Orwell, Geological Periods
Orwell is a Garden World in the Chronos Cluster with complex indigenous life and a long, complex geological history.

Geological Periods

Although there have been a number of periodization systems for Orwell's prehistory, the most common comes from a popular series of educational virches for modosophonts, released 6644 AT. Later researchers have complicated and subdivided this scheme, but it will be used as the basis for this outline.
Geological timescale for Orwell
Image from Steve Bowers
The NovaTerran Period: 8 Gya-7.3 Gya
(named after Nova Terra, originally a lifeless EoGgaian world)
This covers Orwell's formation and the development of its atmosphere and ocean before the origin of life (precise date unknown).

The Eostremonathan Period: 7.3 Gya-3.5 Gya
Named after Eostromonath, which had anaerobic life and was similar to Orwell in this period before terraformation)
This covers the period when only simple bacterial life existed.

The Penglaian Period: ~3.5 Gya - 3 Gya
(Named after Penglai, which was at a similar stage with microscopic anaerobic life, including stromatolite-analogues before colonisation)
This sees the development of anaerobic photosynthesis, eukaryotes, and complex microbial chemotrophic ecologies.

The Euripidian Period: 3.0 Gya -1.4 Gya
(Named after Euripides Mey, a garden world with similar characteristics)
This begins with the development of aerobic photosynthesis. Oxygen levels slowly climb. Life remains microscopic, but the three multicellular kingdoms develop. As carbon dioxide is converted to oxygen, temperatures begin to descend, but stay largely within the realm of wet greenhouse.

The Tohulian Period: 1.4 Gya -1.26 Gya
(Named for Tohul, a wet greenhouse world)
This begins with the first fossilized evidence of macroscopic life. Comparable to a steamy version of Earth's Vendian; life is sparse at the equator. Tissue structure remains basic, but hard parts exist.

The Panthalassic Period: 1.26 Gya - 1.14 Gya
(Named for Panthalassa, a water world)
Starting 1.26 billion years ago, with the first major radiation of Northern swimmers, this sees substantial diversification of life. Temperatures drop sharply in the latter half, with the first ice forming at the period's end; as a result, significant exchanges occur between the northern and southern hemispheres.

The Darwinian Period: 1.14 Gya- 1 Gya
(Named after Darwin, a world resembling Earth in the Cambrian period)
1.14 billion years ago, the ice recedes, shallow seas open up, and oxygen arrives at an Earth-like density. The result is a massive adaptive radiation almost as dramatic as Earth's Cambrian Explosion, though coming at a point when life is a bit more complex. The fauna finally develops sophisticated organ systems and body plans. It also experiments with the first gliding and amphibious forms.

The Rajasekaran Period: 1 Gya - 825 Mya
(Named after Rajaseka, a planet with intermittent ice ages)
1 billion years ago, the first of the periodic ice ages hits. This leads to a mass extinction, pruning the exuberance of the Darwinian Period, but as the ice comes and goes, life readapts.

The Dantean Period: 825 Mya -775 Mya
(Named after Dante, a planet with persistent ice cover)
Not very well defined in beginning and end, this straddles about fifty million years on either end of 800 million years ago, give or take. Containing Orwell's harshest ice age, it doesn't quite reach Snowball Earth proportions, but does manage to lock much of the globe in ice, eliminate shallow seas, and leave much of the land cold desert. The location of the ice cap moves as the axis wobbles. This extreme is unstable, however, as photosythesizer die-offs lead to resurgences of carbon dioxide and repeated oscillations. Naturally, intense mass extinctions and instability result. However, this does give rise to full-scale colonization of the land, although land fauna remains small. A supercontinent almost forms, but not quite.

The Paulan Period: 775 Mya- 600 Mya
(Named after Paula, a world with diverse sealife)
Temperatures settle at an equilibrium (partly because the star is getting warmer all this time, but mostly because life develops feedback systems). Towards the end of the period, oxygen levels spike, while carbon dioxide remains high. Biodiversity flourishes; separate continents go their own ways.

The Khitaian Period: 600 Mya- 570 Mya
(Named after Khitai VI, a cold world)
Another major ice age hits, and the continents coalesce into the first true supercontinent, assembled about 600 million years ago. This leads to conditions similar to Earth's Permian extinction.

The Macrystian Period: 570 Mya- 504 Mya
(Named after Macrystis, a somewhat forested world)
Reestablishment in the aftermath of the Khitaian extinctions. A long stable period (apart from temperature fluctuations from the unstable axis tilt).

The Araratan Period: 504 Mya- ~469 Mya
(Named after Ararat, a world with significant forest cover)
This opens with a minor extinction event 504 million years ago (cause unknown, possibly volcanism). Major rifting of the supercontinent sets in, exposing new shallow seas. Vascular plants return to the sea, developing into the earliest ancestors of Orwell's bamboo seas.

The Treeish Period: ~469 Mya - 393 Mya
(Named after Trees, a planet with heavy forest cover)
Two major mass extinction events fall right next to each other, 469 million years ago and 456 million years ago; scholars vary as to which they use as the boundary. Both were likely impact events, though this is uncertain. The second invasion of the land occurs as sea fauna adapts to the bamboo forests and then follows the trees onto the land. These will develop into the first large-scale terrestrial fauna on Orwell.

The MeditatingIdols Period, the Earthly Period, and the Ridgewellian Period: 393 Mya - 203 Mya
(These periods are named after The World of Meditating Idols, Old Earth, and Ridgewell. Conditions on Orwell resembled those on the first planet to some extent in the first period, while the Earthly period is probably the period when conditions most resembled Earth. Ridgewell has a sophisticated biosphere, and this was a particularly diverse and specialized period.)

There's no clear division between these three periods, which together last from 393 to 203 million years ago. No significant mass extinctions occur (only a minor one 234 million years ago), and continents remain fairly separate, with only a few collision or rifting events. As a result, life diversifies and specializes, while only slowly developing significant change. Temperatures slowly rise, and although ice ages recur throughout, they become less frequent. Carbon dioxide levels also slowly lower as a result of the carbon cycle, which leads to some floral turnover towards the end, as more carbon-efficient plants, as well as carbon-fixing organisms, become more prominent, but the effect as of yet is minimal.

The NewMontanan Period: 203 Mya- ~120 Mya
(Named after New Montana, a world with a newly-formed supercontinent)
An impact event 203 million years ago coincides with the onset of a new supercontinent and increased temperatures (which end the recurring ice ages for good). This results in the most massive extinction event since the Dantean Period. The supercontinent continues to coalesce as the period continues, releasing massive lava traps. Successor groups spread across the newly-joined land.

The Elmoic Period: ~120 Mya- 90 Mya (No clear dividing line.)
(Named after Elmo, a dry Gaian world)
The supercontinent stabilizes; minor rifts form by the end of the period. The carbon cycle is starting to break down during warm spells. Major upheavals in flora as they scramble for carbon dioxide; changes are slow enough that innovative groups have time to adapt. Oxygen levels also start to decline due to floral die backs. Nonetheless, some of the most massive fauna arises in this period, encouraged by the wide open spaces and nomadic corridors.

The NewGaian Period: 90 Mya- 50 Mya
(Named after New Gaia, a gradually warming world)
A minor impact event 90 million years ago sets off runaway feedback loops, as the most inefficient plants go extinct and oxygen levels plummet (albeit briefly), and biodiversity bottoms out, which further disrupts the carbon cycle and triggers the first big carbon crisis. The best adapted flora bounces back readily, followed by marginalized remnants of the old flora, as the carbon crisis wanes, while new innovations pave the way for biota which would be familiar in the present day. The first biokarst landscapes form.

The Orwellian Period: 50 Mya - Present
(The geologically modern Orwellian period)
Starting around 50 million years ago, the carbon crisis has become acute even during the cooler times when the axis tilt is greater. All remaining flora adapts or perishes. Meanwhile, the new continents separate in earnest and move to their present positions. As temperatures approach wet greenhouse, photodissociation becomes noticeable, but the sea levels still haven't fallen far. Despite the impending doom, this is a prime time for biodiversity, as the slow change has given life ample opportunity to adjust to the new conditions, and it indeed revels in the fertility of wet warmth and plentiful shallow seas.

See Also

The Evolution of Life on Orwell
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Development Notes
Text by TSSL
Initially published on 17 November 2014.