Index to this page

The history of life as revealed by the fossil record.

ErasPeriodsEpochsAquatic LifeTerrestrial Life
With approximate starting dates in millions of years ago in parentheses. Geologic features in green
Cenozoic (65)
The "Age of
Mammals"
Neogene (23)HoloceneHumans in the new world
PleistocenePeriodic glaciationFirst humans
Continental drift continues
PlioceneHominids and pongids
MioceneMonkeys and ancestors of apes
Paleogene (65) Oligocene All modern groups present
EoceneAdaptive radiation of birds and mammals
Paleocene
Mesozoic (251)
The "Age
of Reptiles"
Cretaceous (146)Still attached: N. America & N. Europe; Australia & Antarctica
Modern bony fishesFirst modern birds; extinction of dinosaurs and pterosaurs
Extinction of ammonites, plesiosaurs, ichthyosaursRise of woody angiosperms, snakes; first placental mammals (Eutheria)
Africa & S. America begin to drift apart
Jurassic (200)Plesiosaurs, ichthyosaurs abundant; first diatomsDinosaurs dominant; first angiosperms
Ammonites again abundantFirst mammals; Archaeopteryx; first lizards
Skates, rays, and bony fishes abundantAdaptive radiation of dinosaurs; insects abundant
Pangaea splits into Laurasia and Gondwana
Triassic (251)First plesiosaurs, ichthyosaursAdaptive 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, placodermsReptiles 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"
Ammonites, bony fishesFirst reptiles
Coal swamps
Mississippian (359) Adaptive radiation of sharksForests of lycopsids, sphenopsids, and seed ferns
Amphibians abundant
Land snails
Periodic aridity
Devonian (416)
The "Age of Fishes"
Extensive inland seasPlacoderms, cartilaginous and bony fishes. Ammonites, nautiloidsFerns, lycopsids, and sphenopsids
First gymnosperms
First amphibians
Adaptive radiation of ostracoderms, eurypteridsArachnids (scorpions)
Silurian (444)Mild climate; inland seas Nautiloids, Pilina, other mollusksFirst insects
Ordovician (488)Mild climate, inland seasTrilobites 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 invertebratesNo 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

The Geologic and Evolutionary Record

A remarkable feature of the table above is how often evolutionary changes coincided with geologic changes on the earth. But consider that changes in geology (e.g., mountain formation or lowering of the sea level) cause changes in climate, and together these alter the habitats available for life. Two types of geologic change seem to have had especially dramatic effects on life:

Continental Drift

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.

Shape of the Continents

The east coast of South America and the west coast of Africa and are strikingly complementary. This is even more dramatic when one tries to fit the continents together using the boundaries of the continental slopes (e.g., 6000 feet down) rather than the shorelines.

Geology

Fossils

The Impact Hypothesis

The Cretaceous period, the last period of the Mesozoic, marked the end of the Age of Reptiles. It was followed by the Cenozoic era, the Age of Mammals. Although extinctions have occurred throughout the history of life, an extraordinary number of them occurred in a relatively brief period at the end of the Cretaceous. Why?

The Alvarez Theory

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.

Other Impacts?

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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6 April 2005