The Geological Timescale Chart
----------------------------------------------------------------------
| Era | Period | Epoch1 | Millions of years ago |
|-------------|----------------|-------------|-----------------------|
| | | Holocene | 0.01|
| | Quaternary |-------------|-----------------------|
| | |Pleistocene | 1.8|
| |----------------|-------------|-----------------------|
| | | Pilocene | 5.3|
| | |-------------|-----------------------|
| Cenozoic | | Miocene | 23.7|
| | |-------------|-----------------------|
| | Tertiary | Oligocene | 36.6|
| | |-------------|-----------------------|
| | | Eocene | 57.8|
| | |-------------|-----------------------|
| | | Paleocene | 66.4|
|-------------|----------------|-------------|-----------------------|
| | Cretaceous | 144|
| |------------------------------|-----------------------|
| Mesozoic | Jurassic | 208|
| |------------------------------|-----------------------|
| | Triassic | 245|
|-------------|------------------------------|-----------------------|
| | Permian | 286|
| |------------------------------|-----------------------|
| | Pennsylvanian/ | |
| | Upper Carboniferous2 | 320|
| |------------------------------|-----------------------|
| | Mississippian/ | |
| | Lower Carboniferous | 360|
| Paleozoic |------------------------------|-----------------------|
| | Devonian | 417|
| |------------------------------|-----------------------|
| | Silurian | 443|
| |------------------------------|-----------------------|
| | Ordovician | 495|
| |------------------------------|-----------------------|
| | Cambrian | 544|
|-------------|------------------------------|-----------------------|
| Precambrian3| N/A | 4600|
----------------------------------------------------------------------
Footnotes
1: It's true that all periods are divided into epochs. However, with the exception of the
Cenozoic, these are all referred to as "Early", "Middle", and "Late" sections of the period. I don't really see a need to subdivide them this way for our purposes here.
2: The period from 360 to 286 million years ago is called the Carboniferous in
Europe and elsewhere (named after the
coal beds found in that
rock layer).
American geologists instead split it into two periods, the Mississippian and
the Pennsylvanian.
3: Technically speaking, the Precambrian (or
Cryptozoic) is not an era, but an
aeon. It's often treated as an era because, quite simply, it's easier
considering how relatively little happened during that time (except, you know,
the emergence of
life as we know it). I've put it this way on the chart
because making another column for aeons would needlessly consume space.
Brief Overview of Periods
Some of the events I'm listing here are also listed in the various small
writeups written for each of the periods. For the sake of simplicity, I've
consolidated them into this writeup.
Precambrian
Formation of the Earth (duh)
First lifeforms emerge around 3.5 billion1 years ago -
unicellular, anaerobic, heterotropic prokaryotes.
Modern atmosphere and ocean form in mid- to late Precambrian
Two major glaciation periods (or "Ice Ages"). First is the Gowgonda
glaciation in the Paleoproterozoic (c. 2500 m.y.a.), second is the Varangian in
the Neoproterozoic (c. 600 m.y.a.). The Varangian Ice Age was indirectly
responsible in part for the Cambrian Explosion (see below).
First multicellular lifeforms appear near the end of the Precambrian.
1: Note that I am using the American definition of billion
(109), not the British definition, which is equal to the American trillion (1012).
Paleozoic
Cambrian
Rhodinia, the original supercontinent, breaks up.
The Cambrian Explosion. In a matter of 10 to 15 million years (very short
in geological time - or evolutionary time for that matter), known lifeforms
grow from about 15 families to 100 or so. The Varangian Ice Age may
have contributed to this by causing the extinction of archaeocyathids (small
plants which attached themselves to rocks or other lifeforms), opening up
niches for new species. Other contributing factors: Increase of oxygen in
the atmosphere (essential to multicellular lifeforms); near-extinction of
stromatolites (another form of primitive plant life); possible rise in
nutrient levels (there is currently little evidence for this hypothesis).
Trilobites are arguably the dominant lifeform at this point.
The Ross Orogeny begins, creating the Antarctic mountain ranges.
Ordovician
Further adaptive radiations; by the Ordovician there are 400 families of
fauna.
Brachiopods (which first appeared in the Cambrian) flourish. Appearance of
crinoids, echinoids, bryozoans, others.
First signs of primitive vertebrate fish.
Ross Orogeny ends.
The end of the Ordovician marks the second-worst mass extinction in
Earth's history (in terms of numbers of species). It is theorized that this came
about due to mass glaciation.
Silurian
Little change in animal life, though some new families and genera appear.
Plants begin to grow on dry land.
Devonian
Fish become the dominant lifeforms. Many species of fish evolve.
First land vertebrates appear - primitive amphibians.
Insects and spiders also make their first appearance here.
There is relatively little orogenic activity during this period.
Carboniferous
Amphibians and insects continue to develop. Primitive reptiles appear.
Coral, brachiopods, crinoids, and other aquatic invertebrates flourish.
Seed ferns and other primitive trees proliferate, creating huge forests
which would later form the major coal beds.
Permian
The Appalachian/Hercynian orogeny takes full effect; Laurentia
(essentially North America) and Baltica (essentially Western Europe) hit
Gondwanaland (essentially everything else), forming the supercontinent of
Pangaea.
First turtles appear.
First "mammal-like reptiles" (Synapsids), e.g. Dimetrodon.
Sea reptiles appear: Icthyosaurs and Plesiosaurs.
Primitive conifers become the dominant plant life.
The end of the Permian marks the worst mass extinction in Earth's history.
As much as 95% of marine life died out, including trilobites, bryozoans, and
many species of fish. Land animals suffered as well, especially amphibians. The
causes of the Permian extinction are not fully understood, but it's generally
believed that a major eustatic sealevel change (in this case, a major
regression of the ocean) caused sudden climate changes that many species could
not adapt to.
Mesozoic
Triassic
The beginning of the Age of Reptiles. Emergence of first dinosaurs:
coelosaurs, prosauropods.
First appearance of crocodiles and alligators.
First pterosaurs.
Ammonites flourish as the dominant marine invertebrates.
Appearance of earliest known bird, Protoavis (note: whether Protoavis was an
early bird, and even whether it was a real species, is still a matter of
controversy).
Pangaea breaks apart into Laurasia and a new Gondwanaland.
Jurassic
Dinosaurs continue to develop: Sauropods, stegosaurs, carnosaurs, etc.
The largest known land animal in Earth's history appears: the sauropod
Ultrasaurus. Based on fossil remains, this dinosaur is believed to have been
as much as 60 feet tall, 100 feet long, and weighed 80 tons.
First mammals appear, in the form of early rodents.
The much better-established bird ancestor, Archaeopteryx, appears in this
period.
Antarctica, Australia and the Indian subcontinent break away from
Gondwanaland.
Cretaceous
Even more new dinosaurs: Tyrannosaurs, anklyosaurs, ceratopsians, etc.
The first flowering plants (angiosperms) appear.
First "true" birds appear.,
K-T extinction occurs. Most reptile species die out, as do ammonites, most
plankton, and many other species. The most common hypothesis concerning this
extinction is that a meteorite or asteroid hit the Earth, wreaking havoc
with the climate.
South America and Africa break apart; Africa collides with Laurasia.
Cenozoic
Tertiary
Beginning of the Age of Mammals. With many of the ecological niches filled
by the various dinosaurs and other reptiles gone, the mammals underwent a series
of adaptive radiations producing a wide variety of new mammalian families. The
two main groups were carnivores and hooved herbivores.
In the Eocene, many new mammals appear: rhinoceroses, horses,
mastodons, etc.
Birds become more prolific.
In the Pilocene, one of our earliest hominid ancestors appears:
Australopithecus.
Continental drift continues, until the continents look roughly as they do
today.
Quaternary
Appearance of modern humans, homo sapiens sapiens.
The latest Ice Age covers much of the northern regions of the world with
glacial ice until a few thousand years ago.
Most of the large mammalian species, such as the wooly mammoth and the giant sloth, die out after several thousand years.
Humans invent nuclear weapons, the internal combustion engine, and
television, and are now well on their way to out-doing the Permian extinction. (I kid, I kid.)
Please note: In the above summary, I'm trying to limit the number of events I list. I suppose I could write down every orogeny and the names of all the animal families that existed in a given period (at least until we get into Carboniferous or so), but I don't think that would be very effective.
That said, I *am*, after all, working mostly off of my notes from Geology 102 and 104, so if you think I'm missing an important event (or that some of my above information is incorrect), please /msg me about it. Unless, of course, you're some wacky Young-Earther or something, in which case I direct you to read Hutton's Theory of the Earth. Granted, it's somewhat out-of-date, being from the early 18th century; but then, so are you. (It's also about a thousand pages long, so it'll keep you busy for awhile.)
Thanks to Tiefling for pointing out my (now fixed) error in the British definition of 'billion'