Life expanded rapidly during the Cambrian period
Cambrian period (452-488 mya) marks the beginning of the Palaeozoic era.
The O2concentration 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 period.
THE ORDOVICIAN (488-444 MYA)
o The continents, located primarily in the S. Hemisphere, still lacked multicellular plants.
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 much.
o Marine life rebounded
o Animals able to swim and feed above the ocean bottom appeared for the first time.
o No new major groups of marine life evolved.
o The tropical sea was uninterrupted by land barriers
o Most marine organisms were widely distributed.
o First vascular plants appeared late in Silurian period; less than 50cm tall and lacked
roots and leaves.
o First terrestrial arthropods appeared at about the same time.
DEVONIAN (416-359 MYA)
o Rates of evolutionary change accelerated.
o The northern land mass (Laurasia) and the southern land mass (Gondwana) moved
slowly toward each other.
o Great evolutionary radiations of coral and shelled squid like cephalopods.
o Fishes diversified as jawed forms replaced jawless ones and less rigid outer covering of
modern fishes evolved.
o All current major groups of fishes were present by the end of the period.
o Terrestrial communities also changed dramatically; club mosses, horsetails, and tree
ferns became common.
o Their deep roots accelerated the weathering of rocks, resulting in the development of
the first forest soils.
o Ancestors of gymnosperms, first plants to produce seeds, appeared later in the era.
o Extinction of about 75 percent of all marine species marked the end of this era.
THE CARBONIFEROUS (359-297 MYA)
o Large glaciers formed over high-latitude Gondwana, but extensive swamp forests grew
on the tropical continents.
o Dominated by giant tree ferns and horsetails with small leaves.
o Fossilized remains of those trees formed the coal.
o The diversity of terrestrial animals increased greatly.
o Insects evolved wings (first animals to fly) and gained access to tall plants. o Amphibians became larger and better adapted to terrestrial existence after splitting
from the lineage leading to the amniotes, vertebrates with well-protected eggs that can
be laid in dry places.
o In seas, crinoids reached their greatest diversity, forming “meadows” on the sea floor.
THE PERMIAN (297-251 MYA)
o Continents coalesced into the supercontinent Pangaea.
o Near the end, reptiles greatly outnumbered the amphibians.
o Late in the period, the lineage leading to mammals diverged from one reptilian group.
o In fresh waters, the Permian period was a time of extensive diversification of ray-finned
o Massive volcanic eruptions resulted in outpourings of lava that covered large areas of
Earth; ashes produced blocked the sunlight and cooled the climate forming largest
glaciers in history.
o Atmospheric temperature dropped; about half of the Permian land would have been
o Resulted in the most drastic mass extinction even in Earth’s history.
Geographic differentiation increased during the Mesozoic era
Few organisms that survived found themselves in a relatively empty world at the start of the
Mesozoic era (251 mya).
Pangaea slowly separated into individual continents; oceans rose and reflooded continent
shelves, forming huge, shallow inland seas.
Atmospheric oxygen concentrations gradually rose to their former levels.
Different groups of organisms came to dominate the Earth:
o Three groups of phytoplankton---dinoflagellates, cocccolithophores, and diatoms—
became ecologically important at this time.
Earth’s biota became increasingly provincialized –distinct terrestrial biotas evolved on each
Biotas of the shallow waters bordering the continents also diverged from one another.
By the end of the era, the continents were close to their present positions and many organisms
looked similar to those living today.