The present study introduces a new classification of the family Eostaffellidae, a group of discoid to rhomboid, mainly Mississippian to Lower Pennsylvanian, primitive Ozawainellacea, intermediate between the ancestral, mainly Mississippian, superfamily Endothyracea and the descendant, mainly Pennsylvanian to Permian, superfamilies Ozawainellacea, Fusulinacea and Staffellacea.
The new classification is based on descriptions and illustrations of previously introduced species and (sub)genera and their assumed relationships. The following (sub)genera are accepted in the present paper: Eoparastaffella (Eoparastaffella), Eoparastaffella (Eoparastaffellina), Eostaffella, Ikensieformis, Mediocris (Mediocris), Mediocris (Chomatomediocris), Endostaffella, Eostaffellina, Plectostaffella, Plectomillerella, Millerella (Millerella), Millerella (Seminovella), Millerella (Pseudonovella), Novella, Paramillerella (Paramillerella), and Paramillerella (Acutella).
The genus Paramillerella, although generally considered a synonym of Eostaffella, is accepted here because Mississippian Eostaffella probably descended from Eoparastaffella, whereas Pennsylvanian Paramillerella may have descended via Plectomillerella from Endostaffella. Moreover, species of Paramillerella exhibit more advanced pseudochomata than species of Eostaffella.
In the past Eoparastaffella and Endostaffella, the probable precursors of the Eostaffellidae, were commonly assigned to two separate families, the Pseudoendothyridae and the Endothyridae, respectively. In the present classification Eoparastaffella and Endostaffella are considered primitive genera of the Eostaffellidae. There is some evidence that the family Eostaffellidae including the genera Eostaffella and Millerella has a stem species close to the Tournaisian/Visean boundary and possibly contains some features of both Eoparastaffella and Endostaffella or Mediocris.
A comparison with genera of the ancestral Endothyridae points to repetitions in the evolutionary pattern. Examples are the extremely evolute, very slender Novella and the streptospiral Plectostaffella, both of Pennsylvanian age, which are quasihomeomorphs of the Mississippian (Lower Visean) endothyrids Loeblichia and Dainella, respectively.
Eurasia exhibits a relatively high diversity of eostaffellid species in the upper Upper Visean (upper Middle Mississippian) and the Bashkirian to Lower Moscovian (Lower Pennsylvanian to lower part of Middle Pennsylvanian). The diversity apparently peaks in the Lower Bashkirian. The early appearance in Eurasia (close to the Tournaisian/Visean boundary) and the numerous eostaffellid species described from Eurasia suggest that the family’s roots may lie in Eurasia rather than in North America.