Geological eras

The Phanerozoic eon ('visible life'), the last half-billion years, is the eon from which we can find fossils. Life existed before, but did not have bones, and was probably(?) not as complex. For that reason, we often talk as if life really got started in this eon. The eon is divided into these geologic periods (first column says how many million years ago the period started):

The lines interrupting the above table mark the Permian-Triassic (P-T) extinction event and the Cretaceous-Paleogene (K-Pg) extinction event. There have been others, but these two are the largest, and amateur geologists can detect these lines in rock sediment.

The periods in between the two major extinctions – the Triassic, Jurassic and Cretaceous periods – composed the age of dinosaurs. It is also called the Mesozoic Era ('middle life'), with the Paleozoic ('old life') coming before and Cenozoic ('new life') coming after.

Note that geology assigns specific meanings to the words eon, era and period. The Phanerozoic Eon is made up of the Paleozoic, Mesozoic and Cenozoic Eras, which in turn are made up of Periods like the Triassic and Jurassic Periods. To go down another level, each Period is divided into a number of Epochs.

It is easy to remember details about the Mesozoic and Cenozoic because they bear relevance to us, because "dinosaurs are cool", because they're recent, and because they're shorter than the Paleozoic. The Paleozoic may need more deliberate memorization. There are six periods in it, abbreviated as Є O S D C P: Cambrian-Ordovician-Silurian-Devonian-Carboniferous-Permian.

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The Cambrian is the time of the "Cambrian Explosion", the proliferation of a lot of new species, and is the time of the earliest fossils (with exceptions – there are some older fossils from the "Precambrian" times, usually imprints of soft tissue, not skeletons). In particular, an extinct class of animals called trilobites was very common in the Cambrian. All life was in the sea – the land was lifeless. The supercontinent Pannotia had just broken up.

The Ordovician was a time with a hundredfold as many meteor strikes compared to today. An asteroid field passing by? The sea was dominated by invertebrates, like in the Cambrian, but fish had started to evolve.

The Silurian saw the diversification of jawed and bony fish. Small plants appeared beside lakes and streams.

The Devonian has been dubbed the "Age of Fish" because fish reached substantial diversity in this period, thus beginning the age of vertebrates. It could also be called the age of forests: the lands got covered in forest (though a different kind of forest from what we're used to). No animals on land yet. The Devonian forest would've been a safe place to be for a time-traveller, it would just have been him/her and the plants. We're now at 400 Ma, so it's been over 100 million years since the Cambrian Explosion. By the middle of the period, plants evolved leaves and true roots, and by the end, the first seeds appeared.

The Carboniferous, if you have a time machine, would be the period to visit if you want to see bugs as large as cats. The period is so named because many coal beds were formed in this period – from the constant forest fires due to high oxygen levels. There were land animals, primarily amphibians (part-time water-dwellers) and arthropods (insect-likes). The insects were enormous. A fossil we've named Meganeura was basically a 70 cm wide dragonfly. The reason for such size was the record oxygen levels, 35% compared to 21% today. The continents started merging.

The Permian saw some amphibians become fulltime land-dwellers, forming the ancestors of the mammals, turtles, lepidosaurs (reptiles) and archosaurs (dinosaurs). This is partly thanks to the Carboniferous Rainforest Collapse, which gave an advantage to creatures that could handle dry conditions, over amphibians. We're now at 300 Ma, so it's been over 200 million years since the Cambrian explosion. It took that long for animals to get out of the ocean properly. There were two continents: Pangaea and Siberia, surrounded by the ocean Panthalassa. The Permian period ended with the largest mass extinction in history, the Great Dying, making extinct 90% of all species, and thus the Paleozoic Era as a whole ends.

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The Triassic began after the P-T extinction event, so the period had very poor biodiversity. It took 30 million years for ecosystems to recover. This also starts the Mesozoic Era: a lot of new and different species formed to replace the lost. The supercontinent Pangaea merged all land mass.

The Jurassic

The Cretaceous

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The Paleogene

The Neogene

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To zoom out rather than in, the whole of the Phanerozoic Eon ('visible life'), i.e. the time from the Cambrian to present, is one of four eons in the Earth's lifetime:

1. the Hadean Eon (4500–4000 Ma), when the Earth and Moon took shape
2. the Archean Eon (4000–2500 Ma), when microbes appeared
3. the Proterozoic Eon (2500–540 Ma), when oxygen appeared due the activities of some microbes, and eukaryotes appeared
4. the Phanerozoic Eon (540 Ma to present), when large (visible to the naked eye) multicellular species became common

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• 2022-01-25
• Evolution