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Re: Biogeochemistry!



>I'm after some opinions regarding a paper by Bocherens et al. (1994).  It
>states that via the use of biogeochemical techniques on fossil plants and
>collagen from a hadrosaur, the presence of C4 (or CAM) plants is hard to
>dismiss.  If they were present then why do they not fossilise until Ma's
>down the track?
>
>Also, can anyone give me any references on the Proctor Lake Dinosaur Site,
>Texas; apart from Winkler & Murry (1989).
>
>
>References:
>
>Bocherens, H., Friis, E.M., Mariotti, A., & Pederson, K.R. (1994b).  Carbon
>Isotope Abundances in Mesozoic and Cenozoic Fossil Plants: Palaeoecological
>Implications.  Lethaia  26:347-58.
>
>Winkler, D.A., & Murry, P.A. (1989).  Paleoecology and Hypsilophodontid
>Behavior at the Proctor Lake Dinosaur Locality (Early Cretaceous), Texas. 
>In Farlow, J.O. (Ed.) Paleobiology of the Dinosaurs (pp. 55-61). 
>Geological Society of America, Special Paper 238.
>
>
>Thanks,
>
>
>Darren R. Grocke
>Dept. of Earth Sciences
>Monash University
>Clayton   Victoria      3168



        Since I am one of the authors of the quoted paper, I may as well
give some extra information connected to this article.
        First, I will just give a summary of the paper for the ones who
could not read it. The study is based on the analysis of the carbon
isotopic ratio (13C/12C) in fossilized plant fragments, ranging in age from
the Middle Triassic (around 240 Ma) to the Miocene (around 7 Ma). These
isotopic ratios have been shown to reflect environmental conditions and
photosynthesis type (C3, C4, CAM) in modern plants, and the data from the
paper strongly suggest that the analyzed fossil plants also have preserved
their original isotopic ratios. For those who are not familiar with the
photosynthetic types, C3-photosynthesis is used by almost all plants on
Earth, i.e. all the trees, and all the plants in temperate and cold areas.
C4-plants are grasses and sedges from hot and dry areas, such as savannas
and grasslands (Maize and sugar cane are C4-plants). CAM plants use both
photosynthetic types according to the environmental conditions, they are
mainly succulent plants. The 13C/12C values are very different in C3 and C4
plants. Actually it is the easiest way to determine the photosynthetic type
of a plant. 
        To come back to the paper itself, the goal was to investigate
isotopic ratios in fossil taxa with morphological adaptations to dryness
(such as thick cuticles), or found in deposits with evidence of dry
conditions (such as evaporites), in order to eventually find  evidence for
C4 photosynthesis during the geological times. All the measured values are
within the range of C3-plants, except for one locality in the latest
Cretaceous of Wyoming (Lance Formation). In that case, the organic matter
extracted from a dinosaur bone fragment ("Anatosaurus") had been analyzed
as well for the isotopic ratio. If this ratio is the one recorded by the
dinosaur during its life-time, the value suggest C4-plants in its diet.
However, there are still some discussion about the preservation of isotopic
ratios in organic matter that old, and more work is necessary to clear that
point.
        More generally, the fact that C4-plants may be present as far back
as the Cretaceous, but do not spread before the late Miocene (10 Ma) is not
impossible. One should not forget that the only evidence we have to analyze
is the fossilized fragments of plants, or paleosoils. C4-plants are
restricted today to dry environments, where fossilisation is very unlikely.
There is a strong preservation bias for plants from wet environments, i.e.
C3-plants. Today, all coals and peats have a C3 isotopic signature,
although we know that C4-plants are very abundant in savannas and
grasslands. Thus, the isotopic signature of fossil herbivores may give a
wider view of the ecological diversity around the locality where the fossil
has been found, since the living animal will collect plants coming from a
large area, where C4-plants may be present, but will not fossilize as
plants.

        I hope it helps a little.



Herve Bocherens

Laboratoire de Biogeochimie Isotopique
Case 120
Universite Pierre et Marie Curie
4 Place Jussieu
F-75252 PARIS Cedex 05 (FRANCE)

Tel: (1) 44 27 72 82
Fax: (1) 44 24 41 64