Per Ahlberg

Cambrian trilobites mainly lived on the sea floor, and up till now few, if any, unequivocally planktonic trilobites have been reported from earlier than the Ordovician. The late Cambrian (Furongian) to late Ordovician olenids are a distinctive group of benthic (sea-floor dwelling) or nekto-benthic trilobites. Here we show, however, that one recently described, miniaturized and very spiny olenid species, Ctenopyge ceciliae must have been planktonic ( passively drifting or feebly swimming in the upper waters of the sea). This interpretation is based not only upon body form but also on the analysis of its visual system and may be one of the earliest records of the planktonic realm being invaded by trilobites.