Life History of an Ordovican Trilobite Triarthrus Eatoni

Triarthrus eatoni (Hall) was a trilobite common in Late Ordovician marine environments of the Appalachian region. New aspects of its growth, development, and population dynamics are surmised from a rare census fossil assemblage. Its development from protaspis larva (three stages separated by molts) through meraspis juvenile (at least 14 stages) to adult halospis (many stages) was very gradual. Size—frequency distributions reveal that reproduction was seasonal, probably having occurred in a single burst per year. This provides a means of measuring time in the life cycle. The growth curve for overall length is approximately linear. The protaspid period lasted on the order of a week or so, the meraspid period through most of the rest of the first year, and the holaspid period to a maximum longevity of at least 4 years. The survivorship curve is essentially an exponential decay curve; the probability of an individual's survival was between 0.3 and 0.4 per year. Through its earlier juvenile stages, very roughly through the first month of life, the trilobite was a planktonic suspension—feeder. This was probably a dispersal phase in the life cycle, and its long duration helps explain the cosmopolitan distribution of the genus. Afterward, the trilobite settled to live on or near the bottom as an epifaunal deposit—feeder.