Biology Reference
In-Depth Information
of relative pause. It is not generally commented on but this seemingly un-
eventful transitory period—its relative quiescence suggested by the Oligo-
cene interval on the paleotemperature curve (fi g. 3.4)—was important. If
the trend in extinctions and redistributions from older times had continued
unabated into the ice ages, without the benefi t of a transition interval, we
would likely be living under a very different set of global ecosystems.
NORTH AMERICA (NORTH OF MEXICO)
To the far north, these adjustments are evident in fossil fl oras from Alaska
(Wolfe 1972, 1977, 1992). Data presented in table 6.1 show a decrease in
MAT between the late Paleocene to the early Oligocene from about 12.3°C
to 9.0°C, and a decrease in growing season precipitation from around
1450 mm to 600 mm. The boreotropical fl ora developed a less tropical and
more boreal aspect, with
Alnus
(alder) and
Ulmus
(elm), for example, and
there were an increasing number of deciduous, temperate-habitat plants
shared with eastern Asia. In the early to middle Eocene McAbee fl ora of
British Colombia, the oldest remains of
Fagus
are recorded (Dillhoff et al.
2005; see also Mindell et al. 2009). Many older fossils referred to
Ulmus
are now treated as
Chaetoptelea
, and the oldest elm fossils in North America
are from the late part of the early Eocene (Denk and Dillhoff 2005). By the
beginning of the Oligocene, the vegetation of the far north was essentially a
deciduous forest—
Acer
(maple),
Betula
(birch),
Carya
(hickory, pecan),
Cas-
Table 6.1.
Alaskan Paleogene fl oras and inferred climates
Assemblage
Age
MAT
CMMT
WMMY
MART
MGSP
Redoubt
E Oligo
9.0
-
4.0
22.0
26.0
60?
Rex Creek
L Eoc
12.3
0.3
24.3
24.0
40?
Katalla
L Eoc
16.2
6.6
25.6
19.2
120
Carbon Mt.
M-L Eoc
8.7
-
0.8
18.2
19.0
?
Aniakchak
M Eoc
13.7
3.3
24.1
20.8
>
145
Charlotte Rdg.
M Eoc
17.2
8.6
25.8
17.2
>
145
Kulthieth
E Eoc
19.4
12.2
26.2
13.6
>
145
Chickaloon
L Paleo
12.3
1.1
23.5
22.4
>
145
Source:
Adapted from Wolfe 1994.
Note:
MAT
=
mean annual temperature (°C); CMMT
=
cold-month mean temperature (°C);
WMMT
=
warm-month mean temperature (°C); MART
=
mean annual range of temperature
(°C); MGSP
=
mean growing season precipitation (cm); E
=
early; M
=
middle; L
=
late.
