ERA
|
PERIOD
|
AGE
(of start)
|
Cenozoic
|
|
1.6 my
|
|
65 my
|
Mesozoic
|
|
140 my
|
|
205 my
|
|
250 my
|
Paleozoic
|
|
290 my
|
|
355 my
|
|
410 my
|
|
438 my
|
|
510 my
|
|
540 my
|
|
Cretaceous 140 - 65 million
years ago
Life
Dominant life includes:
Dinosaurs, birds, gymnosperm and fern
forests
Tyrannasaurus Rex
is alive during this time.
First angiosperms (flowering plants)
At the beginning of the Cretaceous, the
climate was cold with snow and ice at the poles. Later in
the period it became warmer with polar ice melting.
The main rock types/ deposits in PA
are clay and sand.
K-T Boundary Extinction Event
At the end of the Cretaceous (abbreviated
as K) and at the beginning of the Tertiary (T) a meteor hit
the Earth just off of the Yucatan Peninsula in Central America.
All around the world Iridium, an element found mainly
in space, is found in the rocks at this time period. It is
believed that the impact of the meteor caused a massive dust
cloud and disrupted life on Earth. This may have caused the
dinosaurs to become extinct, although by fossils it looked
like the dinosaurs may have been dying out for some other
reason before the meteor hit. About 50% of species become
extinct.
|