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Gillicus in Mexico 215 This work provides the description of the first specimen of the species Gillicus arcuatus (Cope, 1875) collected in Mexico. This ichthyodectiform fossil was collected in a marl flagstone quarry on Los Pilotes Ranch, Muzquiz County, Coahuila. The rocks of the quarry are of Turonian age, and constitute part of the Eagle Ford Formation. This Gillicus arcuatus specimen fits within the known temporal distribution of Gillicus, but extends the geographical distribution of this species to include Northern Mexico, where once the North American Western Interior Sea and the Western Tethys Sea converged. Los Pilotes Ranch is the southernmost fossil locality so far known where Gillicus has been found