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These denizens of the Era seas were surprisingly diverse

Richard A. Fortey

D. W. Miller rugose



Phacops Koneprusia


Cyphaspis Erbenochile

Figure 1. Ancient seafloor of what is now was host to an odd menagerie of trilobites during the Period, more than 350 mil- lion ago. During their 270 million- reign in the Earth’s seas, the trilobites inhabited a broad range of niches as predators, , particle feeders and filter feeders. Some scuttled on the seafloor or swam for short bursts, whereas others cruised at various depths in the wa- ter column. The last died shortly before the great , about 250 million years ago.

446 American Scientist, Volume 92 © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected] f you had been able to scuba dive ton made of the mineral . Not Where trilobite limbs have been pre- I during the Period, some that the whole outside of the body was served in the state it is usually 450 million years ago, you would have calcified—unlike living and lob- because they were coated with a min- seen at once that the seas swarmed with sters, the limbs of trilobites never be- eral—such as an iron or ap- trilobites. A few trilobites were as large came coated with a calcitic crust. And atite—before they had a chance to de- as dinner plates, many more were the so of trilobite legs are exceed- cay. The mineral film remained behind size of modern shrimp, and yet others ingly rare. Instead, it was just their were smaller than peas. They lived al- backs (their dorsal surfaces) that be- Richard A. Fortey is a research scientist at the most everywhere, from shallow waters came covered in a protective shield, Natural History Museum, London, and visiting to deep environments beyond the reach which was tucked around the edges of professor of at the University of Ox- of light. There were spiny ones like fully the in a fold called the doublure. ford. His current research includes molecular in- laden pincushions and smooth ones The three lobes that give trilobites their vestigations into the of the evo- looking rather like large pill bugs. Some name comprise a conspicuous (and lutionary “explosion,” studies on the of carried strange colander-like brims, un- usually convex) axis flanked on either vision and the reconstruction of ancient like any animal living today. Many had side by pleural areas, which are turned continents. In 2003, he received the Lewis Thomas large and obvious ; others again down laterally. Trilobites are also di- Prize from Rockefeller University, which honors were blind. vided crosswise into a (cephalon) scientists for their literary achievements. He is the author of Life: A Natural History of the First Their wonderful range of shapes bearing the eyes, a flexible thorax com- Four Billion Years of Life on Earth, which was suggests that the trilobites must have prising a number of articulated seg- named as one of the 11 “books of the year” in 1998 occupied many different ecological ments and a tail () formed of by the Times. Address: Department of roles. We have a good fossil record of several fused segments. On this com- , The Natural History Museum, them because they were the first paratively simple theme the trilobites Cromwell Road, London, SW7 5BD, United King- to secrete a hard exoskele- played a host of variations. dom. Internet: [email protected] www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2004 September–October 447 with permission only. Contact [email protected] dorsal ventral

cephalon cephalic doublure

pleural thorax doublure

pleural lobes pygidium axial lobe


branch thoracic leg

spiny limb base Figure 2. Anatomy of a trilobite gives testimony to its arthropod kin- Figure 3. Ventral view of the Devonian trilobite reveals sev- ship with spiders, scorpions and horseshoe crabs. The dorsal shell eral pairs of legs and gill branches. Trilobite legs were not protected by protected both the soft body of the animal and the delicate gill an exoskeleton, so they are rarely preserved. This exceptional fossil branches that sat atop each leg (bottom). A typical trilobite is repre- was excavated from the Hünsruck Shale in . (All photographs sented here. (Adapted from “A Guide to the Orders of Trilobites,” by courtesy of David L. Bruton, University of Oslo, and Winfred Haas, Sam Gon III.) University of Bonn.) after bacteria had destroyed the soft trilobite limbs that have been found, all the trilobites were both abundant and tissue. We know that trilobites had an- were built to this basic pattern. So it is varied. More than 5,000 different gen- tennae at the front of the head, like likely that much of the extravagant va- era have now been named, and doubt- many of their arthropod relatives, such riety in trilobite design was played out less many more remain to be discov- as the and . Behind upon the hard calcite of the exoskele- ered. They ultimately died out about the antennae, the numerous limbs, ton. Abundant fossils give us a good 270 million years later, near the end of arranged in pairs, were all rather simi- idea of the variety of form adopted by the Permian period, during the “great lar along the length of the body; a pair these extinct . extinction” that wiped out 95 percent of limbs was attached to each thoracic Trilobites appeared rather suddenly of all in the Earth’s oceans. segment. On each limb, a jointed walk- in the fossil record, low in Cambrian They could not have survived so pro- ing leg was overlain by a gill branch, strata laid down about 522 million lifically for so long had they not been with which the animal breathed. Of the years ago. Within a few million years well adapted for life in the Paleozoic seas. It is important to understand what these adaptations might have been if we are to build a picture of life in the early marine biosphere. But how can one reconstruct the lives of organ- isms that have been extinct for several hundred million years? This is not an impossible task, but we must acknowledge that we will never know for sure that we are right about our deductions. There are several ways of going about the . We might, for example, look among the living to see if there are arthropods with distinc- tive structures that are similar to those of Figure 4. Compound eyes of trilobites were usually holochroal (left) or schizochroal (right). a particular trilobite. The similarity Holochroal eyes generally consisted of hexagonal, closely packed lenses—as many as 15,000 might suggest that they shared similar per eye. Schizochroal eyes were typically made of larger, fewer lenses that were separated by life habits. Or we might examine the exoskeletal material. structure of the trilobite as a piece of bio-

448 American Scientist, Volume 92 © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected] logical engineering to see what the ani- mals could and could not do. Then, too, the geological strata in which the fossils are found can tell us much about the habitat the trilobites formerly occupied. Were they living in deep water? Or were they adapted to life on a ? In fortu- nate cases, different lines of evidence all point to the same conclusion, and we can be confident that we are converging on some truth about these organisms.

Windows to the Trilobite Soul The eyes can tell us much about an an- imal’s life habits. But since these struc- tures are often made of soft tissues, they are rarely present in the fossil record. Not so for the eyes of trilobites. Comparable in some respects to the compound (multi-lensed) eyes of many living arthropods, trilobite eyes differed from those of all their living (if distant) relatives in having lenses made of calcite—they recruited the hard mineral of their exoskeleton for optical purposes. While calcite pre- serves well in the fossil record, making Figure 5. Devonian trilobite Erbenochile erbeni of Morocco had spectacular schizochroal eyes, it a boon for the optically inclined pale- which rose up in large columns above the animal’s head. Views from behind (upper left), the ontologist, it also provided some perks side (upper right) and above (lower left) indicate that the animal may have had nearly a 360-de- for trilobites. These animals exploited gree view of its world. A detailed view of one (lower right) reveals an over- the peculiar property of calcite that al- hanging eye shade that would have blocked sunlight coming from above. (Reprinted with lows light to pass unrefracted along permission from R. Fortey and B. Chatterton, Science 301:1689, 2003.) one of the mineral’s crystallographic axes. This optical axis was aligned nor- Many trilobites secondarily lost their rocks of Spitsbergen. These trilobites mal to the surface of each , so we eyes, and it has been shown that this were peculiar in other ways too. They can deduce the direction in which a happened several times in the history tended to have smallish elongate bod- given trilobite lens was looking. of the group. Field studies on their ge- ies, and the lateral pleurae on the tho- Most trilobite eyes had many, even ological occurrence show that some of rax were much reduced, although they thousands, of tiny hexagonal lenses these blind species probably lived at seem to have retained strong muscula- (called holochroal eyes), which could reg- considerable depths, below the photic ture. Some species had spines on the ister small movements as first one, and (sunlit) zone, where eyes were often re- head which are directed downward. then a neighboring, lens was stimulat- dundant. It has been shown that the Since this is hardly a sensible adapta- ed. A plot of their field of view shows loss of vision was progressive among tion for a bottom-dwelling animal, it that most trilobite eyes looked lateral- some of these deep-water inhabitants, seems likely that these trilobites were ly—in other words, over the sediment with the gradual loss of lenses until the pelagic, swimming actively in the open surface on which the bottom-dwelling visual area became sealed over. There ocean. They economized on the weight animal lived. Another type of trilobite is no known example where eyes, once of the exoskeleton by truncating their eye (called schizochroal) had fewer, larg- lost, were regained. lateral regions, while most of the later- er, biconvex lenses separated by baffles. At the other extreme, there were a al parts of the head were occupied by These lenses were highly sophisticated; few trilobites in which the eyes became huge eyes. There are living amphipod they even had internal variations in enormously enlarged. In some species crustaceans with similar habits, and their refractive index to correct for such these huge eyes became quite globular, they too have hypertrophied eyes. optical problems as spherical aberra- with lenses distributed over the whole Pelagic trilobites were particularly tion. These, too, had predominantly lat- visual surface. It is obvious that these common in the Ordovician period, eral fields of view. In some species the species had a more comprehensive when they may have occupied some of eyes were fantastically modified as tow- field of view than the average trilobite, the niches inhabited today by krill. er-like structures. These were capable of since the lenses faced forward, up- Some of these species are extremely seeing accurately over long distances ward, downward, even backward— widespread, far more so than any of since the optical axes are parallel to one giving the animal almost 360-degree their bottom-dwelling contemporaries. another. One species, recently de- vision. In 1975, I described an extreme My colleague Tim McCormick and I re- scribed, even had a kind of eye-shade version of these animals—named cently analyzed the morphology of one overhanging the visual surface to keep Opipeuter, “one who gazes”—which of these Ordovician species, Carolinites out distracting light rays from above. was discovered in the Ordovician genacinaca, in different parts of the www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2004 September–October 449 with permission only. Contact [email protected] comparable to those of modern insects and crustaceans that live in dim light. So it seems likely that cyclopygids were animals of the mesopelagic re- gion, the “twilight zone” just below the ocean’s photic zone, and their remains accumulated in death alongside bot- tom-living trilobites that had lost their eyes. Curiously, a few of these deep- water pelagic animals had square lens- es, rather than the more-typical hexag- onal ones, for reasons that have yet to be explained.

Lifestyles of the Benthic and Pelagic Figure 6. Cornuproetus had holochroal eyes that looked laterally over the seafloor, which may Most pelagic trilobites were rather poor- have been typical of benthic-dwelling trilobites. ly streamlined, and it is unlikely that they swam very fast. Alongside them globe. We concluded that the same However, some other strange, large- there are a few, larger trilobites, such as species was found in what is now the eyed trilobites may have lived at some Parabarrandia, which have a smoothed- United States, the Arctic, , Aus- depth in the water column rather than out profile, with the head end pro- tralia and China—a range spanning near the surface. Cyclopygid trilobites longed into an elongate “nose,” some- the entire equatorial realm at the time. are bug-eyed forms that were generally what in the fashion of some modern, This kind of distribution is also matched wedded to a deep-water habitat, prob- small sharks. These larger trilobites are by some species of living planktonic ably below 200 meters. They are often modified into a hydrofoil shape, which animals. The species Carolinites genaci- found in the company of blind trilo- probably helped them to swim much naca represents an example in which bites that lived on the seafloor in deep faster than their smaller contempo- the animal’s functional morphology, its waters, within sediments that accumu- raries. Although it seems likely that the geologic distribution and the compar- lated at the edges of former continents smaller, swimming trilobites fed on isons with living fauna all point to a or deep shelf sites. Studies of the eyes phytoplankton and zooplankton, it is pelagic way of life. of these trilobites show that they are possible that these larger species were predators on some of the earliest crus- taceans. The trilobites appear to have been capable of occupying several nich- es, even in the pelagic realm. The vast majority of trilobites lived on or around the seafloor. They never seem to have invaded freshwater habi- tats—maybe if they had, there might still be species alive today. These bot- tom dwellers vary from tiny species, which reached mature size at just over a millimeter, to giants nearly a meter long. Such widely diverging animals must certainly have had different roles in the Paleozoic ecology. It was once epipelagic Carolinites extends thought that all trilobites were primi- across all sediments tive particle feeders, but it is now con- sidered likely that trilobites spanned almost the whole gamut of livelihoods mesomesesopelagiceso available to living marine arthro- only found in offshore pods—with the possible exception of sediments , although even this has been claimed for one group. Some trilobites, especially the largest of the tribe, had important roles as Figure 7. Ordovician pelagic trilobites, Carolinites killaryensis (top, left) and Pricyclopyge bin- predators and scavengers. This is be- odosa (top, right), had large bulbous eyes, which would have given the animals enormous lieved to have been the primitive fields of view. Based on the nature of the sediments in which the fossils were found, Carolin- lifestyle for trilobites. Many studies of ites appears to have been an epipelagic species, living near the surface of the ocean, whereas Pricyclopyge was a mesopelagic animal, generally living below a depth of 200 meters (bottom). their phylogenetic relationships place This is consistent with measurements of their eyes, which predict that these two species would them in a larger that includes the have lived at different depths in the ocean, with differing amounts of light filtering down from stem group of the living arachnoids— the surface (see Figure 8). (Adapted from McCormick and Fortey 1998.) of which the horseshoe , ,

450 American Scientist, Volume 92 © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected] luminance (log scale) ∆Φ –2 –1 0 123 4 10

deep-sea amphipod crustaceans 8 D diurnal marine crustaceans dimlight insects diurnal insects 6


2 √ ∆Φ eye parameter = D 3 2 eye parameter (radian-micrometers) D = lens facet diameter (micrometers) 0 night street interior light daylight ∆Φ = interommatidial angle (radians) light

Figure 8. “Eye parameter” measures some basic properties of a compound eye (left), and provides insight into the relative luminance of an an- imal’s habitat (right). The eye parameter values of Carolinites and Pricyclopyge (see Figure 7) are consistent with the hypothesis that they lived at different depths of the ocean. Eye parameter values of Carolinites were typically below 4 radian-micrometers, comparable to those of mod- ern diurnal marine crustaceans. Eye parameter values for Pricyclopyge were mostly above 3 radian-micrometers, in the range found for modern deep-sea amphipod crustaceans. (Adapted from McCormick and Fortey 1998.) scorpions and spiders are familiar ex- American paleontologists Loren Bab- dovician, the list of potential preda- amples. The great majority of this cock and Richard Robison showed tors is augmented with nautiloid mol- group make a living from scavenging that in some populations of Cambrian lusks and, by the Devonian, with or hunting other animals. Where trilobites, bite marks were more fre- jawed fish. It is tempting to see the known, the limbs of the early Cambri- quent on the right side of the animals, proliferation of spectacularly spiny an fossils have strong spiny bases that suggesting that they were prey to an and prickly trilobites at this time in may have been employed in shredding organism with a ritualized plan of at- certain parts of the world as a re- soft-bodied worms and other prey. tack. As to the attacker, it is likely that sponse to an increasingly precarious There are fossil examples where the other arthropods were the culprits in existence by the later Paleozoic. When track of a worm is approached by the some cases. The gut of a Cambrian some of these trilobites rolled up, their track of a trilobite—and the trilobite enigmatic arthropod has been found spines poked out in all directions, alone walks away! On the underside of filled with the mangled remains of making them a risky mouthful. the trilobite head there was a calcified tiny agnostid trilobites. The compara- Altogether less showy, many trilo- ventral plate called the hypostome. The tively huge and spectacularly be- bites were small animals, only a cen- mouth lay near the posterior end of the clawed, -sized “proto-arthro- timeter or two long, which scuttled over hypostome. In predatory trilobites, the pod” , first described muddy seafloors often in the thou- hypostome was rigidly braced. Many from the famous , may sands. A quarry located in the Middle species developed special structures on have been the tiger of the Cambrian Cambrian strata of has been the hypostome around the mouth area, seas, and was also equipped with “mined” for a little trilobite of this kind such as forks or thickenings. Both fea- good, if uncalcified, eyes. By the Or- called kingi; it can be found as tures probably helped with the dis- patch of bulky prey. A few species even had a kind of rasp on the inside of the fork. A large trilobite must have been a formidable foe to an annelid worm. Its well-developed sight doubt- less helped the animal to locate its prey, assisted by chemical sensors on the antennae. These predatory trilo- bites even left behind their digging traces—called —the imprint of their daily lives on the hunt. There is plenty of evidence that trilobites were themselves the prey of other animals. They are quite com- monly found with what appear to be bite marks—I am tempted to say a Figure 9. Dicranurus monstrosus from the Devonian of Morocco bore an elaborate armor of “trilo-bite”—but the fact that these are spines, which would have protected it from potential predators, such as the “recently” evolved often healed over shows that the at- jawed fishes. The hypostome of Dicranurus suggests that it may have been predatory, proba- tacks were frequently not fatal. The bly feeding on various worms. www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2004 September–October 451 with permission only. Contact [email protected] 6 7 5 8 4 3 5 6 3


7 1

Figure 10. Devonian trilobite was probably a particle feed- Figure 11. Feeding trail—the ichnofossil semiplicata—was er that scoured the seafloor for organic debris. It sported some long left by a sediment-grazing trilobite of the upper Cambrian. Individual spines on its back, including a pair of devil-like horns, of unknown counter-clockwise traces are identified by number. (Adapted from function, on its cephalon. Fortey and Seilacher 1997.) small “medallions” in museum shops braided tire-tracks (called Cruziana the field. The trilobites in question, es- around the world. With a flattened semiplicata) as they plowed their shal- pecially those belonging to the family shape, and often with large numbers of low furrows through the sediment sur- , had a very thin exoskeleton thoracic segments, these animals were faces. Some species even seem to have in relation to their size. Many of them seafloor dwellers that retained a broadly adopted particular feeding techniques, probably had rather feeble muscles similar morphology for more than 200 like circling around and around, to ef- and were sluggish bottom-dwellers. million years. They were a successful, if ficiently graze the sediments. But, in complete contrast to the pelagic conservative, design. Several groups of trilobites, closely trilobites, they had wide, flat lateral Such small trilobites have hypos- related to particle feeders, adopted a (pleural) areas and numerous thoracic tomes that are not rigidly attached at peculiar life habit in environments that segments. This allowed for the exten- the front, so they were supported by a were low in oxygen. The rocks that sion and multiplication of their gill flexible ventral membrane. Such a hy- were deposited in this dysaerobic habi- branches, and presumably helped in postome may have worked as a kind of tat include sulfide-rich black shales coping with low oxygen tension. scoop, helping the ingestion of soft and —the latter sometimes There are many examples in the liv- sediment, from which organic particles referred to as “stinkstones” because of ing fauna where specialists in low-oxy- could be extracted. The same trilobites the unpleasant sulfurous odor they gen habitats survive and prosper by en- have left grazing traces that look like emit when broken with a hammer in tering into a symbiotic relationship with

Figure 12. tesselatus created a filter-feeding chamber under its body. It is believed that the trilobite captured particles of food in wa- ter currents (hypothetical path, red arrows) that it generated with its legs. The currents may have exited the chamber through openings in its cephalon. Cryptolithus lived during the Ordovician in . (Drawing adapted from Fortey and Owens 1999.)

452 American Scientist, Volume 92 © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected] colorless sulfur bacteria. These animals length of exoskeleton (millimeters) “farm” the bacteria, which serve as a 0 10 20 30 40 50 60 70 nutritious food source. Some clams grow the bacteria in modified and pelagic absorb nutrients from them directly. It Microparia broeggeri Degamella evansi seems plausible that some of the trilo- bites that lived in oxygen-depleted en- streamlined feeders vironments had adopted similar life habits, thus taking the symbiotic rela- Pricyclopyge binodosa eurycephala tionship with sulfur bacteria all the way Cyclopyge grandis back to the Cambrian period. These brevirhachis kinds of trilobites can be found in great numbers when conditions were just sluggish plankton feeders right. Usually no more than one or two benthic species are found together, covering the Ormathops nicholsoni Illaenopsis harrisoni rock surfaces in “graveyards”—and Dindymene saron normal trilobites were absent. A few species of this kind also have degener- taylorum ate hypostomes, suggesting that they predators / scavengers may have been able to absorb bacterial nourishment directly from their gills. linleyoides Even more specialized trilobites of this general kind developed spectacular in- Bergamia rushtoni flated bulbs on the front of the filter chambers cephalon. These bulbs may have func- tioned as brood pouches to help their Shumardia crossi larvae through the early stages of life, range of sizes until they were big enough to acquire particle feeders their own symbionts. Another group of bottom-living trilo- Figure 13. Assemblage of 11 contemporaneous trilobite species from a quarry in Whitland, bites developed extraordinary brims South , reveals the remarkable diversity that can be found at a single site. The trilobites around the front of the head, which do were able to co-exist because they exploited different feeding habitats (as indicated by their not correspond to anything seen in any body size and structure) and different depths in the water column. The species lived during living arthropod. In many species, these the lower Ordovician. (Adapted from Fortey and Owens 1999.) brims are covered with small pits, and they are linked by a fine canal to oppos- matching the resting profile of the trilo- Fortey, R. A., and A. Seilacher. 1997. The trace ing pits arising from the doublure and bite, each one with a pair of scoops fossil Cruziana semiplicata and the trilobite the dorsal surface. The brims are fre- recording the action of the limbs. One that made it. Lethaia 30: 105–112. Fortey, R. A., and R. M. Owens. 1999. Feeding quently extended backwards into long can even see where the trilobites shifted habits in trilobites. Palaeontology 42(3):429–465. spines alongside the thorax, so that the from one patch to another in search of Gon III, S. “A Guide to the Orders of Trilobites.” animal could clearly rest gently on the better sediment. www.trilobites.info sediment surface supported by its mar- The Paleozoic seas swarmed with Jell, P. A., and J. M. Adrain. 2003. Available gin. It is a puzzling adaptation, but ob- trilobites with all manner of life habits: generic names for trilobites. Memoirs of the viously a successful one because an Or- Many large species crawled over the Queensland Museum 48:331–553. dovician family of trilobites (the seafloor in pursuit of prey, using their Lane, P. D., R. A. Fortey and D. J. Siveter (eds.). trinucleids) with this design are often great visual acuity; hordes of others 2003. Trilobites and their Relatives. Special Pa- pers in Paleontology, 70. found in enormous numbers. Some hint stirred up the sediment to filter out ed- McCormick, T., and R. A. Fortey. 1998. Inde- of what the design was for can be ible particles, while nearby another pendent testing of a paleobiological hy- gleaned by looking at the animal side- group of species plowed through the pothesis: The optical design of two Ordovi- ways. The thorax was held well above surface layers in search of food. We are cian pelagic trilobites reveals their relative the sediment surface, so that there was a beginning to understand how a group paleobathymetry. Paleobiology 24:235–253. chamber beneath the animal, flanked on of animals sometimes regarded as either side by the broad spines. The hy- “primitive” were actually sophisticat- postome (and therefore the mouth) was ed and varied. Had it not been for the held well above the sediment surface. It Permian they might be seems very likely that these trilobites with us still. For relevant Web links, consult this lived by stirring sediment up into sus- issue of American Scientist Online: pension and filtered out edible particles Bibliography http://www.americanscientist.org/ from the muddy cloud. If this idea is Fortey, R. A. 2000. Trilobite: Eyewitness to Evolu- correct, these little trilobites would have tion. New York: Knopf. IssueTOC/issue/641 had to “hop” from one patch to another Fortey, R. A., and B. Chatterton. 2003. A De- to stir up another feed. This is support- vonian trilobite with an eyeshade. Science ed by the discovery of traces exactly 301:1689. www.americanscientist.org © 2004 Sigma Xi, The Scientific Research Society. Reproduction 2004 September–October 453 with permission only. Contact [email protected]