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Unusual Pennsylvanian bivalves recovered from the Buckhorn Asphalt Quarry of south-central Oklahoma are members of the Order Praecardioida, Family Lunulacardiidae. This is the youngest occurrence of praecardioids in North America. These praecardioids have an ontogeny characterized by a veneriform juvenile shell, changing abruptly to an elongate adult shell with sharp carinae, accompanied by strong rotation of the juvenile shell and change in ligament. These shells provide new evidence of dentition and ligament for lunulacardiids and the first documentation of praecardioid shell microstructure. The Buckhorn taxa possess stout hinge teeth, indicating that lunulacardiids are not edentulous. Preserved ligament in multiple growth stages indicates that beaks are prosogyrous and the flattened truncated portion of the shell is posterior. A revised diagnosis of the Lunulacardiidae is presented. New taxa: Buckhornia carteri n. gen., n. sp.

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