I was interested in the comments about Sr/Ca ratios in fossils and
stable isotope work in general. Andrew Millard suggested that there are
difficulties in interpreting the signals. I may not be up to date on
this but I remember that sequential washes were used to remove the
diagenic signal and the remaining signal was taken to be an indication
of the biogenic ratios. This seemed to make sense because there were
differences between definte herbivores and definite carnivores from the
same deposits (presumably subjected to the same diagenic regime). This
leaves the possibility that bones are entering the deposit with
different signals based less on their trophic level but more on
differences between the background levels of trace elements/ rare
isotopes between the localities where they 'grew' their bone/ laid down
the mineral signal; i.e. a suprious signal ante-mortem, or there are
some other post-mortem processes such as different bone chemistry
unrelated to diet leading to a 'false' signal in the fossils even after
all the washes.  Both these 'spolier' hypotheses seems unlikely and in
any case, the first one would show up with more sampling... are their
grounds for the second?  I would love to know more as it is an
intriguing line of evidence. Can Dr Millard, (or anyone else) fill us in
or cite some of the up to date references so we (I) can read some of the
criticism of this provoking work.

Mark

--
Mark Leney
New College
University of Oxford