Biology Reference
In-Depth Information
surfaces. The Gulf Coast in the middle Miocene at 17 Ma now runs just
south of Jackson, Mississippi, along a line about halfway between Austin
and Houston, Texas, and into Mexico between Reynosa and Matamoros.
These factors combine to create a tipping point in the middle Miocene, as
shown on the paleotemperature curve (fi g. 3.4), toward cooling tempera-
tures, general drying, and greater seasonality. The evolution and radiation
of lineages adapted to drier and seasonal conditions will benefi t from the
change at the middle Miocene.
The vegetation in the far north has changed from the megathermal, sub-
tropical forests of the early Eocene (notophyllous evergreen broad-leaved
forest, paratropical rain forest, and thermophilous deciduous angiosperms
and gymnosperms). In the Miocene, it is now a deciduous forest of
Celtis
,
Fagus
,
Juglans
,
Liquidambar
,
Magnolia
,
Nyssa
,
Platanus
,
Pterocarya
, and
Zel-
kova
(Seldovia Point fl ora, 16.8 Ma). The trend is toward an increase in
microthermal elements (
Acer
,
Betula
,
Populus
,
Salix
), a decrease in present-
day Asian components, and an expanding boreal coniferous forest associa-
tion (
Abies
,
Picea
,
Tsuga
). Ice was locally present in the late Eocene, and by
the end of the late Miocene it will be present in the continental interior, at
high elevations, and in the adjacent seas. Some tundra elements (grasses,
sedges, small alder, birch, and willows) appear in the far north by 17 Ma,
and they are beginning to coalesce. The MAT is 6°C-7°C (CLAMP 9°C),
with an annual warm-season to cold-season range of 26°C.
Drier vegetation is encountered toward the south. If one were to climb
the southeastern Sierra Nevada near Tehachapi, Nevada, at 17.5 Ma, the
vegetation in the lowlands would still include some evergreen subtropical
holdovers, but higher up it is a oak-piñon pine woodland. At Buffalo Can-
yon in western Nevada at 16.6 Ma, there is dry vegetation on the slopes,
and a mixture of conifers and deciduous hardwoods starting at elevations
around 1280 m. The estimated MAT is 10°C, and the MAP is 1000 mm.
In addition to the trend toward drying and greater seasonality, edaphic fac-
tors, slope, and exposure play a signifi cant role, as shown by the vegetation
at Middlegate and Eastgate in western Nevada at 15.5 Ma. The former is a
shrubland-chaparral of
Arbutus
(madrones),
Cedrela
,
Cercocarpus
(mountain
mahogany),
Lithocarpus
(tan oak), and
Quercus
growing on drier south-
facing slopes (MAT 10.2°C, CLAMP). The latter grows on moister north-
facing slopes and has a better representation of mesic
Abies
,
Larix
,
Amel-
anchier
(serviceberry), and
Aesculus
(buckeye, horse chestnut). The regional
MAP at midelevations is about 900 mm.
In the Plains area, the vegetation is shrubland-savanna with some
Acer
,
Crataegus
(hawthorn),
Robinia
(locust), and
Ulmus
, but trending to-
