Part 1—Nonmarine vertebrates and marine-nonmarine tie-ins by Donald E. Savage.
With the conviction that an agreement regarding precise Miocene-Pliocene and Oligocene-Miocene boundaries within the stratal succession of the West Coast region of North America is less important than the establishment of refined age and subage correlations within this region, we assign the following to the Miocene Epoch:
Latest Miocene (10-12 m.y. ago) Clarendonian mammal “age,” Cerrotejonian and Montediablan mammal ages, San Pablo (Cierbo-Neroly) of Santa Margarita mega-invertebrate “ages,” Mohnian and? Delmontian (in part?) foraminiferal ages.
Late to middle Miocene (12-17 m.y. ago) Barstovian mammal “age,” Briones and upper part of Temblor mega-invertebrate “age,” later part of Saucesian plus Relizian and Luisian foraminiferal ages.
Middle to early Miocene (17-21 m.y. ago) Hemingfordian mammal “age,” earlier part of Temblor mega-invertebrate “age,” middle part of Saucesian foraminiferal age.
Earliest Miocene (21-26 m.y. ago) Arikareean mammal “age” (early part may be Oligocene), Vaqueros-Temblor transition plus Vaqueros or Vaquerosian mega-invertebrate “ages,” earliest part of Saucesian plus later part of Zemorrian foraminiferal ages.
The Arikareean, Hemingfordian, Barstovian, and Clarendonian “ages” are easily recognized from joint occurrence of fossils representing certain genera of insectivores, rodents, carnivores, mastodonts, horses, rhinos, oreodonts, camels, and other groups of mammals. We are now concerned chiefly with deciphering a more precise time-stratigraphic range for each of the species. We intend to establish a well-disciplined (vertebrate) paleontologic stratigraphy, which can lead to zonation within the basins of nonmarine deposition.
Unfortunately, fossils of land vertebrates are scarce in districts of littoral deposition, but the “classic” sections in the Tejon Hills, San Francisco East Bay, North Coalinga, Shark-tooth Hill-Pyramid Hill, Caliente Range-Cuyama Valley, Tecuya-San Emigdio Range, South Mountain, and Santa Ana Mountains districts provide tie-ins between the generalized marine and nonmarine paleontologic stratigraphies and geochronologies of the Miocene.
Part 2—Marine vertebrates by Lawrence G. Barnes.
Emphasis on faunal studies and intensified fossil collecting have shown that marine vertebrates in Miocene strata of the West Coast of North America are useful in paleontologic correlation and in geochronology. Mammals, sharks, and bony fishes, in that order, are probably the most useful groups for correlation and chronology; birds and turtles are less useful at present because they are less well studied. Associations of marine vertebrate fossils with terrestrial mammals and marine invertebrates at several localities have permitted correlations between land-mammal “ages” and marine ages. There are three major chapters in the evolutionary history of marine vertebrates in the West Coast Miocene. These are termed early, middle, and late and are roughly equivalent to “Vaqueros,” “Temblor,” and “Santa Margarita” ages respectively. Early Miocene faunas are characterized by archaic mammals (eurhinodelphid dolphins, squalodonts, early sea lions) and birds, and mixed types of sharks. Middle Miocene faunas are characterized by relict archaic mammals (eurhinodelphids, squalodonts, primitive sea lions), some highly specialized mammals (desmatophocine sea lions, desmostylians), and the earliest ancestors of living groups (modernized dolphins, cetotheres). Middle Miocene fishes are tropical, the birds are related to modern taxa, and the sharks are noticeably different from early Miocene species. Late Miocene faunas closely resemble middle Miocene faunas, with similar sharks and birds, but usually lack most of the archaic mammals and turtles and show increased numbers of modernized mammals (dolphins, baleen whales, modern sea lions). Transitions between these three major marine faunas of the Miocene are rarely found.