The Dove Spring Formation (DSF) is an 1,800-m-thick succession of fluvial, lacustrine, and volcanic rocks that contains a nearly continuous sequence of diverse vertebrate fossil assemblages. When the North American provincial mammalian ages were originally defined in 1941, the fossil fauna of the DSF (now, part of the Ricardo Group) was one of four fossil assemblages named as principal correlatives of the Clarendonian mammal age. Early radiometric work yielded a maximum age of 10.0 Ma for this fossil assemblage, and by correlation to the Great Plains of the United States, this date was considered representative of Clarendonian time.

Detailed geologic mapping and precise stratigraphic control of fossils collected in the DSF permit the development of a biostratigraphy consisting of four mammalian fossil assemblage zones. The biologic characterization of the Clarendonian mammal age in California is expanded on the basis of this succession of assemblage zones.

Radiometric and paleomagnetic results provide a detailed chronologic framework that indicates a time span from at least 13.5 Ma to 7.3 Ma for the fossils of the DSF. Furthermore, the new sequence of assemblage zones helps to fill faunal gaps that appear to exist, at the upper and lower boundaries of the Clarendonian faunal record in the Great Plains. On the basis of the biochronology developed in the DSF and on the faunal correlation with the Great Plains sequence, the Barstovian/Clarendonian boundary, as recently characterized by some workers, is dated at 12.5 Ma, and the Clarendonian/Hemphillian boundary is constrained to 8.9 Ma. This lower boundary is about 0.9 m.y. older than previously proposed, whereas the upper boundary is in close agreement with previous proposals.

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