The Story of Life in 25 Fossils: Tales of Intrepid Fossil Hunters and the Wonders of Evolution

Trilobites tell me of ancient marine shores teeming with budding life, when silence was only broken by the wind, the breaking of the waves, or by the thunder of storms and volcanoes. The struggle of survival already had its toll in the seas, but only natural laws and events determined the fate of evolving life forms. No footprints were to be found on those shores, as life had not yet conquered land. Genocide had not been invented as yet, and the threat to life on Earth resided only with the comets and asteroids. All fossils are, in a way, time capsules that can transport our imagination to unseen shores, lost in the sea of eons that preceded us. The time of trilobites is unimaginably far away, and yet, with relatively little effort, we can dig out these messengers of our past and hold them in our hand. And if we can learn the language, we can read the message.

One of the most popular of all fossils for amateur collectors and professional paleontologists alike are the trilobites. These creatures lasted from 550 to 250 million years ago and, over those 300 million years, evolved more than 5000 genera and 15,000 species, all of which are now extinct (figure 4.1). They range from the tiny Acanthopleurella (barely 1 millimeter [0.04 inch] in length) to the giant Isotelus rex (more than 70 centimeters [2.3 feet] long). Since they are relatively easy to collect in many places, and extraordinarily abundant almost everywhere in beds of the Early Paleozoic (especially the Cambrian), they often become the core of many amateur fossil collections. Their wonderfully complex shapes, elaborate ornamentation, bizarre structures of the eyes and many other parts of their anatomy, and surprising features make them irresistible to most fossil collectors.

Even more important, trilobites were the first large shelled organisms on Earth. There is abundant evidence from the genetic divergence times of their close relatives that soft-shelled trilobites were around in the earliest Cambrian and developed mineralized shells in the Atdabanian, the third stage of the Early Cambrian (see figure 3.4). This may be because atmospheric oxygen levels were finally high enough that trilobites could crystallize calcite in their shells. Most of the creatures that preceded them either were soft-bodied, with no hard parts or shells, or had tiny and inconspicuous shells (chapters 2 and 3); therefore, they were fossilized only in environments with conditions that favored preservation, not decomposition (chapter 5). Not only did trilobites have a large complex shell made of the protein chitin (as do crabs, lobsters, shrimps, insects, spiders, scorpions, and all other arthropods), but this relatively soft and easily decayed shell was fortified by layers of the mineral calcite. Thus trilobites were much more likely to be fossilized than any other Cambrian creature, since they were one of the few groups with mineralized shells. The appearance of hard-shelled trilobites in the Atdabanian makes them overrepresented in the fossil record, and they give the false impression that there was a “Cambrian explosion” of life between the Tommotian and the Atdabanian (see figure 3.4). Instead, it was an “explosion” of animals with mineralized skeletons.

The “head” of most arthropods is called the cephalon (Greek for “head) in trilobites (figure 4.2A). It is usually a broad structure with two “cheek regions” on each side of a central lobe (“nose”) called the glabella. On each side of the glabella are typically two eyes. Some trilobites had tiny eyes or none at all, so they had limited vision or were blind; others had huge eyes that wrapped around and gave a 360-degree range of view to spot any predators. Many advanced trilobites had lenses made of two crystals of calcite, forming a doublet lens structure that corrected for spherical aberration in thick lenses. About 400 million years after trilobites evolved these features, they were reinvented by Christian Huygens, the great Dutch scientist, in the sixteenth century. Even more important, trilobites were probably the first creatures on Earth to have true eyes and to use visual clues to find food and avoid predators.

The cheek regions broke off from the central part of the cephalon (cranidium) during molting, so most trilobite fossils consist of just the center of the cephalon. A good trilobite specialist often needs only a cranidium to identify the species. There are great variations in the details of the eyes, the glabella, the shape of the cheeks, and the spines on the edge of the cephalon. On the front of well-preserved specimens are two antennae that trilobites used for feeling their way around in dark muddy waters. On the bottom is a plate that partially covered the mouth, with mouthparts that were used for sucking up food-rich mud and digesting the nutrients out of it (see figure 4.2B). Most trilobites were deposit feeders or mud grubbers.

The middle part of the body of a trilobite is called the thorax, as in most arthropods. In trilobites, the thorax is divided into segments, allowing the middle of the body to flex and curl as the animal moved or to roll up for protection. Each segment has two lobes on the sides (pleural lobes) and one that runs down the central axis (axial lobe). It is this side-by-side division of the body into three lobes that gives the group their name. Some trilobites have just two or three thoracic segments, so they were not very flexible and must have lain flat nearly all the time. Others have many segments, so they could roll up tightly into a ball to deter predators, such as the isopod crustaceans known as “roly-polies” or “sowbugs” do today. Well-preserved fossils show that beneath each thoracic segment was a pair of walking legs, and on each side a pair of feather-like gills attached to the base of the legs.

Through the rest of the Paleozoic, the trilobites were hammered by a series of mass extinction events (figure 4.4). They include the multiple minor extinctions in the Late Cambrian, when several pulses of disasters wiped out the diversity of trilobites, wave after wave. During the Ordovician, trilobites experienced their first encounter with large predators, probably gigantic nautiloids. Trilobites quickly became more specialized and easier to tell apart as they soon adopted a variety of shapes and lifestyles that made them less vulnerable to predation. These adaptations included burrowing (the smooth “snowplow” trilobites known as Asaphida or Illaenida), rolling up into a ball (the Calymenida), or becoming tiny (the Trinucleida, such as Cryptolithus, the thumbnail-size “lace collar” trilobite). Then came the Late Ordovician extinction (about 450 million years ago), and only a few lineages survived into the Silurian and Devonian. The final flourishing of trilobites occurred in the Devonian, when the complex-eyed Phacopida were common, along with large spiky trilobites like Terataspis (about 0.5 meter [1.5 feet] long). The Late Devonian extinctions at 375 million years ago and 357 million years ago wiped out all but one order of trilobites, the relatively small and simple Proetida, which persisted in the background for another 125 million years.

Finally, the trilobites disappeared during the great Permian extinction, some 250 million years ago, the largest mass extinction in Earth’s history, when 95 percent of all marine species vanished. This huge event wiped out not only the last of the stragglers among trilobites, but also the two dominant groups of Paleozoic corals (the tabulates and the rugosids), as well as the blastoids (relatives of the crinoids, or “sea lilies”) and the fusulinid foraminiferans (incredibly abundant protozoans with shells shaped like rice grains). There have been many controversies about what caused “the mother of all mass extinctions,” but current data indicate that the largest volcanic eruption in geological history, the Siberian lava flows, helped trigger an extremely rapid greenhouse climate that made the oceans too hot and acidic to support much life, and overcharged the atmosphere with too much carbon dioxide and not enough oxygen. These, along with some other catastrophic events, destroyed all but a tiny percentage of life on Earth.

A number of museums have trilobite fossils on display, although most do not show very good complete specimens of olenellids. Some that do include the Denver Museum of Nature and Science; Field Museum of Natural History, Chicago; Geology Museum, University of Wisconsin, Madison; and National Museum of Natural History, Smithsonian Institution, Washington, D.C.

Olenellids are so abundant at some localities around the world that they are easy to collect for yourself. Here are three famous and easy-to-reach localities in the United States. Consult fossil-collecting guidebooks and Web sites for more such areas.

◉ MARBLE MOUNTAINS, CALIFORNIA. Take Interstate 40 (either eastbound or westbound) to exit 78, Kelbaker Road; leave the interstate; and drive 1 mile south to the “T” junction. Turn left (east) and drive to the ghost town of Chambless along the National Trails Highway (former U.S. Route 66). Turn southeast on the road to Cadiz. After the paved road curves due east and just as the paved road goes due south to cross the railroad tracks, turn onto a dirt road on the left that goes due north. Drive about 1 mile north on the dirt road until you reach a junction with a well-traveled east–west dirt road, and then turn east. About half a mile along this road, you will see the dirt road heading northeast toward an old quarry. Follow this road as far as it is passable, and then hike up to the Latham Shale (the brown shale unit) below the gray cliff-forming Chambless Limestone. Look for old “glory holes” of serious collectors, and turn over the larger shale pieces. You will see many good cephala of every size, although complete trilobites are extremely rare. Two good Web sites are “Trilobites in the Marble Mountains, Mojave Desert, California” (http://inyo.coffeecup.com/site/latham/latham.html) and “Trilobites of the Latham Shale, California” (http://www.trilobites.info/CA.htm).

◉ EMIGRANT PASS, NOPAH RANGE, CALIFORNIA. Take Interstate 15 to Baker, California; leave the interstate; and drive north for 48 miles on California State Route 127 (Death Valley Road) to Old Spanish Trail. Turn right on Old Spanish Trail and proceed through Tecopa to Emigrant Pass. The exposure is just to the west of the summit of the pass on the south side of the road (GPS coordinates = 35.8856N, –116.0603W). Two good Web sites are “Trilobites in the Nopah Range, Inyo County, California” (http://inyo.coffeecup.com/site/cf/carfieldtrip.html) and “Ollenelid Trilobites at Emigrant Pass, Nopah Range, CA” (http://donaldkenney.x10.mx/SITES/CANOPAH/CANOPAH.HTM).

◉ OAK SPRING SUMMIT, LINCOLN COUNTY, NEVADA. From Caliente, Nevada, take U.S. Route 93 west for 10 miles, or east for 33 miles from the junction with Nevada State Route 375 and 318 (between Hiko and Ash Springs). Look for a turnoff on the north side of the highway, signaled by a prominent Bureau of Land Management sign that reads “Oak Springs Trilobite Site.” Turn north, drive along the dirt road, park in the gravel lot, and then hike up the Trilobite Trail from the trailhead across the sagebrush until you arrive at a flat area covered with pieces of the Pioche Shale. Turn over shale, and you will find many fine cephala, occasionally better specimens, of every age and size. A good Web site is “Oak Spring Summit” (http://tyra-rex.com/collecting/oaksprings.html).