The fossils were found by a team headed by Bruce Lieberman, senior curator of invertebrate paleontology at the University of Kansas.His
group's report on the discovery appears in the current issue of PLoS One
, an international online scientific journal published by the Public Library of Science
in San Francisco.
According to Lieberman
, the ones his
group found must have sunk onto the soft sand on the bed of a shallow sea and then lay there as tiny particles of extremely fine sand buried them to preserve their delicate imprints.
"The fossil record is biased against soft-bodied life forms such as jellyfish because they leave little behind when they die," Lieberman
And the fact that those jellies were so complex, and apparently so successful that their counterparts exist today, means that they either evolved very rapidly by 500 million years ago, or that the group is even older and had evolved even longer ago, he
team said their fossils are "exquisitely preserved" but they were careful not to claim that they belong to the exact same orders or classes as their modern look-alikes.