Every major catastrophe seems to provide opportunities for new biological life forms. So it was with the Ordovician–Silurian Extinction. Up to that point, life had been...
The Early Paleozoic: Major Expansions and ExtinctionsLast Updated on 2010-12-19 00:00:00
By 0.57 Ga, Earth’s climate became warm, and the glaciers retreated. The continents were primarily low-lying deserts and alluvial plains. Rising sea levels encroached upon these areas and covered about 85% of Earth’s surface (today, water covers 71%). Without continental landmasses at the poles, oceans circulated freely, and ice formation was negligible.
Photosynthetic organisms, which had somehow escaped extinction during the severe glaciation of the Cryogenian, proliferated, and atmospheric O2 concentrations climbed to about 15%.  Multicellular organisms appeared. Again, this was not a coincidence. At higher O2 concentrations, parts of organisms could be situated deeper within the organism and still receive sufficient O2 to conduct aerobic respiration.  Also, the thicker ozone (O3) layer in the upper atmosphere afforded greater UV protection, so organisms... More »
The Late Paleozoic: Responding to CatastropheLast Updated on 2010-12-19 00:00:00
Every major catastrophe seems to provide opportunities for new biological life forms. So it was with the Ordovician–Silurian Extinction. Up to that point, life had been bound to the seas, an environment that strongly limits both the penetration of sunlight and the diffusion of carbon dioxide (CO2). In the mild climate that followed the extinction, plants emerged from the aquatic habit at about 0.43 Ga and colonized land.
The ascent to land was a formidable undertaking. In their ancestral aquatic habitat, all cells were in close proximity to water and the nutrients dissolved in it. The restlessness of the surrounding fluid promoted mixing as well as diminished the boundary layer between an aquatic organism and its medium. Thus, absorption of water and nutrients by an aquatic organism did not quickly deplete their immediate surroundings.
On land, a desiccating atmosphere... More »
The Age of the DinosaursLast Updated on 2010-12-18 00:00:00
Reptiles occupied the void left by the Great Dying and dominated animal life from 251 megayears (Ma) to 66 Ma (one megayear = one million years ago), an era known as the Mesozoic, or the Age of Dinosaurs. Angiosperms, true flowering plants, also first appeared and became prevalent. All of this biological activity undoubtedly benefited from a relatively stable, warm climate.
The Triassic period (251 Ma to 200 Ma), like the periods that preceded it, was dry and had large seasonal variations in temperature and precipitation. Low sea levels probably intensified seasonal variations. During the Jurassic period (200 Ma to 146 Ma), Earth’s crust expanded on the ocean floor, displacing water and causing sea levels to rise. Pangea began to rift into smaller continents, bringing more land area in contact with the ocean. With closer proximity to water, these continents had more stable... More »
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