Triarthrus - Preserved Legs and Antennae

Often when we imagine a "typical" trilobite we picture an animal something like this genus, Triarthrus. Known from the Upper Ordivician soils of North America (midwestern states as well as New York and regions of Canada), China, and Scandinavia, Triarthrus is one of the final lineages of Olenid trilobites which was highly successful and very diverse during the Cambrian. Triarthrus is so well represented in the fossil record and considered so highly "typical" of trilobites that it is often used as the textbook example of what a trilobite looks like. One of the best preserved species within this genus is T. eatoni, is known from the state of New York and Canada (multiple sites across the provinces of Ontario and Quebec). Shale and iron pyrite deposits in these areas, especially the Franklin Shale in the state of New York, have produced specimens so exceptionally preserved that legs and associated appendages, gills, and antennae are perfectly retained in the fossils. A single location in New York, the Beecher's Trilobite  Bed, is world known; Triarthrus makes up approximately 85% of all of the material that has been recovered in that location. An entire community, including the shed and discarded remains of younger life stages, of T. eatoni are known from the Franklin Shale. 

This community graveyard of Triarthrus has allowed for a great deal of interpretation concerning the life cycle of these animals. The discarded exoskeletons of members that could float about with plankton were interspersed with the exoskeletons of 2mm sea floor dwelling individuals and, of course, a large number of larger exoskeletons of adults. Though 2mm may appear to be minuscule, adults of T. eatoni were not very large overall either, at approximately 5cm (50mm). Triarthrus individuals are preserved in a variety of configurations; lateral dorsal, and ventral views are all well represented. 

Other discoveries of T. eatoni have led to the discovery of trilobite eggs as well. Because of all of these exquisitely preserved Triarthrus specimens, we know much about its life from birth to death. The image below shows a ventral view of a well preserved T. eatoni with eggs preserved near the cephalon shield. The eggs were photographed in the study of this fossil by Thomas A. Hegna using a scanning electron microscope. 

Image credit: Thomas A. Hegna et al, doi: 10.1130/G38773.1.