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Classification and Systematics
Fossil and Living Horseshoe Crabs

Liomesaspis laevis Raymond, 1944

Xiphosurans are marine aquatic arthropods which are grouped together with the Chelicerates. Other more familiar Chelicerates include spiders and scorpions (arachnids), and eurypterids (extinct aquatic arthropods). As more and more sites yielding exceptionally preserved fossil arthropods turn up in rocks of Cambrian age, it seems less likely that xiphosurans and trilobites shared an immediate common ancestor. There are far too many other better suited candidates.

The listing below is modified from a research paper which I co-authored with my research supervisor, Dr Paul A. Selden (University of Manchester, England). This only deals with the Palaeozoic Xiphosura; I decided that the Permian extinction event was a good place to end the study! Hopefully, I'll get time to do a similar revision of the Mesozoic and Recent Xiphosura. I have included here the classification down to species level. However, fossils just keep getting found so the list is bound to increase but you've got to make a start somewhere. :)