The fossil represents a new genus and species called Haootia quadriformis. Liu and his colleagues classified it as a type of cnidarian - a phylum of aquatic animals that includes corals, sea anemones and jellyfish - because of its resemblance to modern cnidarians, especially the stalked jellyfish Lucernaria quadricornis.
The researchers ruled out other possible explanations for the fiber bundles, such as the movement of tectonic plates or sediments, or the conditions of fossilization. The complexity and arrangement of the bundles, and the fact that some of them appear to be contracted, all suggest that the structures are indeed muscles, Liu said. The researchers compared the bundles to muscle tissue in modern cnidarians and found them to be similar, he added.
In addition to being the oldest fossil with muscles, the newfound cnidarian also serves as a calibration point for the molecular clocks that show when different species diverged from each other, Liu said.
The development of muscles was a critical event in animal evolution. Except for sponges, all animals rely on muscles to move from place to place, to escape from predators, to feed and to reproduce. Vertebrates are the most extreme example: "Everything is dependent on muscle tissue being able to contract and extend," from breathing to digestion, Liu said.