The Edmonton Journal reports that University of Alberta researchers have discovered that there were creatures crawling along sea floor at least 585 million years ago, about 30 million years earlier than previously known. These slug-like creatures left marks in sediment, which are preserved in rocks located in Uruguay. These are what are known as “trace fossils”. Since these creatures had soft bodies, preservation of fossils of the actual organism is highly unlikely. Before the Cambrian explosion, most lifeforms did not have carbonate shells, so the only evidence of their existence comes from the marks they left on the ocean floor.
The research, published in this week’s edition of Science, shows that the rock unit has numerous trails left by these organism. These creatures lived in shallow water, at a period of time when the Earth was in glacial conditions. A modern analogue might be shelf regions of Antarctica. The age of the rocks were determined by dating a granitic intrusion that cut through the rocks (giving a minimum age of 585 million years), and detrital zircons found within the sediment (giving a maximum age of 600 million years). The finding pushes back the evolutionary clock and gives insight into early complex life.