Re: what did the soft parts on trilobites do?

The preservation of soft parts in ancient organisms is very exciting.  We 
can see things about them that people once did not think would ever be 
possible.  The rare preservation of soft parts in trilobites is generally 
limited to appendages (legs, antennae, a few other things). There are no 
known living trilobites, but trilobites are arthropods and they have many 
living relatives: insects, crabs and shrimp, spiders, and so on.  All 
arthropods have jointed limbs that bear very small hairs. The shapes their 
limbs take are strongly affected by what they use them for.  For example, 
the big claws that some crabs display are obviously for fighting, opening 
things that are hard to open, and activities like that.  By contrast, 
delicate limbs that go down both sides of the body and have more or less a 
common shape are almost always legs used for walking. Centipedes, 
cockroaches, and spiders all have walking legs of this kind. Fossil 
trilobites that have preserved soft parts usually show the ends of legs 
sticking out from one or both sides of the shell (exoskeleton) and these 
legs look very much like the walking legs of living arthropods.  That is 
how we interpret their function.  

Some fossil trilobites show very slender, long, straight appendages 
sticking out from under the front of the "head" (cephalon).  These look 
like the antennae of modern arthropods and we interpret them to have been 
similar to modern antennae.  Antennae are generally used for sensing the 
environment and trilobites probably used their antennae the same way.  The 
whiskers of cats are made quite differently but they serve a similar 
function.  

Preservation of soft parts in trilobites is an excellent example of how 
paleontologists can compare modern organisms to ancient ones and understand 
something about how the ancient creatures lived.  If you want to learn more 
about trilobites you could start by visiting the web sites of major natural 
history museums.  Type in trilobites in the search function and you will 
probably find some fascinating things.

David C. Kopaska-Merkel
Geological Survey of Alabama
box 869999
Tuscaloosa, AL 35486-6999