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.
THE TRIASSIC (251-200 mya)
o Pangaea began to break apart
o Many invertebrate groups became more species-rich; many burrowing animals evolved
from groups living on the surfaces of seafloor sediments.
o Conifers and pteridosperms became the dominant trees.
o First frogs and turtles appeared.
o Great radiation of reptiles began.
o End of this period was marked by mass extinction of about 65% of the species on Earth.
THE JURASSIC (200-145 mya)
o Land was once again divided into two large continents.
o First salamanders and lizards appeared, and flying reptiles evolved.
o Dinosaur lineages evolved into predators that walked on two legs and large herbivores
that walked on four.
o Several groups of mammals first appeared during this time.
o Plant evolution continued with the emergence of the flowering plants that dominate
Earth’s vegetation today.
THE CRETACEOUS (145-65 mya)
o Laurasia was completely separated from Gondwana.
o A continuous sea encircled the tropics.
o Sea levels were high, and Earth was warm and humid.
o Marine invertebrates increased in diversity and in number. o Dinosaurs continued to diversify and first snakes appeared.
o Early in this period, flowering plants began the radiation that lead to their current
dominance on land.
o By the end, many groups of mammals had evolved; most were small, few were large.
The modern biota evolved during the Cenozoic era
By early Cenozoic ear (65 mya), the positions of the continent