8
Neogene (Miocene
And Pliocene)

The Neogene Period spans over 20 million years, more than ten times the span of the whole Quaternary (Table 3). The fossil record reveals only glimpses separated by great gaps in space and time. However, there are some rich macrofossil deposits with floras that are almost all identifiable with living genera; many members appear to be closely related or identical to living species. In paleobotany, Tertiary and older fossils are usually given discrete species names in separate time levels, partly because their fragmentary nature prevents complete comparisons. Since the fossil binomials are meaningless to nonpaleobotanists, however, a common practice (which will be followed here) is to give the name of the closest living counterpart in parentheses, for example, Pinus (strobus) .

Most of the fossil floras are from freshwater sediments and are thus biased in favor of local lakeshore or stream bank plants. Also there is bias in favor of species producing large, tough leaves or other organs capable of leaving a good impression in sediment. On the basis of sediment bedding and texture, Axelrod (1958a, 1980) has in some cases been able to discriminate between fossils from local riparian vegetation and material transported from elsewhere by floods and mudflows.

Dating of Neogene floras is currently in a state of flux. Recent advances in dating marine sediments have led to drastic revision of the Neogene time scale and to a shift of the Miocene–Pliocene border to a much later date. The terrestrial and marine chronologies are not yet well integrated. Time correlations between fossil floras of different sites are often controversial except in the few cases where radiometric dates have been established.

By the beginning of the Neogene, the global map of continents and

oceans was beginning to resemble its modern pattern, but some fossil floras deposited in Neogene time have since been significantly displaced in latitude or elevation. Also, major changes in ocean temperatures, rain shadows behind mountain barriers, and coastlines make reconstruction of paleoclimates a formidable problem, one which will not be considered here.

North-Central Europe

(Axelrod 1983b ; Butzer 1971; Ferguson 1971; Frenzel 1968; Hammen et al. 1971; Mädler 1939; Szafer 1966)

North-central Europe had a wonderfully rich Neogene forest flora of both conifers and broadleaf hardwoods. Listing of genera (Table 4) understates the richness because many genera were represented by multiple species. Nearly all these genera are considered members of the Arcto-Tertiary Geoflora, which was shared among all the northern continents. Where its members originated and their directions of migration are not known. Very few of them appear to be capable of ocean crossings. A few probably had been shared between Europe and eastern North America before formation of the North Atlantic Ocean in the Paleogene. Exchanges between Europe and Asia may have occurred repeatedly during both Paleogene and Neogene.

Various floras from Poland, Germany, and the Netherlands, from which Table 4 is compiled, suggest that this rich forest flora persisted into the early Pliocene, when it was decimated during the Villafranchian, a time with predominantly heath vegetation. Reforestation occurred during the succeeding warmer Tegelen period, but few tree species returned, although the high alpine barrier to migration did not yet exist. In the Netherlands, about 80% of the species present in the rich early Pliocene Reuverian flora but only about 20% of the species in the impoverished late Pliocene Tegelen flora are now extinct in the region. The late Pliocene Ludhamian flora of Britain was dominated by genera still common there, for example, Pinus, Alnus, Betula, and Quercus ; it had only a few genera that have since become extinct in the region, for example, Tsuga and Pterocarya . Thus, the so-called modernization of the European flora by regional extinction, although not completed until the Pleistocene, was well underway during the Pliocene. Most of the extinct species or close relatives survive elsewhere; for example, North America has living counterparts of the fossil European Pinus (strobus) and Populus (balsamifera) and eastern Asia has living counterparts of fossil Acer (palmatum) and Quercus (serrata) .

Freshwater aquatic species may have suffered less Pliocene and later extinctions in north-central Europe than forest trees did. Szafer (1946) reported

early Pliocene fossils in Poland of 23 species of freshwater aquatic and marsh plants. One water lily, a species of Euryale, is now extinct in Europe and survives only in eastern Asia. All the other species belong to genera that survive in the region, for example, Ceratophyllum, Najas, Sagittaria, Trapa, Carex, and Polygonum ; Szafer assigned 16 of these fossils to living northern European species.

Western North America

(Axelrod 1958a,b, 1973, 1980, 1983b, 1985a, Axelrod and Raven 1985; Cross and Taggart 1982; Leopold 1969; Raven and Axelrod 1974; Thorne 1978; Wolfe 1972; Wolfe and Tanai 1980)

The rich macrofossil record of western North America has long been studied by an outstanding corps of paleobotanists. Parts of the region have fossil beds preserved in ash falls and lake basins created by volcanic and

tectonic activity. These may record not only stable vegetation controlled by climate and topography but also temporary successional series. The region is so vast and diverse and the fossil record so discontinuous that only a tentative and sketchy migrational story has yet emerged.

In the Miocene time, the woody flora of the region included a great majority of the genera that are important there today and many others that no longer survive there (Table 5). For broad generalization, it is useful to separate Arcto-Tertiary and Madro-Tertiary floras, although the fossil record indicates that, then as now, species ranged individually and formed kaleidoscopic rather than constant associations. Many species, however, evidently had broader and more continuous ranges then because the present great mountain ranges did not yet exist.

Arcto-Tertiary Flora

All but a few of the gymnosperms and deciduous angiosperms in Table 5 are classified in the Arcto-Tertiary group. These genera had spread from uncertain origins all around the northern hemisphere during Paleogene time. Few if any of them appear to have been capable of ocean crossings. Some presumably had crossed between Europe and North America before the North Atlantic Ocean barrier developed in the Paleogene. Contacts across Beringia may have continued intermittently during the Miocene. Especially close relationships between eastern Asian and western North American members of the Arcto-Tertiary forest floras have long been recognized. Recent chemotaxonomic study has shown that Miocene fossils from Oregon of several Arcto-Tertiary genera have organic compounds more similar to living eastern Asian than to eastern North American congeners (Niklas and Giannasi 1978). Different authorities, including Batchelor (1979) and Frakes (1979), have irreconcilably different concepts of eustatic sea level changes during this time.

During the early Miocene, from about 25 to 15 million years ago, a rich assortment of Arcto-Tertiary conifers and deciduous hardwoods ranged from Alaska, and perhaps a Bering land bridge, down to northern California and inland to Idaho and northern Nevada. They did not comprise a homogeneous vegetation. Lowlands were presumably dominated by broadleaved evergreen and deciduous hardwoods, many of which are no longer present in the region. Also present were some conifers, such as Glyptostrobus and Metasequoia , that are no longer present there. Higher elevations were dominated by confier genera, some of which are still present there, such as Abies, Picea, Pinus, Pseudotsuga , and Sequoia. Abies, Picea , and Pinus forests were more extensive inland in Montana and Wyoming, although the major uplift of the modern Rocky Mountains had not yet begun. A few deciduous Arcto-Tertiary genera, including Metasequoia, Carya, Pterocarya, Sassafras , and Ulmus , that have since become extinct in the region were still present locally in early Miocene time.

At the same time, in central California and west central Nevada, the Arcto-Tertiary and Madro-Tertiary genera mingled in a mosaic, with the Arcto-Tertiary flora presumably occupying the cooler north slopes and higher elevations. Pure conifer forests may have been confined to isolated high volcanic peaks. In Miocene southern California, the Arcto-Tertiary flora was evidently represented only by riparian deciduous hardwoods, such as Acer, Alnus, Fraxinus, Platanus, Populus , and Salix , all of which have close modern relatives in the region, and by one extinct species, Ulmus (americana) .

Neogene migrations of the Arcto-Tertiary flora consisted mainly of retreat to disjunct relict areas and local extinctions, which Axelrod (1985a ) has

attributed mainly to decreasing summer rainfall due to ocean cooling reinforced in places by rising topography and rainshadows. Retreats were most drastic in Alaska, where few hardwood trees were left by late Miocene time except riparian Alnus and Salix . By late Miocene time, parts of Alaska had glaciers down to sea level, and above 65°N, taiga, tundra, and heath had taken over. Although probably open, the Bering land bridge was evidently no longer a pathway for intercontinental tree migration. Beginning about 14 million years ago forests in the Pacific Northwest, from Washington and Idaho to central California and central Nevada, became impoverished as many species retreated to maritime coastal regions, rising interior mountains, and riparian sites. Of the Arcto-Tertiary genera that have become extinct in the Pacific Northwest since the early Miocene, none of the conifers and only a few of the deciduous hardwoods are known to have survived after the Pliocene. However, the fossil record is so fragmentary that relicts may have persisted without being recorded.

Madro-Tertiary Flora

The woody flora at the start of Neogene time also included many genera that evidently originated and remained in North America and Mexico, being unable to penetrate high enough latitudes to use Tertiary land connections to Eurasia around either the North Atlantic or North Pacific. This Madro-Tertiary group includes nearly all the evergreen angiosperm genera listed in Table 5, which are still important members of the modern sclerophyll woodlands and chaparral in the region. The group also includes the deciduous angiosperms that survive in interior arid areas of the region (Table 5); these are members of modern thorn scrub and riparian desert vegetation. This flora also includes a few miscellaneous angiosperm genera no longer locally present but surviving in Mexico. Some of these genera are strictly confined to tropical lowlands today, such as Cedrela , a tall, compound-leaved deciduous tree of Mesoamerica.

During early Miocene time, some Madro-Tertiary species ranged much farther north than their surviving relatives. For example, in the Pacific Northwest's present Cascade Mountain region, the Arcto-Tertiary flora commonly had an admixture of Madro-Tertiary evergreen broadleaf trees and chaparral species, including Quercus (chrysolepis), Lithocarpus (densiflora), Arbutus (arizonica), Persea (borbonia), Heteromeles (arbutifolia), Ceanothus (cuneatus), Cerocarpus (betuloides) , and the broadleaf deciduous Cedrela (odorata) . During the Pliocene in central California, some Madro—Tertiary sclerophyll species still grew north of the ranges of their present relatives, Quercus (engelmannii, tomentella), Rhus (laurina, ovata), Ceanothus (spinosus) , and Lyonothamnus (floribundus) .

During early Miocene time, Madro-Tertiary sclerophyll woodland and chaparral species also ranged farther east than their surviving relatives. For example, in west-central Nevada, areas now dominated by desert shrubs and sagebrush had early Miocene floras including all the species noted in the preceding paragraph except Cedrela and Quercus (tomentella) . These fossil floras also included several other sclerophyll tree and shrub genera now confined to the west side of the Sierra Nevada.

In late Miocene time, the general level of the incipient Sierra Nevada was probably still 2,000 m lower than today. The 7-million-year-old Mount Reba flora, from a site now near timberline at 2,650 m, was dominated by Madro-Tertiary sclerophyll woodland species whose modern equivalents are present at about 600 m elevation in the adjacent foothills. Evidently, development of the intense Great Basin rainshadow and spread of desert flora postdate the Miocene. There is no Pliocene fossil record from Nevada.

Most of what is now southern California lay beneath the sea through the Neogene. The few Miocene floras known from the region all evidently grew at low or medium elevations and lack the montane conifers now present in the region. Along with the few deciduous riparian trees noted above, Miocene floras included a variety of chaparral, sclerophyll woodland, and desert shrub genera, mostly with close relatives surviving in the region (Table 6). Considering how much time and tectonic change has ensued, the Miocene floras were amazingly modern; by comparison the contemporary fossil fauna

included many strange, extinct genera. Also, Miocene floras found west of the San Andreas fault actually grew in much lower latitudes. For example, the Mint Canyon flora may have been displaced 200 km northward from its original location. This might be expected to explain the presence of xerophytic fossil plants north of the present limits of Mexican relatives (Table 6), but the same xerophytic species were present in Miocene floras found east of the fault and not displaced. Two species, Lyonothamnus (floribundus) and Prunus (lyonii) that were present on the southern California mainland during the Miocene have surviving relatives only on the offshore islands. When the retreat of the xerophytic and island species began is not known. Early, middle, and late Miocene floras from the region are very similar, and there is no Pliocene record from the region.

Middle America

(Graham 1973, 1976S, 1982)

Pollen records from Mexico and Central America suggest southeastward migration during the Miocene along the Cordilleran axis of many Arcto-Tertiary tree genera: Abies, Picea, Pinus, Alnus, Fagus, Juglans, Liquidambar, Populus , and Ulmus . All of these appeared in Vera Cruz, on the Mexican Gulf coast, during the Miocene. They seem to have formed a lowland forest of mixed conifers and broadleaf deciduous trees adjacent to coastal mangroves with no sign of intervening tropical lowland vegetation. Little work has been done, however, on the pollen signatures of modern tropical lowland forests. It is conceivable that insect-pollinated trees could be present in a region and leave little trace in the pollen record. In Vera Cruz at present, the mangroves grow next to extensive rainforest; the Arcto-Tertiary genera are still present in southern Mexico but grow high in the mountains and in different associations.

The Cordilleran system was being uplifted during the Miocene, and by late Miocene time the Panama land bridge had probably formed. Fossil pollen indicates that Alnus and Juglans had reached Panama during the Miocene and entered northern South America during the Pliocene.