If a horseshoe crab is neither horse nor crab, what is an evolutionary explanation?

Horseshoe crabs are prime examples of “living fossils” that have evolved little since their appearance in the fossil record hundreds of millions of years ago,* but now some evolutionary paleontologists want to shout that a fossil horseshoe crab “illuminates the evolution of arthropod limbs.”  Indeed, Derek Briggs and his Yale gang, publishing in PNAS ( September 11, 2012, doi: 10.1073/pnas.1205875109), are convinced that we can all learn oodles of wisdom by observing that some relatives had single legs and others had split legs.  Of less concern to them is why horseshoe crabs (Limulus) remained basically unchanged for 445 million years (or more, if Cambrian arthropods with similar body plans are considered).  Joined or single limbs might seem minor compared to eyes, guts, muscles, nerves, sexual organs and all the other complex features of these enduring animals that appear abruptly in the fossil record.

PhysOrg jumped right on the evolutionary bandwagon, posting the Yale press release uncritically and with a jazz riff by Briggs himself:

“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.

What apparently was not as remarkable to Briggs was that this creature, threatened by man today, survived all the major mass extinctions that wiped out 90% of organisms in the Permian and all the dinosaurs in the Cretaceous, and other mass extinctions in the evolutionary timeline.  Yet somehow it never evolved much in far more Darwin Years than it took for cows to become whales and chimps to become humans.  Evolution works in strange ways….

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