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Open Access Publications from the University of California
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Cover Caption: The carapace of Derrisemys sterea gen. nov. et sp. nov. (UCMP 130000) in (from left) dorsal, right lateral, and ventral views.

Articles

New soft-shelled turtles (Plastomeninae, Trionychidae, Testudines) from the Late Cretaceous and Paleocene of North America

Two new genera (Derrisemys and Plastomenoides) and three new species (D. sterea, P. lamberti, P. tetanetron) of plastomenine trionychids from Montana and Wyoming are described. They are unique within the Trionychidae in having the entoplastron locked into notches in the hyoplastra and restricting midline kinesis. D. sterea occurs in the Lancian NALMA (North American Land Mammal Age) of Montana and Wyoming and Puercan NALMA of Montana. P. tetanetron occurs in the Puercan NALMA of Montana. P. lamberti occurs in the Torrejonian NALMA of Montana and Tiffanian NALMA of Wyoming and Utah. Plastomenus acupictus Hay 1907 from New Mexico is referred to Derrisemys. The age of D. acupictus is uncertain but is likely from early Paleocene (Torrejonian NALMA).

Revised large mammal biostratigraphy and biochronology of the Barstow Formation (Middle Miocene), California

A new biostratigraphic zonation for the middle Miocene Barstow Formation based on the mammalian megafauna is presented. Four biostratigraphic zones are outlined, two containing fossil assemblages of early Barstovian (Ba1) age, and two with late Barstovian (Ba2) assemblages. Recommendations are made for defining and characterizing the Barstovian North American Land Mammal Age (NALMA). The Ba1 biochron is defined based on the first occurrence of Plithocyon, and the base of the Ba2 biochron is revised based on the first appearance of the antilocaprid Ramoceros.

The ursid Plithocyon remains a valid defining taxon for the base of the Barstovian NALMA. The use of both gomphotheriid and mammutid proboscideans to define the base of the Ba1 or Ba2 is abandoned due to the diachrony of first appearances across North America. The Ba1 biochron is additionally characterized based on the first appearance of the equid Scaphohippus. The base of the Ba2 biochron is revised based on the first appearance of Ramoceros. The Ba2 can also be characterized by the first appearance of the borophagine canid Protepicyon and the anchitherine equid Megahippus.