However, the new discovery known as "Ikaria wariootia" has those features.
For 15 years, scientists agreed that fossilized burrows found in 555 million-year-old deposits in Nilpena, South Australia, belong to creatures that had two symmetrical sides. But there was no sign of the creature that made the burrows.
According to the study, which was published in the Proceedings of the National Academy of Sciences journal, the researchers noticed oval impressions near some of these burrows.
With funding from a NASA exobiology grant, they used a three-dimensional laser scanner that revealed the regular, consistent shape of a cylindrical body with a distinct head and tail and faintly grooved musculature. The animals ranged between 2-7 millimeters long and about 1-2.5 millimeters wide, with the largest has the size and shape of a grain of rice- just the right size to have made the burrows.
In a report published on the University's website, Mary Droser, a professor of geology, said: "We thought these animals should have existed 500 million years ago, but always understood they would be difficult to recognize. Once we had the 3D scans, we knew that we had made an important discovery."
"In spite of its relatively simple shape, Ikaria was complex compared to other fossils from this period. It burrowed in thin layers of well-oxygenated sand on the ocean floor in search of organic matter, indicating rudimentary sensory abilities," she added.
The burrows also preserve crosswise, "V"-shaped ridges, suggesting this organism moved by contracting muscles across its body like a worm, known as peristaltic locomotion. Evidence of sediment displacement in the burrows and signs the organism fed on buried organic matter reveal Ikaria probably had a mouth, anus, and gut, explained Droser.