"(nytimes.com, July 7, 2012) Bilateral animals, or bilaterians - so called because unlike, say, jellyfish, they have left, right, back and front sides - are probably the first animals that could move on their own. Until now, the oldest fossil evidence for bilaterians dated back 555 million years.
However, scientists have found fossil burrows of a segmented slug that are about 30 million years older.
The burrows, found in northeastern Uruguay, have fine details that offer hints about the appearance and behavior of the animals that made them. The slugs were less than four-tenths of an inch long, and apparently moved in search of food, grazing on various kinds of organic material found on the sea bottom in shallow water. They had primitive "feet" that they could extend into the mud to help them slide along.
The researchers dated the fossils by measuring uranium decay over time, which allows them to estimate their age quite accurately, geologically speaking: within six million years..."