Biology Reference
In-Depth Information
6
Tra ns i tion
Middle Eocene through the Early Miocene
Momentous events occurred during the interval of the middle Eo-
cene through the early Miocene (50-16.3 Ma), including a precip-
itous drop in temperature beginning around 52 Ma and continuing
through the end of the Eocene (35-34 Ma). This period was fol-
lowed by a less eventful transition time during the Oligocene and
early Miocene, 35-34 Ma through 16.3 Ma (fi g. 3.4). The cumula-
tive effect of the changes was to signifi cantly alter the old Paleo-
gene world and set the stage for the next inevitable series of de-
velopments in the Earth's biota later in the Neogene. Atmospheric
CO
2
concentration diminished to the point that climates cooled,
seasonality increased, and temperate conditions spread. This was
due in part to a slowing of plate movement, from an annual average
of about 12-15 cm in the Jurassic and Early Cretaceous to about
2-3 cm at present. With a reduced output of CO2, less heat re-
mained trapped in the Earth's atmosphere, and temperatures fell.
Superimposed on this trend was a period of signifi cant uplift
of major mountain systems, which increased the weathering and
