Muscle systems and motility of early animals highlighted by cnidarians from the basal Cambrian

Although fossil evidence suggests that various animal groups were able to
move actively through their environment in the early stages of their
evolution, virtually no direct information is available on the nature of
their muscle systems. The origin of jellyfish swimming, for example, is of
great interest to biologists. Exceptionally preserved muscles are
described here in benthic peridermal olivooid medusozoans from the basal
Cambrian of China (Kuanchuanpu Formation, ca. 535 Ma) that have direct
equivalent in modern medusozoans. They consist of circular fibers
distributed over the bell surface (subumbrella) and most probably have a
myoepithelial origin. This is the oldest record of a muscle system in
cnidarians and more generally in animals. This basic system was probably
co-opted by early Cambrian jellyfish to develop capacities for
jet-propelled swimming within the water column. Additional lines of fossil
evidence obtained from ecdysozoans (worms and panarthropods) show that the
muscle systems of early animals underwent a rapid diversification through
the early Cambrian and increased their capacity to colonize a wide range
of habitats both within the water column and sediment at a critical time
of their evolutionary radiation.