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Cleveland-Lloyd, Dry Mesa, Bakker (was RE: Tarbosaurus?)



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Tracy Ford
>
> Cleveland-Lloyd is NOT a flood plain deposit. It is a seep or
> pond in which
> dinosaurs got stuck in, then the theropods went for a seemingly easy meal
> and they themselves got stuck. This was an ongoing process, unlike a
> floodplain that would leave skeletons in a single layer to be
> covered up by
> the next flood.

Actually, Sue Ann Bilbey did her disseration (at Univ. Utah) on the geology
and taphonomy of the Cleveland-Lloyd.  These results were summarized in the
Morrison double volume of Modern Geology:
Bilbey, S.A.  1998.  Cleveland-Lloyd dinosaur quarry - age, stratigraphy and
depositional environments.  Modern Geology 222: 87-120.

Her conclusion as to the dinosaur-bearing part of the quarry: The layer is a
lake deposit, and there is strong indication of volcaniclastic sediment.
There is no single clear explanation for the death assemblage, however.
Dodson et al.'s idea of it being an oxbow lake is not supported by the
sedimentological evidence: the layers immediately below the lake are
paleosols.  Periodic minor flooding was not large enough to have been
catastrophic. Possibly an influx of the volcaniclastic debries might have
temporarily absorbed the water of the lake, leaving a restricted body.
There is evidence of scavenging and trampling of the carcasses.

Incidentally, the next paper in the volume is a study of Dry Mesa:
Richmond, D. R. & T. H. Morris.  Stratigraphy and cataclysmic deposition of
the Dry Mesa dinosaur quarry, Mesa County, Colorado.  Modern Geology 22:
121-143.

This site seems to be the result of an extreme drought and subsequent flash
flood.

Dinogeorge wrote:
>Bakker's thesis that predator-prey ratios can provide a handle on dinosaur
>metabolism is perfectly good, but everybody disregards his argument because
>of taphonomic "analysis paralysis." They can't get by the taphonomy and
>collection biases to see the underlying truth of his argument. (Never
thought
>I'd be defending Bakker's ideas in an open forum, but here I am.) The fact
>that predator-prey ratios on the whole are overwhelmingly on the side of
>endothermic dinosaurs despite the taphonomic differences at individual
>localities seems to have fallen by the wayside. Bakker pointed out a simple
>pattern in the dinosaur fossil record and a good reason for it, but
>old-fashioned obscurantism has defeated it.

To be fair, there are other reasons why the predator:prey ratio argument
hasn't been as widely accepted.  The first three are operational, and the
most series.  The other two are paleoecological, and less important but have
been raised as concerns.

I) The raw data were never published.  We have the plots, but we don't have
(among other things) the number of specimens represented in each site: this
makes a big difference in the confidence in the stats.  There IS a
manuscript out there, which some of the dino paleoecologists (such as Jim
Farlow) have seen and reported on, but that manuscript was never published.

II) The data themselves were based on museum collections.  There are other
biases than preservation in museum collection: mainly, collection bias.
Some taxa might be preferentially collected (i.e., theropods), others (in
particular, anything common) might be preferentially left in the field and
thus under-represented in museum collections.  Other biases include
logistical (what is easier to get out, how to get "more bone for the buck"
as it were) and personal/professional interest (for example, a team in
Mongolia might leave ceratopsians and ornithopods and tyrannosaurids in the
field, while preferentially collecting small coelurosaurs and lizards and
mammals).

III) The mass estimates.  As Bakker rightly pointed out, it isn't the number
of predators vs. the number of prey that's important; its the biomass of
predators vs. the biomass of prey that is significant.  However, good mass
estimates are only obtainable from relatively complete skeletons, and the
best of these estimates have only been done in the last few years (by Paul,
Henderson, and others).  Granted, you could then scale other fragmentary
specimens relative to these.

IV) The trophic relationships.  Okay, not so bad for the level of analysis
that is being worked on, but if it turned out (for example) that
_Ceratosaurus_ only fed on _Camptosaurus_ (extremely unlikely, I grant you),
that would greatly skew the results.

V) The fact that predator:prey ratios tell you nothing about the physiology
of the herbivores.  Now it might be possible to do the equivalent study for
herbivores (biomass of herbivores vs. productivity of the vegetation), but
this is even more problematic: herbivores are often selective feeders, we
don't know the ranges over which any of these individuals fed, and we
honestly don't know as much about productivity of many of the plant groups
of interest (although the latter is being worked on, specifically in the
context of paleoecology).

I do agree, however, it is intriguing that (at least as reported) ratios for
fossil dinosaurian assemblages are comparable to fossil mammalian
assemblages.  In fact, given the phenomenal growth in our coverage of fossil
communities in the quarter century since Bakker's studies, a new attempt (or
rather series of attempts) at the same type of analysis might be extremely
profitable.

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): 301-314-7843