Coprolites were collected 74 m above the base of the Upper Triassic Ischigualasto Formation of northwestern Argentina and their chemistry, mineralogy, and textures were studied to infer their biological source and taphonomy. They were found to contain a few fragments of bone and much crystalline apatite, and so can be assigned to carnivores able to digest bone. Primary minerals derived from endogenous materials were apatite and pyrite, and secondary minerals derived from ground water were chiefly calcite and glauconite in one coprolite and glauconite in another. Primary apatite, presumed originally to have been mostly dahllite (a carbonate hydroxyapatite) precipitated from digested bone, was converted to francolite (a carbonate fluorapatite) during diagenesis through the introduction of fluoride from ground water. The chemistry, mineralogy, and structure of the coprolites suggest an early onset of mineralization, early anaerobic burial, and a recent anaerobic-to-aerobic transition during weathering. The chemistry of the coprolites is consistent with the stratigraphic identification of the coprolite horizon as an ancient flood plain.