Conodonts are zoologically enigmatic, toothlike phosphatic microfossils occurring in marine sedimentary rocks ranging in age from Cambrian to Triassic. Dimpled spheres of less than 1 millimeter in diameter are sporadic associates of conodonts and have identical chemical composition and microstructure. Mineralogy, morphology, and occurrence of these spheres suggest that they are pearls secreted by the conodont-bearing animal.

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The Glen Park Member of the Sulphur Springs Formation (Devonian) in east-central Missouri contains abundant phosphate nodules. Dissolution of several nodules in hydrochloric acid yielded a diverse, well-preserved Ordovician fauna, containing diagnostic graptolites, ostracodes and conodonts, demonstrating that the Glen Park phosphate nodules are reworked from the Ordovician. Faunal similarities and the in situ occurrence of phosphorite in nearby exposures of the Maquoketa Shale (Ordovician) strongly suggest that phosphate nodules in the Glen Park were reworked from the Maquoketa.

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