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The epochs of a period in the Tertiary time scale are, supposedly, convenient terms of expression, and as such, have been altered somewhat from the original designations of Lyell by different authors. Many controversies have arisen in the arbitrary assignment of boundaries to these divisions. This article is concerned with Mio-Pliocene boundary, particularly in California.
From a study of certain living mammals, it is recognized that similar fossil forms realized a wide and rapid dispersal without appreciable evolutionary change and are useful in Holarctic correlation. Furthermore, the evidence shows that different plant associations were not effective barriers to
these mammals. Hence it is again suggested that the equid genus Hipparion be accepted as an indicator of the beginning of the Pliocene.
The late Tertiary time scale now in use on the Pacific Coast is based, primarily, on the early identifications of fossil vertebrates from the Coalinga area in California, and in part on the percentage methods as applied to the invertebrates. The horse teeth, Neohipparion molle Merriam, from the Jacalitos, which were used to mark the beginning of Pliocene time on the Pacific Coast, we now find to be very close in structural characters to Middle Pliocene forms elsewhere in America. From the evidence now available and in accordance with the above principles in intercontinental correlation, it is suggested that Pliocene time in California began with the Orinda, lower Chanac, upper Nerola and their equivalents.
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