The Lifestyles of the Trilobites

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446 American Scientist, Volume 92

The Lifestyles of the Trilobites

These denizens of the Paleozoic Era seas were surprisingly diverse

Richard A. Fortey

rugosecorals

annelid

ammonite

Hollardops

Koneprusia

ErbenochileCyphaspis

Phacops

Dicranurus

Walliserops

Figure 1. Ancient seafloor of what is now Morocco was host to an odd menagerie of trilobites during the Devonian Period, more than 350 mil-lion years ago. During their 270 million-year reign in the Earths seas, the trilobites inhabited a broad range of niches as predators, scavengers,

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 trilobite died shortly before the great Permian extinction, about 250 million years ago.

The Lifestyles of the Trilobites

These denizens of the Paleozoic Era seas were surprisingly diverse

Richard A. Fortey

2004 Sigma Xi, The Scientific Research Society. Reproductionwith permission only. Contact [email protected].

D. W. Miller


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I f you had been able to scuba diveduring the Ordovician Period, some450 million years ago, you would haveseen at once that the seas swarmed withtrilobites. A few trilobites were as largeas dinner plates, many more were thesize of modern shrimp, and yet otherswere smaller than peas. They lived al-most everywhere, from shallow waters

to deep environments beyond the reachof light. There were spiny ones like fullyladen pincushions and smooth oneslooking rather like large pill bugs. Somecarried strange colander-like brims, un-like any animal living today. Many hadlarge and obvious eyes; others againwere blind.

Their wonderful range of shapessuggests that the trilobites must haveoccupied many different ecologicalroles. We have a good fossil record ofthem because they were the first

arthropods to secrete a hard exoskele-

ton made of the mineral calcite. Notthat the whole outside of the body wascalcifiedunlike living crabs and lob-sters, the limbs of trilobites never be-came coated with a calcitic crust. Andso fossils of trilobite legs are exceed-ingly rare. Instead, it was just their backs (their dorsal surfaces) that be-came covered in a protective shield,

which was tucked around the edges ofthe animal in a fold called the doublure.The three lobes that give trilobites theirname comprise a conspicuous (andusually convex) axis flanked on eitherside by pleural areas, which are turneddown laterally. Trilobites are also di-vided crosswise into a head (cephalon)bearing the eyes, a flexible thorax com-prising a number of articulated seg-ments and a tail (pygidium) formed ofseveral fused segments. On this com-paratively simple theme the trilobites

played a host of variations.

Where trilobite limbs havebeen pre-served in the fossil state it is usuallybecause they were coated with a min-eralsuch as an iron pyrite or ap-atitebefore they had a chance to de-cay. The mineral film remained behind

2004 SeptemberOctober 447www.americanscientist.org 2004 Sigma Xi, The Scientific Research Society. Reproductionwith permission only. Contact [email protected].

Richard A. Fortey is a research scientist at theNatural History Museum, London, and visiting

professor of paleobiology at the University of Ox-ford. His current research includes molecular in-vestigations into the nature of the Cambrian evo-lutionary explosion, studies on the evolution ofarthropod vision and the reconstruction of ancientcontinents. In 2003, he received the Lewis ThomasPrize from Rockefeller University, which honorsscientists for their literary achievements. He is theauthor ofLife: A Natural History of the FirstFour Billion Years of Life on Earth, which wasnamed as one of the 11 books of the year in 1998by the New York Times. Address: Department ofPaleontology, The Natural History Museum,Cromwell Road, London, SW7 5BD, United King-

dom. Internet: [email protected]


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after bacteria had destroyed the softtissue. We know that trilobites had an-tennae at the front of the head, likemany of their arthropod relatives, suchas the insects and crustaceans. Behindthe antennae, the numerous limbs,arranged in pairs, were all rather simi-lar along the length of the body; a pairof limbs was attached to each thoracicsegment. On each limb, a jointed walk-ing leg was overlain by a gill branch,with which the animal breathed. Of the

trilobite limbs that have been found, allwere built to this basic pattern. So it islikely that much of the extravagant va-riety in trilobite design was played outupon the hard calcite of the exoskele-ton. Abundant fossils give us a goodidea of the variety of form adopted bythese extinct animals.

Trilobites appeared rather suddenlyin the fossil record, low in Cambrianstrata laid down about 522 millionyears ago. Within a few million years

the trilobites were both abundant andvaried. More than 5,000 different gen-era have now been named, and doubt-less many more remain to be discov-ered. They ultimately died out about270 million years later, near the end ofthe Permian period, during the greatextinction that wiped out 95 percentof all species in the Earths oceans.They could not have survived so pro-lifically for so long had they not beenwell adapted for life in the Paleozoicseas. It is important to understandwhat these adaptations might havebeen if we are to build a picture of lifein the early marine biosphere. But howcan one reconstruct the lives of organ-

isms that have been extinct for severalhundred million years?

This is not an impossible task, but wemust acknowledge that we will neverknow for sure that we are right aboutour deductions. There are several waysof going about the science. We might, forexample, look among the living fauna tosee if there are arthropods with distinc-tive structures that are similar to those ofa particular trilobite. The similaritymight suggest that they shared similarlife habits. Or we might examine the

structure of the trilobite as a piece of bio-

448 American Scientist, Volume 92 2004 Sigma Xi, The Scientific Research Society. Reproductionwith permission only. Contact [email protected].

dorsal ventral

cephalon

tho

rax

pygidium

axial lobe

eye hypostome

pleurallobes

gill branch

spiny limb base

dorsal exoskeleton

thoracic leg

cephalicdoublure

pleuraldoublure

Figure 2. Anatomy of a trilobite gives testimony to its arthropod kin-

ship with spiders, scorpions and horseshoe crabs. The dorsal shellprotected both the soft body of the animal and the delicate gill

branches that sat atop each leg (bottom). A typical trilobite is repre-sented here. (Adapted from A Guide to the Orders of Trilobites, bySam Gon III.)

Figure 3. Ventral view of the Devonian trilobite Phacops reveals sev-eral pairs of legs and gill branches. Trilobite legs were not protected byan exoskeleton, so they are rarely preserved. This exceptional fossil

was excavated from the Hnsruck Shale in Germany. (All photographscourtesy of David L. Bruton, University of Oslo, and Winfred Haas,

University of Bonn.)

Figure 4. Compound eyes of trilobites were usually holochroal (left)or schizochroal (right).Holochroal eyes generally consisted of hexagonal, closely packed lensesas many as 15,000

per eye. Schizochroal eyes were typically made of larger, fewer lenses that were separated by

exoskeletal material.


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logical engineering to see what the ani-mals could and could not do. Then, too,the geological strata in which the fossilsare found can tell us much about thehabitat the trilobites formerly occupied.Were they living in deep water? Or werethey adapted to life on a reef? In fortu-nate cases, different lines of evidence allpoint to the same conclusion, and we

can be confident that we are convergingon some truth about these organisms.

Windows to the Trilobite SoulThe eyes can tell us much about an an-imals life habits. But since these struc-tures are often made of soft tissues,they are rarely present in the fossilrecord. Not so for the eyes of trilobites.Comparable in some respects to thecompound (multi-lensed) eyes ofmany living arthropods, trilobite eyesdiffered from those of all their living (if

distant) relatives in having lensesmade of calcitethey recruited thehard mineral of their exoskeleton foroptical purposes. While calcite pre-serves well in the fossil record, makingit a boon for the optically inclined pale-ontologist, it also provided some perksfor trilobites. These animals exploitedthe peculiar property of calcite that al-lows light to pass unrefracted alongone of the minerals crystallographicaxes. This optical axis was aligned nor-mal to the surface of each lens, so wecan deduce the direction in which agiven trilobite lens was looking.

Most trilobite eyes had many, eventhousands, of tiny hexagonal lenses(called holochroal eyes), which could reg-ister small movements as first one, andthen a neighboring, lens was stimulat-ed. A plot of their field of view showsthat most trilobite eyes looked lateral-lyin other words, over the sedimentsurface on which the bottom-dwellinganimal lived. Another type of trilobiteeye (called schizochroal) had fewer, larg-er, biconvex lenses separated by baffles.

These lenses were highly sophisticated;they even had internal variations intheir refractive index to correct for suchoptical problems as spherical aberra-tion. These, too, had predominantly lat-eral fields of view. In some species theeyes were fantastically modified as tow-er-like structures. These were capable ofseeing accurately over long distancessince the optical axes are parallel to oneanother. One species, recently de-scribed, even had a kind of eye-shadeoverhanging the visual surface to keep

out distracting light rays from above.

Many trilobites secondarily lost theireyes, and it has been shown that thishappened several times in the historyof the group. Field studies on their ge-ological occurrence show that some ofthese blind species probably lived atconsiderable depths, below the photic(sunlit) zone, where eyes were often re-dundant. It has been shown that theloss of vision was progressive amongsome of these deep-water inhabitants,with the gradual loss of lenses until thevisual area became sealed over. Thereis no known example where eyes, oncelost, were regained.

At the other extreme, there were a

few trilobites in which the eyes becameenormously enlarged. In some speciesthese huge eyes became quite globular,with lenses distributed over the wholevisual surface. It is obvious that thesespecies had a more comprehensivefield of view than the average trilobite,since the lenses faced forward, up-ward, downward, even backwardgiving the animal almost 360-degreevision. In 1975, I described an extremeversion of these animalsnamedOpipeuter, one who gazeswhich

was discovered in the Ordovician

rocks of Spitsbergen. These trilobiteswere peculiar in other ways too. Theytended to have smallish elongate bod-ies, and the lateral pleurae on the tho-rax were much reduced, although theyseem to have retained strong muscula-ture. Some species had spines on thehead which are directed downward.Since this is hardly a sensible adapta-tion for a bottom-dwelling animal, itseems likely that these trilobites werepelagic, swimming actively in the openocean. They economized on the weightof the exoskeleton by truncating theirlateral regions, while most of the later-al parts of the head were occupied by

huge eyes. There are living amphipodcrustaceans with similar habits, andthey too have hypertrophied eyes.Pelagic trilobites were particularlycommon in the Ordovician period,when they may have occupied some ofthe niches inhabited today by krill.Some of these species are extremelywidespread, far more so than any oftheir bottom-dwelling contemporaries.My colleague Tim McCormick and I re-cently analyzed the morphology of oneof these Ordovician species, Carolinites

genacinaca, in different parts of the


Figure 5. Devonian trilobite Erbenochile erbeni of Morocco had spectacular schizochroal eyes,which rose up in large columns above the animals head. Views from behind (upper left), theside (upper right)and above (lower left)indicate that the animal may have had nearly a 360-de-gree view of its world. A detailed view of one compound eye (lower right)reveals an over-hanging eye shade that would have blocked sunlight coming from above. (Reprinted with

permission from R. Fortey and B. Chatterton, Science 301:1689, 2003.)


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globe. We concluded that the samespecies was found in what is now theUnited States, the Arctic, Siberia, Aus-tralia and Chinaa range spanning

the entire equatorial realm at the time.This kind of distribution is also matched by some species of living planktonicanimals. The species Carolinites genaci-naca represents an example in whichthe animals functional morphology, itsgeologic distribution and the compar-isons with living fauna all point to apelagic way of life.

However, some other strange, large-eyed trilobites may have lived at somedepth in the water column rather thannear the surface. Cyclopygid trilobites

are bug-eyed forms that were generallywedded to a deep-water habitat, prob-ably below 200 meters. They are oftenfound in the company of blind trilo-bites that lived on the seafloor in deepwaters, within sediments that accumu-lated at the edges of former continentsor deep shelf sites. Studies of the eyesof these trilobites show that they are

comparable to those of modern insectsand crustaceans that live in dim light.So it seems likely that cyclopygidswere animals of the mesopelagic re-gion, the twilight zone just below theoceans photic zone, and their remainsaccumulated in death alongside bot-tom-living trilobites that had lost theireyes. 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 tobe explained.

Lifestyles of the Benthic and PelagicMost pelagic trilobites were rather poor-ly streamlined, and it is unlikely thatthey swam very fast. Alongside themthere are a few, larger trilobites, such asParabarrandia, which have a smoothed-out profile, with the head end pro-longed into an elongate nose, some-

what in the fashion of some modern,small sharks. These larger trilobites aremodified into a hydrofoil shape, whichprobably helped them to swim muchfaster than their smaller contempo-raries. Although it seems likely that thesmaller, swimming trilobites fed onphytoplankton and zooplankton, it ispossible that these larger species werepredators on some of the earliest crus-taceans. The trilobites appear to havebeen capable of occupying several nich-es, even in the pelagic realm.

The vast majority of trilobites livedon or around the seafloor. They neverseem to have invaded freshwater habi-tatsmaybe if they had, there mightstill be species alive today. These bot-tom dwellers vary from tiny species,which reached mature size at just overa millimeter, to giants nearly a meterlong. Such widely diverging animalsmust certainly have had different rolesin the Paleozoic ecology. It was oncethought that all trilobites were primi-tive particle feeders, but it is now con-sidered likely that trilobites spanned

almost the whole gamut of livelihoodsavailable to living marine arthro-podswith the possible exception ofparasitism, although even this hasbeen claimed for one group.

Some trilobites, especially the largestof the tribe, had important roles aspredators and scavengers. This is be-lieved to have been the primitivelifestyle for trilobites. Many studies oftheir phylogenetic relationships placethem in a larger clade that includes thestem group of the living arachnoids

of which the horseshoe crab, Limulus,


Figure 6. Cornuproetus had holochroal eyes that looked laterally over the seafloor, which mayhave been typical of benthic-dwelling trilobites.

Figure 7. Ordovician pelagic trilobites, Carolinites killaryensis (top, left)and Pricyclopyge bin-odosa (top, right), had large bulbous eyes, which would have given the animals enormousfields of view. Based on the nature of the sediments in which the fossils were found, Carolin-ites appears to have been an epipelagic species, living near the surface of the ocean, whereasPricyclopyge was a mesopelagic animal, generally living below a depth of 200 meters (bottom).This is consistent with measurements of their eyes, which predict that these two species would

have lived at different depths in the ocean, with differing amounts of light filtering down from

the surface (see Figure 8). (Adapted from McCormick and Fortey 1998.)

Iapetus Ocean

epipelagic Carolinitesextendsacross all sediments

opelagic Pricyclopygeonly found in offshore

sediments


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scorpions and spiders are familiar ex-amples. The great majority of thisgroup make a living from scavengingor hunting other animals. Whereknown, the limbs of the early Cambri-an fossils have strong spiny bases thatmay have been employed in shreddingsoft-bodied worms and other prey.There are fossil examples where thetrack of a worm is approached by thetrack of a trilobiteand the trilobitealone walks away! On the underside ofthe trilobite head there was a calcifiedventral plate called the hypostome. Themouth lay near the posterior end of thehypostome. In predatory trilobites, thehypostome was rigidly braced. Manyspecies developed special structures onthe hypostome around the mouth area,such as forks or thickenings. Both fea-tures probably helped with the dis-patch of bulky prey. A few specieseven had a kind of rasp on the inside ofthe fork. A large trilobite must havebeen a formidable foe to an annelidworm. Its well-developed sight doubt-

less helped the animal to locate itsprey, assisted by chemical sensors onthe antennae. These predatory trilobites even left behind their diggingtracescalled Rusophycusthe imprintof their daily lives on the hunt.

There is plenty of evidence thattrilobites were themselves the prey ofother animals. They are quite com-monly found with what appear to bebite marksI am tempted to say atrilo-bitebut the fact that these areoften healed over shows that the at-

tacks were frequently not fatal. The

American paleontologists Loren Bab-cock and Richard Robison showedthat in some populations of Cambriantrilobites, bite marks were more fre-quent on the right side of the animals,suggesting that they were prey to anorganism with a ritualized plan of at-tack. As to the attacker, it is likely thatother arthropods were the culprits insome cases. The gut of a Cambrianenigmatic arthropod has been foundfilled with the mangled remains oftiny agnostid trilobites. The compara-tively huge and spectacularly be-clawed, lobster-sized proto-arthro-pod Anomalocaris, first describedfrom the famous Burgess Shale, mayhave been the tiger of the Cambrianseas, and was also equipped withgood, if uncalcified, eyes. By the Or-

dovician, the list of potential preda-tors is augmented with nautiloid mol-lusks and, by the Devonian, with jawed fish. It is tempting to see theproliferation of spectacularly spinyand prickly trilobites at this time incertain parts of the world as a re-sponse to an increasingly precariousexistence by the later Paleozoic. Whensome of these trilobites rolled up, theirspines poked out in all directions,making them a risky mouthful.

Altogether less showy, many trilo-bites were small animals, only a cen-timeter or two long, which scuttled overmuddy seafloors often in the thou-sands. A quarry located in the MiddleCambrian strata of Utah has beenmined for a little trilobite of this kindcalled Elrathia kingi; it can be found as


Figure 8. Eye parameter measures some basic properties of a compound eye (left), and provides insight into the relative luminance of an an-imals habitat (right). The eye parameter values of Carolinites and Pricyclopyge (see Figure 7)are consistent with the hypothesis that they livedat 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 moderndeep-sea amphipod crustaceans. (Adapted from McCormick and Fortey 1998.)

Figure 9. Dicranurus monstrosus from the Devonian of Morocco bore an elaborate armor ofspines, which would have protected it from potential predators, such as the recently evolvedjawed fishes. The hypostome of Dicranurus suggests that it may have been predatory, proba-

bly feeding on various worms.

eye parameter = D3

D

10

8

6

4

2

night daylight0

2 1 0 1 2 3 4

luminance (log scale)

eyeparameter(radian-micrometers)

eep-sea amphipod crustaceansiurnal marine crustaceansimlight insectsiurnal insects

2

D= lens facet diameter (micrometers)

= interommatidial angle (radians)streetlight

interior light


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small medallions in museum shopsaround the world. With a flattenedshape, and often with large numbers ofthoracic segments, these animals wereseafloor dwellers that retained a broadlysimilar morphology for more than 200million years. They were a successful, ifconservative, design.

Such small trilobites have hypos-tomes that are not rigidly attached atthe front, so they were supported by aflexible ventral membrane. Such a hy-postome may have worked as a kind ofscoop, helping the ingestion of softsediment, from which organic particlescould be extracted. The same trilobiteshave left grazing traces that look like

braided tire-tracks (called Cruzianasemiplicata) as they plowed their shal-low furrows through the sediment sur-faces. Some species even seem to haveadopted particular feeding techniques,like circling around and around, to ef-ficiently graze the sediments.

Several groups of trilobites, closelyrelated to particle feeders, adopted apeculiar life habit in environments thatwere low in oxygen. The rocks thatwere deposited in this dysaerobic habi-tat include sulfide-rich black shalesand limestonesthe latter sometimesreferred to as stinkstones because ofthe unpleasant sulfurous odor theyemit when broken with a hammer in

the field. The trilobites in question, es-pecially those belonging to the familyOlenidae, had a very thin exoskeletonin relation to their size. Many of themprobably had rather feeble musclesand were sluggish bottom-dwellers.But, in complete contrast to the pelagictrilobites, they had wide, flat lateral(pleural) areas and numerous thoracicsegments. This allowed for the exten-sion and multiplication of their gill branches, and presumably helped incoping with low oxygen tension.

There are many examples in the liv-ing fauna where specialists in low-oxy-gen habitats survive and prosper by en-tering into a symbiotic relationship with


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5

3

6

4

3

1

2

7

Figure 10. Devonian trilobite Cyphaspis was probably a particle feed-er that scoured the seafloor for organic debris. It sported some longspines on its back, including a pair of devil-like horns, of unknown

function, on its cephalon.

Figure 12. Cryptolithus 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 North America. (Drawing adapted from Fortey and Owens 1999.)

Figure 11. Feeding trailthe ichnofossil Cruziana semiplicatawasleft by a sediment-grazing trilobite of the upper Cambrian. Individualcounter-clockwise traces are identified by number. (Adapted from

Fortey and Seilacher 1997.)


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colorless sulfur bacteria. These animalsfarm the bacteria, which serve as anutritious food source. Some clamsgrow the bacteria in modified gills andabsorb nutrients from them directly. Itseems plausible that some of the trilo-bites that lived in oxygen-depleted en-vironments had adopted similar lifehabits, thus taking the symbiotic rela-

tionship with sulfur bacteria all the wayback to the Cambrian period. Thesekinds of trilobites can be found in greatnumbers when conditions were justright. Usually no more than one or twospecies are found together, covering therock surfaces in graveyardsandnormal trilobites were absent. A fewspecies of this kind also have degener-ate hypostomes, suggesting that theymay have been able to absorb bacterialnourishment directly from their gills.Even more specialized trilobites of this

general kind developed spectacular in-flated bulbs on the front of thecephalon. These bulbs may have func-tioned as brood pouches to help theirlarvae through the early stages of life,until they were big enough to acquiretheir own symbionts.

Another group of bottom-living trilo-bites developed extraordinary brimsaround the front of the head, which donot correspond to anything seen in anyliving arthropod. In many species, thesebrims are covered with small pits, andthey are linked by a fine canal to oppos-ing pits arising from the doublure andthe dorsal surface. The brims are fre-quently extended backwards into longspines alongside the thorax, so that theanimal could clearly rest gently on thesediment surface supported by its mar-gin. It is a puzzling adaptation, but ob-viously a successful one because an Or-dovician family of trilobites (thetrinucleids) with this design are oftenfound in enormous numbers. Some hintof what the design was for can begleaned by looking at the animal side-

ways. The thorax was held well abovethe sediment surface, so that there was achamber beneath the animal, flanked oneither side by the broad spines. The hy-postome (and therefore the mouth) washeld well above the sediment surface. Itseems very likely that these trilobiteslived by stirring sediment up into sus-pension and filtered out edible particlesfrom the muddy cloud. If this idea iscorrect, these little trilobites would havehad to hop from one patch to anotherto stir up another feed. This is support-

ed by the discovery of traces exactly

matching the resting profile of the trilo-bite, each one with a pair of scoopsrecording the action of the limbs. Onecan even see where the trilobites shiftedfrom one patch to another in search ofbetter sediment.

The Paleozoic seas swarmed withtrilobites with all manner of life habits:Many large species crawled over theseafloor in pursuit of prey, using theirgreat visual acuity; hordes of othersstirred up the sediment to filter out ed-ible particles, while nearby anothergroup of species plowed through the

surface layers in search of food. We arebeginning to understand how a groupof animals sometimes regarded asprimitive were actually sophisticat-ed and varied. Had it not been for thePermian extinction event they might bewith us still.

Bibliography

Fortey, R. A. 2000. Trilobite: Eyewitness to Evolu-tion. New York: Knopf.

Fortey, R. A., and B. Chatterton. 2003. A De-vonian trilobite with an eyeshade. Science

301:1689.

Fortey, R. A., and A. Seilacher. 1997. The trace

fossil Cruziana semiplicata and the trilobitethat made it. Lethaia 30: 105112.

Fortey, R. A., and R. M. Owens. 1999. Feedinghabits in trilobites. Palaeontology 42(3):429465.

Gon III, S. A Guide to the Orders of Trilobites.www.trilobites.info

Jell, P. A., and J. M. Adrain. 2003. Availablegeneric names for trilobites. Memoirs of theQueensland Museum 48:331553.

Lane, P. D., R. A. Fortey and D. J. Siveter (eds.).2003. Trilobites and their Relatives. Special Pa-pers in Paleontology, 70.

McCormick, T., and R. A. Fortey. 1998. Inde-pendent testing of a paleobiological hy-pothesis: The optical design of two Ordovi-

cian pelagic trilobites reveals their relativepaleobathymetry. Paleobiology 24:235253.

2004 SeptemberOctober 453www.americanscientist.org 2004 Sigma Xi, The Scientific Research Society. Reproductionith i i l C t t @ i

pelagic

benthic

length of exoskeleton (millimeters)

0 10 20 30 40 50 60 70

streamlined plankton feeders

sluggish plankton feeders

predators / scavengers

filter chambers

particle feeders

Microparia broeggeri Degamella evansi

Pricyclopyge binodosa eurycephala

Cyclopyge grandisbrevirhachis

Dindymene saron

Ormathops nicholsoni

Colpocoryphe taylorum

Illaenopsis harrisoni

Ampyx linleyoides

Bergamia rushtoni

Shumardia crossi

range of sizes

Figure 13. Assemblage of 11 contemporaneous trilobite species from a quarry in Whitland,

South Wales, reveals the remarkable diversity that can be found at a single site. The trilobiteswere able to co-exist because they exploited different feeding habitats (as indicated by their

body size and structure) and different depths in the water column. The species lived duringthe lower Ordovician. (Adapted from Fortey and Owens 1999.)

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