Middle Ordovician Aparchitidae and Schmidtellidae: the significance of ‘featureless’ ostracods

Abstract. Schmidtellidae and Aparchitidae form a common element of North American middle Ordovician ostracod faunas. Characterized by relatively simple morphology with few obvious diagnostic features they are conventionally assigned to the Leiocopa. Their simple morphology has led to numerous species being referred to as ‘bag gencra’ such as Aparchites or Schmidtella. Aparchitids differ markedly from schmidtellids in the nature and degree of valve overlap, shell thickness and development of dorsal and ventral valve modifications. Schmidtellids may be more closely related to leperditellaceans. Kayina, previously assigned to the leiocope Family Jaanussoniidae and recorded from the middle Ordovician of North America, does not conform to leiocope morphology and is probably a leperditellacean. Aparchitaceans (Aparchitidae and Jaanussoniidae) may represent the root-stock from which the Paraparchitacea evolved later in the Palaeozoic. In North America schmidtellids and aparchitids have potential for middle Ordovician stratigraphic correlation. They co-occur in normal marine shelf depositional environments but were absent from marginal marine depositional settings.