Fossils from 465 million years ago recently discovered in Portugal have revealed the huge size reached by trilobites, the most diverse group of extinct marine arthropods. Gutierrez-Marco et al. describe the largest trilobites ever found, which, in life, would have reached up to 90 centimeters (35 inches).
This remarkable record suggests evidence of polar gigantism in an area of Gondwana close to the South Pole during the Ordovician. The Portuguese trilobites also show an astounding array of behavioral clustering -- with some patches reaching groups of over a thousand specimens -- revealing a very diverse social conduct, including hiding from predators and sexual aggregations.
This could have played a major role in the undisputed success of this group through the Paleozoic Era. The original discovery site and its fossils are one of the main attractions of the recently established Arouca Geopark in northern Portugal.
This study, "Giant trilobites and trilobite clusters from the Ordovician of Portugal," by Juan C. Gutierrez-Marco et. al. was published in the May 2009 edition of Geology.
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