The lower 1,000 feet of the Horquilla Limestone of southeast Arizona contains abundant Early and Middle Pennsylvanian fusulinids. The Millerella and Profusulinella Assemblage Zones are relatively thin as their combined thickness is about 100 feet. The Fusulinella Assemblage Zone, varying in thickness from about 100 to nearly 400 feet, is remarkably complete and has transitional species of Fusulinella Moller with Profusulinella Rauser-Chernoussova and Beljaev below and Fusulina Fischer de Waldheim above. The Fusulina Range Zone, reaching 600 feet in thickness, is well represented by a diverse fauna. Unconformities lie at the base of the Horquilla Limestone and at the top of the Fusulina Range Zone.

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