Why are geological periods getting shorter?

[Tom Mazanec]

From Wikipedia:
Name Duration (My)
Siderian 200
Rhyacian 250
Orosirian 250
Statherian 200
Calymmian 200
Ectasian 200
Stenian 200
Tonian 150
Cryogenian 215
Ediacaran 93
Cambrian 53.7
Ordovician 44.6
Silurian 27.7
Devonian 56.8
Carboniferous 60.2
Permian 48.0
Triassic 51.4
Jurassic 54.1
Cretacious 80.0
Paleogene 42.47
Neogene 20.442
Quartenary 2.588 so far
One possibility might be myopia...the details blurring together in early time. But then why were durations like the Rhyacian and Orosirian made Periods instead of eras? Or advanced life might evolve faster, so that geologic time is actually speeding up. Or perhaps some thing I did not think of is shortening the durations.
Thoughts?

[Jeff Root]

I never heard of the first eight periods on your list before.
They must have been added relatively recently.

-- Jeff, in Minneapolis

[Ara Pacis]

The farther back you go, the less we may be able to discern as a means of differentiation. As the tree of life emerged and spread out change probably took more time, or looks that way because of limitations in their ability to be fossilized. A lot of the recent stuff is probably based in other forms of data that still exist from that time, such as ice sheets. And some of it is probable about hominid relevance.

[grapes]

Yeah, it doesn't look so much like its getting shorter as there are just two different frames--the pre-cambrian, where the periods are about 200My, and since, when the periods are about a fourth of that.

[Romanus]

1.) Note that, when you get down to it, the geologic time scale is almost entirely based on biostratigraphy, not tectonic events or what the atmosphere was like, etc.
2.) The Precambrian featured (relatively) little biological change, so the differentiation is necessarily broad.
3.) The fossil / geologic record for older periods is poorer to begin with, which also requires a broader brush.

Boom! The Cambrian explosion, and the beginning of the Phanerozic. Now we have:

4.) Much more rapid faunal turnover of macroscopic organisms.
5.) More abundant fossil evidence from terrestrial and marine environments.
6.) Superior biostratigraphic data which leads to finer geologic divisions.
7.) More preserved rocks, which lead to finer geologic divisions, and so on.

In short: The periods are shorter because we're better at seeing what changed, and thus requiring finer distinction; we know a lot more about what happened over the 23 million years of the Neogene than we do over the 2 billion years of the Proterozoic.