What is the age of the oldest known non-permineralized shell?

[Eye in the Sky]

Recently, a group of paleontologists were having a discussion regarding fossil shells after a woman produced one for identification. Discussion turned to the difference between a shell and a fossil shell (that being a shell which has permineralized). There was debate as to whether or not there are any shells that are older than any fossil shells in the general paleo record. One of the paleontologists knows a shell expert and she reported back that the oldest known shell is approximately 18,000 years old and that all shells older than that have permineralized. However, some found this difficult to believe, as shell material, being mostly calcium carbonate, should be able to survive for very long periods in non-acidic deposition environments. Can anyone confirm this? Thanks!

[Ludwigia]

I don't by any means profess to know the answer here, but I do know that shells can be composed of two different versions of calcium carbonate. Aragonite is the one mostly used by molluscs and since this is relatively unstable, it tends to become permineralized into calcite within a geologically short period of time. However, I do know that the shells of most oysters, for instance, are composed of the much more stable version calcite, so might it not be possible, since oysters go back quite a long way, that their substance may have remained unchanged for much longer than 18,000 years?

[Eye in the Sky]

In Feburary of this year it was widely reported that Archeologists had discovered the oldest known seashell instrument, an 18,000 year-old conch shell that had been previously languishing in a museum collection. It had been found in a cave known to shelter hunter-gatherers in southern France. But that implies that conch shells can be at least that old if preserved in a cave environment. It would be difficult to believe that this is one of the oldest known shells, if indeed they preserved well in cave environments.

[Oxytropidoceras]

Some calcitic and phosphatic brachiopods from early Paleozoic, late Cambrian and Ordovician, strata have been found to have remained relatively unaltered. Because of their basically unaltered state, they have been used to reconstruct temperature and seawater stable isotope fluctuations. For examples, go see:

 

Bergmann, K.D., Finnegan, S., Creel, R., Eiler, J.M., Hughes, N.C., Popov, L.E. and Fischer, W.W., 2018. A paired apatite and calcite clumped isotope thermometry approach to estimating Cambro-Ordovician seawater temperatures and isotopic composition. Geochimica et Cosmochimica Acta, 224, pp.18-41.

 

and

 

Robles, M., 2012. Geochemical and Taphonomic Analysis of Very Well-Preserved Late-Middle Cambrian Lingulid Brachiopods From Laurentia. Master's thesis, University of California, Riverside.

 

An example of relatively unaltered Silurian brachiopods is:

 

Azmy, K., Veizer, J., Bassett, M.G. and Copper, P., 1998. Oxygen and carbon isotopic composition of Silurian brachiopods: implications for coeval seawater and glaciations. Geological Society of America Bulletin, 110(11), pp.1499-1512.

 

A general discusssion of both the preservation and alteration of calcitic shells through time is:

 

Brand, U. and Morrison, J., 1987. Paleoscene# 6. Biogeochemistry of fossil marine invertebrates. Geoscience Canada, 14(2), pp.85-107.

 

Yours,

 

Paul H.

 

Edited December 19, 2021 by Oxytropidoceras
corrected grammar