LATE CRETACEOUS – 70 to 65 mya
A shallow sea extended from the Gulf of Mexico to the Arctic Ocean. This sea is called an epeiric sea.
Present north America was physically divided into western and eastern regions.
The Appalachian mountains rose during the Paleozoic and were the highest mountains in
Pangaea, during the Ordovician (~480 to 430 May).
The Rocky Mountains rose between 80 – 50 Mya. They were much lower than the present at the end of the Cretaceous.
Sierra Nevada was low hills at the end of the Cretaceous.
Modern Cascades did no exist at this time. They appeared during the Pliocene, 5 – 7
Mya.
Temperatures were much higher than the present, with mean temperatures above 20°C.
The period from the Cretaceous through the middle Eocene (70--35 Ma) included some of the warmest temperatures in all of Phanerozoic time, perhaps 5°--10° C warmer than at

present at low midlatitudes (30° N) and 30° C warmer at high latitudes (80° N) (J. T.
Parrish 1987).
With the continent physically divided by the epeiric sea into western and eastern regions, two principal floristic provinces characterized the late Cretaceous vegetation of North
America.
Southeastern North America was covered with evergreen tropical woodland. The estimated mean annual temperature was above 25°C.
The west was covered with broadleaf evergreen forests and conifers. The estimated mean annual temperature was 20 - 25°C.
The epeiric sea began to retreat at the end of the Cretaceous and allowed the dispersal between the two regions.
The impact of a large asteroid could have caused climatic changes responsible for the extinction of marine and terrestrial animals, and to a lesser extent, of terrestrial vegetation.
TERTIARY – 65 TO 2.6 Ma.
Tectonic activity continued as Gondwana finally split completely apart, and India collided with the Eurasian plate . South America was connected to North America toward the end of the Tertiary. Antarctica — which was already separate — drifted to its current position over the South Pole . Widespread volcanic activity was prevalent.
Climates during the Tertiary slowly cooled, starting off in the Paleocene with tropical-tomoderate worldwide temperatures and ending before the first extensive glaciation at the start of the Quaternary.
Tertiary: Paleocene through Middle Eocene (65--35 Ma)
Generally warm temperatures, and possibly increased but seasonal (winter-dry) rainfall regimes, characterized the Paleocene, 65-55 Ma. and early and middle Eocene, 55-37 Ma.
(J.A. Wolfe and G.R. Upchurch 1987b).
Tropical dry-season vegetation first appeared; the Betulaceae, Fabaceae, Fagaceae,
Juglandaceae, and Ulmoideae diversified in the middle and higher latitudes.
Araucarian conifers became extinct in the Northern Hemisphere at the end of the
Cretaceous.

Glyptostrobus and Metasequoia became abundant, the latter the most frequent and widespread member of the Taxodiaceae (Cupressaceae in the flora) in North America from Cretaceous to Miocene times.
Acer , Ailanthus , Celtis , Hydrangea , Liquidambar , Populus , and Rhus made their first appearance.
The family Poaceae was present in North America by the Paleocene/Eocene (W.L. Crepet and G.D. Feldman 1991).
Tropical forests (Annonaceae, Lauraceae, Menispermaceae) in the Southeast reached their maximum northern expansion (to ca. 50°--60° N).
In general, the tropical to warm temperate component of Paleocene and early Eocene floras from the eastern Rocky Mountain region shows affinity with eastern Asia, while by the middle Eocene the affinity is with seasonally dry vegetation of northern Latin
America
Several paleo-subtropical taxa were introduced into North America from the Old World tropics over the North Atlantic land bridge during the late Paleocene and early Eocene (
Lowland Malaysian genera are prominent in some Alaskan strata of early Eocene age, e.g. Barringtonia (a tropical Pacific beach plant), the dipterocarp Parashorea.
Most of the land south of the Denali Fault in Alaska were formed at sites far to the south and have been accreted to the North American Plate.
The Eocene Beringian land bridge was primarily occupied by a broad-leaved deciduous forest (J.A. Wolfe 1985, cited in B.H. Tiffney 1985b) and may not have been a major route of migration for megathermal elements.
To the northeast, North America was connected to Europe by the now sundered landmass of Euramerica, which existed at the end of the Paleocene but was beginning to fragment by the early Eocene.
There were two routes of migration:
1.
The Scotland-Greenland southern route.
2.
The De Geer route, about the 10°- 15° farther north, through Spitsbergen and
Greenland.
The migrants were for the most part temperate deciduous plants.

The major limiting of migration during the Tertiary was climatic conditions rather than physical barrier
The early Eocene was the warmest interval of the Tertiary, and this was a time of rapid migration from Europe to North America over the North Atlantic land bridge.
Late Eocene to Late Miocene (35--10 Ma)
There was a significant drop in temperature in North America and elsewhere during the mid-Tertiary. The drop was particularly notable for winter minimum temperatures near the end of the Eocene, ca. 33 Ma. The lower temperature had an influence on the flora.
The sequence of floras from western North America suggests the following general elevational zonation:
1.
lowlands (below ca. 300 m)---warm temperate, broad-leaved evergreen forest;
2.
300--1000 m---temperate, mixed deciduous hardwood forest;
3.
1000--1300 m---cool temperate, conifer--deciduous hardwood forest;
4.
above 1300 m---cold temperate montane conifer forest with few deciduous hardwoods.
Some genera present at that time are: Abies , Picea , Pinus , Chamaecyparis , Acer ,
Ailanthus , Amelanchier , Bursera , Cardiospermum , Carpinus , Carya , Castanea , Cedrela ,
Celastris , Celtis , Cercis , Crataegus , Hydrangea , Mahonia , Persea , Platanus , Populus ,
Quercus , Salix , Sambucus , Sassafras , Tilia , Ulmus , Vitis , Zelkova and Ulmus
The break of Gondwana, the large southern continent, greatly influenced the northern hemisphere.

Australia separated further from Antarctica, and the Drake Passage between
Antarctica and South America opened (middle Oligocene). As a result, cold water flowed northward into the southern oceans and circulated into equatorial latitudes, in turn strengthening high-pressure systems and the drier climates associated with them in North America.

Drier climates and colder winters initiated the decline of tropical elements from the North American flora.

The expansion of the mesothermal broad-leaved temperate forests occurred at midlatitudes.

The early appearance of taxa adapting to these drier habitats was particularly evident in southwestern North America
Other geological events influenced the flora during the Eocene:

As the Rocky Mountains uplifted, coniferous forests characteristic of higher elevations expanded.


The area of the southern Rocky Mountains and the Sierra Madre Occidental was an important center for the evolution of the Madro-Tertiary geoflora (D.I. Axelrod
1958) and for pines (D.I. Axelrod 1986b).

During the mid-Tertiary, a significant lowering of sea level was evident at about
30 Ma.

Major volcanic activity occurred in western North America, the Andes, and the
Philippines.

The Mississippi Embayment, a remnant of the epeiric sea, was rapidly retreating southward.

Temperate conditions continued to prevail across the North Atlantic and Beringia.
Tectonic events caused the further disruption of the North Atlantic land bridge.
Late Miocene through Pliocene (10--2 Ma)
Cooling in the middle Miocene was due to northward drifting of continents.

All through this period, a general cooling occurred.

The geographic extent of the temperate deciduous hardwood forests declined from their former extensive ranges across the medium and high latitudes.

Extensive grasslands appeared during this time, and at high latitudes and elevations coniferous forests continued to expand.

Polar ice formed in the Arctic, the tundra elements appeared.

The Sierra-Cascades reached substantial heights, and a rain shadow developed in the Great Basin.

There was additional uplift in the Rocky Mountains and, at their southern end, the further development of a sclerophyllous element occurred.
By the middle Miocene the Rockies was an effective barrier to biotic interchange between eastern and western North America.
The flora of Nebraska at this time suggests reduced rainfall but no well-developed prairie; open pine-oak woodlands.
Components of the flora are Carya , Liquidambar , Nyssa , Platanus , Populus , and Tilia .
Other plants included Acer , Celtis , Fraxinus , Prunus , Pterocarya , Ulmus , and a more southern element of Cedrela , Cordia , Diospyros , and Meliostoma .
Associated fossil faunas suggest frost-free climate.
Rainfall began to decrease about 16 Ma at the eastern foothills of the Rockies.
In the middle to late Miocene, the western American, eastern Asian, and western
European floras, as well as the few floras from mid- and eastern North America, record an extensive, temperate, mixed deciduous hardwood forest across the middle and high latitudes. This is the much-debated Arcto-Tertiary geoflora.

Drier sclerophyllous vegetation continued to develop in the southern Rockies due to reduced rainfall and lower minimum temperatures.
With the late middle Miocene cooling, the west Antarctic Ice Sheet developed, and polar ice first appeared in the Arctic.
Tundra vegetation began to evolve.
Broad-leaved deciduous forest reached their maximum southern expansion in eastern
Mexico.
By the beginning of the late Miocene (7--5 Ma), many Asian, neotropical, and paleotropical elements had disappeared from eastern North America.
The rain shadow created by the Rockies continued to influence the development of herbaceous vegetation and restricts trees to valleys. Grasslands, however, developed extensively in the Quaternary.
Extensive grasslands probably began at the Miocene-Pliocene transition (7--5 Ma). This was the driest part of the Tertiary. Forests and woodlands were restricted, and grasses and forbs rapidly radiated (D.I. Axelrod 1985).
In the Pliocene, the Cascade-Sierra Nevada and the Coast Ranges reached sufficient heights to create an effective rain shadow over the Basin and Range Province, resulting in a trend from mesic and summer-wet to xeric and summer-dry conditions.
About 4.5 Ma, modern grasslands developed in the Columbia Plateau. Browsers and grazers dominated the fauna.
Deciduous gymnosperms ( Ginkgo , Glyptostrobus , and Metasequoia ) and angiosperms
( Pterocarya ), now restricted to East Asia, disappeared in the Pliocene.
The Pliocene vegetation of Alaska was largely coniferous forest.
Conifers disappeared from what is now the Bering Sea and were replaced by herbaceous and shrubby vegetation dominated by Poaceae, Cyperaceae, Salicaceae and Rosaceae.
There were no typical tundra plants here at this time.
The appearance of widespread prairie vegetation in midcontinental North America, plus sclerophyllous and coniferous vegetation in western and northern North America, was the first major disruption of the broad-leaved deciduous forest, the Arcto-Tertiary geoflora that had extended across temperate latitudes of the Northern Hemisphere since late
Eocene times.
Elements of the broad-leaved deciduous forest that had extended into eastern Mexico during the middle to late Miocene became isolated in climatically comparable zones. The continuity of the flora that was established during the lat Miocene cooling became

disrupted during the Pliocene resulting in the present existing relationship between
Eastern North America and Eastern Mexico.
The broad-leaved deciduous forests of eastern North America and Eastern Asia are floristically related and it resulted from the maximum extension of the temperate deciduous forest in the mid-Tertiary and its disruption in western North America during the Pliocene and in western Europe during the Quaternary.
SUMMARY
In the Miocene,

The Neotropical-Tertiary geoflora retreated south of the United States into Central
America. The Great Plains became savannas and grasslands in the rain shadow of the
Rocky Mountains.

The Arcto-Tertiary geoflora moved south and occupied most of present day United
States. As the Rocky Mountains rose the Arcto-Tertiary forest gave way to grasslands in the central part of the United States.

The Madro-Tertiary geoflora extended northward in the southwest of United States to form sclerophyllous woodland ancestral to the pine-oak woodland of today. Several other types of vegetation developed and are present to this day: chaparral, desert and woodland. This change was brought on by the uplift of the southwestern region of
North America.
The mesophytic deciduous forest that was continuous with Eurasia became fragmented as a result of continental drift in the last 20 million years. Some species became extinct, and other disappeared in one region of the world but not in another.

Magnolias, sweet gum, hickory, walnuts, etc. disappeared in Europe but not in North
America; Metasequouias and ginkgoes disappeared in North America but still exist in
China.

A relic of the Arcto-Tertiary forest, poor in species was left in California and the
Pacific Northwest, e.g. sequoias.
In the Pliocene (7-2 m.y.a.) continuous climatic cooling and mountain building brought on continental glaciations.