ANOMALOCARIDIDS FROM THE ORDOVICIAN FEZOUATA BIOTA OF MOROCCO

Anomalocaridids, characterized by a pair of anterior, segmented appendages, a circlet of plates around the mouth, an elongate trunk lacking true tergites, and a pair of flexible lateral lobes per segment, are iconic representatives of Cambrian Burgess Shale-type faunas, the largest animals of their time. These top predators were long believed to have gone extinct by the end of the Middle Cambrian. However, recent discoveries from the Early Ordovician Fezouata Biota of south-eastern Morocco have shown that anomalocaridids were an important element of deeper marine communities for at least 30 Ma longer than previously realized. The Moroccan specimens represent at least two different taxa. The first, which is known from complete body fossils, attained a size well in excess of 1 m, making it the largest anomalocaridid known. It is comparable to Laggania from the Middle Cambrian Burgess Shale, with a long head and a broad, flat body, and a dorsal array of flexible blades attached to a transverse rachis on the trunk segments. These blades probably functioned as gills; their dorsal position would have assured continued oxygenation when in close proximity to anoxic bottom conditions. A second, smaller taxon is represented by a median triangular and lateral spinose valve-like elements of a tripartite carapace, a unique feature hitherto only recorded in Hurdia from the Middle Cambrian Burgess Shale. The Fezouata discoveries provide the only temporal link between the Cambrian occurrences and the Early Devonian great appendage arthropod Schinderhannes, which retains some anomalocaridid characters. The Moroccan specimens show that anomalocaridids were among the largest organisms in some ecosystems even during the Ordovician, and were presumably at the top of the food chain. The demise of anomalocaridids may have been associated with the rise of large predatory eurypterids and stem cephalopods during the Great Ordovician Biodiversification Event.