Biomineralization

[Mike Simmons <gmi267@xxxxxxxxxx>]

I'm wondering if subscribers to PaleoNet or Micropal might be able to 
help and advise me. 

Along with colleagues at University College London, the Natural 
History Museum and Moscow State University, I'm looking at the 
earliest planktonic foraminfera (Middle Jurassic - Early Cretaceous). 
Taxonomic revision and synthesis is well in hand, but in the course 
of our work, we've been analysing the composition of the tests of 
these forms. Following on from observations from Russian workers, 
we're able to confirm that (some) of these taxa had aragonitic tests. 
Others are calcitic, but this may well we secondary (work is ongoing). 
As far as I know (correct me if I'm wrong) all modern planktonic 
foraminifera are primary calcite as are Late Cretaceous and Tertiary 
forms. 

My major question here is what governs if an organism secretes a 
calcitic or aragonitic shell? I've tried to investigate this topic but so 
far all the references I've seen are vague. Perhaps this is an area in 
which palaeontologists should be talking more with cell biologists? 
Maybe someone out there is working on this problem or can point me 
in the direction of someone who is. Is there an obvious reference I 
have missed?

There are some questions of taxonomy and phylogeny. For example 
outside the planktonic foraminifera there are examples of calcitic and 
aragonitic homeomorphs - Trocholina (aragonite) and Neotrocholina 
(calcite) in the benthic foraminifera; Salpingoporella (aragonite) and 
Hensonella (calcite) in the dasyclad algae. Should these forms really 
be seperated?

Sorry for such a long posting, but if anyone can help I'd appreciate it. 
It seems to me that the issue of biominealization is a topic worthy of 
more study in all aspects of palaeontology not just my little bugs! 

Thanks,

Mike

Mike Simmons
University of Aberdeen
m.d.simmons@abdn.ac.uk