The Ecology of the Cambrian Radiation - Andrey Zhuravlev - Chapter 17 docx

This trilobite had elongated eyes, a relatively wide axis permitting the attachment of large muscles, spinose posterior thoracicpleurae, and a distribution spanning a wide range of litho

Nigel C Hughes

Ecologic Evolution of Cambrian Trilobites

Skeletonized Cambrian trilobites are both varied and abundant and provide tial proxies for understanding the evolution of nonskeletonized arthropod groups Soft- and hard-part morphology suggests that Cambrian Trilobita pursued a variety

poten-of feeding habits, ranging from predator-scavenger activity to sediment ingesting and suspension feeding They occupied habitats ranging from infaunal to probably pelagic and lived in ecosystems that were structured in a manner comparable to those of marine habitats today The range of ecologic diversity among skeletonized Cambrian trilobites is similar to that exhibited by nonskeletonized Cambrian arthropods Data

on taxonomic, morphologic, and size diversity, in combination with information about abundance and occurrence, suggest that considerable ecologic diversity was es- tablished by the appearance of trilobites in the fossil record Species richness and the absolute abundance of individuals increased during the remainder of the Cambrian, but in at least some biogeographic provinces the rate of morphologic diversification was constrained after the Early Cambrian This constraint may have been related to the demise of carnivorous redlichiid trilobites and the radiation of primitive libristo- mate trilobites with a primary consumption feeding mode Many of the phylogenetic and ecologic components of Ordovician trilobite communities appeared no later than the Middle Cambrian but did not rise to dominance until the establishment of the Paleozoic fauna.

THE BIOMASS OF TRILOBITESin scientific collections far exceeds that of all otherCambrian metazoans put together This fact reflects the volumetric and taxonomicabundance of trilobites in a wide range of Cambrian sediments, their intricate and la-bile morphology, and their occurrence throughout the majority of Cambrian time.These attributes have given the group unrivaled utility as zonal fossils in Cambrianstrata, and as the principal faunal element used to assess Cambrian paleobiogeogra-

phy Paradoxically, while trilobites serve as the timekeepers by which we gauge the

ecologic evolution of other Cambrian metazoans, the ecology of Cambrian trilobites

remains poorly resolved This chapter summarizes current knowledge of the ecology

of Cambrian trilobite species and their place in Cambrian communities, outlines thedifficulties in making paleoecologic inferences in this group, explores a number of in-

direct measures of ecologic diversity, and presents an overview of the ecologic

evolu-tion of these fossils

Recent interest in the Cambrian radiation has been fueled by the redescription ofCambrian soft-bodied organisms and by the discovery of new ones Advances inarthropod systematics have constrained the taxonomic position of the Trilobita (e.g.,Wheeler et al 1993; Wills et al 1994) The trilobites are a monophyletic constituent(Fortey and Whittington 1989) of a larger clade of arachnate arthropods that werecommon in Cambrian marine environments and that exceeded other Cambrianarthropods clades in terms of taxic diversity (at least within individual Burgess Shale –type Lagerstätten) Furthermore, schizoramid arthropods (arachnates crustaceano-morphs marrellomorphs) apparently dominated Cambrian communities in terms

of numbers of taxa, individuals, and biovolume (Conway Morris 1986) Trilobites arethus important not only in their own right, but also as possible proxies for under-

standing patterns of ecologic evolution in other soft-bodied Cambrian arthropods,

which played a dominant role in Cambrian ecologies

Despite the good fossil record of trilobites, interpretation of their life habits is ten difficult We are unable to use modern representatives for direct insights into theecology of Cambrian relatives because trilobites are extinct Although extant arach-nate horseshoe crabs can provide some pointers about possible trilobite lifestyles, this

of-information does little to resolve the ecologic significance of particular trilobite

mor-photypes or characteristic features Hence knowledge of Cambrian trilobite ogy is based on case studies of particularly well-preserved or morphologically distinc-tive trilobites

autecol-INSIGHTS INTO THE AUTECOLOGY OF CAMBRIAN TRILOBITES

Although trilobite exoskeletal morphology is not intimately linked to feeding

strat-egy, as in some Paleozoic groups (e.g., Wagner 1995), major morphologic differences

likely imply different ecologies, and many aspects of trilobite form and habits bear

on the ecologic evolution of the group These include sensory systems (e.g., son 1973), locomotion (e.g., Whittington 1980), molting behaviors (e.g., McNamara1986; Whittington 1990), and reproductive strategies (e.g., Hughes and Fortey 1995)

Clark-Of the “economic” aspects of ecology (sensu Eldredge 1989), inferences on feedingbehavior are most important because these may indicate the role of trilobites in thetrophic structure of Cambrian marine communities and the habitats that they oc-

cupied Direct evidence for feeding strategies comes from appendage morphology,known in some exceptionally preserved faunas Indirect indicators such as exoskele-tal shape, trace fossils, and functional modeling provide additional information.

Direct Evidence for Feeding: Exceptionally Preserved Material

Soft-part preservation in Cambrian deposits has permitted reconstructions of theprincipal external features of several taxa, including (1) Early Cambrian redlichiids

Eoredlichia intermedia and Yunnanocephalus yunnanensis (Shu et al 1995; Ramsköld and Edgecombe 1996); (2) Middle Cambrian corynexochides Olenoides serratus and Kootenia burgessensis (Whittington 1975; Whittington 1980) and soft-bodied nectas- pid trilobites Naraoia compacta (Whittington 1977) and Tegopelte gigas (Whittington 1985); and (3) the Late Cambrian agnostid Agnostus pisiformis (Müller and Walossek

1987) These studies, and others of post-Cambrian trilobites, suggest that trilobiteslacked specialized feeding appendages All trilobites apparently fed by passing food

to the midline and then moving it forward to the mouth, which in A pisiformis was

posteriorly directed This movement was achieved by rotating the basis, the plate towhich both endopodites and exopodites are attached, in the horizontal plane Hence,locomotion and feeding were combined processes, as in other arachnomorphs (Mül-ler and Walossek 1987)

Naraoia compacta and O serratus possessed spinose gnathobases on the basis that

likely shredded food These trilobites also had spinose endopods and are interpreted

as predators or scavengers on benthic organisms (Whittington 1975; Whittington

1980; Briggs and Whittington 1985) Agnostus pisiformis also possessed a spinose gnathobase (Müller and Walossek 1987), but because adult Agnostus was so much smaller than Naraoia or Olenoides, the type of food particles macerated by Agnostus

must have differed Based on the structure of the thorax and of the appendages,

Müller and Walossek (1987) concluded that A pisiformis lived partially enrolled and

fed by collecting suspended detrital particles while actively swimming or by ing material at the sea floor

process-Exceptional preservation of gut morphology in Late Cambrian Pterocephalia from

British Columbia (Chatterton et al 1994) provides details of both the alimentary canaland the food source The composition of the gut contents suggests that this trilobitewas a deposit feeder and that it ingested fine-grained sediment Similar structures have

been reported in Eoredlichia, and the putative absence of spines on the endopods of

this animal (Shu et al 1995) might suggest that the food particles ingested were small

Further investigations of the limb structure in Eoredlichia, however, suggest a level of endopod spinosity comparable to that of Naraoia (Ramsköld and Edgecombe 1996) This at least suggests that food particles handled by Eoredlichia were larger Negative allometry of the hypostome in Eoredlichia with respect to overall size supports the

idea of a relatively small food particle size throughout growth In conclusion,

excep-tionally preserved material indicates a variety of feeding strategies among Cambriantrilobites ranging from predator-scavenger activity to sediment ingesting and suspen-sion feeding.

Recent analyses have shown that the soft-bodied forms Naraoia and Tegopelte may

not be the closest relatives of skeletonized trilobites or of each other (Edgecombeand Ramsköld 1999) Additional discoveries of anatomically disparate Early Cam-brian trilobite-like arachnates (e.g., Ivantsov 1999) further strengthen the impres-sion of broad morphologic and, by proxy, ecologic diversity among Early Cambrianarachnates

Indirect Evidence for Feeding

The Generalized Trilobite Body Plan

Although Cambrian trilobites displayed a wide variety of form, features general totheir morphology provide broad indicators of life habits On the basis of functionaldesign, analogy with living arthropods, and homology with extant arachnates, thegeneralized body plan common to most trilobites, consisting of a rigid dorsal exo-skeleton with eyes perched on the dorsal surface and homopodous walking legs, sug-gests a vagile benthic or nektobenthic life Marked departures from this basic mor-phology suggest alternative lifestyles

Specialized Morphologies and “Morphotypes”

In some cases, more-detailed inferences on ecology can be deduced from exoskeletalmorphology Fortey (1985), using explicit criteria based on occurrence, analogy, andfunctional morphology, presented strong arguments for pelagic life habits among

some Ordovician trilobites The convergence of a set of morphologic and occurrence

features (particularly related to the form of the eye) among members of several ferent clades permitted the recognition of a generalized pelagic trilobite morphotypeand the recognition of specializations within this broad habit Fortey (1985 : 227) alsosuggested a candidate Cambrian pelagic morphotype, exemplified by the Late Cam-

dif-brian primitive libristomate Irvingella This trilobite had elongated eyes, a relatively

wide axis (permitting the attachment of large muscles), spinose posterior thoracicpleurae, and a distribution spanning a wide range of lithofacies and paleocontinents.This generalized morphotype and a similarly widespread distribution were found in

the Middle Cambrian redlichiid Centropleura and the latest Cambrian olenid pis, and each of these forms may have been pelagic in adult life However, as the eye

Jujuyas-structure of these animals is poorly known, and the functional significance of the tended pleurae unclear, the case for a pelagic habit remains incomplete

ex-An alternative example of a derived, specialized morphology is found in severalLate Cambrian trilobites that are characterized by an inflated and effaced cephalon

with small eyes, angular articulation of cephalon and thorax, and postcephalic

seg-ments with wide axes This morphotype is epitomized by Stenopilus pronus and is

in-terpreted to be the result of a shallow infaunal habit (Stitt 1976) Both the overall form

of the animal, and minor modifications such as the surface sculpture, suggest that

Stenopilus occupied the sediment by adopting the bumastoid stance (Fortey 1986),

with the cephalon resting horizontally on the sediment surface, and the thorax andpygidium extending vertically down Trilobites adopting this morphology are thought

to have been suspension feeders (Stitt 1976; Westrop 1983), although there is no pendage evidence to support this feeding mode This is a case in which the mor-phology of the animal is modified such that its life mode can be directly inferred fromfunctional morphology Unfortunately, such cases are rare among Cambrian trilobites.The nature of attachment of the hypostome to the remainder of the cephalon mayprovide a feature of importance in interpreting broad feeding habits for many trilo-bites (Fortey 1990) Natant or “floating” hypostomes, which are not attached by

ap-calcified exoskeleton to the remainder of the dorsal shield, show morphologic

conser-vatism through the Cambrian and beyond Based on the style of attachment, smallsize, and evolutionary conservatism, Fortey (1990 : 553) suggested that natant hy-postomes characterize trilobites that consumed small organic particles extracted bythe gnathobases or that directly ingested sediment Conterminant trilobites, with hy-postomes attached to the remainder of the exoskeleton, display a wider variety of hy-postomal forms, some of which may have been specialized for processing larger fooditems, including prey Evidence for this interpretation includes the greater strength ofthe buttressed hypostome in conterminant forms, and the presence of special adap-tations such as posterior forks on the hypostomes (in post-Cambrian forms) that mayhave assisted in food maceration The recognition of these two basic feeding typesamong Cambrian trilobites is important because it links the feeding habits deduced

from exceptionally preserved taxa to morphologic characters that can be recognized in

the majority of Cambrian trilobites

Fortey and Hughes (1998) argued that a sagittal swelling anterior to the glabella insome primitive libristomate trilobites, most common in the Cambrian, may represent

a brood pouch The ideas of Fortey (1990) on broader aspects of trilobite feedingecology have been significantly expanded, notably providing stronger support forfilter feeding in post-Cambrian trilobites (Fortey and Owens 1999)

Major “morphotypes” have been recognized on the basis of the form of the dorsalshield ( Jell 1981; Repina 1982; Fortey and Owens 1990a), and attempts have been

made to link these morphologies to particular ecologic strategies The morphotypes of

Fortey and Owens (1990a) were defined as similar morphologies that arose gently among different clades of trilobite (figure 17.1) They suggested that conver-

conver-gence on a common morphology argued for a common ecologic strategy, even if the

nature of that strategy remained unresolved Recognition of these morphotypes isbased on either the overall form of the animal (such as the miniaturized morphotype)

Figure 17.1 Cambrian and Ordovician occurrences of eight common trilobite morphotypes

plotted against time Note that most morphotypes are represented in the Cambrian

Modified from Fortey and Owens (1990a: figures 5.4 –5.6).

or a specific character state (such as the atheloptic morphotype, which had reduced

eyes) This approach provides a way of assessing the ecologic diversification of

trilo-bites that is partially independent both of taxonomy and of the need to identify cific niches for each form The results of this approach are discussed below

spe-Trace Fossils

The direct association of trilobites with trace fossils proves that they were the makers

of some lower Paleozoic traces (e.g., Osgood 1970; Draper 1980; Geyer et al 1995).Such direct associations are unknown in the Cambrian Nontrilobite arthropods are

known to have produced Cruziana-like tracks (Seilacher 1985), and given the

diver-sity of Cambrian homopodous arachnomorphs, many of these traces could have been

made by organisms other than trilobites The common occurrence of Rusophycus lonensis in the pretrilobitic Cambrian suggests that organisms making Rusophycus were not always preservable as body fossils Nevertheless, Cruziana/Rusophycus mak-

ava-ers likely occupied niches similar to those of trilobites, and the supposed parallel

trends in size and abundance of Cruziana/Rusophycus and trilobites argue that

trilo-bites were the principal architects of these traces (Seilacher 1985; but see also tington 1980) An alternative interpretation is that the evolutionary history of trilo-bites was mirrored by that of other cruzianaeform trace producers, but in either casethe evolutionary history of trilobites is likely representative of that of the trace maker

Whit-The case for an association of the Late Cambrian trace fossil Cruziana semiplicata

and the trilobite Maladioidella cf colcheni, found in adjacent beds, was made recently

by Fortey and Seilacher (1997), but no direct association was observed

Despite the abundance of cruzianaeform trace fossils, there is little strong evidence

as to their function An exception is the association between Rusophycus and

teich-ichnian burrows in the Early Cambrian of Sweden (Bergström 1973; Jensen 1990),which provides evidence that burrowing arthropods preyed on infaunal worms Thelarge size of the burrows and the form of a cephalic impression are consistent with

the makers’ being olenelloid trilobites, which are associated with these deposits ziana and Rusophycus provide unequivocal evidence of infaunal activity; some formed

Cru-interstratally (Goldring 1985), while others suggest surficial burrowing (Droser et al.1994)

Functional Modeling

Experiments with models of trilobites have provided insights into the ics of Ordovician trilobites (Fortey 1985) Cambrian trilobites with morphologiessimilar to those modeled presumably behaved in similar fashions, and on this basisHughes (1993) suggested a bottom-hugging life mode of the Late Cambrian asaphide

hydrodynam-Dikelocephalus.

INSIGHTS INTO CAMBRIAN TRILOBITE SYNECOLOGY

The Burgess Shale fauna provides the clearest evidence of the role of trilobites in brian marine communities (Briggs and Whittington 1985; Conway Morris 1986)

Cam-Trilobites from that assemblage include free-swimming suspension feeders (e.g., chagnostus), benthic primary consumers (e.g., Elrathina), and carnivores (e.g., Naraoia and Olenoides) These broad lifestyles were shared with a wide variety of other schizo-

Pty-ramid arthropods Hence, trilobite morphology did not constrain the group to a

lim-ited range of ecologic opportunities; rather the group explolim-ited the same broad range

of niches available to other arthropods The presence of benthic primary consumers

(e.g., Eoredlichia), carnivores (e.g., Naraoia), and a possible free-swimming eodiscid

from the Early Cambrian Chengjiang fauna (Shu et al 1995), suggests that, mally, this pattern was in place shortly after the advent of skeletonization, and possi-bly prior to that time Identifying specific synecologic relationships within “normal”assemblages of Cambrian trilobites is more difficult, but specific size and habitat par-titioning relationships have been suggested for Early and Middle Cambrian agnostidtrilobites (Robison 1975), based on differences in maximum sizes of individual taxaand their relationship to lithofacies

mini-Abnormalities of various kinds also provide direct evidence of Cambrian bite synecology A variety of skeletal abnormalities have been described in trilobites,resulting either from developmental anomalies, disease, infestation, or injury (e.g.,

trilo-Figure 17.2 Abnormalities in a Cambrian

trilobite, possibly related to parasitism (see Hughes 1993 : 15) Divisions on scale bars in millimeters; arrows mark positions of struc-

tures of interest A, Swelling on glabella of

Briggs 1985 on Anomalocaris) and may even occur on large trilobites (Hughes 1993:

plate 7, figure 8), which were themselves likely predators

Pratt (1998) has argued that extinction of a major predator on trilobites occurredduring the Late Cambrian, based on changes in sclerite fracture in the lower Rabbit-kettle Formation Although imaginative, it remains unclear why the putative preda-tor should have actively fractured exuvae, which likely formed the large majority ofspecies examined Furthermore, no candidate predator capable of smashing calcifiedexoskeletons in the manner envisaged by Pratt (1998) has yet been identified amongthe Burgess Shale – type faunas A nonbiological explanation for the change in frac-turing, such as a longer time interval prior to shell bed cementation, remains a viablealternative

The discussion above indicates that Cambrian trilobites likely occupied a range ofhabitats from infaunal to probably pelagic realms Indirect and direct evidence con-sistently suggests a number of feeding strategies among Cambrian trilobites, includ-ing sediment ingestion, suspension feeding, and active predation Other feeding strate-gies, such as filter feeding, have been proposed (e.g., Bergström 1973; Stitt 1983) but

are less firmly established Evidence that a wide variety of trilobites were hosts forparasites, and prey for other organisms, suggests that they lived in ecosystems thatare at least comparable to those found in marine habitats today Trilobites apparently

exploited a range of ecologic strategies similar to those employed by other Cambrian

arthropods

LIMITS ON ECOLOGIC RESOLUTION IN CAMBRIAN TRILOBITES

Despite progress toward understanding feeding and habitats of Cambrian trilobites,several major problems remain unsolved The inability to infer specific life habits andniches for the majority of Cambrian trilobites presents the greatest challenge to un-derstanding the ecologic evolution of these forms Even though it is obvious thatdistinctive morphotypes must have had specific functional constraints, we are often

at a loss to identify these constraints An example is the multisegmented

Cermatops-like pygidium This morphotype is characterized by reduced propleurae and a widedoublure (Hughes and Rushton 1990; Rushton and Hughes 1996) and evolved in-dependently in peri-Gondwanan early Late Cambrian iwayaspinids and idahoiidsand in latest Cambrian dikelocephalids from Laurentia Pygidia are indistinguishableamong certain species belonging to distantly related groups Repeated convergence

on this morphology suggests a specific function for this pygidium, but that functionremains unknown Paleoenvironmental distributions offer no clues: taxa bearing the

Cermatops-like pygidium appear in a wide variety of lithofacies, ranging from

carbon-ate shelf environments to clastic submarine fan deposits and deeper-wcarbon-ater dysaerobicenvironments They also occur at a wide range of paleolatitudes and around severalCambrian landmasses (Rushton and Hughes 1996) The same difficulty extends across

a wide variety of morphologies For example, the distinctive catillicephalid type ( Jell 1981), consisting of a bulbous glabella, a small pygidium, and a small num-ber of segments, was almost certainly related to a specific feeding habit —yet, beyond

morpho-a genermorpho-al resemblmorpho-ance to Stenopilus, thmorpho-at hmorpho-abit is unknown (see the different

interpre-tations offered by Stitt [1975] and Ludvigsen and Westrop [1983])

The Cermatops-like pygidium reflects another broad difficulty in studies of

Cam-brian trilobites: rampant convergent evolution Although convergent structures mayindicate functional constraints, they can also confound attempts to assess phyloge-netic relationships Some, but not all, Cambrian trilobites show marked intraspecificvariation (e.g., Westergård 1936; Rasetti 1948; Hughes 1994) and mosaic patterns ofvariation among related species (Kiaer 1917; Whittington 1989) This plasticity pre-sents problems for systematics because of the difficulty of recognizing discrete taxa,distinguished by stable character sets The “ptychopariid problem” (e.g., Lochman1947; Schwimmer 1975; Ahlberg and Bergström 1978; Palmer and Halley 1979;Blaker 1986), which is the seemingly intractable systematics of a paraphyletic group

of primitive libristomates, is an expression of this phenomenon Reasons for the high

levels of homoplasy among Cambrian trilobites are poorly known They may reflectprocedural or preservational artifacts, such as the desire to recognize stratigraphicallydiagnostic species (Hughes and Labandeira 1995), or greater absolute abundance oftrilobites during the Cambrian than at later times (Li and Droser 1997) These factorscould increase the range of intermediate morphotypes relative to units that are poorlystudied or sampled Alternatively, high levels of homoplasy may reflect a develop-mental or ecologic constraint that reduced the numbers of viable character statesamong primitive libristomate trilobites (see the section “A History of Cambrian Trilo-bite Ecology” below).

TRILOBITE DIVERSITY, ABUNDANCE, AND OCCURRENCE

AS TOOLS FOR ECOLOGIC ANALYSES

Given the ignorance of the specifics of trilobite ecology, we must find alternative ways

of estimating ecologic diversity A comparative approach can provide useful mation on the ecologic evolution of the group Estimates of taxic and morphologic di-versity, and patterns of trilobite occurrence and abundance, can serve to indicate as-pects of the ecologic structure of the group By assessing these parameters throughCambrian time, the comparative ecologic evolution of the group can be charted, eventhough we lack details of the role of each form within its own community The skele-tonized Trilobita are the only Cambrian clade sufficiently common to permit thiskind of broad-scale analysis, and hence the group provides a unique perspective onCambrian ecologic evolution Furthermore, Burgess Shale – type faunas suggest thatCambrian trilobites occupied a range of niches similar to those of other Cambrianarthropods Hence it is possible that the evolutionary history of the trilobites may berepresentative of the history of schizoramid arthropods as a whole A discussion ofmeasures of ecological diversity follows

infor-Taxonomic Diversity

Taxonomic diversity provides a rough measure of morphologic variety It is mate because it is impossible to standardize systematic judgments in groups with di-vergent morphologies, patterns of variation, and preservational styles (see Lochman1947; Rasetti 1948) and because other factors, such as stratigraphic position, paleo-geography, and taxonomic philosophy, have influenced systematic placement (Fortey1990; Hughes and Labandeira 1995) Given that morphologic variety reflects eco-logic diversity, the taxonomic history of trilobites provides insights into their ecologicevolution The diversity of trilobites increased through the Cambrian at all taxonomiclevels, and Cambrian ordinal-level diversity is likely to increase further as systematicstudies are refined and additional basal sister taxa of post-Cambrian clades are iden-tified (see Fortey and Owens 1990b) Generic and species-level diversity increased

approxi-dramatically through the Cambrian (e.g., Foote 1993: figure 5; Zhuravlev and Wood1996: figure 2), reaching higher levels in the later Cambrian than at any other time intrilobite history (figure 17.3A) This sharp increase in later Cambrian diversity partlyreflects high species turnover rates in the Late Cambrian (Foote 1988) Estimates ofthe number of taxa alive at any one time can be computed by calculating speciesturnover rates Foote (1988) calculated that turnover rates were three times higher inthe Cambrian than in the Ordovician, but revised estimates of Cambrian duration

Figure 17.3 Species diversity in Cambrian

and Ordovician trilobites based on the

compi-lation of Foote (1993) A, Raw diversity data

for the intervals earlier and later Cambrian, and earlier and later Ordovician Standard error bars are smaller than the symbols Note the sharp peak in species diversity in the later

Cambrian B and C, Estimates of trilobite

stand-ing taxonomic diversity (i.e., those alive at any

one time) B, Cambrian species richness

di-vided by 3 to account for Cambrian trilobite species turnover rates being estimated at

3 times greater in the Cambrian than in the

Ordovician (Foote 1988) C, Cambrian species

richness divided by 6 This figure was chosen because of updated estimates of the duration

of the Cambrian, which is now thought to cupy a shorter span than used in Foote (1988).

oc-Source: Figures computed by Mike Foote.

suggest that the average turnover rate could have been up to six times that of the dovician (M Foote, pers comm., 1997) These results confirm that although trilobitespecies diversity was greatest in the later Cambrian (figure 17.3A), the standing di-versity of species at any one time may have been similar to (figure 17.3B) or signifi-cantly lower than (figure 17.3C) that during Ordovician times.

Or-Despite the overall increase in taxonomic diversity during the Cambrian, the EarlyCambrian contained a wide diversity of trilobite forms, typified by olenelloids, other

“redlichiids,” agnostids, corynexochids, primitive libristomates, and possibly alsoodontopleurid trilobites Although the earliest collections of Cambrian trilobites atvarious sections worldwide usually contain only a single taxon (e.g., Brasier 1989a,b),the global diversity of the earliest trilobites and their well-established provincialismsuggest that the appearance of trilobites in the rock record was not congruent withthe earliest evolution of the group (Fortey and Owens 1990b; Fortey et al 1996).While trilobite taxonomic diversity continued to increase rapidly in the Cambrian at

a variety of taxonomic levels, many of the most distinctive Cambrian trilobite photypes were established by the close of Early Cambrian time Trilobite higher taxa

mor-of Middle and Late Cambrian age have commonly been erected on the basis mor-of bers of constituent lower taxa rather than specified quanta of morphologic variation(Hughes and Labandeira 1995) Hence it is unclear whether the increase in taxo-nomic diversity in the later Cambrian is related to the abundance of trilobites in the

num-rock record (and its relationship to taxonomic practice) or to continued rapid phologic diversification.

mor-Trilobite taxonomic diversity peaked in the Ordovician (Stubblefield 1959; Foote1993), suggesting that trilobites reached their maximal ecologic diversity at that time.This argument is strengthened by (1) “morphospace” analyses, which assess aspects

of trilobite diversity independently of taxonomy (Foote 1991, 1992); and (2) photype” approaches, which estimate the numbers of clades contributing to distinc-tive recurrent morphotypes that presumably shared common life habits (Fortey andOwens 1990a) (see figure 17.1) Both these approaches indicate that the maximumdiversity of trilobites occurred during the Ordovician and that it was coupled with theradiation of clades of distinctive and disparate trilobites Morphospace approaches tothe diversification of trilobites have proved particularly instructive in this regard andare discussed further below

of taxonomy Here the term morphospace rather than morphologic is used here, because

these studies consider the relative placement of individuals within a space that is fined by the same set of individuals Because morphospaces are based on a sample ofthe overall morphology, the extent to which they summarize the group’s morphologicvariation depends on the degree to which the sample is representative of total mor-phology Morphospace approaches have been used to address a variety of questionswithin Cambrian trilobites (e.g., Ashton and Rowell 1975; Schwimmer 1975), butthe most relevant application to the ecologic evolution of the Trilobita has been at-tempts to assess the morphologic diversification of trilobites throughout their evolu-tionary history (Foote 1990, 1991, 1992, 1993)

de-Using an analysis of the outline of the cranidium in trilobites that have a dorsal ture and the outline of the cephalon in forms that do not, Foote (1989) suggested thatthe morphologic diversity of polymerid trilobites increased from the earlier to laterCambrian, followed by a sharp increase in diversity in the later Ordovician (Foote1991) (figure 17.4A) Although the earlier Cambrian shows the lowest overall diver-sity, the transition to the later Cambrian is not marked by a significant jump in area

su-of occupied morphospace, despite the large increase in numbers su-of species sampled.Furthermore, the variance of earlier Cambrian trilobites apparently exceeds that oflater Cambrian forms (Foote 1993) (figure 17.4B) The transition from Cambrian tolater Ordovician was marked by the appearance of several distinct trilobite morpho-types, which went on to dominate the remainder of trilobite evolutionary history.Given the roughly similar volume of morphologic space occupied by earlier and laterCambrian trilobites, the greater variance of earlier forms, and the profound difference

in numbers of species in each interval, estimates of diversity must be corrected to sess the effects of differing sample sizes These analyses showed that earlier Cambrianmorphologic diversity might actually have been higher than that of the later Cam-brian (Foote 1992) Alternatively, if the increased sample size of later Cambrian trilo-bites reflects an absolute increase in taxic diversity during that time, it may suggestthat the later Cambrian diversification of trilobites was morphologically constrained.Foote’s work permits an improved understanding of the morphologic diversifica-tion of trilobites, but interpretation of his data is complicated by the fact that thecephalic structures studied were not homologous among all the trilobites surveyed(Foote 1991) Early Cambrian olenelloids lacked a dorsal facial suture, and so the out-line of the cephalon was used as a proxy for cranidial form The cephalic outline in-cludes the genal spine, a character showing considerable variation, and the presence

as-of this spine contributes to the high disparity among olenelloid taxa (Foote 1991:text-figure 3) relative to forms in which cranidial outline was used The argument thatinclusion of the genal spine increases intragroup variability is supported by the pat-tern shown in the Ordovician cheirurids, which also occupied a larger proportion ofmorphospace than other groups and show a greater intrataxon disparity Cheiruridshad a proparian facial suture, with the result that their genal spines were also in-

cluded in the data set Whether this anomaly explains the relatively high variance inEarly Cambrian trilobites as revealed by rarefaction analysis (Foote 1992) is unclear.Nevertheless, high variation in the Early Cambrian is consistent with the appearance

of five trilobite orders during that time, each with a distinctive morphology

Size Ranges

Estimates of the range of maximum sizes within a group provide a measure of ecologicdiversity, because maximal body size is directly related to ecologic activity (McKinney1990) Analysis of the size ranges of Cambrian trilobites was attempted using data on

Figure 17.4 A, Morphological diversification

of Cambrian and Ordovician trilobites as pressed by the first two principal components

ex-of Fourier coefficients ex-of the outlines ex-of phalic structures (Foote 1989, 1993) Note the relatively constant area of morphological space occupied from the earlier Cambrian through

ce-earlier Ordovician B, Morphological variance

of trilobites Note that the variance of earlier Cambrian trilobites is slightly higher and shows greater error estimates than that of the

later Cambrian Source: Figures computed by

Mike Foote.

Figure 17.5 Maximum glabellar lengths of

253 species of skeletonized trilobites from China and Australia, as measured from system- atic illustrations (see text for discussion) Note

that maximal size diversity is found during the Early Cambrian, and the dominance of primi- tive libristomate trilobites in Middle and Late Cambrian faunas.

253 Gondwanan Cambrian trilobite species from China and Australia (figure 17.5).Maximum occipital-glabellar lengths were calculated using the largest cranidia of eachspecies, illustrated in two extensive monographs (Zhang and Jell 1987; Bengtson et al.1990), and a supplementary paper (Zhu and Jiang 1981) Trilobites from each of theChinese Cambrian stages (or its correlatives) were sampled and assigned to the fol-lowing age classes: Early, Middle, and Late Cambrian This area was chosen becausethe large monograph by Zhang and Jell (1987) includes trilobites from each Cam-brian epoch, and because Jell was also coauthor of the paper on Australian Early Cam-brian forms (Bengtson et al 1990) By limiting the sources to comprehensive workswith a common author, I have attempted to maximize the consistency of the sampleanalyzed

Results indicate that maximum size diversity occurred in the Early Cambrian (n

36, mean  10.7 mm, standard deviation [SD]  7.9 mm) This finding is due to thepresence of several large redlichiid trilobites in the Early Cambrian data set TheMiddle Cambrian shows reduced size ranges, despite having by far the largest num-

ber of species sampled (n 153, mean  6.6 mm, SD  3.8 mm) The Late Cambrian

shows a slight increase in the numbers of larger trilobites (n 65, mean  8.9 mm,

SD  4.8 mm) Several biases affect this data set, including different numbers of taxasampled within each time interval, variable durations among the time intervals, dif-ferences of paleoenvironment both within and between time intervals, variation inglabellar structure among the taxa sampled, and inconsistent underestimation ofmaximal glabellar lengths of the taxa analyzed Nevertheless, the overall pattern dem-onstrates that trilobites achieved a broad distribution of maximal sizes during theEarly Cambrian, with many large redlichiid trilobites present at that time In this data

set the Middle Cambrian shows a relatively restricted range of sizes, related to the cline of redlichiids and dominance of primitive libristomate forms The Late Cam-brian shows a slight expansion in the number of larger libristomate trilobites.The results accord with data from the taxonomic diversification of trilobites andwith the morphospace analyses of Foote (1991, 1992, 1993) Small agnostid andeodiscid trilobites were both present during the Early Cambrian, as were large olenel-loids, many of which are significantly larger than the largest Early Cambrian trilobites

de-in the data set above (e.g., Geyer and Palmer 1995) Some paleogeographic areas, such

as the Mediterranean sector of peri-Gondwanaland, have large redlichiid trilobitespersisting well into the Middle Cambrian, with an expansion in the range of maximalsizes at that time This finding is due to the large Middle Cambrian paradoxidids (e.g.,Bergström and Levi-Setti 1978) In other areas, this pattern would be mirrored bylarge Middle Cambrian forms such as the xystridurids (e.g., Öpik 1975) However,all these forms were redlichiids, a group with attached hypostomes that appeared inthe Early Cambrian and had become extinct by Late Cambrian time Faunal provincesmay differ in patterns of maximal size distribution, but in at least some regions, EarlyCambrian size distribution was more diverse than at later Cambrian times The de-cline in size-range diversity in the Middle Cambrian in the data set presented (figure17.5) relates to the rise to prominence of primitive libristomate trilobites (commonlycalled “ptychoparioids”), which were generally quite small This result is consistentwith Foote’s suggestion of a morphologically constrained diversification in later Cam-brian times, which was based on data from Laurentia (Foote 1992) After the extinc-tion of redlichiids, the increased number of larger trilobites in the Late Cambrian wasrelated to the advent of advanced trilobite groups with attached hypostomes such asthe asaphids, some of whose latest Cambrian members are among the largest of allCambrian trilobites (e.g., Hughes 1994)

The analysis of trilobite size diversity presented herein contrasts with the results of

an analysis of a Treatise-based Cambrian Ptychopariina and Asaphina (Trammer andKain 1997), in which greatest size diversity was found late in the Cambrian The con-trast is explained by the fact that large Lower and Middle Cambrian trilobites con-sidered in my analysis were excluded from that of Trammer and Kaim (1997) becausethese species are members of other higher taxa Neither their nor my analyses arecomprehensive, and both should be viewed as exploratory

Abundance

Few data exist on the abundance of trilobites during Cambrian time, but analyses ofthe nature and frequency of shell accumulations through the Cambrian of the GreatBasin provide insight in this regard (Li and Droser 1997) The overall thickness andabundance of shell beds increased during the trilobite-bearing Cambrian, as did thephylum-level diversity of these concentrations After attempting to assess the influ-ence of the depositional history on these trends, Li and Droser (1997) conclude that

increase in the absolute abundance of skeletonized animals was the major influenceresponsible for this trend Increased abundance is compatible with the rise in trilo-bite taxonomic diversity during the same time period, because larger species num-bers are likely to produce more fossils However, it is also possible that the numbers

of individuals within species also increased The relationship between specimen dance and taxic diversity will repay further study, because it is important to assesswhether the numbers of specimens analyzed per species show variation through theCambrian (Hughes and Labandeira 1995) Late Cambrian species turnover rates arethree times higher than in the Early Ordovician (Foote 1988) This may reflect a fun-damental difference in speciation patterns during the two periods, or alternatively itmay be an artifact of different taxonomic practices applied during the two intervals.Trilobites are the most abundant macroinvertebrates found in Upper Cambrian rocks,and consequently there may have been a tendency to proliferate the numbers of spe-cies for the purpose of biostratigraphic resolution

abun-Linkages between numbers of specimens preserved in sedimentary rocks and covered for analysis, and the numbers of described taxa and absolute abundances ofindividuals within species, are incompletely understood Extracting detailed informa-tion on relative abundance of individuals from a myriad of taphonomic and preserva-tional influences could be intractable (Westrop and Adrain 1998), but at the biofacieslevel at least it appears that specimen abundance contains information of biologicalimport

re-Occurrence

Trilobite taxa differ in their temporal and geographic distributions The study of bite distributions provides information on the ecologic evolution of the group, even ifthis information cannot be directly related to specific niches Occurrence, along withanalogy and functional considerations, can constrain hypotheses about trilobite lifehabits (Fortey 1985) For example, the widespread geographic occurrence of agnos-tid trilobite species, upon which much of intercontinental Cambrian biocorrelationrests, supports morphology-based arguments that these animals were free-swimmingand possibly pelagic (see the section “Specialized Morphologies and ‘Morphotypes’ ”above) Pioneering studies of the global distribution of Cambrian trilobites (e.g.,Richter and Richter 1941; Repina 1968, 1985; Cowie 1971; Jell 1974; Taylor 1977;Shergold 1988) indicate broad faunal provinces during Cambrian time Faunal dataare broadly consistent with other indicators of Cambrian global paleogeography Lau-rentian shelf faunas are apparently the most distinctive, a characteristic consistentwith the notion that Laurentia was geographically isolated during Cambrian times Awidespread shelf fauna occurs about the peri-Gondwanan margin, although the re-striction of many elements to specific regions suggests some paleolatitudinal control

trilo-of faunal distribution Faunas adapted to cooler waters had more widespread