The most primitive member of the arthropods, which include spiders, insects and crabs, might have evolved a jointed exoskeleton initially as an adaptation for swimming.
David Legg at Imperial College London and his colleagues studied Nereocaris exilis (pictured), which was discovered in the Burgess Shale, a rich fossil field in Canada. The creature is the most primitive arthropod yet found and dates back to the Cambrian period around 500 million years ago, the researchers say. The animal had a bivalved main shell and an elongated abdomen covered with a hard, jointed exoskeleton — characteristic of arthropods. However, the creature's limbs were too thin for walking, which suggests that the exoskeleton was probably used to aid swimming. These jointed suits of armour were adapted only later to support life on the ocean floor, the authors propose.
The anatomy of this fossil also implies that the first arthropods were prey, rather than active predators as they are today.
Proc. R. Soc. B. http://dx.doi.org/10.1098/rspb.2012.1958 (2012)