The Ecology of the Cambrian Radiation - Andrey Zhuravlev - Chapter 15 pot

True pelecypods diversified even later on, in the Ordovician.Discussion of the paleoecology of these Cambrian groups figures 15.1 and 15.2 willfocus on their morphologic adaptations, possi

THE CONTINUOUSPhanerozoic history of marine mollusks that bore mineralizedskeletons began in the Early Cambrian Molluskan remains constitute an importantpart of the earliest skeletal assemblages (Bengtson and Conway Morris 1992; Dzik1994) In the present chapter, hyoliths, stenothecoids, and coeloscleritophorans aretreated together because even now most of these are considered to be mollusks(Marek and Yochelson 1976; Bengtson 1992; Starobogatov and Ivanov 1996) None-theless, the systematic position among the class Mollusca of many of these Cambriangroups is still disputed (Runnegar and Pojeta 1974, 1985; Yochelson 1978; Linsleyand Kier 1984; Missarzhevsky 1989; Peel 1991; Geyer 1994; Runnegar 1996) In thischapter, I follow the systematics of the principal groups of Early Paleozoic mollusksdeveloped by Peel (1991), which is supported by morphologic-functional analyses aswell as by the observed diversification pattern (Wagner 1996; Zhuravlev, this volume).

This pattern displays the highest diversity of helcionelloids, as well as some minorgroups treated as Early Cambrian paragastropods, pelecypods, and rostroconchs, inthe Tommotian, followed by continued steady decline during the Cambrian, inter-rupted by almost complete elimination during the early Botoman Sinsk event (Zhu-ravlev and Wood 1996) In contrast, indisputable rostroconchs, gastropods, ter-gomyans, polyplacophorans, and cephalopods started to diversify at the end of theCambrian and achieved their first peak of diversification in the latest Sunwaptan (Zhu-ravlev, this volume) True pelecypods diversified even later on, in the Ordovician.Discussion of the paleoecology of these Cambrian groups (figures 15.1 and 15.2) willfocus on their morphologic adaptations, possible trophic orientations, and organism-substrate relationships.

MOLLUSKS

Polyplacophorans

The first probable multiplated mollusks appeared during the latest Late Cambrian

(Bergenhayan 1960; Stinchcomb and Darrough 1995) Early Cambrian Triplicatella,

previously interpreted as the earliest chiton (Yates et al 1992), is an operculum way Morris and Peel 1995) The morphology of the Late Cambrian multiplated mol-lusks, probable members of the class Polyplacophora, is the subject of some debate.They may be reconstructed as metamerized sluglike animals bearing about eight mid-dorsal plates (Pojeta 1980; Stinchcomb and Darrough 1995) A Late Cambrian mul-

(Con-tiplated mollusk, Matthevia, has been described in detail, based on co-occurrence of

three morphologic types of matthevian shells (valves) (Runnegar et al 1979) Eachshell possesses two large ventral holes; no multiple muscle scars were found All thevalves, when clustered in situ, are of essentially the same shape The armor might

have consisted of more or less than eight shells Hemithecella and Elongata, which were

described by Stinchcomb and Darrough (1995), differ from representatives of the

post-Cambrian order Paleoloricata (class Polyplacophora) and Matthevia The

assign-ment of such forms to the Polyplacophora is questionable because the number andarrangement of scars are similar to those of monoplacophorans

Conical shells of the multiplated mollusks were robust enough to withstand wave activity Like Recent chitons, the Late Cambrian multiplated mollusks possiblywere scrapers or grazers that fed on algal and bacterial mats (figure 15.2 : 9, 10) (Tay-lor and Halley 1974; Runnegar et al 1979) Shells of multiplated mollusks are as-sociated with stromatolite cores that show little abrasion and rarely breakage Thissuggests that they occupied stromatolitic reef areas and may well have lived on firmsubstrates of stromatolitic buildups (Runnegar et al 1979; Stinchcomb and Darrough1995) Like Recent chitons, they possibly lived in intertidal and shallow subtidalenvironments

storm-Helcionelloids and Paragastropods

The majority of Cambrian univalves (helcionelloids) fall into three main morphologiccategories These reflect adaptive strategies but are also important evolutionarily, giv-ing rise to pelecypods, rostroconchs, and, subsequently, scaphopods

The earliest helcionelloid, Bemella, is a small caplike shell, with the apex usually lying outside by a slightly elongate apertural ring Planispirally coiled Latouchella-like and Bemella-like shells, with relatively broad apertures, are abundant and diverse in

the lowermost Lower Cambrian and also subsequently They exhibit a compromise

Figure 15.1 Generalized reconstruction of the

Early Cambrian community of mollusks, liths, stenothecoids, and coeloscleritophorans (background  calcimicrobial-archaeocyathan

hyo-mounds) Helcionelloids: 1, Oelandiella; 2,

Ana-barella; 4, Yochelcionella; 5, Ilsanella

Paragastro-pod: 3, Aldanella Stenothecoid: 6,

Stenothe-coides Rostroconch: 7, Watsonella Pelecypod:

8, Fordilla Orthothecimorph hyoliths: 9, datheca; 10, Conotheca Hyolithomorph hyolith:

La-11, Burithes Coeloscleritophorans: 12, loria 13, Halkieria.

Chancel-Figure 15.2 Generalized reconstruction of

the Late Cambrian community of mollusks and hyoliths (background  stromatolithic

mounds) Gastropods: 1, Sinuopea; 2,

Strepso-discus; 3, Matherella; 4, Spirodentalium

Tergo-myans: 5, Proplina; 7, Hypseloconus

Helcionel-loid: 6, Scenella Cephalopod: 8, Plectronoceras Polyplacophorans: 9, Matthevia; 10, Hemithe-

cella Rostroconchs: 11, Pleuropegma; 12, kila; 13, Ribeiria Orthothecimorph hyolith:

Oepi-14, Tcharatheca Hyolithomorph hyolith: 15, Linevitus.

between a flattened shell with broad aperture and a tightly coiled shell with a smallaperture (Runnegar and Pojeta 1985) They occur in various facies worldwide and,based on their low-spired and widely expanded shell (Linsley 1978), would have had

a broad foot, which characterizes sluggish epifaunal deposit feeders (figure 15.1 : 1, 5)(Kruse et al 1995; Gubanov and Peel 1999) They were probably an ancestor for othermorphologic-adaptive lineages of helcionelloids

The principal morphologic trend among helcionelloids is lateral compression of theshell and aperture and loss of strong comarginal ornamentation, often followed by thedevelopment of emarginations such as sinus, internal ridges, and snorkel Such shellshave an elongate narrow aperture and a high rate of expansion, with rather smooth but

often plicate walls (e.g., Anabarella, Stenotheca) Peel (1991) has reconstructed

Eote-benna as a transitional range of forms from sinus-bearing to elongated with snorkel.

These emarginations are assumed to have had an exhalant function (sometimes bothexhalant and inhalant) and were oriented posteriorly (Peel 1991) Some reconstruc-tions place them anteriorly (Runnegar and Jell 1980), but the small cross-sectional

area of the snorkel in Yochelcionella, and the development of the snorkel in Eotebenna and Oelandia, suggest its posterior direction and exhalant function (Peel 1991) Lat-

eral compression of the shells may be consistent with a vagrant semi-infaunal livingmode and with suspension or detritus feeding (Runnegar and Pojeta 1985) Using thecriteria of Linsley (1978) and McNair et al (1981), laterally compressed and widely

umbilical helcionelloids with a long aperture, such as Bemella, Anabarella, and

Yochel-cionella, are inferred to have been actively mobile on soft substrate in low-energy

con-ditions and thus to have been semi-infaunal filter feeders (Peel 1991; Kruse et al 1995;Gubanov and Peel 1999) (figure 15.1 : 2, 4)

However, the distinction between suspension and deposit feeding, as well as tween semi-infaunal and epifaunal habitats, may be meaningless in such small animals,approaching interstitial sizes Among modern macrofauna, deposit-feeding inverte-brates feed principally upon bacteria, whereas suspension feeders ingest phytoplank-ton (Levinton 1974) For diminutive Early-Middle Cambrian mollusks, such a dis-tinctive difference might be inappropriate

be-Another main adaptive strategy of helcionelloids is shell elongation and subsequentcompaction by means of coiling into a bilaterally symmetric, or dissymmetric, spiral

This mode of development is seen in low-spired bilaterally symmetric Latouchella-like forms when the beak deviates to the left (e.g., Pseudoyangtzespira) or to the right (e.g.,

Archaeospira), giving rise to dextrally or sinistrally coiled forms, respectively (Qian

and Bengtson 1989) Together with increase in the number of revolutions, sculptural

relief becomes lower in a succession of dextral forms: Aldanella crassa –A operosa –

Paraaldanella (Golubev 1976) The shell becomes involute or tightly coiled evolute,

with more revolutions in groups of sinistral mollusks (Barskovia hemisymmetrica –B.

rotunda; Beshtashella –Yuwenia –Kistasella) (Missarzhevsky 1989; Bengtson et al 1990)

and planispiral forms (Khairkhania n.sp.–K evoluta –K rotata) (Esakova and Zhegallo 1996) Hook-shaped forms (e.g., Ceratoconus) probably often precede loosely and

tightly coiled symmetric or asymmetric conchs with low rates of expansion Uncoiled,tall, small-apertured shells have a high pressure point and center of gravity (Linsley1978) To balance such a shell when moving, it is necessary to obtain a lower center

of gravity and pressure point and to minimize the frontal cross-sectional area ture and coiling enable a shell held by a snail to be balanced, because movement with

Curva-a tCurva-all or loosely coiled shell is difficult in Curva-agitCurva-ated wCurva-ater Achievement of Curva-a ately small cross-sectional area, low pressure point, and low center of gravity favors ac-tive locomotion

proportion-Because they were compact, strong, and able to contain a relatively voluminousbody, tightly coiled shells could successfully compete with other forms and invadevarious ecologic niches Detritus-feeding or grazing is usually assigned to the Cam-brian coiled mollusks (Runnegar and Pojeta 1985) Minute shell size, especially in theEarly Cambrian, suggests that many Cambrian paragastropods may well have used al-gae as substrates Peel (1991) concluded this for Recent and Silurian gastropods of 1–2 mm in size On the other hand, small paragastropods, with their elongated tan-gential aperture, have also been inferred to have been mobile epifaunal deposit feed-ers on soft substrates (Linsley and Kier 1984) (figure 15.1 : 3)

It is possible that small or large individuals of the same species occurred in differentenvironments in the Early Cambrian, depending on water energy A large helcionel-

loid, Randomia aurorae, was common in microbial mud mounds of the Fosters Point Formation (Landing 1992) Another large helcionelloid, described as Bemella jacutica,

was recovered in the vicinity of calcimicrobial-archaeocyath reefs of the PestrotsvetFormation (Dzik 1991) Peribiohermal facies of the Selinde River calcimicrobial-

archaeocyath reefs are surrounded by limestones with abundant Helcionella with

di-ameters up to 1.5 cm These occur with their apex upright, which is suggestive of

their in situ life position (Repina and Zhuravleva 1977) Tannuella elata, a large (2–

3 cm) Atdabanian helcionelloid, occurs in interbiohermal and peribiohermal facies ofthe Medvezh’ya River archaeocyathan reefs (Sundukov and Fedorov 1986) Shallow

subtidal wackestones of the Medvezh’ya Formation abound with Aldanella costata

(pers obs.) This organism probably dominated subtidal muddy soft substrates ofthe Tommotian Yudoma-Olenek Basin, Siberian Platform (Vasil’eva and Rudavskaya1989) Very shallow level-bottom environments are indicated by condensed peritidallimestones that form, for example, the tops of shoaling cycles in Member 4 of the EarlyCambrian Chapel Island Formation (Myrow and Landing 1992), with numerous firmsurfaces containing abundant but small helcionelloids In general, mollusks that in-habited reefal areas were relatively sizable forms with robust conchs

Rostroconchs

Primitive riberiid rostroconchs of the Cambrian had laterally compressed bivalvedshells with a univalved protoconch The development of this morphology was prob-ably the result of a change in living habit, from mainly epifaunal to semi-infaunal sus-

pension /detritus feeding of an Anabarella-like ancestor (Runnegar 1978) Watsonella

(?Heraultipegma) is the earliest-known rostroconch Landing (1989) noted that

although some small (

crawlers in spite of their laterally compressed condition, a quarter of Watsonella

spec-imens collected were oriented vertically in situ; a position more compatible with an

infaunal or semi-infaunal habit Kruse et al (1995) suggested that Watsonella, based

on its morphologic similarity to Anabarella-like helcionelloids, was more probably a

semi-infaunal suspension feeder (figure 15.1 : 7)

Rostroconchs occurred throughout the Cambrian and constitute an important part

of latest Late Cambrian fossil assemblages from China and Australia (Pojeta et al.1977; Druce et al 1982) Rostroconchs became diverse and abundant at the very end

of the Late Cambrian (latest Sunwaptan-Datsonian), before the first diversity sion of bivalves This time interval was previously placed in the Early Ordovician, andthe major rostroconch diversification was therefore assigned to that epoch (Runnegar

explo-1978; Pojeta 1979) A variety of life habits, ranging from epifaunal seston feeding

(Eu-chasma) to infaunal seston or deposit feeding (Ptychopegma), appeared by the end of

the Late Cambrian, but semi-infaunal deposit feeding or suspension feeding was ably the most common life strategy until the Permian, when rostroconchs died out (fig-ure 15.2 : 11–13) (Runnegar and Pojeta 1985)

prob-The paleoecology of Early Cambrian Watsonella crosbyi is relatively well known.

It has been recovered from various lithofacies, including subtidal siliciclastic stones, intertidal stromatolites, and peritidal wacke-packstones of warm- and cool-water environments of various depths and probably of normal salinity (Landing1989) Late Cambrian rostroconchs are known from warm-water environments,where they seem to have preferred quiet conditions in offshore muds and carbonates(Pojeta and Runnegar 1976; Runnegar 1978)

mud-Pelecypods

A massive radiation of the Bivalvia, which effectively competed with rostroconchs, curred in the Ordovician They evolved from mostly polar onshore infaunal depositfeeders (nuculoids) and suspension feeders (conocardiids, babinkiids, cycloconchids)

oc-in the Early Ordovician to proc-incipally epifaunal suspension feeders oc-in the MiddleOrdovician (Pojeta 1971; Babin 1995) A byssus was a key adaptation for elaboration

of sessile modes of life among Ordovician pelecypods, both infaunal and epifaunal(Stanley 1972)

Several genera of Cambrian bivalved mollusks have been described (Pojeta et al.1973; Jell 1980; MacKinnon 1982; Shu 1986; Krasilova 1987; Hinz-Schallreuter1995; Geyer and Streng 1998) It seems possible that the bivalved condition appearedindependently several times within the Cambrian According to Runnegar and Pojeta(1985), a ligament was the critical point in the origin of the Bivalvia

The Early Cambrian Pojetaia and Fordilla (and their numerous synonyms) are

usu-ally referred to as the Bivalvia (divided valves, adductor muscles and ligament), even

though there are no intermediates between them and Ordovician clams Pojetaia and

Fordilla occur, with rare exceptions, in articulate closed mode, which may well

sig-nify infaunal habitation (Runnegar and Bentley 1983; Ermak 1986, 1988) Otherwise,valves would be disarticulated because of bottom current action The author has about

a hundred specimens of Fordilla sp from the Siberian Platform and of Pojetaia

runne-gari from Australia, entirely in closed mode However, a shell hash in thin section may

well correspond to their detritus, and it is likely that there has been dissolution of articulated carbonate valves and selective preservation of phosphatized internal molds

dis-in residues Even if the valves were dis-in closed condition, it does not appear to showconvincingly their infaunal lifestyle; some small (

open after their death on the sediment surface The disarticulation process depends

on decay rate of the adductor muscles and ligament and on intensity of sediment

dis-turbance (Tevesz and McCall 1985) The growth lines on the ligamental area of

Poje-taia, arranged parallel to the hinge, indicate that the ligament was composed of

mul-tiple layers and was probably very weak Therefore the valves would not necessarilyhave sprung open after death A weak ligament is additional evidence that the clamsperhaps did not burrow at all, because an elastic ligament is essential for burrowing.Recent infaunal clams possess a deep pallial sinus, which is lacking among theirCambrian relatives This seems to be in agreement with a supposed absence of si-phuncles due to the unfused mantle of the earliest pelecypods (Stanley 1975) Thebeaks of most burrowing forms are directed forward (prosogyrous) Such an adapta-tion increases burrowing efficiency (Stanley 1975) and might explain the prosogyrate

shape of Fordilla-like mollusks, but the lack of a blunt anterior contradicts this

inter-pretation Thus, their size and morphology are not incompatible with an epifaunalmode of life (Tevesz and McCall 1985) Again, the distinction between epifaunal andinfaunal life modes is difficult to make, given that the size of the animal approachesthat of sediment grains (MacKinnon 1982, 1985) Recent juvenile and adult bivalves,less than 3 mm in size, pick up individual food particles with the foot but do not filterwater for food (Reid et al 1992) Supposedly, these bivalves were either inhalant de-posit feeders, using ciliated body and mantle surfaces to collect and sort particles offood (Runnegar and Bentley 1983), or epifaunal suspension feeders (Tevesz and Mc-Call 1976, 1985) (figure 15.1 : 8)

Tergomyans

Another adaptive lineage of univalves is represented by bilaterally symmetric conic or cyrtoconic tergomyans, more or less flattened or tall, with the apex inside theapertural ring Most of these have a rather large whorl expansion rate and relativelyisometric broad aperture, providing stability to the shell on the substrate In this case,the substrate functioned as a “ventral valve” to protect the animal Linsley (1978) noted

ortho-that shell shape is significantly correlated with rate of locomotion Flattened shells,

like Proplina and Kalbiella, had low position of both pressure point and center of

grav-ity (figure 15.2 : 5, 6) Recent tergomyans with flat shells inhabit quieter environmentsfeeding on detritus Well-developed radular and pedal muscular scars in the Late

Cambrian tergomyan Pilina from North China indicate that it was indeed a clamping

and crawling grazer (Yu and Yochelson 1999)

Gastropods

Torsion may be regarded as an initial adaptation for living in an elongate coiled shellwith a rather narrow aperture In this case, Cambrian coiled forms could be torted,partially torted, or untorted Asymmetric Early-Middle Cambrian gastropod-like mol-lusks may well have been incompletely torted and thus were not gastropods but para-gastropods (Runnegar 1981a; Linsley and Kier 1984), the taxonomic rank of which,however, is relatively low The global lack of predominance of dextral over sinistralforms in the Early Cambrian raises even more suspicion about their gastropod affin-ity If it is admitted that the exogastric shell might have had an adaptive significancefor the planktotrophic larva, when torsion occurred at the end of the veliger stage,then torsion would be merely an aftereffect of size increase Late Cambrian gastropodswere indeed quite sizable animals in comparison with other Early Cambrian coiledmollusks, including even the largest representatives quoted by Dzik (1991)

True archaeogastropods with a deep sinus and slit appeared in the Late brian and include the orders Pleurotomariida, Bellerophontida, and Macluritida (fig-

Cam-ure 15.2 : 1–3) The first probable gastropod was Middle Cambrian Protowenella, with

an ultradextral shell coiling and bellerophontid muscle scar position Judging fromthe scar position, deep inside the conch on the umbilical shoulder, Brock (1998) sug-gested that the animal was capable of retracting in the shell as gastropods do Based

on spire heights and apertural inclinations, the Late Cambrian Macluritacea and rotomariacea were restricted to clear water and hard substrates, since a large amount

Pleu-of suspended fine sediment would have easily fouled the complex aspidobranchial gill(Vermeij 1971) On the other hand, poorly balanced shells, a diffuse nervous system,and weak radulae would have restricted Cambrian archaeogastropods, slow and un-streamlined animals, to a diet of mud (Yochelson 1978; Hickman 1988) Throughoutthe entire Paleozoic, archaeogastropods were indeed confined to soft sediment en-vironments (Peel 1985) Nonetheless, filter feeding is postulated for sinistral open-coiled macluritid gastropods reported from the late Late Cambrian (Yochelson 1987;Yochelson and Stinchcomb 1987) and even from the late Middle Cambrian (Peel1988) Their open-coiled shape is not compatible with movement (figure 15.2 : 4)(Yochelson and Stinchcomb 1987) There is a similarity in form and morphologic gra-

dation between Cambrian apparently sinistral (ultradextral?) forms (e.g., Scaevogyra,

Matherella, Kobayashella) and operculate hyperstrophic macluritacean gastropods of

the Ordovician (Palliseria, Teiichispira, Maclurites) The Ordovician Maclurites has also

been interpreted as immobile filter feeders living on reef flats (Webers et al 1992).Among gastropods, gross shell morphology often reflects basic trophic strategy andfunction Thus, the concentration of such a large number of major transitions pertime interval in the Late Cambrian –Middle Tremadoc (Wagner 1995) may indicatethat the principal trophic groups had already evolved by then

Late Cambrian assemblages include abundant and relatively large hypseloconidsand macluritids, predominantly in high-energy bioclastic carbonates deposited innearshore medium to high-energy environments, often on reef flats (Webers et al.1992)

Cephalopods

Forms with tall, cyrtoconic, slightly coiled septate shells of the order Hypseloconida

(Knightoconus, Hypseloconus, Shelbyoceras) could be ancestors of the first pods (Teichert 1988) (figure 15.2 : 7) Early cephalopods, such as Plectronoceras, were

cephalo-mainly endogastric and rarely exogastric The direction of coiling does not appear to

be of high taxonomic value but might have had an adaptive significance, becauseendogastric shells could be more suitable for benthic forms (figure 15.2 : 8) A largenumber of cephalopods have been described from the Late Cambrian of North China(Chen et al 1979a,b; Chen and Qi 1982), and about 150 species, 40 genera, 8 fami-lies, and 4 orders were recognized (Chen and Teichert 1983) An additional but muchless diverse cephalopod fauna is known from Kazakhstan, Siberia, and Laurentia Thissurprising diversity of early cephalopods, most of which had become extinct by theend of the Late Cambrian, is consistent with the explosive record of other Late Cam-brian mollusks However, the data are restricted to mostly Chinese localities and needmore investigation

Early cephalopods might have been carnivores, although, this has not been quately demonstrated The earliest forms were basically benthic, with their shells ver-tical in life The elaboration of a regulatory mechanism controlling buoyancy made itpossible to inhabit an ecologic niche with very good prospects Further evolution led

ade-to increase of mobility: “From what is known of the early straight shelled pods, they were not restricted to a benthonic mode of life It is far more likely thathard parts supplied a balancing mechanism which permitted active swimming, lead-ing to nektonic existence and in rare cases even perhaps a planktonic mode of life”(Yochelson et al 1973 : 296)

cephalo-Cambrian cephalopods favored warm-water environments but do not seem to havebeen restricted to a single type of substrate They occurred from shallow water to theouter shelf and continental slope The first cephalopod fauna, which includes only

species of Plectronoceras, inhabited well-oxygenated, more or less turbulent

shallow-water environments Cephalopods then became dominants and occupied all availableecologic niches in the inner and outer shelf and the continental slope (Chen and

Teichert 1983) They occurred in turbulent water on the seaward side of stromatolitereefs and in quieter waters of level-bottom environments Nektonic cephalopods oflow diversity occurred in deposits of stagnant basins with euxinic bottoms, which isnot compatible with a benthic cephalopod fauna.

COELOSCLERITOPHORANS

Metazoan communities during the Early-Middle Cambrian abounded in problematicorganisms called coeloscleritophorans (Bengtson and Missarzhevsky 1981), bearingcalcareous sclerites of various size, shape, and degree of mineralization Sluglike coe-

loscleritophorans, so far as is known from scleritomes of Wiwaxia corrugata and

Hal-kieria evangelista, were bilaterally symmetric and probably metameric forms (Conway

Morris and Peel 1995) Paired arrangements of elongate sclerites might correspond to

an eight- or nine-segmented body (Dzik 1986) There is a certain analogy between waxiidan weakly mineralized leaflike scales and the elitra of segmented annelids (But-terfield 1990) The group might be closely related to the Annelida, but similarities withMollusca and Brachiopoda also exist (Conway Morris and Peel 1995) On the otherhand, Starobogatov and Ivanov (1996) consider that differentiation of the body into

wi-a dorswi-al surfwi-ace with “metwi-americ” orgwi-anizwi-ation of trwi-ansverse sclerite rows, wi-and wi-a tral surface without cuticularization, as well as the presence of a cuticular radula-like

ven-apparatus, still allow Wiwaxia to be ascribed to the subphylum Aculifera (Mollusca) Furthermore, the anterior and posterior shells of Halkieria may be homologous with

the first and last plates of chitons, and consequently, Starobogatov and Ivanov (1996)

assign Halkieria to the class Polyplacophora.

Comparative functional morphology of sluglike coeloscleritophorans, such as

Wi-waxia and Halkieria, bears on their ecology According to reconstructions (Conway

Morris 1985), Wiwaxia had a slightly elongate, almost isometric body covered with

imbricated rows of flattened sclerites, and additionally carrying two sets of

elon-gate spinose sclerites Halkieria had a more elonelon-gate and flattened form Its scleritome included about 2,000 imbricate sclerites, which are smaller than those of Wiwaxia

(Conway Morris and Peel 1990, 1995) Elongate spinose sclerites are believed to haveserved in defense, judging from their upright position (Conway Morris 1985) Im-

bricate sclerites and two terminal shells of Halkieria evangelista possibly had a

pro-tective function Wiwaxiids were probably able to shed their sclerites (Conway ris 1985), although halkieriids may have grown without molting, because the twoterminal shells grew accretionally, and there were several zones where new scleriteswere generated

Mor-Locomotion of Halkieria and Wiwaxia could have been effected by locomotory

waves along the muscular sole, rimmed by lateral sclerites No discrete locomotory pendages have been observed The halkieriid body was very flexible, could shorten,and possibly enroll (Conway Morris and Peel 1995) A vagrant epifaunal lifestyle hasbeen suggested for both genera (figure 15.1 : 13)

ap-Two or three rows of posteriorly directed teeth are recognized within the anterior

part of Wiwaxia (Conway Morris 1985) A similar feeding apparatus might have been concealed under the anterior shell of Halkieria but has not yet been identified (Con-

way Morris and Peel 1995) Such a position of the mouth is typical for deposit

feed-ers, scavengfeed-ers, and grazers Surprisingly, Halkieria bodies show bradoriids stacked

in the stomach

Scaly structures of various shapes (from small groups of fused sclerites and scalyplates to relatively high scaly and tuberculate cones) are common in Early Cambrianstrata (Qian and Bengtson 1989; Bengtson et al 1990) They can all be referred toscleritophoran animals called siphogonuchitids (Bengtson 1992) Apart from an un-restricted and fairly large basal opening, siphogonuchitid sclerites are superficiallysimilar to those of halkieriids Siphogonuchitids presumably lacked terminal shellswith growth lines, but some bilaterally symmetric caplike shells, often confused withmonoplacophorans sensu lato, might belong to siphogonuchitid sclerotomes (e.g.,

Maikhanella, Purella) However, none has yet been found.

Spongelike chancelloriids are referred to as coeloscleritophorans, because they bearsclerotomes consisting of hollow calcareous sclerites (Bengtson and Missarzhevsky1981) Recent investigations of chancelloriids suggest that they were attached to thesubstrate (A Zhuravlev, pers comm., 1996) and that sclerites were hardly used toprovide grip on the sediment (cf Bengtson 1994) The most completely preserved

specimens referable to chancelloriids recovered in the Burgess Shale are of Allonnia

sp and are up to 20 cm high Chancelloria eros (up to 10 cm high) and C pentactina

were also described as bag-shaped forms from the Middle Cambrian Wheeler Shale

of Laurentia (Rigby 1978) They show that the sclerotome consists of a different type

of rosettes These animals have funnel-shaped bodies covered with sclerites and a thinskinlike layer between sclerites (Mehl 1996) A group of small sclerites located at oneend might represent a growth zone and /or mouth Rare specimens form groups ofindividuals of various growth stages Mehl (1996) inferred from the type of biomin-eralization and growth that chancelloriids represent an early branch of the Deuteros-tomia rather than mollusks, but Butterfield and Nicholas (1996) compare the micro-structure of organic-walled chancelloriid sclerites with that of the fibers in certainhorny sponges However, sessile conical pelecypods, rudists, should be kept in mind.Even if chancelloriids were not sponges, in spite of their overall body shape andthe presence of spiny corolla (similar to that of hexactinellids and archaeocyaths) sur-rounding an osculum-like opening, they would still most probably be sessile suspen-sion feeders (figure 15.1 : 12)

Coeloscleritophorans inhabited various environments Complete specimens of

Halkieria evangelista have been found in relatively deep subtidal deposits This is

sug-gestive of in situ preservation or minimal transport from an adjoining shallow tidal setting of a carbonate platform reminiscent of many other Burgess Shale – typefaunas (Conway Morris and Peel 1995; Butterfield 1995) The earliest diverse shellyassemblage containing siphogonuchitids (Bokova 1985; Rozanov and Zhuravlev

sub-1992) has been reported and described from the mouth of the Kotuykan River bar Uplift, Siberian Platform) by many authors (Rozanov et al 1969; Missarzhevsky1989; Khomentovsky and Karlova 1993) This unit occurs between two calcimicrobialbiostromes in the uppermost Manykay Formation and includes rare calcimicrobial

(Ana-mounds (Luchinina 1985) Mounds and adjacent rocks contain scaly caps of Purella

and disarticulated siphogonuchitid sclerites

STENOTHECOIDS

Stenothecoids represent a group of enigmatic Early to Middle Cambrian bivalved ganisms The overall shape of the skeleton resembles that of brachiopods or pelecy-pods, and even hinge teeth are found (Pel’man 1985) Their adult shells are usuallyslightly inequivalve, and individual valves are often curved or have asymmetric mar-gins Stenothecoids are, to some extent, similar to monoplacophorans sensu lato,considering the valve shape and metameric paired possible muscle scars on the in-ternal surface Based on this, Runnegar and Pojeta (1974) regarded them as possiblebivalved monoplacophorans

or-Stenothecoid soft-body anatomy is unknown, and the plane of symmetry is tionable In this chapter the viewpoint of Yochelson (1969) is accepted — that is, thatstenothecoids are a distinctive class of brachiopod-like animals with the plane of sym-metry crossing the valves Nonetheless, Aksarina (1968) has proposed a pelecypod-like bilateral symmetry, and Rozov (1984) established a new phylum Stenothecatabased on the two planes of symmetry

ques-Stenothecoids (Stenothecoides? kundatensis) are first reported from the late

Tom-motian of the Altay Sayan Foldbelt (Pel’man et al 1992) but definitely occur inthe Atdabanian (Rozanov and Zhuravlev 1992) Similar shells were found in a sub-Tommotian part of the Manykay Formation (Missarzhevsky 1989), where they weredescribed as the earliest stenothecoids by Bokova (1985) Their steinkerns are slightlyasymmetric and bear concentric plication, growth lines, and small tubercles No teeth

have been found There is a similarity in shape to smooth steinkerns of Purella cf.

antiqua from the same sample, which normally has scaly walls and co-occurs with

siphogonuchitidan sclerites

It can be speculated that stenothecoids might have evolved from a halkieriid-likeancestor with two terminal shells, by reduction of intermediate sclerites This processmay have been accompanied by transition from vagrant to sessile lifestyle Stenothe-coids probably were suspension feeders inhabiting calm silty interreef environments(Spencer 1981; Zhuravlev 1996) (figure 15.1 : 6)

HYOLITHS

Cambrian elongate cone-shaped calcareous conchs are ascribed to hyoliths An culum (or the second valve) sealed the conch in true hyoliths Two taxonomic subdi-