Haikoucaris ercaiensis, a Putative Chelicerate Fossil from Chengjiang

Description: This unusual arthropod is known as Haikoucaris ercaiensis. It was described in 2004 from only THREE examples. The authors posited it as a very rare member of the Chengjiang Biota. The diversity of soft-tissue fossils is astonishing: algae, medusiforms, sponges, priapulids, annelid like worms, echinoderms, arthropods (including trilobites), hemichordates, chordates, and the first agnathan fish make up just a small fraction of the total. Numerous problematic forms are known as well, some of which may have represented failed attempts at diversity that did not persist to the present day.

The taxon is allied with the “great appendage” arthropods, but had a most unusual claw as preserved in the holotype. The similarity of these claws to those of the Chelicerata led the authors to hypothesize that it was ancestral to the Chelicerates, which today encompasses the spiders, scorpions, mites, ticks, and horseshoe crabs. They may have used their appendage to attack other arthropods in much the same way as modern-day mantis shrimp. The slender body shape and large swimming appendages seem to indicate that swimming was the major mode of locomotion. This is a truly unusual specimen, one that few will ever be able to hold in a collection, whether private or that of a museum.