Although both foraminiferal and conodont zonations have been proposed for Lower and Middle Pennsylvanian strata, the current state of the art indicates that several proposed Morrowan conodont zones and one important Derryan(?) fusulinid zone (21) and the ranges of several important associated conodonts and fusulinids are in question. The conodont zones in question are the Gnathodus girtyi simplex, the Streptognathodus expansus–S. suberectus, the Idiognathodus humerus–I. Sinuosis, and the Streptognathodus parvus–Adetognathus spathus Zones of Dunn and the Idiognathodus sinuosis, I. Klapperi, and I. Convexus Zones of Lane and Straka. The conodonts whose ranges are in question are Gnathodus girtyi simplex, Rhachistognathus, muricatus, Declinognathodus noduliferus, Adetognathus spathus, Neognathodus bassleri bassleri, and Idiognathoides convexus.
Evidence based on conodonts suggests that there is a time overlap between uppermost type Morrowan and lowermost type Derryan strata. Because of this, there is as yet no precise paleontological definition of a single Morrowan-Derryan (or Morrowan-Atokan) boundary.