Horseshoe Crabs – An Ancient Ancestry Revealed

Abstract

The fossil record of the basic xiphosurid horseshoe crab body plan has been extended back to the Late Ordovician Period, about 445 million years ago, demonstrating an origin that lies outside of the paraphyletic ‘synziphosurines.’ Horseshoe crab body fossils are exceptionally rare and are found mostly in shallow coastal and marginal marine Konservat-Lagerstatten deposits. Their sporadic occurrences document a post-Cambrian history of low overall diversity with a modest morphological and taxonomic peak in the Late Paleozoic Era. Survival of a single xiphosurid lineage through the end-Permian mass extinction events was followed by a minor secondary radiation during the Triassic Period. The Jurassic to Recent fossil record of horseshoe crabs is relatively impoverished in both taxa and known occurrences. Overall, the rarity of fossil xiphosurids reflects both taphonomic biases inherent in the unusual conditions required for preservation of their non-biomineralized exoskeletons and complex ecological factors related to a long-term association with shallow marginal aquatic habitats. Focused paleontological investigations should yield additional fossil horseshoe crab discoveries that will in turn inform research on their phylogeny, morphological stasis, and ecological persistence.