Precambrian life was small and aquatic
Life was confined to oceans and all organisms were small.
Over the Precambrian the shallow seas slowly began to teem with life.
For most of the Precambrian, life consisted of microscopic prokaryotes (eukaryotes
evolved 2/3 of the way through the era).
Unicellular eukaryotes and small multicellular animals fed on floating photosynthetic
Small floating organisms, plankton, were eaten by slightly large animals.
Other animals ingested sediments on the seafloor and digested the remains of
organisms within them.
By late Precambrian, many kinds of multicellular soft-bodied animals had evolved.
Life expanded rapidly during the Cambrian period
Cambrian period (452-488 mya) marks the beginning of the Palaeozoic era.
The O 2oncentration was approaching its current level; the continents had come
together to form several large land masses.
The largest, Gondwana.
A rapid diversification of life took place Cambrian explosion.
Most of the major groups of animals that have species living today appeared during this
THE ORDOVICIAN (488-444 MYA)
o The continents, located primarily in the S. Hemisphere, still lacked multicellular
o Evolutionary radiation of marine organisms during early stages.
o At the end, massive glaciers formed over Gondawa, sea levels were lowered
about 50 meters, and ocean temperatures dropped.
o About 75% percent of the animal species became extinct, probably because of
these major environmental changes.
SILURIAN (444-416 MYA)
o Northernmost continents coalesced, but the general positions did not change
o Marine life rebounded
o Animals able to swim and feed above the ocean bottom appeared for the first
o No new major groups of marine life evolved.
o The tropical se