How Do Scientists Date Ancient Events?
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
The largest, Gondwan.
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
o About 75% percent of the animal species became extinct, probably because of these major environmental
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
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 rea