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Giant trilobites and trilobite clusters from the Ordovician of Portugal

¹Departamento de Paleontología, Instituto de Geología Económica (CSIC-UCM), Facultad de Ciencias Geológicas, 28040 Madrid, Spain
²Departamento de Geologia, Universidade de Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal; and Centro de Geociências da Universidade de Coimbra, 3000-272 Coimbra, Portugal
³Museo Geominero–IGME, Ríos Rosas 23, 28003 Madrid, Spain
⁴Centro de Interpretação Geológica de Canelas, 4450-252 Canelas, Portugal

Correspondence: *E-mail: asa{at}utad.pt.

Large quarrying surfaces of roofing slate in the Arouca Geopark (northern Portugal), formed under oxygen-depleted conditions, have yielded a unique Ordovician fossil lagerstätte that reveals new information on the social behavior of trilobites. It provides several of the world's largest trilobite specimens (some reaching 70 cm), showing evidence of possible polar gigantism in six different species, as well as numerous examples of monotaxic and polytaxic size-segregated autochthonous trilobite clusters, some of which contain as many as 1000 specimens. These reveal a very diverse social behavior, which includes temporary refuge from predation and synchronous molting and reproduction, demonstrated for the first time in five contemporary families of three different trilobite orders from a single formation.