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Re: Idle question on Cretaceous chronostratigraphy

The Cretaceous is the longest period of the Phanerozoic, yet also the
only one divided into only two epochs (Lower and Upper Cretaceous) in
the ICS chart. Is there a rational explanation for this somewhat
surprising state of affairs, or did it just "happen"?

Yay! Time for Geology 101 (something I teach often)! (Also: forgive me if this post parallels what Lindsay wrote -- her message on the archives got the dreaded "TRUNCATED" thing, so I couldn't see what it said!)

The divisions of the Geologic Time Scale are neither (a) wholly arbitrary nor (b) there for any sort of artifical "convenience." Each division (with one notable exception, the Jurassic-Cretaceous boundary) is supposed to be based on a relatively major faunal change -- the bigger the change, the bigger the division (smaller changes get Substage or Stage-level divisions, larger one Epoch, Period, or Eon divisions, sequentially). This is also why divisions of the time scale are not of uniform length -- these changes have not occurred regularly in geologic history. The Cretaceous as a Period is marked by few major turnovers -- just one big enough to warrant an Epoch-level division...hence, Early Cretaceous and Late Cretaceous (although as Dann mentioned, there have been enough references to an informal "middle Cretaceous," at least in the vertebrate literature, that one might be able to make a semi-plausible case for formally erecting one...except that the changes used in the record are typically those of marine invertebrates simply because their fossil record is both more abundant and has much higher resolution than the terrestrial record usually affords). Several smaller-scale changes allow the Cretaceous to be divided into numerous Epochs, though (e.g., Barremian, Aptian, Albian, Cenomanian, etc.).

More recently, other periodic geological events -- magnetic reversals, geochemistry, etc. -- have been used alongside the biostratigraphic data to calibrate the time scale, but it is fundamentally based on appearances and disappearances of various taxa.

There's nothing magical about many Periods having three (Lower, Middle, Upper) divisions -- some have four (e.g., Cambrian, Silurian, Neogene); the Cretaceous happens to have two.

This is understandable, since calling something around 100 million years old 'Late Early Cretaceous' or 'Early Late Cretaceous' is fairly cumbersome.

Agreed, though it's still correct to do so (although one would say "early Late Cretaceous" or "late Early Cretaceous" -- the capitalization is a clue to what's a descriptive adjective and what's part of a proper noun). Better than this, though, is just resorting to Stage names -- saying "Aptian-Cenomanian" beats "middle Cretaceous" in my book!

Jerry D. Harris
Director of Paleontology
Dixie State College
Science Building
225 South 700 East
St. George, UT  84770   USA
Phone: (435) 652-7758
Fax: (435) 656-4022
E-mail: jharris@dixie.edu
and     dinogami@gmail.com

"Trying to estimate the divergence times
of fungal, algal or prokaryotic groups on
the basis of a partial reptilian fossil and
protein sequences from mice and humans
is like trying to decipher Demotic Egyptian with
the help of an odometer and the Oxford
English Dictionary."
-- D. Graur & W. Martin (_Trends
in Genetics_ 20[2], 2004)