HORSESHOE CRABS AS MODERN ANALOGS OF TRILOBITES: CONTRARY EVIDENCE FROM A MERASPID CLUSTER IN THE UPPER CAMBRIAN CONASAUGA FORMATION, WESTERN GEORGIA

Horseshoe crabs and related fossil xiphosurans are generally recognized as the closest living relatives of trilobites, based largely on their mutual attributes of biramous appendages, lack of jaws, and generally basal arthropod morphology. Juvenile horseshoe crabs also resemble first stage meraspid trilobites. Since genetic material for trilobites is absent, an additional test of relatedness could be ontogeny in the respective clades. Recent published evidence confirms that at least some trilobites were oviparous, storing eggs in the cephalic genal regions, like horseshoe crabs. What has been unknown is the subsequent ontogeny; specifically, it has not been documented whether trilobites buried their eggs in sandy intertidal to supratidal environments, like horseshoe crabs. Also unknown is whether the newly-hatched larvae dispersed, as with horseshoe crabs, or remained at the egg deposition site as with some other Arachnomorpha.

A new ~6 cm² clayshale specimen from an upper Cambrian (Furongian, Paibian) Conasauga Formation site in western Georgia illuminates this question. It contains a cluster of >180, mostly complete, fifth-stage meraspid trilobites, identified as juvenile Aphelaspis brachyphasis. The meraspids are in multiple orientations and were evidently not emplaced by ocean currents. This cluster appears to be a homing association at a nest site. Given that the specimen occurs in thick-bedded clayshales, it does not reflect the sandy, shallow-water depositional site selection of limulid xiphosurans. Neither does the homing behavior indicated by this specimen reflect the post-hatch behavior of horseshoe crabs.