Horseshoe Crabs – An Ancient Ancestry Revealed

  • D. M. RudkinEmail author
  • G. A. Young


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-Lagerstätten 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.


Fossil Record Horseshoe Crab Late Ordovician Taphonomic Process Burgess Shale 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



The authors gratefully acknowledge the National Sciences and Engineering Research Council (NSERC), The Manitoba Museum Foundation, the Board of Governors of the Royal Ontario Museum, and the Churchill Northern Studies Centre (CNSC) for financial and logistical support of fieldwork and research on Ordovician xiphosurids and Lagerstätten. Rudkin’s invited attendance at the 2007 International Symposium on the Science and Conservation of Horseshoe Crabs was generously funded by conference sponsors and organizers, and he is indebted to all involved for the opportunity to participate.


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Authors and Affiliations

  1. 1.Department of Natural History – PaleobiologyRoyal Ontario MuseumTorontoCanada
  2. 2.The Manitoba MuseumWinnipegCanada

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