Credit: M. VALÉRIO/AROUCA GEOPARK

Geology 37, 443–446 (2009)

Fossils of some of the largest trilobites ever found have been located in Portugal, and they bear witness to a very active social life in these extinct marine arthropods.

Artur Sá at the University of Trás-os-Montes and Alto Douro in Vila Real and his colleagues found fossils reflecting mass-mating, moulting and furtive manoeuvres among trilobites that lived during the Middle Ordovician period, 470 million–460 million years ago. The arthropod assemblage, from a roof-slate quarry in the Arouca Geopark, captures five contemporary families of three trilobite orders in a single formation for the first time.

Sá's team notes gigantism among six of these species, with one specimen reaching 70 centimetres long, and estimates for incomplete remains suggesting a possible 90 centimetres in another. The researchers suggest that their large size might be an adaptation to cold water.