Sister relationship of the Neoephemeridae and Caenidae (Ephemeroptera: Pannota)

A consistent structural characteristic has been found to be unique to the mayfly families Neoepbemeridae and Caenidae. It is termed a sutural ommation and is present on the adult mesonotum. The structure is described and illustrated. Its unique presence supports the hypothesis of a sister relationship of Neoephemeridae and Caenidae among the pannote mayflies, a cladistic arrangement that has been somewhat debatable in the past. The Baetiscidae and Prosopistomatidae, which previously have been hypothesized to be closely related to either Neoephemeridae or Caenidae by various authors, are considered to constitute an aberrant clade of mayflies with still dubious relationships within the Ephemeroptera.