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The Campanian Pierre Shale Group contains a diverse assemblage of marine reptiles, including the sea turtle Toxochelys. Twenty-one specimens from South Dakota include cranial or mandibular material useful for taxonomic and stratigraphic analyses. Twenty of the toxochelyids were collected from three formations of the lower to middle Campanian Pierre Shale in western South Dakota: 1 from the basal Gammon Ferruginous Formation, 15 from the overlying Sharon Springs Formation, and 4 from the Mitten Black Shale Formation. Examination of the specimens suggests identification as Toxochelys sp. cf. T. browni. These toxochelyids reveal previously undescribed characters and are defined by snout emargination, a hooked beak of the mandible, and labial and lingual ridges of the dentary.

One toxochelyid was recovered from the DeGrey Formation of the upper Campanian Pierre Shale Group in central South Dakota and is identified as Toxochelys sp. A. This toxochelyid is distinguished by a smooth snout without emargination, a posterolateral margin of the pterygoid that extends to the base of the condylus mandibularis, and absence of the depressor mandibulae muscle attachment site.

Qualitative and quantitative stratigraphic analyses of the South Dakota toxochelyids indicate that Toxochelys sp. cf. T. browni could serve as the basis for a bio-stratigraphic local range taxon zone, spanning the time from the late early Campanian to the middle Campanian, between 81 and 78 Ma.

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