The new species of single tooth-rowed captorhinid reptile Captorhinus magnus n. sp. from the Lower Permian fissure fillings at Richards Spur, Oklahoma differs from Captorhinus aguti in body size and dental and femoral morphology. Linear measurements of fully mature C. magnus elements range in size from 1.5 to 2.3 times as great as those of C. aguti, and the proximal articular surface of the femur, which is convex in C. aguti, is concave throughout ontogeny. C. magnus possesses ogival cheek teeth aligned in a single row, indicating that ogival dentition can no longer be considered unique to C. aguti. A phylogenetic analysis of captorhinid interrelationships indicates that C. aguti and C. magnus form a clade that possesses a sister-group relationship with Captorhinus laticeps. Incompletely ossified astragali referred to C. magnus provide unequivocal evidence that the astragalus of Captorhinus formed through the fusion of three, originally separate ossifications, the tibiale, intermedium, and proximal centrale, rather than from a single ossification center. At the Richards Spur locality, C. magnus is the most abundant Captorhinus species produced from the deeper, stratigraphically lower sediments of the quarry. It is rare, however, in the uppermost, presumably younger deposits, where C. aguti represents the most numerous Richards Spur captorhinid.

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