The known fossil fruits and leaves of Acer from western North America represent 91 species and 28 sections, 12 of which are extinct and are described as new sections of Acer. Sixty-four species are described as new, 2 new combinations are proposed, and 6 species are left unnamed; 21 have been previously described. The most diverse sections of Acer in the Tertiary of western North America are the extinct Glabroidea (at least 13 species), Negundo (9 species), Macrophylla (8 species), and Eriocarpa (8 species), Descriptions of almost all the species are presented, and all species are illustrated. Although Aceraceae are considered to be derivatives of an early, extinct group of Sapindaceae, Paullinieae (rather than Harpullieae) are considered 10 be the extant tribe of Sapindaceae most closely related 10 Aceraceae. A cladistic analysis of Aceraceae and of Acer includes Sapindaceae, Dipteronia, and the "Acer" arcticum complex, which is thought to represent an extinct genus of Aceraceae. The cladistic analysis based on extant Acer results in the subdivision of Acer into 4 informal groups: Spicata Group, Macrantha Group, Macrophylla Group (including section Acer and allies), and the Platanoidea Group. Timing of first appearances of the various groups and sections in the fossil record generally parallel the cladistic analysis. The Spicata Group is the oldest (latest Paleocene); this group includes three extinct sections in the early middle Eocene, all of which became extinct by the late middle Eocene. First known in the early middle Eocene are extinct sections of the Macrantha and Macrophylla groups; extant sections of these groups appear by the late middle to early late Eocene. The Platanoidea Group appeared in the late middle Eocene, and extant sections appeared by the latest Eocene. A fifth group, the Orba Group, is known only as fossil and represents sections that diverged between the divergences of the Macrantha and Macrophylla groups. Diversification of Acer at the sectional level appears to have taken place in a volcanic upland region in western North America during the Eocene. Although possibly a mesothermal genus during the late Paleocene and early Eocene, Acer diversified greatly during the middle and late Eocene as microthermal climates increased in area. During the early middle Eocene, 10 sections (all extinct) and 11 species of Acer are known. During the late middle to late Eocene, Acer reached maximal diversity in western North America: at least 34 species and 15 sections are known, and occurrences of other species and sections can be inferred. Acer, however, was apparently a very minor element in Eocene microthermal vegetation. Sectional diversification of Acer was largely completed by the end of the Eocene, although a few derivative sections may be of post-Eocene age. Acer reached maximum abundance in western North America during the early and middle Miocene: at least 29 species and 10 sections are known. Following the middle Miocene, Acer underwent a major decline in diversity and abundance in western North America; this decline was due primarily to declining summer temperatures at high latitudes and increasing aridity at middle latitudes. Present distributions of sections and species of Acer have resulted from a complex history of dispersals and vicariant events, most of which are related to climate. Probable origin of many extant Asian sections of Acer in western North America during the Eocene implies many dispersals from North America to Asia during the Eocene. Many extinct and extant sections of Acer became extinct on North America during the late Eocene and early Oligocene; some of these extant sections re-entered North America during the late Oligocene and Miocene but again became extinct during the Miocene. Cladistic relationships of series Saccharodendron strongly indicate an origin in western Eurasia. Appearance of this section in North America during the early Miocene and absence of a Beringian fossil record indicate long-distance dispersal across the Atlantic Ocean. Absence of a Tertiary record in western North America of Palmata indicates a long-distance dispersal from eastern Asia
