Redescription ofAlokistocare subcoronatum(Hall and Whitfield, 1877), the type species ofAlokistocare, and the status of Alokistocaridae Resser, 1939B (Ptychopariida: Trilobita, Middle Cambrian)

Abstract

Alokistocare subcoronatum(Hall and Whitfield, 1877) is the type species ofAlokistocareLorenz, 1906, the nominal genus for the diverse Middle Cambrian Alokistocaridae Resser, 1939b. New collections from theEhmaniellaBiozone contain cranidia, librigenae, hypostomes, rostral plates, thoracic segments, and pygidia, which allow a more complete understanding of the morphological range and character of this important species.Cladistic analyses using this new information aboutA. subcoronatumsuggest that: 1) Ehmaniellidae Sundberg, 1994, is synonymous with Alokistocaridae; 2) Alokistocaridae is a monophylectic clade and not synonymous with Papyriaspididae Whitehouse, 1939; 3)PseudoalokistocareSundberg, 1994, is synonymous withAlokistocare; and 4) Taxa previously assigned to the older“Alokistocare”/Amecephalusgroup (PlagiuratoGlossopleurabiochrons) form a separate clade and should not be included inAlokistocareor the Alokistocaridae. These older species are assigned toAmecephalusWalcott, 1924, which is restricted to the Delamaran Stage. Alokistocaridae as recognized here is restricted to the Marjuman Stage.A morphometric analysis ofAlokistocare subcoronatumshows minor but significant differences among collections, indicating population differences. In addition, the cranidium displays both positive and negative allometric growth as well as isometric growth. Allometric growth is present in both meraspid and holaspid stages. The pygidium shows only isometric growth.