Now that I've reached that "ripe old age" of over 80, I find myself enjoying fossils more than I did years ago. Do you suppose someone's trying to tell me something? Be that as it may, for more years than I care to remember, I've heard the old axiom, "Some day, someone will find a trilobite in Oregon." Well, maybe.
One place to look is in the fossil-bearing Permian limestone of Coyote Butte near the Nevada border in southeast Oregon. "Hold on," you say, "We have a Coyote Butte along China Hat Road, southeast of Bend." Sorry, that's a cinder cone, no trilobites there, it's volcanic in origin and several millions of years to young.
The Coyote Butte near the Nevada border has yielded some fossil trilobites look-a-likes, but they're really chitons, (kite-uns), similar to what we see today alive-and-well on the Oregon Coast. The chitons are fossilized in rocks of the right age, but not even close to what a real trilobite is, like the one pictured above. That, oh best beloved, is the real thing; I dug it out of limestone in Utah in a BLM quarry open for public digging. And, if you want to find out how they got to Utah, read John McPhee's Basin and Range.
As you can see, trilobites have eyes with compound lenses, and there's also an axis in a central lobe running the length of the trilobite body. The legs of arthropods (which is what a trilobite is) may be either biramous (two-branched) or uniramous (several branched). Trilobites fall into the two-branched bunch. A trilobite's exoskeleton (skeleton on the outside, muscles on the inside, the reverse of you and me) is made of chitin; the same stuff as our fingernails. And as their name suggests, the trilobite's body is made of three separate body parts (but joined): head, thorax and abdomen, just like today's insects.
Some trilobite impostors can look enough like them to fool even an expert, like the water pennies, but they're really larvae of aquatic beetles. Isopods, which are actually crustaceans, can also resemble a trilobite. A huge, palm-sized deep-sea isopod has fooled enough people that some scientists use it as "proof" of living trilobites - despite the fact they are crustaceans.
Actually, trilobites comprise a complex and huge class of arthropods that resemble today's insects, spiders and other invertebrates with multiple legs, like horseshoe crabs, for example. Despite their extensive fossil record, however, extinct trilobites remain problematic within the arthropod phylum.
The Paleozoic (going back roughly 542 to 251 million years ago) is often called the "age of trilobites," at which time they radiated repeatedly, expanding in diversity and distribution, beginning with and after Cambrian times, but they, like so many other life forms on planet Earth, also suffered periodic declines in major extinction events.
The number of families of trilobites actually peaked in the late Cambrian when another extinction event removed many of them. The morphological diversity actually peaked at the end of the Ordovician, 440 million years ago, during a great ice age where ice sheets advanced to the equator, which brings us to the questions regarding the alleged climate change of today. Makes me wonder if we're next for extinction...
Regrettably, trilobites never truly recovered in the Carboniferous period, with but a handful of genera extant by the Permian, a time period that lasted from about 299 to 251 million years before the present. Failing to adapt to environmental changes led to their disappearance prior to yet another great mass extinction at the end of the Permian, and with all the extinctions, the age of the trilobite yielded to the age of the insect.
Darwin was confident in his conjecture that trilobites descended from one pre-Cambrian crustacean ancestor. But, the trilobite's position in the universal tree of life remains confounded, with debate remaining whether their closest cousins are, for example, a crustacean - the horseshoe crab - or among the spiders and scorpions of the present.
Despite setbacks during numerous Paleozoic extinction events, the trilobite lineage persisted for some 300 million years before finally becoming extinct at the end of the Permian period. So, if you go searching for trilobites in the Coyote Butte limestone, look at the base of the formation, it's older down there. Or, you can come along with me next summer to Utah; I know where the big ones are! But we'd better get on it, according to the Mayan calendar, and some people I know, there's a big event headed our way in 2012.