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
|
|
Ordovician 510
- 438 million years ago
Life
Dominant life includes:
Molluscs, Bryozoa, Graptolite, Brachiopods,
Conodonts (early vertebrates)
The first vertebrates and primitive fish
evolve during this time period.
During the early- middle Ordovician, the
climate was relatively warm and moist.
During the late Ordovician, Gondwana positioned
itself over the South Pole, resulting in massive glaciers
and a drop in sea level. A mass extinction event occurred
at the end of the Ordovician, killing about 60% of marine
genera.
Supercontinent over South Pole in late Ordovician
called Gondwana.
The main rock types/ deposits in PA are
limestone, dolomite, and shale.
|