During the Precambiran period, life consisted of microscopic prokaryotes; eukaryotes probably evolved about two-thirds of the
way through the era.
o Unicellular eukaryotes & small multicellular animals fed on floating photosynthetic microorganisms
Shallow Precambrian seas teemed with life, including protists and algae.
By the late Precambrian, many kinds of soft-bodied invertebrates had evolved, some of which may be members of animal
lineages that have no living descendants.
Cambrian period marks the beginning of the Paleozoic era
By the early Cambrian period (543–490 mya), atmospheri2 O levels had nearly reached current levels.
The continental plates came together in several masses. Gondwana was the largest land mass.
The rapid diversification of life that took place at this time is referred to as the Cambrian explosion.
The best fossils of Cambrian animals are found in China.
A mass extinction occurred at the end of the Cambrian period.
Fossils tell us only about the hard parts of organisms
o 3 Cambrian fossil beds- the Burgess Shale (British Columbia), Sirius Passet(Northern Greenland) & Chengjiang
o Arthropods are the most diverse group in the Chinese fauna
Divided into the Ordovician, Silurian, Devonian, Carboniferous, and Permian periods
o Each period is characterized by the diversification of specific groups of organisms
o Mass extinctions marked the ends of the Ordovician, Devonian, and Permian
The Ordovician (488-444 mya)
o Continents mostly in the S. Hemisphere.
o Intense evolutionary radiation of marine organisms. Animals lived on the sea floor or burrowed in sediments.
o Ancestors of club mosses and horsetails colonized wet terrestrial environments.
o Mass extinction at end: 75% of marine species lost. Sea levels dropped about 50 meters, and glaciers formed over
The Silurian (444-416 mya)
o Northern continents coalesced (united as one) , but their general position did not change.
o Marine life rebounded from previous mass extinction from the Ordovician time; animals that can swim and feed above
the ocean appeared for the first time, but no new major groups of marine animals evovled
o The first known tracheophytes (vascular plants) appeared on land in the late Silurian.
o First terrestrial arthropods (scorpions, millipedes) evolved
The Devonian (416-359 mya)
o rates of change accelerated in many groups of organisms during this period
o the northern land mass called Laurasia, and the southern land mass called Gondwana moved slowly toward eachother
o fishes diversified as jawed forms replaced jawless ones, heavy armor gave way to the less rigid outer coverings of
all current major groups of fishes were present by the end of the period
o the ancestors of gymnosperms, the first plants to produce seeds, appeared later in the Devonian
o Fishlike amphibians began to occupy the land
o A mass extinction of about 75% of all marine species marked the end of the Devonian
The Carboniferous (359-297 mya)
o Large glaciers at high latitudes and extensive swamp forests grew in tropical areas.
o Fossilized remains of the forests formed the coal we now mine for energy.
o Diversity of terrestrial animals increased greatly.
o Snails, scorpions, centipedes, and insects were abundant, first winged insects.
o Amphibians became larger, and reptiles evolved from one amphibian lineage.
o Crinoids (sea lilies and feather stars) were plentiful on the seafloor
The Permian (297-251 mya) o During the Permian period, the continents united to form the supercontinent Pangaea
o Massive volcanic eruptions: ash blocked sunlight and cooled the climate => largest glaciers in Earth’s history.
o By the end, reptiles greatly outnumbered amphibians.
o The lineage leading to mammals diverged from one line of reptiles. Bony fishes radiated in the oceans.
o Mass extinction at end: 96% species lost (volcanoes, meteorite & ocean turnover implicated)
o At the end of the Permian, a large meteorite crashed into northwestern Australia.
o Volcanic eruptions poured lava into the oceans, which dep2eted O in deep oceans. Oceanic turnover then carried the
depleted water to the surface where it released t2xic CO2and H S.
o O2concentrations dropped to about 12 percent—most animals would have been unable to survive at elevations above
o About 96% of all species on Earth became extinct.
Mesozoic Era (251–65 mya)
o At the start of the Mesozoic era (248 mya), the few surviving organisms found themselves in a relatively empty world.
o Pangaea slowly separated, glaciers melted, and shallow inland seas formed.
o Life proliferated and diversified.
o Earth’s biota diversified and became distinct on each continent (previously more homogeneous).
Earth’s biota became increasingly provincialized: that is distinct terrestrial biotas evolved on each continent
o By end of the era, the continents were close to their present positions and many organisms looked similar to those
o Divided into The Triassic, Th