>I have two people with good questions requiring help.
>
> 1. Kathy: fossil vs bleached shell:
>http://www.manandmollusc.net/Mystery_shell_pages/kathy-mystery.html
How do you know if a shell you have is just a shell, worn and
weathered, or if it is a fossil shell? I would assume that a shell
that has obviously changed into another material like stone or
mineral or filled with or part of a rock hard sediment is a fossil.
Those types of fossils are obvious. But I have seen many internet
listings for "fossil shells" that just look chalky, worn, and faded.
So what makes these fossils? How do I distinuish them from old, over
bleached shells you find on the beach? For example, this website
lists this as a fossil. It looks like and old worn shell to me.
Lovely; but what makes it a fossil??
I have a number of shells like that in my personal collection and
would love to know if they would be considered fossils.
What about coral? Isn't all coral technically a fossil? I also have a
lot of coral that I found inland near caves, far from the water in
Aruba. I would love to know if they are fossils or not.
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Many Cenozoic molluscan fossils (ie those under 65 million years old)
are original shell material. If they occur in rock being eroded by
the sea, you may not be able to tell whether they are Recent or
fossil unless you know the fauna(s). There is a locality about 100km
north of here where early Miocene fossils are washing out of a beach
outcrop and others are washing in from Pliocene rocks on the shallow
seabed... so the beach has Miocene, Pliocene and Recent shells
washing-up... and the fossils look as good as the Recent ones.
To be a fossil a shell must have been buried in a rock unit as the
unit was being formed... the rock can be sedimentary (sandstone,
limestone, musdtone etc), volcanosedimentary (airfall ash deposits ie
tuffs) or volcanic (lava flows). Tuffs can have really good mollusc
faunas! The rock need not be hard... indeed, it can be loose
(unconsolidated). I guess, though there is no set rule, the shells
should be fairly old... say over 500,000 years. However crabs in
burrows which are hardening to concretions in harbor sediments near
here right now would have to be considered fossils, even though they
are only a few years old. And a shell which died 1 million years ago
and has been lying loose on the seafloor ever since is not a fossil
(these occur off the SE New Zealand shore too).
Coral skeletons are biogenic rocks... and old uplifted reefs can
definitely be considered to be fossil. These occur in the Caribbean
and SE Asia.
--
Andrew Grebneff
Dunedin
New Zealand
Fossil preparator
Seashell, Macintosh, VW/Toyota van nut
‚ Opinions stated are mine, not those of Otago University
"There is water at the bottom of the ocean" - Talking Heads
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