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Re: The Time Scale (was RE: Land Plants Origins Pushed Back)



James,
I agree with Tom Holtz completely that the lower boundary of the Cambrian (beginning of the Phanerozoic) is set, and we shouldn't be messing around with that. That would be a horribly confusing can of worms to open up.
This is especially true as more and more bilateral metazoan groups are found in the Late Precambrian (echinoderms, sachitids, brachiopods, hyoliths, various arthropods, various "worms", and so forth). The important thing to recognize is that the term "Cambrian Explosion" is a confusing misnomer, since it was almost certainly more of an "explosion of hard parts", not the phyletic explosion it is often made out to be.
Personally I would like to rename the phyletic diversification of metazoans something like "The Vendian Expansion" (perhaps with an Ediacaran acceleration as it approached the Cambrian)----and the relative "explosion" of hard parts appeared here and there "phyletically scattered" across an already established and diverse tree of metazoans.
Whether there was a concurrent expansion of bryophytes on land during the Vendian remains to be seen (too bad they didn't evolve harder parts as well, but there would have been no need to, since there were no metazoans on land yet to eat on them).
-------Ken Kinman
P.S. Although many acritarchs are dinoflagellates, many others are apparently chlorophytes (green algae). I'll try to find out if both types are used in Precambrian stratigraphy, and will let you know offlist.
*******************************************
From: "Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>
Reply-To: tholtz@geol.umd.edu
To: <Apollo@MLink.net>, "Dinosaur Mailing List" <dinosaur@usc.edu>
Subject: The Time Scale (was RE: Land Plants Origins Pushed Back)
Date: Thu, 16 Aug 2001 14:54:09 -0400

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> JAMES ARONIS
>
> Yes, I was referring to the geologic time scale. I was
> contemplating that the
> discovery of indisputable evidence for the existence of land
> plants predating the
> advent of the Paleozoic era, might compel a restructuring of the
> classic time scale
> we all know. A possible alteration would be to push back the
> Cambrian period to at
> least 700 MY. This would reflect the new findings. Of course I'm
> no geologist either,
> and I'm certain that there would be a host of other
> considerations determining the
> justification of such a restructure. (There is a greater emphasis
> in the evolution of
> animal life as opposed to plant life in the geologic time scale,
> for one thing.)

Some things to clear up:
Although index fossils were crucial to the development of interregional
correlations, and two ranks in the scale (Eon and Era) derive their names
from animal life, the boundaries of the geologic time scale are time
boundaries.  They are not meant to reflect major episodes in the history of
life per se.  For example, early geologists did not place the
Permian-Triassic boundary because they recognized that this was a major
turning point in the divesity of life, but only that there was a change in
the types of fossils used as markers.

There are multinational commissions devoted to clarifying the various
stratigraphic boundaries, selecting criteria and type sections. The base of
the Cambrian uses the earliest appearance of the trace fossil Trycophycus
pedum; not because the latter represents some hugely significant innovation
in the history of life (although it could be argued it does), but because it
is easy to recognize and is widespread. Now that the base of the Cambrian
(and hence the Paleozoic Era and Phanerozoic Eon) is set, discovery of
earlier trilobites or archaeocyathids or land plants will not be sufficient
reason for moving that boundary.


For an exceedingly brief description of the geologic time scale, see
http://www.geol.umd.edu/~tholtz/G102/102Lec09.htm. For more detail, check
out most any historical geology or stratigraphy text book.


So, to get back to the chase: the base of the Cambrian (and thus the
Paleozoic, and thus the Phanerozoic) is set, and will not move with these
new discoveries.

Also, to address Kinman's uncertainty: there is a LOT of work on the
Precambrian time scale, with particularly detailed work in the
Neoproterozoic Era (the Era just prior to the Paleozoic).  Acritarchs
(calcareous microfossils possibly representing resting stages of
dinoflagellates) are major index fossils for the latter Precambrian.

Hope this helps.

              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



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