ANOMALOCARIDID DIVERSITY IN THE EARLY ORDOVICIAN FEZOUATA BIOTA OF SOUTHEASTERN MOROCCO

Anomalocaridids are a major clade of stem arthropods, which, until recently, were considered to have disappeared after the middle Cambrian. While generally regarded to have been fierce apex predators, the wide morphological variation exhibited by the grasping appendages of different taxa is testimony to their ecological diversity; in fact, it was recently shown that the early Cambrian Tamisiocaris borealis from the Sirius Passet fauna of Greenland was a filter-feeder. The Fezouata Biota from the Early Ordovician of Morocco has provided the first undoubted post-middle Cambrian anomalocaridid record, and is home to at least four different taxa. The most conspicuous of these is a giant hurdiid reaching a length in excess of 2 m. In addition to abundant disarticulated material, consisting of central and lateral elements of the frontal carapace, bands of setal blades, and anterior appendages, several three-dimensionally preserved bodies are known from concretions. The latter have provided unequivocal evidence for the presence of a second set of lateral flaps in anomalocaridids; the lower flaps are homologous to walking limbs. Exceptionally well-preserved anterior appendages show this giant taxon to represent another example of a suspension feeding anomalocaridid. Another, much rarer hurdiid is represented by a large isolated anterior appendage, which exhibits a specialised predatory morphology, being optimised for shredding soft prey. Other material includes disarticulated H- and P-elements of a Hurdia-like frontal carapace, specimens of anterior appendages that show similarities to Peytoia, and the ventral spines of a smaller filter-feeding form. A handful of cephalic shields bear a resemblance to those recently described for the middle Cambrian Anomalocaris canadensis from the Burgess Shale of Canada. Articulated oral cones have so far proven to be extremely rare in the Fezouata Biota. However, a single minute, complete specimen with a diameter of only ca 4 mm belongs to either a miniature taxon, or a juvenile. The presence in the Fezouata Biota of at least four anomalocaridid taxa exhibiting widely differing, specialised appendage morphologies further underscores the ecological diversity and continued importance of this group in post-Cambrian Paleozoic ecosystems.