Olenimorph Trilobite

Another piece for ArtEvolved.
One group of fossil trilobites, the Olenina, include several species from rock layers that were once sulfur-rich deep sea habitats. Modern-day animals living in these kinds of habitats often have symbiotic relationships with bacteria that use sulfur to create foodstuffs for themselves and their hosts. It's quite possible that deep-dwelling Olenids had similar relationships. Further evidence for this symbiosis can be found in the large gills found in some well-preserved fossils, since gills are often where the sulfur-eating bacteria live in modern organisms. These fossils frequently also have an increased number of body segments (and thus an increased number of gills) as well as highly-reduced mouthparts (since most of their nutrition would come from the bacteria).

Here I've depicted one such symbiotic trilobite crawling over the sea floor near a colony of prehistoric tube-worms. The brown creatures in the foreground are brachiopods, animals which resemble, but are completely unrelated to, clams, mussels and other bivalves. The blue fish swimming overhead are Ctenurella, a species of placoderm.

Most of this information came from Samuel Gon's excellent trilobite website: The Orders of Trilobites [link]