Geoscience Reference
In-Depth Information
liths, or bio-geo-gastroliths. But either of those terms would be needlessly pedantic
andcausescienceeditorseverywheretoflinchandtwitchuncontrollably,especially
if these bio-geo-gastroliths are touted as “missing links” in a “living fossil.” And
we wouldn't want to do that.
Withbio-gastroliths,theanimalsmakethesethemselves,secretingmineralsin-
ternally and forming concretions in their body cavities. Sound strange? Not really.
If you've ever heard of gallstones or kidney stones, or been unfortunate enough to
haveexperiencedthemdirectly,thenyouknowaboutbio-gastroliths.Ioncelearned
moreaboutbio-gastrolithsandtheireffectsonhumanhealthwhilewaitinginahos-
pital emergency room with a fractured right radius, a consequence of a bicycle ac-
cident. Meanwhile, the patient lying next to me passed a kidney stone, which took
him about an hour or so. (Let's just say that after that, I stopped feeling so sorry
for myself.) Later, once the memory of his screams faded enough for me to think
more objectively, I read about kidney stones. These bio-gastroliths are deposits of
calcium oxalate, which form in people's kidneys as a result of calcium imbalances
and insufficient fluid intake. (Fortunately for many of us, myself included, regu-
lar beer drinking is excellent preventative medicine.) Gallstones, on the other hand,
are more organic than mineral and are normally composed of cholesterol, although
these can sometimes have calcium mixed in as well.
Remarkably, some crustaceans, such as marine crabs and freshwater crayfish,
secrete their own bio-gastroliths of calcium carbonate. The purpose of these secre-
tions is to absorb calcium from their exoskeletons just before they molt, then put
thatcalciumbackintonewexoskeletons.Hence,thesebio-gastrolithsarenotpatho-
logical but act more like tiny biochemical ATMs, from which these crustaceans can
deposit or withdraw calcium as needed. These concretions are not only in modern
crustaceans but also are preserved in the geologic record, trace fossils that let us
know about a former crab or crayfish presence.
In contrast, geo-gastroliths are rocks made outside of animals' bodies by nor-
mal geological processes—whether igneous, metamorphic, or sedimentary—but
