|Index to this page|
|Eras||Periods||Epochs||Aquatic Life||Terrestrial Life|
|With approximate dates in millions of years ago in parentheses. Geologic features in green|
|Cenozoic (66 to present)
The "Age of
|Quaternary (2.6 to present)||Holocene||Humans in the new world|
|Pleistocene||Periodic glaciation||First humans|
|Continental drift continues|
|Neogene (23–2.6)||Pliocene||Atmospheric oxygen reaches today's level (21%)||Hominids|
|Miocene||Adaptive radiation of birds; continued radiation of mammals|
|Paleogene (66–23)||Oligocene||All modern groups present|
|Cretaceous (146–66)||Still attached: N. America & N. Europe; Australia & Antarctica; Mass extinction of both aquatic and terrestrial life at the end|
|Modern bony fishes||Extinction of dinosaurs and pterosaurs; first snakes|
|Extinction of ammonites, plesiosaurs, ichthyosaurs||Rise of angiosperms|
|Africa & S. America begin to drift apart|
|Jurassic (200–146)||Plesiosaurs, ichthyosaurs abundant; first diatoms||Archaeopteryx; dinosaurs dominant but mammals (Eutheria) begin to diversify|
|Ammonites again abundant||First lizards|
|Skates, rays, and bony fishes abundant||Adaptive radiation of dinosaurs|
|Pangaea splits into Laurasia and Gondwana; atmospheric oxygen drops to ~13%|
|Triassic (251–200)||Mass extinctions at the end.||Mass extinctions at the end.|
Adaptive radiation of reptiles: thecodonts, therapsids, turtles, crocodiles, first dinosaurs
|Ammonites abundant at first|
|Adaptive radiation of bony fishes|
|Paleozoic (542–251)||Permian (299–251)||Periodic glaciation and arid climate; atmospheric oxygen reaches ~30%. Volcanic eruptions killed off 90% of marine species at end.|
|Extinction of trilobites||Reptiles abundant. Cycads, conifers, ginkgos|
|Pennsylvanian (318–299)||Warm, humid climate
make up the
also called the
"Age of Amphibians"
|Great diversity of marine invertebrates||First reptiles|
|Mississippian (359–318)||Adaptive radiation of sharks||Forests of lycopsids, sphenopsids, and seed ferns|
Adaptive radiation of the insects (Hexapoda)
|Atmospheric oxygen begins to rise as organic matter is buried, not respired|
The "Age of Fishes"
|Extensive inland seas||Cartilaginous and bony fishes abundant. Ammonites, nautiloids, ostracoderms, eurypterids||Ferns, lycopsids, and sphenopsids|
|Silurian (444–416)||Mild climate; inland seas||First bony fishes||First myriapods and chelicerates|
|Ordovician (488–444)||Mild climate, inland seas||Trilobites abundant||Fungi present|
First plants (liverworts?) First insects
First vertebrates (jawless fishes). Eurypterids, crustaceans|
mollusks, echinoderms, sponges, cnidarians, annelids, and tunicates present. Trilobites dominant.
|No fossils of terrestrial eukaryotes, but phylogenetic trees suggest that lichens, mosses, perhaps even vascular plants were present.|
|Fossil evidence of multicellular algae, fungi, and bilaterian invertebrates|
|Evidence of eukaryotes|
~1.8 x109 years ago
|Archaean (4000–2500)||Evidence of microbial life
3.5–3.7 x109 years ago
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 (~1800 meters).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.
|View an animation of the breakup of Pangaea from the Jurassic (200 million years ago) to the present.|
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Louis Alvarez, his son Walter, and their colleagues proposed that a giant asteroid or comet striking the earth some 66 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 Paleogene period (called the K-Pg boundary after the German word for Cretaceous). Iridium is a rare element on earth (although often discharged from volcanoes), 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 1990s 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.
A mass extinction of non-dinosaur reptiles occurred earlier, at the end of the Triassic. It was followed by a great expansion in the diversity of dinosaurs. The recent discovery of a layer enriched in iridium in rocks formed at the boundary between the Triassic and Jurassic suggests that impact from an asteroid or comet may have been responsible then just as it was at the K-Pg boundary.
The largest extinction of all time occurred still earlier at the end of the Permian period. There is evidence off the coast of Australia that a huge impact there may have contributed to the extinctions at the Permian-Triassic (P-T) boundary.