In horseshoe crab history, legs come and go

Horseshoe crabs, including the iconic Limulus we know today, have existed for more than 450 million years. Over that long history, evolutionary change has particularly affected the nature of their legs. A new fossil discovery in Britain captures a previously unseen stage in the evolution of these ancient arthropods - the transformation of two-branched legs into nearly identical but separately attached limbs, one of which was destined to disappear. "This fossil provides remarkable confirmation of the loss of a limb branch during horseshoe crab evolution, a change predicted by the common presence of two branches in the arthropods that appeared earlier, during the Cambrian explosion," said Derek E. G. Briggs, director of the Yale Peabody Museum of Natural History and lead author of a paper to be published online the week of Sept. 10 in the journal PNAS. The fossil dates from the Silurian period, about 425 million years ago.