Elizabeth, you'll be amused by this one. I haven't seen gypsum on a fossil
or archaeo mammal skull, but years ago when out prospecting in the Idaho
desert ("fossil Lake Idaho" beds, Plio-Pleistocene in age) I came across a
whole horizon of bioturbated sediment composed of worm poop, each discrete
little ball nicely cemented to the rest by gypsum.

I interpreted this as an effect of groundwater flux, i.e. the water table
moving up and down through the very permeable sand of the particular
formation. How the gypsum came to be particularly attracted to the worm
poop, i.e. biogenic material, I think is by the same mechanism, called
'adsorption', which when you have groundwater that is rich in silica
rather than gypsum, causes permineralization by molecular replacement in a
wide variety of fossils.

This is all set forth in a very old paper of mine, published in 1976 if I
remember right, in Paleo-Cubed. The authors are Martin and Bennett and it
was on the so-called 'daemonelices' or Devil's Corkscrews, which are
actually the burrows of the desert-adapted beaver Paleocastor fossor.

The process is, the beaver digs himself a spiralling stairwell down to the
groundwater table -- which in the desert is liable to be ten or more feet
straight down -- so that he can have the same kind of water-trap safety
entrance to his den that an ordinary stream-dwelling beaver creates when
he builds his dam in a lake, i.e. he lives where the water-table is
exposed above the ground surface -- otherwise there's no difference.

Because evaporation of the groundwater keeps the corkscrew 'stairwell'
continually humid, plant roots are as magnetically attracted to it as
Cottonwood roots are to your sewer line, and if the beaver did not
maintain his corkscrew by continually nipping off invading roots, the
thing would soon clog up just like the lead-line to an old septic tank.

Obviously when the beaver and all his relatives came to die, the corkscrew
ceased to be maintained. By that time, the walls of the corkscrew would
have become thick with roots, so that almost, the entire wall of the
corkscrew would be 'made of' roots. But as soon as the thing was not
maintained, the roots would grow across and entirely fill it in. This
produced not a hollow spiral pipe but a solid corkscrew (they're called
'Devil's Corkscrews' because when the Mormon immigrants saw them exposed
in bluffcuts, they swore it was the marks made when the Devil and his dark
angels were hurled out of heaven -- evidently they were sure that God puts
some topspin on the ball when He throws things).

The last step in the process then occurred, by which silica in the
groundwater comes out of solution (silica is not very soluble, and
although gypsum is more soluble than silicon dioxide, it is still easy to
pull that salt out too) and 'adsorbs' to the root molecules. With
thousands of years of periodic silica baths with annual or bi-annual
fluctuations in the height of the groundwater table, all 'adsorption'
sites get taken and then, as the lignin which originally structured the
root degrades, the silica molecules start to bump the biogenic molecules
and one by one replace them. In teeth of course the substructure is not
lignin but collagen, which in fact is an even stronger attractant to
minerals dissolved in groundwater than lignin is.

Hope this is somewhat helpful. Cheers -- Deb Bennett

> Hello all. I wonder if anyone has even come across gypsum deposits on
> domestic animal teeth? Sandwiched between upper and lower teeth on a
> complete skull.
>
> Our current assumption is that this is a post-depositional issue but given
> the context of recovery, we are wondering about alternative explanations.
>
> Any thoughts and ideas are welcome.
> Thank you!
>
> Elizabeth R. Arnold, PhD
> Associate Professor
> Department of Anthropology
> 225 Lake Michigan Hall
> Grand Valley State University
> Allendale, MI 49401
> U.S.A.
> Office: (616) 331 8936<tel:%28616%29%20331%208936>
> Fax: (616) 331 2328<tel:%28616%29%20331%202328>
>
> www.gvsu.edu/anthropology/<http://www.gvsu.edu/anthropology/>
>