Recognition of the Morrowan-Atokan Series boundary of the Pennsylvanian Subsystem in North America is complicated by the presence of unconformities succeeded by poorly fossiliferous lithologies at that level in both type regions. Abandonment of the name Atokan in favor of the alternatively used name Derryan would not resolve the problem. The appearance of the fusiform fusulinid Profusulinella has been taken to mark the base of the Atokan Series since its proposal, yet there is no evidence to support that conclusion. A competing definition based on the appearance of the primitive fusulinids Pseudostqffella and Eoschubertella seems to define better a boundary that is consistent with what is known of the Morrowan and Atokan type regions and of other faunal groups. A boundary stratotype is needed to fully resolve the problem.

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