>One technical definition is to call everything Pleistocene and older
>(over 10,000 years) a fossil and younger but long-dead material
>subfossils.  However, telling the difference between a
>well-preserved specimen just over 10,000 years and a highly altered
>younger specimen may be difficult.  If the age is unknown, using the
>condition of the shell as a practical division makes sense.

Shells of 70 million years age can be in perfect preservation, so
condition is not necessarily an indicator; bones are even more
resistant. Generally speaking, subfossils will not be encased in hard
rock (though exceptions certainly can occur, and true fossils
commonly occur in utterly unconsolidated rocks).

>     Doesn't "fossil" indicate that the material makeup is replaced
>by some other mineral? If so, wouldn't a sub-fossil be a long-dead
>critter to which that process had not begun---or proceeded very far?

>Not necessarily; there are shells about 300 million years old that
>still have original aragonite, and original calcite is known back to
>about 450 million; some of these may also have traces of the organic
>components as well.  However, these do show some alteration from the
>original.  On the other hand, some fossils have no mineral at all
>and are just impressions where the original shell dissolved away.
>Only a few years in a poorly-buffered stream can be enough for a
>unionid shell to dissolve away.  Some modification in the mineralogy
>can also take place quickly, depending on the chemistry of the
>environment.

For sure. And fossil bone is normally still the original calcium
phosphate, often with organics (the cell-spaces and cancellation may
well be infilled with other minerals, but that's something else
entirely). The bone is often in good enough condition that it can be
examined for traces of strontuim and other isotpoes, which can yield
data on paleotemperatures and even icecap volume at the time the
animal died.

Chemistry in the animal's environment can affect mode of
preservation; so can subsequent conditions within the rock, which is
when shells are often dissolved by groundwater. If this happens
before the rock lithifies (hardens), if it ever does lithify, the
fossil is reduced to a matrix "ghost", often slightly phosphatized,
which may have internal and external details superimposed eg Latiarca
with muscle scars and hingeteeth impressed upon the radial ribbing
and the finer commarginal sculpture. If the rock has lithified before
solution the shell may be preserved as an ampty mold, or it may be
infilled by a mineral (calcite, silica, worst-case being marcasite,
which breaks down nastily and messily).

>  >    NOTE: I hope you tell the proper authorities at Alabama that
>you can still kick extra points.<
>
>Naming a species for Bear Bryant might help funding around here.  In
>fact, the collections building is named for Mrs. Bryant.  However,
>fans of fossil mollusks may be more familiar with the namesakes of
>Tuomey Hall and Smith Hall.

What about Quantassaurus and Atlascopcosaurus? Dermochelys
terrypratchetti? There's a trend to paid-for or bribing names...
--
Andrew Grebneff
Dunedin, New Zealand
64 (3) 473-8863
<[log in to unmask]>
Fossil preparator
Seashell, Macintosh & VW/Toyota van nut
I want your sinistral gastropods!
-----------------------
Q: Because it reverses the logical flow of conversation.
A: Why is top posting frowned upon?