The varied fusulinid faunas of the late Pennsylvanian (Virgilian) and early Permian (Wolfcampian) parts of the Naco Group and the Earp Formation can be divided into five assemblage zones. The lowest zone is characterized by Triticites cullomensis and occurs in early Virgilian strata disconformably overlying middle Desmoinesian beds. This early Virgilian sequence is overlain by a zone of middle to late Virgilian fusulinids, Triticites ventricosus sacramentoensis and T. cf. T. plummeri, that occurs in the uppermost beds of the Horquilla Limestone and ranges into the lower 20-235 feet of the Earp Formation. From the fusulinid distribution and the transitional lithologic features it appears that the top of the Horquilla Limestone and the base of the Earp intertongue laterally

The succeeding 540 feet of the Earp Formation contain a fusulinid assemblage of Triticites and Schwagerina characteristic of the Bursum Formation of New Mexico, the Admire Group of Kansas, and Pueblo Formation of north-central Texas. Of particular interest is the nearly complete transition from Triticites into Schwagerina in the lower part of the Earp Formation. Overlying these beds are 1,200 feet of limestone and shale having a Pseudoschwagerina and Triticites assemblage closely similar to that from the Neal Ranch Formation (early Wolfcampian) of the Glass Mountains, Texas. The succeeding 300 feet of limestone beneath the red shale and cross-bedded sandstone at the top of the Earp Formation have Pseudoschwagerina and advanced species of Schwagerina that bear similarity to the fusulin d fauna from the Lenox Hills Formation (late Wolfcampian), Glass Mountains, Texas.

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