Morphology and systematics of the anomalocaridid arthropod Hurdia from the Middle Cambrian of British Columbia and Utah.

In Cambrian fossil Lagerstätten like the Burgess Shale, exceptionally preserved arthropods constitute a large part of the taxonomic diversity, providing opportunities to study the early evolution of this phylum in detail. The anomalocaridids, large presumed pelagic predators, are particularly relevant owing to their unique combination of morphological characters and basal position in the arthropod stem lineage. Although isolated elements and fragmented specimens were first discovered over 100 years ago, subsequent findings of more complete bodies ofAnomalocarisandPeytoia, especially in the 1980s, allowed for a better understanding of these enigmatic forms. Their evolutionary significance as stem group arthropods was further clarified by the recent discovery of a third anomalocaridid taxon,Hurdia. Here, examination of hundreds ofHurdiaspecimens from different stratigraphical layers within the Burgess Shale and Stephen Formation, combined with statistical analyses, provides a detailed description of the taphonomy, morphology and diversity of the genus and further elucidates anomalocaridid systematics.Hurdiais distinguished from other anomalocaridids in having mouthparts with extra rows of teeth, a large frontal carapace complex and diminutive swimming flaps with prominent setal structures. The two original species,H. victoriaWalcott, 1912 andH. triangulataWalcott, 1912, are confirmed based on morphometric outline analyses of the frontal carapace components combined with stratigraphical evidence; a third species,Hurdia dentataSimonetta & Delle Cave, 1975, is synonymized withH. victoria. Morphology, preservation and stratigraphical distribution suggest thatH. victoriaandH. triangulatashare the same type of frontal appendage; a second type of appendage, previously assigned toHurdia(Morph A), belongs toPeytoia nathorsti. These and other morphological differences between the anomalocaridids may reflect different feeding strategies. Appendages and mouthparts ofHurdiaindet. sp. are also identified from the Spence Shale Member of Utah, makingHurdiaandAnomalocaristhe most common and globally distributed anomalocaridid taxa. [ABSTRACT FROM PUBLISHER]