Discovery of new material attributable to Sphenopteris arizonica Daugherty permits a more complete reevaluation of one of the rarest fossil leaves found in the Upper Triassic Chinle Formation in the southwestern United States. This fossil is particularly interesting because it is apparently one of the few surviving Paleozoic floral elements found in the Chinle Formation and also is one of just a few species in the unit showing evidence of insect predation. Sphenopteris arizonica is a small, strongly polymorphic, tripinnate leaf that probably is bipartite. Each division of the leaf has falcate pinnules in the basal portion that grade apically into lobed pinnules. Small foliar excisions caused by one or more phytophagous mandibulate insects grazing occur along some of the pinnule margins on nearly every specimen of the species. These excisions range from small notches on individual pinnules to broadly cuspate traces that cut across several pinnules. In general, they are bordered by a narrow band of reaction tissue, indicating that the grazing occurred while the leaves were still functioning and attached to the parent plant. Although their remains are very rare, the extent of insect predation described here is additional evidence that phytophagous insects were significant members of the Chinle ecosystem.