Recent evidence confirms that trilobites were oviparous; however, their subsequent embryonic development has not been determined. A ∼ 6 cm2 claystone specimen from the upper Cambrian (Paibian) Conasauga Formation in western Georgia contains a cluster of >100 meraspid trilobites, many complete with librigenae. The juvenile trilobites, identified as Aphelaspis sp., are mostly 1.5 to 2.0 mm total length and co-occur in multiple axial orientations on a single bedding plane. This observation, together with the attached free cheeks, indicates that the association is not a result of current sorting. The majority of juveniles with determinable thoracic segment counts are of meraspid degree 5, suggesting that they hatched penecontemporaneously following a single egg deposition event. Additionally, they are tightly assembled, with a few strays, suggesting that the larvae either remained on the egg deposition site or selectively reassembled as affiliative, feeding, or protective behavior.
Gregarious behavior by trilobites (“trilobite clusters”) has been reported frequently, but previously encompassed only holaspid adults or mixed-age assemblages. This is the first report of juvenile trilobite clustering and one of the few reported clusters involving Cambrian trilobites. Numerous explanations for trilobite clustering behavior have been posited; here it is proposed that larval clustering follows egg deposition at a nest site, and that larval aggregation may be a homing response to their nest.