Index to this page |
Eras | Periods | Epochs | Aquatic Life | Terrestrial Life |
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With approximate starting dates in millions of years ago in parentheses. Geologic features in green | ||||
Cenozoic (65) The "Age of Mammals" |
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Neogene (23) | Holocene | Humans in the new world | ||
Pleistocene | Periodic glaciation | First humans | ||
Continental drift continues | ||||
Pliocene | Hominids and pongids | |||
Miocene | Monkeys and ancestors of apes | |||
Paleogene (65) | Oligocene | All modern groups present | ||
Eocene | Adaptive radiation of birds and mammals | |||
Paleocene | ||||
Mesozoic (251) The "Age of Reptiles" | Cretaceous (146) | Still attached: N. America & N. Europe; Australia & Antarctica | ||
Modern bony fishes | First modern birds; extinction of dinosaurs and pterosaurs | |||
Extinction of ammonites, plesiosaurs, ichthyosaurs | Rise of woody angiosperms, snakes; first placental mammals (Eutheria) | |||
Africa & S. America begin to drift apart | ||||
Jurassic (200) | Plesiosaurs, ichthyosaurs abundant; first diatoms | Dinosaurs dominant; first angiosperms | ||
Ammonites again abundant | First mammals; Archaeopteryx; first lizards | |||
Skates, rays, and bony fishes abundant | Adaptive radiation of dinosaurs; insects abundant | |||
Pangaea splits into Laurasia and Gondwana | ||||
Triassic (251) | First plesiosaurs, ichthyosaurs | Adaptive radiation of reptiles: thecodonts, therapsids, turtles, crocodiles, first dinosaurs | ||
Ammonites abundant at first | ||||
Rise of bony fishes | ||||
Paleozoic (542) | Permian (299) | 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 "Carboniferous"; also called the "Age of Amphibians" |
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Ammonites, bony fishes | First reptiles Coal swamps | |||
Mississippian (359) | Adaptive radiation of sharks | Forests of lycopsids, sphenopsids, and seed ferns Amphibians abundant Land snails | ||
Periodic aridity | ||||
Devonian (416) The "Age of Fishes" | Extensive inland seas | Placoderms, cartilaginous and bony fishes. Ammonites, nautiloids | Ferns, lycopsids, and sphenopsids First gymnosperms First amphibians | |
Adaptive radiation of ostracoderms, eurypterids | Arachnids (scorpions) | |||
Silurian (444) | Mild climate; inland seas | Nautiloids, Pilina, other mollusks | First insects | |
Ordovician (488) | Mild climate, inland seas | Trilobites abundant First jawless vertebrates | First fungi First plants (liverworts?) First millipedes? | |
Cambrian (542) |
Trilobites dominant. Eurypterids, crustaceans mollusks, echinoderms sponges, cnidarians, annelids, and tunicates present | No fossils of terrestrial eukaryotes, but phylogenetic trees suggest that lichens, mosses, perhaps even vascular plants were present. | ||
Periodic glaciation | ||||
Proterozoic (2500) | Ediacaran (580) | Fossil eukaryotes rare but evidence of several types of invertebrates | No fossils of terrestrial eukaryotes, but phylogenetic trees suggest that lichens, mosses, perhaps even vascular plants were present towards the end. | |
Paleoproterozoic (1600-2500) | Fossil bacteria 1.9 x109 years ago; perhaps earlier |
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 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 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.
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-T 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.
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