The Ecology of the Cambrian Radiation - Andrey Zhuravlev - Chapter 19 pdf

Early to Middle Cambrian echinoderms are primarily known from soft substrate environments where attached suspension-feeding eocrinoids, crinoids, and edrioas-teroids clung to skeletal de

Echinoderms represent a modest component of the initial metazoan radiation during the Cambrian but responded to global environmental changes across the Cambro-Ordovician boundary with rapid and prolific diversification to more varied lifestyles

in expanded habitats Many attached echinoderms were preadapted to exploit car-bonate hardgrounds and other stable substrates that became abundant on shallow carbonate platforms at that time, whereas other attached — and many new free-living — echinoderms evolved structures to cope with soft substrates.

Early to Middle Cambrian echinoderms are primarily known from soft substrate environments where attached suspension-feeding eocrinoids, crinoids, and edrioas-teroids clung to skeletal debris by suctorial attachment disks or were skeletally ce-mented by a holdfast; helicoplacoids perhaps employed other means Vagile surface deposit-feeding echinoderms included stylophorans, homosteleans, homoiosteleans, and ctenocystoids Echinoderms reached a diversification bottleneck in the Late Cambrian, but stemmed eocrinoids with cemented holdfasts were among the first skeletonized animals to colonize hardgrounds that became common at that time Stylophorans, homoiosteleans, and edrioasteroids were also represented Attached crinoids and free-living rhombiferans led the Early Ordovician radiation among sus-pension-feeding echinoderms and were accompanied by several other newly evolved groups with generally similar lifestyles Vagile herbivorous echinoids and carnivorous asteroids greatly expanded echinoderm ways of life by the Middle Ordovician This overall diversification pattern for echinoderms supports a model of two sequential evolutionary faunas in which shallow-water habitats fostered onshore origination and radiation followed by offshore expansion for many attached forms However, the diversification pattern is not as clear among free-living echinoderm groups, and the expansion direction for several of these could have been from offshore to onshore Bathymetry is a simplification of what must have been a complex list of controls Most Ordovician echinoderms had regular and sturdy construction; these advanced

Thomas E Guensburg and James Sprinkle

Ecologic Radiation of Cambro-Ordovician Echinoderms

designs were versatile and enduring by comparison with Cambrian forms, persisting through the Paleozoic and in some cases to the Recent.

DOCUMENTATION OF ECOLOGIC diversification in the fossil record provides the road map of life’s temporal patterns and the context of evolutionary history Most studies of diversification have emphasized intrinsic biotic driving factors for changes

in diversification patterns and evolutionary pathways (see Sepkoski 1991 for a re-view), but recent field-based studies have emphasized the role of extrinsic causes This latter approach requires extensive field observation and integration of sedimen-tologic, facies, and sequence stratigraphic information with paleobiologic observa-tions (Guensburg and Sprinkle 1992; Rozhnov 1994; Droser et al 1995) Broad-scale linkages are emerging as a result For instance, we have previously correlated global environmental changes with the ecologic expansion and diversification of echino-derms and other metazoans during the Early Ordovician rise of the Paleozoic Evo-lutionary Fauna (Guensburg and Sprinkle 1992; Sprinkle and Guensburg 1995) Echinoderms of the Cambrian remained a modest component of the biota until fa-vorable environmental shifts provided the catalyst for rapid ecologic expansion as part of the Ordovician radiation of metazoans (Sprinkle 1980) The purpose of this chapter is to review the ecologic radiation of Cambrian to Early Ordovician echino-derms and to analyze their diversification patterns Direct associations of echinoechino-derms and substrates are occasionally available when articulated specimens still adhere to at-tachment sites In many other cases, however, life modes must be reconstructed from extensive field correlation of partial specimens and lithofacies, coupled with func-tional morphologic studies and extrapolation from better-preserved close relatives The ecologic radiation for Cambro-Ordovician echinoderms offered here differs from those suggested by Smith (1988: figure 12.3; Smith 1990) and Smith and Jell (1990: figure 53) Many Early Paleozoic echinoderms are interpreted by these authors

to have rested unattached on, or had a distal structure inserted into, soft substrates

In contrast, evidence leads us to conclude that hard attachment surfaces were re-quired and that this was an important limiting factor to the diversification of Cam-brian echinoderms This also implies that most CamCam-brian echinoderms were pre-adapted to exploit the hard substrates that became common by the Late Cambrian These divergent functional interpretations provide an impetus for presentation of our ecologic diversification model below

ENVIRONMENTAL CHANGES DURING THE EARLY PALEOZOIC

The time interval considered here is from the Early Cambrian (Waucoban) through the Early Ordovician (Arenig, Late Ibexian), comprising the Sauk Sequence of Sloss

(1963) The Cambrian period began long after the Varangerian glaciation and breakup

of the supercontinent Rodinia Global environmental shifts at this time can be related

to the early diversification patterns of echinoderms and to the biosphere in general Modeling of Cambrian ocean circulation patterns supports a global warming trend (Golonka et al 1994) Sea levels rose, with interruptions, throughout the Cambrian – Early Ordovician, resulting in widespread and increasingly extensive inundation of cratons ( James et al 1989), potentially enhanced by isostatic and /or thermal subsi-dence of continental margins Generally, configuration of shallow shelves changed from narrow belts with inner detrital, carbonate bank, and outer detrital zones to broad carbonate ramps that extended well into continental interiors (Cook 1989; James et al 1989) Siliciclastic terrigenous sediments dominate Early to Middle Cam-brian sequences, but carbonates compose the majority by the Early Ordovician (see Seslavinsky and Maidanskaya, this volume) This change probably resulted from grad-ual constriction of emergent sediment source areas by rising sea level Evidence of slowed sedimentation during Late Cambrian time includes widespread glauconite for-mation; some Early Ordovician phosphatic-rich sediments have similar implications Seawater chemistry also changed during this time Carbonate deposition of the Early

to Middle Cambrian appears to have been dominated by metastable aragonite, which later altered to calcite (Sandberg 1983) There is little evidence that encrusting organ-isms exploited lithified or firm sea floors at that time In contrast, Late Cambrian to Early Ordovician carbonates were dominantly formed in a primary calcite cementa-tion regime, fostering the formacementa-tion of widespread hardgrounds or lithified substrates (Palmer and Palmer 1977; Wilson et al 1992; Rozhnov 1994) These conditions of-fered ideal habitats for slow-growing (low-metabolic), calcite-secreting, epifaunal or-ganisms such as echinoderms, and they were among the first skeletonized metazoans

to exploit these habitats The first really widespread encrinites, or echinoderm grain-stones, are associated with both intraformational conglomerates and cryptalgal build-ups that served as substrates for hardground formation, although a few echinoderm grainstones have been reported in association with late Early Cambrian reefs ( James and Klappa 1983) Multiplated echinoderm skeletons were rapidly reduced by post-mortem taphonomic processes to concentrations of durable clasts; these significantly increased the volume of sediment available for cementation (Wilson et al 1992) Their porous construction and high-magnesium calcite composition were ideal nu-cleation sites for marine cements in the form of syntaxial overgrowths, thus leading

to rapid lithification and formation of hardgrounds This resulted in a self-perpetuat-ing cycle whereby subsequent generations of echinoderms literally built upon the dis-articulated remains of their ancestors

Paleogeographic reconstructions of the Early Cambrian depict Laurentia, Baltica, and Kazakhstan (in part) separated from Gondwana and other continental masses (Golonka et al 1994; Ruzhentsev and Mossakovsky 1995) Virtually all landmasses were concentrated in the Southern Hemisphere, with Laurentia and parts of

Gond-wana closest to the equator Cratonic seas were widely distributed but covered only continental margins These landmasses retained their identity throughout the Cam-brian and Early Ordovician, and echinoderm faunas remained separate and distinc-tive on these continental blocks (Smith 1988; Sprinkle 1992) Baltica, Kazakhstan, and Laurentia moved slightly farther north into the tropics, then gradually converged (Golonka et al 1994) Baltica and Laurentia collided with the closing of the Iapetus during the Middle Ordovician The reconstructions support merging or linkage of faunal provinces for several continental blocks during the Middle to Late Ordovician, and the echinoderms reflect this greater interchange

TEMPORAL PAT TERNS IN LIFE MODES

Echinoderms constituted a small percentage of the total Cambrian biota, and the ar-ray of basic body constructions and life modes of these organisms was limited rela-tive to younger assemblages Eocrinoids, crinoids, edrioasteroids, and probably heli-coplacoids were sessile low-to-medium-level epifaunal suspension feeders They were either fixed or had minimal movement potential Fossil holothurians are only rarely preserved intact, because of their slightly calcified construction Consequently, we know little regarding their ecologic diversification, except that they were apparently

present by the Middle Cambrian (Eldonia and relatives; undescribed fossils) and could

have had both mobile benthic and planktic life modes by that time All four classes of

“carpoids”— cinctan homosteleans, solutan homoiosteleans, ctenocystoids, and cor-nute stylophorans — are known from the Middle Cambrian They are generally con-sidered to have been vagrant low-level suspension or deposit feeders, although they may constitute a polyphyletic grouping Solutes and cornutes (later joined by mitrate stylophorans) extend into the Late Cambrian and Early Ordovician, where they con-stitute important groups of vagile echinoderms from this time

Most of the life modes established by the Cambrian were carried over and ex-panded with a larger rapid radiation of echinoderms during the Early to Middle Ordovician There was a dramatic increase in faunal diversity, particularly among suspension-feeding echinoderms, and a corresponding increase in fine partitioning according to substrates or attachment sites, tiering or feeding levels, and specialized food particle selection Eocrinoids underwent considerable radiation during the Early Ordovician, giving rise to rhombiferans, diploporans, parablastoids, and paracrinoids (including rhipidocystids) (Sprinkle 1995) Blastoids were added later by the Middle Ordovician

The most spectacular radiation during the Early Ordovician was that of the cri-noids, which eclipsed blastozoans in total diversity and numbers by the Middle Or-dovician No crinoids are known from the Late Cambrian, but they had become abundant and diverse on hard substrates by the Early Ordovician and on soft sub-strates as well by the Middle Ordovician Stelleroids that appeared in the Early

Or-dovician and echinoids and ophiocistioids that appeared in the Middle OrOr-dovician greatly expanded the ecologic diversification of mobile benthic echinoderms to in-clude vagrant scavengers, grazers, and carnivores Certain edrioasteroids continued to diversify with suspension-feeding lifestyles, but only as a relatively minor faunal com-ponent Discussions of specific morphologic changes in echinoderm systems follow

Attachment

A wide range of habitats was exploited by Early Cambrian echinoderms, including deep slope (Poleta Formation, California) to shallow shelf detrital facies, and less com-mon shallow carbonate bioherms and associated facies Based upon functional mor-phology and taphonomy, we believe, contrary to Smith (1988), that most Early Cam-brian echinoderms were attached to firm or hard substrates in life and that the limited availability of these substrates (mostly skeletal fragments) limited the distribution of the echinoderms The fossils commonly occur in siliciclastic-dominated sequences such as fine-grained siltstones and shales that presumably formed soft substrates As-suming that the echinoderms were not usually transported into these settings, the only available attachment sites appear to have been skeletal debris, such as trilobite molts and rare brachiopod or hyolith shells

Specimens associated with attachment sites are rare, and the attachment mecha-nism in other cases is uncertain, although functional morphology and taphonomy provide important clues No known Early to Middle Cambrian echinoderms were

skeletally attached The edrioasteroids Stromatocystites and Camptostroma had basal

disks that are interpreted to have enabled clinging by suction There is a system of ra-diating ridges and plate rings in the loosely plated aboral surface that was capable of being withdrawn upward, forming a partial vacuum (Smith and Jell 1990) Presum-ably the animals released from attachment sites following death (Guensburg and Sprinkle 1994b) Blastozoans are considered to be the sister group to edrioasteroids

(Derstler 1985), and Early Cambrian examples Kinzercystis and Lepidocystis apparently retained attachment disks Specimens of Lepidocystis are rarely attached to trilobite

exoskeletons (Sprinkle 1973: plate 3, figures 1– 4) The paleoecology of helicoplac-oids is more problematic These spindle-shaped echinoderms are most often pre-served flattened parallel to bedding, but a few specimens are vertical, with a thecal pole buried in shale Attachment sites have not been identified

Attachment structures of Middle Cambrian edrioasteroids and eocrinoids are often modified versions of the basal disk described above (figure 19.1) For the most part, these fossils occur in fine-grained siliciclastic and mixed siliciclastic to carbonate (mi-critic) sequences of the outer detrital belt (Sprinkle 1976) The diverse and

wide-spread eocrinoid Gogia and close relatives were the most common echinoderms

dur-ing this time Specimens occasionally occur attached to skeletal fragments (Sprinkle 1973: plate 23, figures 1– 6) by a small multiplated button-shaped holdfast at the end

Figure 19.1 Reconstruction of soft-substrate

echinoderm community from the Middle Cam-brian Burgess Shale (British Columbia, Can-ada) Community is reconstructed at the base

of a carbonate bank in about 150 m of water and includes short- and long-stalked eocrinoids

(Gogia, left, and G radiata, left center), the cri-noid Echmatocrinus (right center), the edrioas-teroid Walcottidiscus (right rear), and tiny

mo-bile Ctenocystis (left front) Echinoderms, which

make up less than 5 percent of the fauna, are shown with other components of the fauna,

in-cluding trilobites, sponges, Marrella, a hyolith, and the priapulid Ottoia Front width of block diagram about 0.5 m Source: Modified from

Sprinkle and Guensburg (1997) by James Sprinkle and Jennifer Logothetti.

of a short-to-long multiplated stalk (figure 19.1) The lower holdfast surfaces are not well known, so it is uncertain whether suction was still used for adherence or if there

was actually skeletal cementation to the attachment surface Lichenoides is a Gogia

relative whose thickened plates of the lower theca as an adult possibly anchored the

animal Cymbionites is a problematic Middle Cambrian taxon known by a greatly

thickened basal plate that must have enabled anchoring in a similar manner

Edrioas-teroids such as Totiglobus and Edriodiscus had basal disks functionally similar to those

of earlier relatives (Guensburg and Sprinkle 1994b) A Totiglobus from southern Idaho

is attached to a trilobite free-cheek The earliest probable crinoid Echmatocrinus

occurs attached to worm tubes (figure 19.1), hyoliths, and possible stalks of other

Echmatocrinus specimens using a medium-length stalk tipped by a low conical

hold-fast that appears to have been cemented to the attachment surface (Sprinkle 1973; Sprinkle and Collins 1998)

Late Cambrian echinoderms are poorly known, but based upon skeletal debris, they were locally common in shallow shelf environments of cratonic seas, and echino-derms were among the first metazoans to attach to widespread hardgrounds

devel-oped on grainstones, intraformational limestones, and cryptalgal biohermal mounds Some hardground surfaces are encrusted by numerous subconical massive cemented holdfasts (figure 19.2), which, based on association with disklike columnals having trilobate lumens and distinctive thecal plates, we assign to eocrinoids No Late Cam-brian crinoids are known This is curious because they commonly encrusted Ordovi-cian hardgrounds (Palmer and Palmer 1977; Brett and Brookfield 1984; Guensburg 1992; Guensburg and Sprinkle 1992; Sprinkle and Guensburg 1995) The eocrinoid

Ridersia may represent a sister taxon to later rhombiferans ( Jell et al 1985) and has

a strongly heteromorphic stem that may indicate a free-living adult life mode Edrio-asteroids continued to attach with a basal disk, but there were modifications that pre-sumably increased efficiency by adding a well-developed peripheral rim that sealed the thecal margin (Smith and Jell 1990) Undescribed edrioasteroids from the Late Cambrian of Missouri have long conical aboral surfaces that could have been inserted

Figure 19.2 Reconstruction of hardground

and soft-substrate echinoderm communities from the Upper Cambrian Snowy Range For-mation (Wyoming, USA) A flat-pebble con-glomerate bed is slowly being covered by soft muddy substrate (right), but thicker parts of the bed (left) have become pitted and corroded during a long period of exposure on the shal-low sea floor Two genera of stemmed trache-locrinid eocrinoids (left center), along with

many additional holdfasts, a biscuit-shaped edrioasteroid (upper left), a sponge (lower

left), and several Billingsella calciate

brachio-pods, are attached to this lithified surface, while two cornute stylophorans (right front),

a solute homoiostelean (right rear), and a trilo-bite feed in the soft muddy sediment Front

width of block diagram about 0.5 m Source:

Modified from Sprinkle and Guensburg (1997)

by James Sprinkle and Jennifer Logothetti.

into firm but plastic siliciclastic substrates or attached to skeletal fragments Early and Middle Ordovician attached echinoderms continued encrusting hardgrounds and other solid surfaces Eocrinoids, paracrinoids, and crinoids all exploited these sur-faces in great numbers Rootlike and radicular holdfasts among crinoids first ap-peared during the Middle Ordovician, corresponding to the rapid ecologic radiation

of this group

Locomotion

“Carpoids” were flattened, more or less bilaterally symmetric, benthic vagrant organ-isms Among these, homosteleans, or cinctans, had a single biserial appendage that perhaps facilitated limited movement Homoiosteleans, or solutes, used the larger of their two appendages in a similar manner Ctenocystoids lacked appendages and pre-sumably moved by means of water pulses channeled through the alimentary canal Cornute stylophorans often have highly asymmetrical thecae, and the nature of loco-motion is difficult to discern A highly flexible appendage, the aulacophore, presum-ably propelled these animals with a wriggling or sculling motion Mitrate stylopho-rans that appeared in the Early Ordovician were bilaterally symmetrical and may have been more active Many rhombiferan cystoids are thought to have broken free or au-totomized from a holdfast as juveniles and been essentially free-living as adults A short flexible proximal stem and a long relatively stiff distal stem perhaps enabled these animals to move across the sea floor Edrioasteroids are rarely skeletally at-tached, and some may have been capable of limited movement

Stem Development and Tiering

Elevation of the feeding appendages and oral surface among early eocrinoids was ac-complished by a stalk or stem This stalk is generally short in Cambrian species, one

or two times the thecal length, but longer in a few taxa (see figure 19.1) Stalks are covered with small irregular plates and terminate at a holdfast below By the late

Middle Cambrian, the eocrinoid Akadocrinus had a stem with polymeric columnals.

Among eocrinoids, the transition to a fully formed stem with holomeric columnals permitting effective elevation of the theca and feeding structures was accomplished

by the latest Middle Cambrian (Sprinkle 1973) Late Cambrian trachelocrinid eocri-noids had stem lengths several times the thecal height, allowing feeding to interme-diate or high levels (see figure 19.2), perhaps as much as 0.5 m above the substrate Other echinoderms appear to have followed a similar pattern, but the record is not as good and the timing was apparently different The earliest fossil we believe to be a

cri-noid, Echmatocrinus, has a medium-length stalk that tapers gradually to a thin zone

immediately above a small encrusting holdfast (see figure 19.1) The next record of crinoids is not until the Early Ordovician, and by then well-developed meric stems

more than 0.5 m in length and attached to large calyces are known, enabling them to reach high feeding tiers Stems tend to be polymeric, usually pentameric, and flexible

to allow advantageous feeding strategies A few crinoids developed stems up to 0.9 m long by the Middle Ordovician (Guensburg 1992; Brower 1994), earlier than

pro-posed by Ausich and Bottjer (1982) The edrioblastoid (edrioasteroid) Cambroblastus

from the Late Cambrian has a short stalklike structure generally similar to an

eocri-noid stalk, and later edrioblastoids such as Lampteroblastus and Astrocystites from the

Ordovician had short stems with columnal-like plates

Feeding

The majority of Cambrian to Early Ordovician echinoderms were suspension feeders Edrioasteroids lacked feeding appendages and probably fed using cilia or tube foot – generated currents, combined with mucous chains along ambulacral food grooves exposed by opening cover plate flaps Beginning in the Late Cambrian, advanced isorophid edrioasteroids apparently had modified or lost the tube feet and perhaps gathered food by cilia-driven mucous on the epithelial lining of the food grooves Blastozoans such as eocrinoids and rhombiferans had thin biserial erect feeding ap-pendages called brachioles arising from short ambulacra on the thecal summit or up-per sides Most Early to Middle Cambrian species had relatively few brachioles, and those formed an open uncoordinated array Brachioles are usually extremely thin and are thought to have lacked tube feet (Sprinkle 1973); feeding is assumed to have been accomplished by the ciliated mucous style Food grooves are narrow, limiting these organisms to small food particles

Many later blastozoans, such as rhombiferans, retained that basic construction, but in other cases there was modification and elaboration Trachelocrinid eocrinoids

of the Late Cambrian have thick erect biserial arms with widely spaced brachioles branching off both sides (see figure 19.2) in a pattern that is functionally similar

to and convergent with crinoids that have a loose filtration fan This basic pattern continued in eumorphocystids, hemicosmitids, and some paracrinoids, but failed to achieve the success of crinoids Early Ordovician cylindrical rhombiferans have

bra-chioles arising from the top of the theca, similar to those of Gogia and as such

prob-ably represent a continuation of the initial blastozoan feeding strategy Pleurocystitids were convergent with many carpoids in their feeding style They have a special-ized and reduced ambulacral system consisting of two large brachioles with the food grooves usually facing the substrate, allowing exploitation of presumably nutrient-rich larger particles at the sediment-water interface Rhipidocystids and some

para-crinoids have small filter-feeding systems Echmatocrinus had short thick

nonbranch-ing arms with wide food grooves and large tube feet (see figure 19.1), indicatnonbranch-ing specialization toward capture of large food particles In general, Early Ordovician cri-noids retained relatively larger food grooves than most blastozoans, indicating that feeding strategies of blastozoans and crinoids remained separate well into the Early

Ordovician Thereafter, early pinnulate crinoids, particularly camerates, were pre-sumably microplankton feeders and competitors to blastozoans

Feeding styles of carpoids apparently differed from those of the suspension feed-ers described above Ctenocystoids presumably strained particulate matter from the sediment-water interface using the ctenoid apparatus surrounding the mouth Cinc-tans apparently had soft tissues protruding from an aperture along the thecal margin, allowing low-level suspension or deposit feeding Mitrate and cornute stylophorans used the aulacophore for surface deposit feeding and /or extended upward, allowing low-level suspension feeding (see figure 19.2) Homoiosteleans had a short feeding arm extending from the anterior thecal margin and presumably used for surface de-posit feeding The first echinoderm carnivores (asteroids) and herbivores (asteroids, echinoids, and ophiocystoids) did not appear until the Early to Middle Ordovician, respectively

Respiration

Cambrian echinoderms have two types of respiratory structures: widespread pores, called epispires, between the thecal plates; and tube feet in the ambulacra that con-nected to the water vascular system and an external hydropore, or madreporite, near the mouth Epispires are found in many Early and Middle Cambrian eocrinoids (Sprinkle 1973), in some Early and Middle Cambrian edrioasteroids ( Jell et al 1985; Smith 1985), and in most Middle Cambrian homosteleans (Friedrich 1993) Epispires occur at the sutures of thick tessellate plates and were apparently occupied by thin outpouchings of epidermis (podia) across which gas exchange could take place Epi-spires were perhaps vulnerable and were either lost by many of these groups after the Middle Cambrian and replaced by taxa with thin tessellate thecal plates, or they evolved into diplopores, which are paired pores within the thecal plates, allowing efficient water flow and better protection on the thecal exterior

Thin-plated echinoderms that respired through the entire plate surface were espe-cially common in the Late Cambrian (see examples in figure 19.2), where only a few epispire-bearing echinoderms have been found, and continued into the Early Or-dovician Thecae with this design were easily disarticulated, contributing to a poor fossil record for echinoderms during this interval (Sprinkle 1973; Smith 1988) Thin-plated eocrinoids were mostly replaced in the Early Ordovician by new groups of blastozoans that developed thicker and stronger thecal plates with specialized respi-ratory structures, such as pectinirhombs and diplopores Other echinoderms that retained thin thecal plates (especially early crinoids and rhombiferans) had stellate plates with one or more strengthening ridges radiating either to the plate sides or less commonly to the plate corners (Paul 1972; Dzik and Oriowski 1993) Several early crinoids that appeared during the Early Ordovician had an anal sac or tube with pore-bearing plates that may also have augmented respiration

Tube feet or podia in the ambulacra were probably important respiratory