Blue-blooded and armored with 10 spindly legs, horseshoe crabs have perhaps always seemed a bit out of place. First thought to be closely related to crabs, lobsters and other crustaceans, in 1881 evolutionary biologist E. Ray Lankester placed them solidly in a group more similar to spiders and scorpions. Horseshoe crabs have since been thought to be ancestors of the arachnids, but molecular sequence data have always been sparse enough to cast doubt. University of Wisconsin-Madison evolutionary biologists Jesús Ballesteros and Prashant Sharma hope, then, that their recent study published in the journal Systematic Biology helps firmly plant ancient horseshoe crabs within the arachnid family tree. By analyzing troves of genetic data and considering a vast number of possible ways to examine it, the scientists now have a high degree of confidence that horseshoe crabs do indeed belong within the arachnids.
Source: Horseshoe crab throws theory of arachnid evolution into disarray