Biology Reference
In-Depth Information
lies such as the Annonaceae, Araliaceae, Burseraceae, and Lauraceae, and
one crocodilian. In addition, however, there are also fossils of
Pinus
,
Cornus
(dogwood),
Nyssa
(tupelo, or sour gum),
Juglans
,
Magnolia
,
Platanus
(syca-
more),
Prunus
(cherry),
Celtis
(hackberry), and
Vitis
(grape). Twenty-four
percent of the genera are found in the early to middle Eocene of Europe, but
strongest affi nities are with Asia (
Ginkgo
,
Actinidia
,
Alangium
,
Ampelocissus
,
Eucommia
,
Iodes
,
Schizandra
), and there are Asian species among other gen-
era. The MAT is estimated at 16°C, at the lower limit of a paratropical forest,
and fossil woods show some seasonality in precipitation. As expected, this
was a vegetation retaining many attributes of the earlier Eocene tropical
communities, but showing the effects of seasonality and cooling tempera-
tures that would next convert the regional biota into a temperate deciduous
forest. In the slightly younger (38.8 Ma) John Day Formation, the MAT is
estimated at about 14°C (Manchester 2000). Present was
Quercus
, among
the oldest records for the genus, and
Pinus
(Cretaceous/Paleocene), that
along with
Juniperus
in the late Eocene fl oras of Montana would differenti-
ate elements available to form dry shrubland/chaparral-woodland-savanna
later in the Tertiary with increased seasonality and rain shadow effects from
the rising mountains. The modern sagebrush (
Artemisia
) desert of the re-
gion would not appear until the Pliocene and Quaternary.
Moving inland from the coast, upward in elevation, and into late Eocene
time (to about 40 Ma), the Copper Basin fl ora of northeastern Nevada,
as expected, is a microthermal mixed deciduous hardwood-gymnosperm
assemblage of
Abies
,
Picea
,
Pinus
,
Larix
, and
Tsuga
, with a warmer, more meso-
thermal fl ora lower on the slopes:
Pseudotsuga
(Douglas fi r),
Mahonia
,
Rhodo-
dendron
, and
Sassafras
(Axelrod 1966). The MAT is estimated at around 11°C.
The middle Eocene Green River fl ora of northern Colorado-Utah on
the east-facing slopes of the Rocky Mountains circa 45 Ma (Smith et al.
2008) provides insight on vegetation developing under a rain shadow. The
mountains in the vicinity were approximately 1.3 km higher than the basin,
and the fl ora grew around a series of lakes covering an area of 65,000 km
2
.
There were seventy-seven depositional cycles that generally conform to the
100,000-year precessional cycle of the Milankovitch variations, and the
sedimentology shows that in twenty-fi ve of these episodes the lakes dried
up to form playas and salt fl ats. The Green River Formation is known for
its exquisitely preserved fi sh fauna, and the fl ora includes plants of drier
habitats like
Ephedra
,
Pinus
,
Bursera
,
Canavalia
(
C. maritima
is a common
pantropical shore plant),
Cardiospermum
,
Celtis
,
Quercus
,
Sapindus
, and
Caesalpinia
. This assemblage suggests an open oak-piñon pine woodland, or
a type of savanna with few grasses.
Typha
(cattail) and
Populus
were present
