>On the other
>hand, dead shells generally give a pretty good idea of the
>local fauna.  Folks have noted the risk of some extraneous
>shell turning up, and there is some degree of transport
>between habitats as well, but the common dead shells
>almost certainly give a good picture of the local live fauna.

Er... well. Fossils can weather out and become mixed into Recent
faunas... such as a South American scallop, Austrochlamys natans,
which I dredged at about 600m depth in a Dunedin canyon.

Sealevels can rise and crust tectonically subside, resulting in
shallow or even land and fluvial shells and the surface they lie upon
becoming deeply submerged.

Also land and fluvial shells can be washed out to sea, at least in
part by unstable slopes.

I have found landsnails (Charopa and close relatives eg Mocella)
washed-up in windrows with what I think was a rain/storm kill, with
scissurellids and billions of tiny Gaimardia plus over 60 other spp.

The example which really brings this to attention is that I have
dredged Potamopyrgus (3 specimens) in a haul in which the dredge ran
upslope from 300-600m. I cannot tell whether these are the fluvial
P. antipodarum or the marginal-estuarine P. estuarinus, but it
matters little. Also the umboniine Antiosolarium egenum,
superabundant in the shallows close to the coast, is common in the
haul... though the shells all look pretty old. I think the hydrobiids
are washing down to the outer shelf and thence, with the trochids
(which may be Pleistocene relicts), slumping down into the upper
canyons.

I have also seen the outer-shelf species Calliostoma blacki,
Cominella nassoides and Provocator mirabilis wash up on beaches...
probably brought up by hermitcrabs, but also possibly by marine lab
bopat dredging activities (due to the latter, i would be surprised if
these species and others are not actually living in the harbor!).
--
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!
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Q: Because it reverses the logical flow of conversation.
A: Why is top posting frowned upon?