Records of the Western Australian Museum 18: 209-217 (1996).

Early Cambrian stenothecoid molluscs from China

Yu Wen

Department of Earth and Planetary Sciences, Western Australian Museum, Francis Street,Perth, Western Australia 6000, Australia

Abstract A new species of Stenothecoides, S. yochelsoni sp. nov., is describedfrom the Early Cambrian Huangshandong Member of the TongyingFormation in Eastern Yangtze Gorge, western Hubei, China. This species isone of the new members of the Yangtze micromolluscan fauna. Theoccurrence indicates that the first appearance of Stenothecoides can be datedto the earliest Cambrian, and it is probably the earliest record of the genus.Two other Early Cambrian species, Bagenovia cf. sajanica Horny andStenothecoides sp., are described from the Xidashan Formation of Kuruktag,Xinjiang, China.

INTRODUCTIONIn 1979, the writer while making a study of the

earliest Cambrian molluscs from the EasternYangtze Gorge of western Hubei, China, recoveredfrom acid-residues from the HuangshandongMember of the Tongying Formation of theTianzhushan Section, an undescribed species ofStenothecoides. The Tianzhushan Section is locatedat the west side of the Dianziliang Primary Schoolat Tianzhushan, Liantuo, about 25 km northwest ofthe city of Yichang. The Huangshandong Member,a richly fossiliferous unit, is composed mainly ofpurplish brown arenaceous-rudaceous dolomites,intercalated with striped siliceous phosphorites,about 1.88 m in thickness. It conformably overliesthe Precambrian Baimatuo Member of theTongying Formation and is overlaindisconformably by the Early Cambrian ShuijingtuoFormation (Zhao et al. 1980).

The material in the arenaceous-rudaceousdolomites was preserved in association with otherelements of the Yangtze micromolluscan fauna,such as Tchangsichiton notabilus Yu, Sinoconusclypeus Yu, Yangtzeconus priscus Yu, Obtusoconusrostriptutea (Qian), Purella tianzhushanensis Yu,Igorella hamata Yu, Archaeospira ornata Yu,Maclurites hubeiensis Yu and Heraultipegmayumumense He and Yang (Yu 1979, 1984a, b, 1985,1987, 1990; Qian, Chen and Chen 1979; He andYang 1982). Of all the micromolluscs, the mostinteresting and characteristic forms are Y. prisCtls,A. Or/lata, O. rostriptutea, 1. hamata and H.ywmanense. These fossils are widely distributed inthe Meishucunian Stage in eastern Yunnan,western Sichuan and in corresponding beds inmany parts of the Yangtze Platform.

Bagenovia cf. sajanica Horny and Stenothecoides sp.were collected in 1978 by Zhang Sengui, Nanjing

Institute of Geology and Palaeontology, AcademiaSinica, from the base of the Early CambrianXidashan Formation on the northern slope ofMohurshan of the Kuruktag Mountains, Xinjiang,China. On the basis of palaeontological andlithological characteristics, the Early Cambrian ofthis area has been divided into two formations: inascending order, the Xishanbuluk Formation andthe Xidashan Formation (Zhang 1983).

The Xidashan Formation is about 88 m inthickness (Zhang 1983). It is composed chiefly ofblackish-grey, arenaceous limestones andargillaceous limestones with a basal bed ofpurplish-grey, arenaceous, lenticular limestone.The basal bed yields a rich fauna of trilobites andarchaeocyathids and some monoplacophorans,gastropods, brachiopods and hyolithids.Stenothecoids are rare within the fauna of theXidashan Formation, only three specimens beingobtained; however these include two speciesplaced in two genera, namely: Bagenovia cf. sajanicaHorny and Stenothecoides sp. In association withthem are the trilobites Metaredlichioides rectangularisZhu and Lin, Chengkouia xinjiangensis Zhu and Lin(Zhu and Lin 1983); the archaeocyathidsAldanocyathus cf. belvederi (Rozanov), Coscinocyathusxinjiangensis Zhang (Zhang 1983) and the molluscsEocyrtolites radiatus Yu and Cyrtodiscus?kuruktagensis Yu (Yu, 1986). Among the associatedfossils, the genera Metaredlichioides and Chengkouiaare zonal fossils of the Metaredlichioides-Chengkouiazone. This zone is widely distributed in the MiddleTsanglangpu Stage of southeastern Shaanxi,northwestern Sichuan and eastern Guizhou (Zhouand Yuan, 1980). Therefore, the basal part of theXidashan Formation is considered to be middleEarly Cambrian, equivalent to the MiddleTsanglangpu Stage in eastern Southwest-Central

210 Yu Wen

., ...........·V\~-----L..--,._.~. )

IAStenothecoides -----~_

• Bagenoviao 4IlO 80lIan, . ,

Figure 1 Sketch map showing the fossil localities.

China (Zhang 1983; Zhu and Lin 1983; Yu 1986).Specimens described in this paper are all

deposited at the Nanjing Institute of Geology andPalaeontology, Academia Sinica.

STENOTIIECOIDES AND BAGENOVIA

Stenothecoides has mainly been reported from theEarly Cambrian of east Greenland (Poulsen 1932),Laborador and Yukon Region of Canada (Resser1938, Yochelson 1968, 1969), Siberia (Horny 1957;Missarzhevsky in Rozanov and Missarzhevsky1966; Aksarina 1968, Aksarina and Pelman 1978),central Kazakhstan (Koneva 1979) and westernMongolia (Zhegallo 1982), and the MiddleCambrian of United States (Walcott 1884, 1886;Robison 1964), British Columbia of Canada (Rasetti1954), central Kazakhstan (Koneva 1979) andcentral North Greenland (Peel 1988). During thelast two decades, this genus has been extensivelyfound in China. It has been collected from theHuangshandong Member of the Early CambrianTongying Formation of Eastern Yangtze Gorge,western Hubei, the Early Cambrian XidashanFormation of Kuruktag, Xinjiang and the EarlyCambrian Tianheban Formation of Xianfeng ofHubei and other places (Figure 1). Of the materialfrom these formations, that from the EasternYangtze Gorge is a bivalved, asymmetrically

inequivalve form. According to previous records,most species of the genus Stenothecoides occur onlyas a single non-articulated valve. Only a fewspecies were preserved in a bivalve condition, e.g.,Stenothecoides knighti Yochelson (1969: 59-60, fig. 2)from the Early Cambrian of Yukon, Canada, S.siberica (Aksarina) (Aksarina and Pelmen 1978: lIS,pI. XVII, figs. 4-6) from the Early Cambrian LenianStage of Siberia, S. bellus Koneva (1979: 26, pI. V,fig. 1) from the Early Cambrian Lenian Stage ofcentral Kazakhstan and Stenothecoides sp.(Yochelson 1969: 51, fig. 1) from the upper EarlyCambrian of Siberia.

So far as I am aware, most of the Early Cambrianspecies of Stenothecoides are associated withtrilobites or archaeocyathids. However, the newspecies of Stenothecoides described herein is foundwith a micromolluscan fauns that predates thetrilobites. It therefore possibly represents theearliest record of Stenothecoides. Therefore, the firstappearance of Stenothecoides can be traced back tothe very earliest Cambrian. Furthermore, it is anew member of the Yangtze micromolluscan fauna,which occurs in the earliest CambrianMeishucunian Stage of the Yangtze Platform inChina. As such it is the first biomineralisationevent in the evolutionary history of invertebratesin the Phanerozoic. It also represents the first stageof the diversification in the history of Phylum

Stenothecoid molluscs from China

Mollusca. The fauna consists of the generaYangtzechiton and Lllyanhaoclziton (Polyplacophora);Yangtzemerisma and Merismoconclza(Merismoconchia); Eosoconlls and Archaeotremaria(Monoplacophora); Yangtzespira and Arclzaeospira(Gastropoda); Yangtzedonta (Bivalvia) andHerallltipegma (Rostroconchia) (Yu 1979, 1984 a, b,1985, 1987, 1990; Jiang in Luo et al., 1982; He andYang 1982). The new species of Stenothecoidesdescribed herein, like other Meishucunianmicromolluscs, is characterized by its tiny shell. Ingeneral, the usual size of Stenothecoides is 4-10 mmin length, though specimens can reach a length of14 mm or so. However, the Meishucunian speciesis 1.1 mm long.

The discovery of Stenothecoides from theMeishucunian Stage provides important materialevidence for study of the origin of Stenothecoidaand the evolutionary relationship with othermolluscs. Of special interest is that most EarlyCambrian species of Stenothecoides possessed adistinct ridge or fold on the dorsal valve or rightvalve of variable expression (Rasetti 1954;Yochelson 1969; Koneva 1979; Peel 1988), while inthe Middle Cambrian, only a few species have aridge on the valve. The new species ofStenothecoides has a thick ridge on the dorsal valve,occupying more than one-third the width of thevalve. These characteristics indicate that thisspecies is a primitive species of the genusStenothecoides.

It is worth noting that the upper part of theH uangshandong Member bears a richmicromolluscan fauna. The polyplacophorans,monoplacophorans, gastropods and rostroconchseither lived a benthonic, creeping life within thesub tidal and intertidal zone or were infaunaldeposit feeders. Most shells are preservedincompletely, some overlap with each other, whilesome are fragmentary. These characters show thatthey were deposited in the turbulent flow of ashallow sea or even in the littoral zone.

One of the forms of stenothecoids in theXidashan Formation at Kuruktag, Xinjiang isBagenovia cf. sajarzica. Horny in 1957 named thegenus taking Bagenovia sajanica Horny as the typespecies. Bagerzovia sajarzica was first reported fromthe Early Cambrian of west Sayan, Siberia. Thisgenus has been found in the Early Cambrian ofSiberia (Radugin 1937; Horny 1957, Sytchev 1960;Aksarina and Pelmen 1978) and the EarlyCambrian Lenian Stage of central Kazakhstan(Koneva 1976, 1979). In China, it has been found inthe Early Cambrian Xidashan Formation ofKuruktag, Xinjiang, the Early CambrianWuxinzhen Formation of Yichun, Heilongjiang andthe Early Cambrian Shipai Formation of Suizhou,Hubei. Two specimens were procured from theXidashan Formation, including one covered with

211

radiating costae and one well-preserved internalmould with internal structures.

The internal structures of the valve consists offour pairs of asymmetrically arranged ridge-likeimpressions and three concentric impressionsvisible on the dorsal valve of the internal mould(Figure 3). The structure may correspond toanatomical features, but the soft tissues ofstenothecoids are quite unknown. Since they haveno expression of the outer surface and the directionof the ridge-like impressions are the reverse of theradiating costae, it seems likely that they reflect theimpressions of some organs of the animal. Thefunction of the internal structures is, however,difficult to interpret. It might be assumed that theyeither had a direct effect on their free movementor, alternatively, relate to the control of the openingand closing of the valves or some otherphysiological action, perhaps representing muscleattachments (Yochelson 1969).

The taxa Bagenovia cf. sajanica and Stenothecoidessp. co-occur with shallow and warm water fossils,such as archaeocyathids and the trilobiteMetaredlichioides and Chengkouia. Most specimensof archaeocyathids, trilobites, brachiopods andmolluscs are preserved incompletely, some of themdisorderly preserved in the limestone, others asfragments (Zhang 1983). These characteristicsindicate that the fauna reflects a shallow watercarbonate facies, perhaps deposited in aepicontinental sea. Their associated sediments,arenaceous limestone, are characteristic of shallowwater deposition.

SYSTEMATIC PALAEONTOLOGY

Class Stenothecoida Yochelson, 1968

Superfamily Cambridiacea Homy, 1957

Family Cambridiidae Homy, 1957

Genus Stenothecoides Resser, 1938

Stenothecoides yochelsoni sp. novo(Figure 2 A-F)

HolotypeNIGP 116387, in the collections of the Nanjing

Institute of Geology and Palaeontology, AcademiaSinica, collected by the writer in 1978 fromHuangshandong Member of the Early CambrianTongying Formation at Tianzhushan of Yichang,Hubei, China.

DiagnosisTiny, bivalved, asymmetrically inequivalved,

narrowly elongated, about one-third as wide aslong. Two valves closed very tight, commissuralline curving slightly downward. Conjoined line notknown in detail. Apical area high, curves

212 Yu Wen

G

E

c

F

IH

Figure 2 A-F. Stenothecoides yochelsoni sp. novo NIGP 116387, Huangshandong Member of Early Cambrian TongyingFormation, Tianzhushan of Yichang, Hubei, A. plan view, x 40. B. enlargement of dorsal valve, showingthick ridge and other internal structures, x 80. C, left side view, x 40. D. enlargement of left side, showingthe commissural line, x 90. E. plan view, x 40. F. enlargement of ventral plan view, x 80. G, right side view,x 40. H-I, Stenothecoides sp. NIGP 116391, Xidashan Formation, Kuruktag Mountains, Xinjiang, plan andlateral views, x 4.

Stenothecoid molluscs from China

anticlockwise during growth, strongly protrudingand overhanging adapical margin. A prominentlythick ridge starting from apical area, extending andwidening to the abapical margin. Ventral valvesmaller and lower than the dorsal one. Apex large,strongly rounded.

DescriptionShell very tiny, 1.1 mm long, 0.45 mm high and

0.5 mm wide; bivalved, asymmetricallyinequivalved and narrowly elongated, about one­third as wide as long. Two valves closed very tight,commissure of valves simple, commissural linecurving slightly downward, in particular anteriorlyand posteriorly. Conjoined line not known indetail. Apical area strongly arched. Dorsal valvehigh, strongly arched, the apex being at about one­third of the length. Apical area high, stronglyprotruding and overhanging the adapical marginand curving anticlockwise during growth. Aprominently thick ridge starting from the apicalarea, extending backward for about two-thirds thetotal length and then abruptly expanding andwidening to the abapical margin. The ridge isbordered by two narrow furrows, occupying aboutone-third the width at the middle part of the dorsalvalve and then almost the whole part at the end.The lateral slopes slightly depressed and thenoblique to the lateral margins.

Ventral valve smaller and lower than the dorsalvalve. Apex large, strongly rounded and convex,gradually sloping toward the abapical margin. Thelateral slopes of the apex abruptly convex and thenflatten to the margin.

The surface character is unknown, except forsome granules in the dorsal valve.

RemarksJudging from the general morphological

characters of the shell, Stenothecoides yochelsoni sp.novo is similar to S. knighti Yochelson (1969: 59-60,figure 2) from the Early Cambrian of Yukon,Canada. They share such common features as: theasymmetrically bivalve shell, the possession of aprominent ridge on the dorsal valve; and simplecommissure of the valves. However, there are stillgreat differences between S. yochelsoni and S.knighti, essentially in the shell being narrowlyelongated in dorsal valve view in the former, butsubelliptical in dorsal valve view in the latter; in S.yochelsoni the commissural line is curving slightlydownward, whereas in S. knighti the commissuralline is curving slightly upward; in the dorsal valveof S. yochelsoni the apical area is stronglyprotruding and overhanging the adapical margin,but in S. knighti, the apical area is low, slightlyoverhanging the ventral valve; in the former, thedorsal valve is strongly arched and has aprominently thick ridge beginning from the apical

213

area, extending backward for about two-thirds thetotal length and then abruptly expanding andwidening to the abapical margin, while in thelatter, the dorsal valve is low and has a narrowridge; in S. yochelsoni the ventral valve is small andlow, with a large and rounded apical area, but in S.knighti, the ventral valve is relatively convex andwith a curved ridge.

In possessing a narrowly elongate valve, S.yochelsoni is closely related to Stenothecoides sp.(Yochelson 1969: 51, figure 1) from the upperLower Cambrian of Siberia, but it is stronglydistinguished from Siberian species in the moreconvex dorsal valve, the less convex ventral valve,in the presence of a distinctly thick ridge on thedorsal valve and the more protruded apical area.

In some respects this species resemblesStenothecoides bel/us Koneva (1979: 26, pI. V, fig. 1)from the Early Cambrian Lenian Stage of centralKazakhstan. It differs from the latter in the morenarrowly elongated shell; the thicker ridge, themore protruded apical area and in the smallerventral valve.

EtymologyThe specific name is in honour of Or Ellis L.

Yochelson of the Department of Paleobiology,National Museum of Natural History, WashingtonD.C., USA.

OccurrenceUpper part of the Huangshandong Member of

the Early Cambrian Tongying Formation atTianzhushan of Liantuo, Yichang, Western Hubei.

Stenothecoides sp.(Figure 2 H-I)

MaterialA single specimen, NIGP 116391, in the

collections of the Nanjing Institute of Geology andPalaeontology, Academia Sinica. Collected byZhang Sen-gui in 1978 from the base of the EarlyCambrian Xidashan Formation on the northernslope of the Mohurshan of the KuruktagMountains, Xinjiang, China.

DescriptionShell of large size, 13 mm long and 6 mm wide;

asymmetric, fairly elongately ovate in plan view.Apex sharply pointed, distinctly protruding andoverhanging the adapical margin and curvinganticlockwise during growth. In plan view thevalve is convex and tends to be most pointed at theadapical margin; narrowly rounded abapicalmargin and somewhat bent to right. Sidesunequally convex, with the greatest width of thevalve at about one-third of the length.

214

Surface of the shell ornamented by prominent,irregularly spaced coarser or finer growth lines.

RemarksAlthough Stenothecoides sp. from the Early

Cambrian Xidashan Formation in Kuruktag is anincomplete specimen, it represents a bivalvedstenothecoid with distinct characteristics: thenarrowly elongated and asymmetrical dorsal valve;the apex curves anticlockwise during growth andwith irregularly spaces growth lines. This speciesshould be unquestionably assigned to the genusStenothecoides.

This form is very similar to Stenothecoides sp.from the upper Lower Cambrian of Siberia(Yochelson 1969: 51, figure 1) in such characters asthe narrowly elongated dorsal valve, the pointedapex and the convex dorsal valve, but differs fromthe latter in the proportion between the length andwidth of the valve, in the more rounded posteriorand in the coarser growth lines. In plan view, thisspecies somewhat resembles Stenothecoides carinatusKoneva (1979: 27-28, pI. VI, figs. 5, 6) from theEarly Cambrian Lenian Stage of centralKazakhstan, differing in the slender and morecurved valve, in the narrower abapical margin andin the irregularly spaced growth lines. It is alsoallied to Stenothecoides elongata (Walcott) (1886: 129,pI. 12, figs. 4 a-h (not figure 4); Resser 1938: 24;Rasetti 1954: 63, pI. 11, figs. 3, 4), but differs fromthe latter in the more bent dorsal valve, in thewider abapical margin and in the irregularlyspaced growth lines.

OccurrenceBasal part of the Early Cambrian Xidashan

Formation of Mohurshan, Kuruktag Mountains,Xinjiang, China.

Genus Bagenovia Horny, 1957

Bagenovia cf. sajanica Horny, 1957(Figures 3, 4 A-I)

Bagenovia sajanica Radugin, 1937: 301, fig. 5,(nomen nudum).

Bagenovia sajanica var. raricostata Radugin, 1937:301, fig. Sa (nomen nudum).

Bagenovia sajanica Horny, 1957: 428, pI. Ill, figs. 1-6;Knight and Yochelson, 1960: 183, fig. 50, 8.

Bagenovia multicostata Yu, in Zhang, 1983: 10(nomen nudum); Gao et al., 1984: pI. IX, figs. 6,9 (nomen nudum); Yu, 1986: 10 (nomennudum).

MaterialThree specimens, NIGP 116388-116390 in the

YuWen

Figure 3 Bagenovia cf. sajanica Horny, showing theinternal structures, X 8.

collections of the Nanjing Institute of Geology andPalaeontology, Academica Sinica. NIGP 116388and 116389 were collected in 1978 by Zhang Sen­gui from the base of the Early Cambrian XidashanFormation on the northern slope of the Mohurshanof the Kuruktag Mountains, Xinjiang, China. NIGP116390 in the Nanjing Institute of Geology andPalaeontology, Academia Sinica, was collected bya geological party from the Geological Bureau ofHubei from the Early Cambrian Shipai Formationof Suizhou, Hubei, China.

DiagnosisSubcircular, slightly asymmetrical, apex pointed,

slightly curved anticlockwise during growth, withtwenty-two pairs of radiating costae from the flatkeel. There are four pairs of asymmetricallyarranged ridge-like impressions and threeconcentric impressions visible on the dorsal valveof the internal mould.

DescriptionValve of medium size, slightly asymmetrical.

Moderately convex, subcircular in plan view. Apexsmall, pointed, distinctly protruding andoverhanging the adapical margin; slightly curvedanticlockwise during growth. In plan view, theindividual valve tends to be narrowly roundedadapically, broad rounded abapical margin, withthe greatest width of the valve at about one half ofthe length. A distinctly flat keel situated near themedian part of the dorsal valve, which graduallyflattens and widens to the abapical margin. Fromthe flat keel about twenty-two pairs of costaeradiate. They are small and short in the adapicalpart, with sharp crests, separated by widerdepressions and curved forward, graduallybecoming larger and longer in the middle part andgradually crowded, indistinct abapically, withobtusely rounded crests and separated by thenarrower depressions. Radiating threads which canbe observed in the flat keel very fine and dense(Fig. 4B). Growth lines very fine and undulating;

Stenothecoid molluscs from China

E

G

,:. ~A

" ~lt• ~;IIlIr , .... -

215

A

D

I

F B

Figure 4 A-I, Bagenovia d. sajanica Horny. A-D, NIGP 116388, Xidashan Formation, Kuruktag, Xinjiang, A.planview, x 5. B. enlargement of dorsal keel, showing the cancellated sculpture, xIS. C-D. adapical and lateralviews, x 5. E-I, NIGP 116389, E, plan view, x 6. F. enlargement of valve, to emphasize the internalstructures, x 12. G-I. adapical, right and left side views, x 5.

216 YuWen

RemarksIn the general shape of the valve, this form

appears to be similar to the type species Bagenoviasajanica Homy (Radugin 1937: 301, figs 5, Sa; Homy1957: 428, pI. Ill, figs. 1-6) from the Early Cambrianof west-Sayan, Siberia, but differs in the presenceof more numerous radiating costae and in thesubcircular outline. In apical view, this species isalso similar to Bagenovia kazakhstanica Koneva(Koneva 1976: 127, Figure 1; 1979: 22, pI. 11, figs. 1­10, pI. Ill, figs. 1-2) from the Early CambrianLenian Stage of central Kazakhstan. It can bedistinguished by the coarser and more numerousradiating costae, and the undulating growth lines.The most important distinguishing feature ofBagenovia cf. sajanica is that the internal structure ofthe valve is characterized by four pairs ofasymmetrically arranged ridge-like impressionsand three concentric impressions on the dorsal

crossed by the radiating threads to form acancellated sculpture. With the increase in shellsize, the growth lines become progressively thicker,and the radiating threads more and more obscureon the margin of the valve.

Shown in Figure 4 E-I is a nearly completelypreserved internal mould. In plan view the valveappears subcircular and the apex is slightly broken.The traces of radiating costae can be seen on themargin of the valve. In the centre of the valve thereis a longitudinal shallow depression, starting fromthe apex and gradually disappearing to theposterior. On the sides of the longitudinal shallowdepression there are about four pairs ofasymmetrically arranged ridge-like impressionsand three concentric impressions (Figure 3). Thedirection of the ridge-like impressions is thereverse of the surface radiating costae. The firstpair of ridge-like impressions are short and veryobscure, situated near the apex. The second pairare situated at about two-fifths of the valve lengthfrom the adapical margin and gradually extendedforward and inclined to the adapical margin; theleft ridge-like impression is longer, and narrowerthan the right one. The third pair are similarmorphologically to the second pairs, situated at themid-length of the valve but longer and larger. Thefourth pair is fainter and more obscure than theothers, which is situated at about two-thirds of thevalve from the adapical margin. The outerconcentric impression is thin, forming a ring on theperiphery. The middle one is stronger and shorterthan the others; while the inner one is faint,gradually extending to the adapical side.

Dimensions (in mm)

NIGP 116388NIGP 116389

LENGTH7.006.80

WIDTH7.504.40

valve of the cast. The internal structure ofBagenovia kazakhstanica possesses a longitudinalcarina and more ridge-like short impressions andthe presence of some small pits, especially near theapex.

OccurrenceBasal part of the Early Cambrian Xidashan

Formation of Mohurshan, Kuruktag, Xinjiang andthe Early Cambrian Shipai Formation of Suizhou,Hubei, China.

ACKNOWLEDGEMENTS

I wish to thank Zhang Sen-gui of the NanjingInstitute of Geology and Palaeontology, AcademiaSinica for sending me the Xinjiang specimens, andK.}. McNamara, E.L. Yochelson and S. ConwayMorris for reviewing the manuscript and makingvaluable suggestions. Thanks also to Song Zhi-yaoand Mao Yong-qiang for preparing thephotographs and to Ren Yu-gao and D. Hendricksfor drawing the text-figures.

REFERENCESAksarina, N.A. (1968). Probivalvia - A new Class of the

oldest molluscs. In: New Data on the Geology andMinerals of Western Siberia (Occasional paper). 3: 77­86, (in Russian).

Aksarina, N.A. and Pelman, Yu. L. (1978). Cambrianbrachiopods and bivalve molluscs of Siberia. TrudyInstituta Geologii i Geofiziki, AN SSSR 362: 180 pp. (inRussian).

Gao Zhenjia, Zhu Chengshun et al. (1984) PrecambrianGeology in Xinjiang, China. Xinjiang People'sPublishing House, Urumuqi (in Chinese).

He Tinggui and Yang Xianhe, (1982). Lower CambrianMeishucun Stage of the western Yangtzestratigraphic region and its small shelly fossils.Bulletin of the Cllengdu Institute of Geology, M.R.,Chinese Academy of Geological Sciences 3: 69-95, (inChinese with an English summary).

Homy, RJ. (1957). Problematic molluscs (? Amphineura)from the Lower Cambrian of south and east Siberia(U.SSR). Sbornik Ustredni Ustav Geologicky, Oddilpaleontologicky 23: 297-432 (in Russian, Czech andEnglish).

Knight, J.B. and Yochelson, E.L. (1960).Monoplacophora. In: RC. Moore (ed.). Treatise ofInvertebrate Palaeontology, part 1, Mollusca 1,Geological Society of America and University ofKansas Press, Lawrence, Kansas: 177-184.

Koneva, S.P. (1976). New members of the ClassStenothecoida from the Lower Cambrian of centralKazakhstan. Paleontologicheskii Zhurnal 2: 230-233 (inRussian).

Koneva, S.P. (1979). Stenothecoids and InarticulateBrachiopods of the Lower Cambrian and Basal Middle

Stenothecoid molluscs from China

Cambrian of Central Kazakhstan. Trudy InstitutaGeologii, AN SSR Kazakhstana, Alma Ata: 123 pp.(in Russian).

Luo Huilin, Jiang Zhiwen, We Xiche, Song Xueliang andOuyang Lin (1982). The Sinian-Cambrian Boundary inEastern Yannan, China. The Yunnan People'sPublishing House, 265 pp. (in Chinese).

Peel, J.S. (1988). Molluscs of the Holm Del Formation(late Middle Cambrian), central North Greenland.Meddeleser om Gronland. Geoscience 20: 145-168.

Poulsen, C (1932). The Lower Cambrian faunas of EastGreenland. Meddeleser om Gnmland 87(6): 1-66.

Qian, Yi, Chen Meng-e and Chen Yiyuan, (1979).IIyolithids and other small shelly fossils from theLower Cambrian Huangshandong Formation in theeastern part of the Yangtze Gorge. ActaPalaeontologica Sinica 18(3): 207-230.

Radugin, K.V. (1937). On the relations of the Cambrianand Precambrian in the Gornaya Shoria. Problems inSoviet Geology 7(4): 295-317 (in Russian).

Rasetti, F. (1954). Internal shell structures in the MiddleCambrian gastropod Scenella and the problematicgenus SterlOthecoides. Journal of Paleontology 28: 59-66.

Resser, CE. (1938). Four contributions to nomenclatureof Cambrian fossils. Smithsonian MiscellaneousCollection 97(10): 43 pp.

Robison, R.A. (1964). Late Middle Cambrian faunasfrom western Utah. Journal of Paleontology 38: 510­566.

Rozanov, A. Yu. and Missarzhevsky, V.V. (1966).Biostratigraphy and fauna of the lower strata of theCambrian. Trudy Geologicheskogo Instituta, AN SSSR,148: 126 pp.

Sytchev, LA. (1960). Phylum Mollusca: Mollusks. ClassLamellibranchiata: Pelecypods. In L.L. Halfina (ed.).Paleozoic Biostratigraphy of Sayano-Altai MountainRegion, 1. Trudy SNIIGGIMS 19: 253-256 (in Russian).

Runnegar, B. (1983). Molluscan phylogeny revisited.Memoir of the Association of AustralasianPaleontologists 1: 121-144.

Voronin, Yu. I., Voronova, L.G., Grigoreva, N.V.,Drozdova, N.A., Zhegallo, E.A., Zhuravlev, A. Yu.,Ragozina, A.L., Rozanov, A. Yu., Sayutina, T.A.,Sysoev, V.A. and Fonin, V.D. (1982). Granitsadokembriya i kembriya v geosinklinal'nykhoblastyakh (opornyj razrez Salany-Gol, MNR). [ThePrecambrian-Cambrian boundary in thegeosynclinal areas (the reference section of Salany­Gol, MPR).] Trudy Sovmestnoj Sovetsko-Mongol'skojPaleontologischeskoj Ehkspeditsii 18, 150 pp. Nauka,Moscow (in Russian).

Walcott, CD. (1884). Palaeontology of the Eurekadistrict, Nevada. Monograph of the United StatesGeological Survey, 8.

Walcott, CD. (1886). Second contribution to studies onthe Cambrian fauna of North America. Bulletin of theUnited States Geological Survey 30: 1-369.

217

Yochelson, E.L. (1968). Stenothecoida, a proposed newclass of Cambrian Mollusca. Abstracts. InternationalPalaeontological Union, Prague, Czechoslovakia, August20-27, 1968: 34.

Yochelson, E.L. (1969). Stenothecoida, a proposed newclass of Cambrian Mollusca. Lethaia 2: 49-62.

Yu Wen (1979). Earliest Cambrian monoplacophoransand gastropods from western Hubei with theirbiostratigraphical significance. Acta PaleontologicaSimca 18(3): 233-270.

Yu Wen (1984a). Early Cambrian molluscan faunas ofMeishucun Stage with special reference toPrecambrian-Cambrian boundary. Academia SinicaDevelopments in Geoscience Contribution to 27thInternational Geological Congress, 1984, Moscow: 21-35,Science Press, Beijing, China, July 1984.

Yu Wen (1984b). On Merismoconchids. ActaPalaeontologica Sinica 23: 432-446.

Yu Wen (1985). Yangtzedonta - a problematic bivalvefrom the Meisuncun Stage, China. ActaMicropalaeontologica Sinica 3: 401-408.

Yu Wen (1986). Lower Cambrian univalved molluscsfrom Kurugtag, Xinjiang. Acta Palaeorltologica Sinica25: 10-16.

Yu Wen (1987). Yangtze micromolluscan fauna inYangtze Region of China with notes onPrecambrian-Cambrian boundary. Stratigraphy andPalaeontology of Systematic Boundaries in China.Precambrian-Cambrian Boundary (I): 19-344. NanjingUniversity Publishing House, Nanjing, China.

Yu Wen (1990). The first radiation of shelled molluscs.Palaeontologia Cathayana 5: 139-170.

Yu Wen (1996). The Yangtze micromolluscan fauna andits significance. In John E. Reptiski (ed.). Sixth NorthAmerican Paleontological Convention Abstracts ofPapers. Smithsonian Institution, Washington, ne. June9-Jlme 12, 1996. The Paleontological Society SpecialPublication 8: 433.

Zhang Sengui (1983). Early Cambrian archaeocyathidsfrom Kuruktag, Xinjiang. Acta Palaeontologlca Sinica22: 9-20.

Zhao Ziqiang, Xing Yusheng, Ma Guogan, Yu Wen andWang Ziqiang (1980). The Sinian System of EasternYangtze Gorges, Hubei. Research on PrecambrianGeology, Sinian Suberathem in China: 31-55. TianjinScience and Technology Press, China (in Chinese).

Zhou Zhiyi and Yuan Jinliang (1980). Lower Cambriantrilobite succession in southwest China. ActaPalaeontologica Sinica 19: 331-339.

Zhu Zhaolin and Lin Huanliang (1983). Some earlyCambrian trilobites from the Xidashan Formation ofKuruktag, Xinjiang. Acta Palaeontologica Sinica 22: 21­30.

Manuscript received 3 July 1995; accepted 20 November1996.