|Index to this page|
|Eras||Periods||Epochs||Aquatic Life||Terrestrial Life|
|With approximate starting dates in millions of years ago in parentheses. Geologic features in green|
The "Age of Mammals"
|Quaternary (1.8)||Recent||Humans in the new world|
|Pleistocene||Periodic glaciation||First humans|
|Continental drift continues|
|Tertiary (65)||Pliocene||All modern groups present||Hominids and pongids|
|Miocene||Monkeys and ancestors of apes|
|Oligocene||Adaptive radiation of birds|
|Eocene||Modern mammals and herbaceous angiosperms|
"The Age of Reptiles"
|Cretaceous (145)||Still attached: N. America & N. Europe; Australia & Antarctica|
|Modern bony fishes||Extinction of dinosaurs, pterosaurs|
|Extinction of ammonites, plesiosaurs, ichthyosaurs||Rise of woody angiosperms, snakes|
|Africa & S. America begin to drift apart|
|Jurassic (205)||Plesiosaurs, ichthyosaurs abundant||Dinosaurs dominant|
|Ammonites again abundant||First mammals; Archaeopteryx; first lizards|
|Skates, rays, and bony fishes abundant||First angiosperms; insects abundant|
|Pangaea splits into Laurasia and Gondwana|
|Triassic (250)||First plesiosaurs, ichthyosaurs||Adaptive radiation of reptiles:|
thecodonts, therapsids, turtles,
crocodiles, first dinosaurs
|Ammonites abundant at first|
|Rise of bony fishes|
|Paleozoic (543)||Permian (286)||Appalachian Mts. formed; periodic glaciation and arid climate|
|Extinction of trilobites, placoderms||Reptiles abundant: cotylosaurs, pelycosaurs. Cycads, conifers, ginkgos|
|Pennsylvanian (320)||Warm, humid climate Together
the Pennsylvanian and Mississippian
make up the
also called the
"Age of Amphibians"
|Ammonites, bony fishes||First reptiles|
|Mississippian (360)||Adaptive radiation of sharks||Forests of lycopsids, sphenopsids, and seed ferns|
The "Age of Fishes"
|Placoderms, cartilaginous and bony fishes. Ammonites, nautiloids||Ferns, lycopsids, and sphenopsids|
|Extensive inland seas|
|Adaptive radiation of ostracoderms, eurypterids||Arachnids (scorpions)|
|Silurian (438)||Mild climate; inland seas||Nautiloids, Pilina, other mollusks|
|Ordovician (490)||Mild climate, inland seas||Trilobites abundant|
First jawless vertebrates
|First fungi and bryophytes|
|Cambrian (543)||First eurypterids, crustaceans|
Sponges, cnidarians, annelids
|No fossils of eukaryotes, but phylogenetic trees suggest that lichens, mosses, perhaps even vascular plants were present.|
|Fossils rare but many protistan and invertebrate phyla toward the end||No fossils of eukaryotes, but phylogenetic trees suggest that lichens, mosses, perhaps even vascular plants were present towards the end.|
A body of evidence, both geological and biological, supports the conclusion that 200 million years ago, at the start of the Mesozoic era, all the continents were attached to one another in a single land mass, which has been named Pangaea.
This drawing of Pangaea (adapted from data of R. S. Dietz and J. C. Holden) is based on a computer-generated fit of the continents as they would look if the sea level were lowered by 6000 feet.During the Triassic, Pangaea began to break up, first into two major land masses:
The present continents separated at intervals throughout the remainder of the Mesozoic and through the Cenozoic, eventually reaching the positions they have today.Let us examine some of the evidence.
Louis Alvarez, his son Walter, and their colleagues proposed that a giant asteroid or comet striking the earth some 65 million years ago caused the massive die-off at the end of the Cretaceous. Presumably, the impact generated so much dust and gases that skies were darkened all over the earth, photosynthesis declined, and worldwide temperatures dropped. The outcome was that as many as 75% of all species - including all dinosaurs - became extinct.
The key piece of evidence for the Alvarez hypothesis was the finding of thin deposits of clay containing the element iridium at the interface between the rocks of the Cretaceous and those of the Tertiary period (called the K-T boundary after the German word for Cretaceous). Iridium is a rare element on earth (although often discharged from volcanos), but occurs in certain meteorites at concentrations thousands of times greater than in the earth's crust.
After languishing for many years, the Alvarez theory gained strong support from the discovery in the 90s of the remains of a huge (180 km in diameter) crater in the Yucatan Peninsula that dated to 65 million years ago.
The abundance of sulfate-containing rock in the region suggests that the impact generated enormous amounts of sulfur dioxide (SO2), which later returned to earth as a bath of acid rain.
A smaller crater in Iowa, formed at the same time, many have contributed to the devastation. Perhaps during this period the earth passed through a swarm of asteroids or a comet and the repeated impacts made the earth uninhabitable for so many creatures of the Mesozoic.