RE: Age division?

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Jordan Mallon
>
> I've heard things like "_T. rex_ was from the Late
> Maastrichtian," but does
> this really mean anything quantitatively?  Are geological ages separated
> according to time like that?  (i.e., would "Late Maastrichtian" mean
> something like 68-65 Ma?)  I'm just looking to put some numbers to these
> distinctions.  I get the impression that anything past the Age
> category is
> fairly subjective.

Well, Mesozoic Stage/Age units have informal substages, based on particular
magnetic chronozone or biochronozone units, sometimes these are informal.
However, "informal" doesn't equal "subjective": these can be pretty secure
as being in the earlier part or later part of the unit, even if
international stratigraphic committees don't officially recognize a
particular boundary.

And PLEASE don't make the (admittedly very common) mistake of thinking that
the geochronological units are defined by numerical time (i.e., 65 Ma)!
Absolute time represents *measurements* of units defined on other data, and
as such are subject to error bars of various sizes.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796