Re: How diefferent are they from Trilobites?

On Jun 4, 6:54 pm, Lorentz <drosen0...@xxxxxxxxx> wrote:
Both trilobites and horshoe crabs are arthropods. In that they
probably are related.

For one thing trilobytes had eyes with lenses of quartz crystal.
Completely unique as far as anyone knows.

I assume that you mean the coverings of the eyes, not the soft parts
of the eyes which are made of pretty much the same material as all
living eyes. Quartz crystal isn't what a living cell is made of. I
assume that you mean that the fossil eyes are made of a quartz-like
material. But that wouldn't have included the internal structure.

I don't think that trilobites had quartz crystal eye coverings.
The trilobite shells we have today are petrified or otherwise replaced
by minerals or casts filled with materials. I suspect that the
fossilized eye coverings that you are talking about (quartz?) may be
merely mineral silica, replacing the original eye-coverings. The
original eye covering were probably chiton, a carbohydrate-enriched
material. Chiton is what the shells of most current arthropods are
made of. I have heard no evidence that trilobites had shells that were
significantly different.

I road home tonight expecting that I was going to have to
humiliate myself before Geoff and admit that I didn't (and
still don't really) understand the difference between quartz
and calcite, but NO!!, lucky me, I get to skewer someone else
instead :-)

From:

http://www.fossilmuseum.net/Evolution/TrilobiteArmsRace.htm

"Though we have no knowledge of the soft-tissue components of
the trilobite's visual systems, we do know that the lenses of
the eyes of living trilobites were unique, being comprised of
inorganic calcite, or as Levi-Setti (1995) calls them, "Crystal
Eyes". There is no other known occurrence of calcite eyes in
the fossil record. This composition has also enabled the
trilobite eyes to survive as exquisite fossils dating back to
the earliest Cambrian."


Still, I seriously doubt that horshoe crabs are descendents of
trilobites. Horseshoe crabs don't have the three-lobed left to right
pattern that trilobites had. Trilobite means three lobed, get it?
Horseshoe crab shells look pretty unsegmented to me, going from the
left to the right. They don't seem to have lobes at all. The split
between trilobites and other arthropods could have been much earlier,
perhaps in the preCambrian. But other scientists, studying the issue
for real, may think otherwise.

I don't remember the details, and am too lazy to look them up
but Richard Fortey the trilobite person from the Natural
History Museum in London expressed basically the same opinion
in his book "Trilobite - An EYEwitness to evolution". Although
he does say that Horseshoe crabs are the nearest living
relative of the Trilobite. He ate one in Thailand to be closer
to the creature he so loves - an act he regrets to this day -
they taste awful.

The book however is a very good read.

People keep all sorts of arthropods as pets. Crabs, crustacean,
shrimp are all part of some peoples aquariums. Cleaning shrimp are
invaluable in maintaining some tanks. I strongly suspect analogous
behavior was found in some trilobites, although unfortunately cleaning
behavior doesn't fossilize well. I suppose a living trilobite would
make a novel pet. Did you check Ebay?

Cordially;

Friar Broccoli
Robert Keith Elias, Quebec, Canada Email: EliasRK (of) gmail * com
Best programmer's & all purpose text editor: http://www.semware.com

--------- I consider ALL arguments in support of my views ---------

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