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Tuarangia from Bornholm (Denmark)and similarities in Baltoscandian andAustralasian late Middle CambrianfaunasVivianne Berg-Madsen aa Geological Institute , Stockholm University , Box 6801,Stockholm, S-10691, SwedenPublished online: 27 Nov 2008.

To cite this article: Vivianne Berg-Madsen (1987) Tuarangia from Bornholm (Denmark) andsimilarities in Baltoscandian and Australasian late Middle Cambrian faunas, Alcheringa: AnAustralasian Journal of Palaeontology, 11:3, 245-259, DOI: 10.1080/03115518708618991

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Tuarangia from Bornholm (Denmark) and similarities in Baltoscandian and Australasian late Middle Cambrian faunas VIVIANNE BERG-MADSEN

BERG-MADSEN, V., 1987:08:31. Tuarangia from Bornholm (Denmark) and similarities in Baltoscandian and Australasian late Middle Cambrian faunas. Alcheringa ll , 245-259. 1SSN 0311-5518. Similarities in late Middle Cambrian trilobite, inarticulate brachiopod and molluscan

faunas are apparent between Bornholm (Denmark) and New Zealand. Tuarangia gravgaerdensis sp. nov. is described from the late Middle Cambrian Andrarum Limestone of Bornholm, Denmark. The Andrarum Limestone is stratigraphically correlated with the Tasman Formation, New Zealand, from where the genus Tuarangia was first described. A single specimen, referred to Tuarangia, has been found in an erratic boulder from NW Poland. The boulder is estimated to be of early Late Cambrian age. The initial taxonomic assignment of Tuarangia to the Bivalvia, Subclass Pteriomorphia, is upheld. The distance between Bornholm and New Zealand, the position at opposite hemispheres, and the palaeogeography in general clearly indicate isocommunities with restricted possibility of exchange of the gene pool at species level. Vivianne Berg-Madsen, Geological Institute, Stockholm University, Box 6801, Stockholm, S-10691, Sweden; received 10 June 1986.

Keywords: Bivalvia, bivalved monoplacophorans, Bornholm, Denmark, Andrarum Lst., late Middle Cambrian, early Late Cambrian, NW Poland, Australia, New Zealand, Tasman Fro., palaeogeography, isocommunities.

THE MIDDLE CAMBRIAN bio- and lithostratigraphy of Bornholm (Denmark) has recently been revised. Previous faunal investigations concentrated on macrofossils such as tri lobites and brachiopods (Linnarsson, 1876; Gr6nwall, 1902)whereas the microfauna was ignored. I have concentrated on the microfauna in which echinoderms and several new molluscan groups have been found (Berg-Madsen, 1981, 1985b, c, 1986a). Of growing interest are the faunal similarities between Bornholm and New Zealand Middle Cambrian sequences (Opik, 1979; Henderson & MacKinnon, 1981).

Palaeogeography Two major faunal provinces characterized by trilobite genera existed during the Early Cambrian: Laurentia, and Gondwana. Baltica comprised a small third province where forms typical of the other provinces are lacking

0311/5518/87/020245-15 $3.00 © AAP

(Cowie, 1974; Bergstr6m & Gee, 1985). When continental rifting began, the plates drifted apart. The first reliable base map shows the status in Late Cambrian and Early Ordovician (Fig. 1). Attached to Gondwana, New Zealand was positioned almost as far distant from Bornholm as today. Both were situated at about 45 degrees latitude, with New Zealand in the northern hemisphere, and Bornholm in the southern (Smith et al., 1973; Scotese et al., 1979; Ziegler et al., 1979; Bambach et al., 1980).

During the Middle and Late Cambrian, both Bornholm and New Zealand thus were located within the temperate zones (Scotese et al., 1979; Berg-Madsen, 1983). The marine environmental conditions seem to have been much the same in both areas, judging from lithology and faunal assemblages (trilobites, inarticulate brachiopods, and molluscs). The faunal relationships usually appear on the generic level; only in the case of cosmopolitan agnostid trilobites do we find similarities on the species level. The similarities are briefly reviewed.

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246 V I V I A N N E B E R G - M A D S E N A L C H E R I N G A

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Trilobites

Polymerid trilobites and agnostids from Australia and Sweden have been compared by Whitehouse (1936, 1939) and Opik (1979). Endemism exists, especially among the polymerid trilobites. The pelagic agnostids are cosmopolitan at species level, and 22 species are common to both regions (Westerg~trd, 1946; Opik, 1979). In New Zealand, late Middle Cambrian trilobites were first recorded from Trilobite Rock (Tasman Fm.) in 1948, and most agnostid genera and species from that locality are also known from Baltoscandia (Benson, 1956). It should be noted that the Andrarum Limestone of Bornholm exhibits many endemic polymerid trilobite species and thus differs from the fauna of the type locality at Andrarum in

Scania (Fig. 2).

Brachiopods Henderson & MacKinnon (1981) described late Middle and early Late Cambrian inarticulate brachiopods from Australasia and correlated the Tasman Fro. with part of the faunal sequence (Ptychagnostus cassis Zone) in the Georgina Basin, Queensland. The inarticulate brachiopods from the Middle Cambrian of Bornholm are under revision but at least four of the nine brachiopod taxa (Acrotreta, Acrothele, Linnarssonia, Micromitra) from the Tasman Fm. and western Queensland are known from the Andrarum Limestone (Bassett & Berg- Madsen, unpublished results). Whether any Bornholm species are conspecific with the Australasian forms has yet to be established.

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Conodonts MacKinnon (1976) r eco rded the p r o t o c o n o d o n t Gapparodus bisulcatus (Miiller) together with bicuspidate and t r icuspidate forms f rom the T a s m a n Formation. Westergaardodina tricuspidata Miiller may well be present among these (David I. MacKinnon, pers. comm., 1986). In the Andrarum Limestone G. bisulcatus is the most common species, but Amphigeisina sp. also occurs. Recently a single, poorly preserved bicuspidate specimen has been

recovered. The Middle-Late Cambr ian boundary on Bornholm is placed 1.5-1.6 m above the Andrarum Lst. on the basis of the first occurrence of W. tricuspidata (Berg- Madsen, 1985c). Whereas IV. bicuspidata occurs in the late Middle Cambrian strata W. tricuspidata has only been recorded from early Late Cambrian strata in Baltoscandia, North America (M~iller, 1959, 1971), and China (Nogami, 1966). Druce & Jones (1971) noted the difficulties in conodont correlation between North America and Australia.

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Fig. 2. Map showing the position of Bornholm in southern Baltoscandia, the sub-Quaternary distribution of Cambrian deposits and the fault block system (slightly modified) in the southern part of Bornbolm. White circles indicate the exposures at Lms~ and OleO.

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248 VIVIANNE BERG-MADSEN ALCHERINGA

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Fig. 3. Correlation chart of Middle Cambrian rock formations, zones and stages of Australia, New Zealand, and Bornholm, Denmark. The Ptychagnostus atavus Zone includes a lower subzone of Tomagnostusfissus and an upper subzone of Hypagnostus parvifrons. In Sweden, the upper part of the P. punctuosus Zone - - the Goniagnostus nathorsti Zone - - is included in the Paradoxissimus forchharnmeri Stage. (Modified from Berg-Madsen, 1985b; Henderson & MacKinnon, 1981; Opik, 1979).

Monoplacophorans and other molluscs Berg-Madsen & Peel (1978) no ted a close re la t ionsh ip be tween the m o n o p l a c o p h o r a n s in the Exsulans Limes tone o f B o r n h o l m (Fig. 3), and those in the C o o n i g a n Fro. o f New Sou th Wales and the C u r r a n t Bush Fm. o f Queens land . Of these, Protowenellaflemingi R u n n e g a r & Jell was la ter f o u n d also in the A n d r a r u m Limes tone o f B o r n h o l m (Fig. 3),

w h e r e i t o c c u r s in t h e So lenop leura brachymetopa Zone (Berg-Madsen , 1985c). T h i s z o n e was c o r r e l a t e d w i t h t h e Ptychagnostus cassis Zone o f A u s t r a l i a by Opik (1979). Mos t recent ly Protowenella was descr ibed f r o m Tr i lob i t e Rock , New Z e a l a n d (Tasman F m . , Ptychagnostus cassis Zone) . The sequence also yie lded the con t rovers ia l b ivalve Tuarangia as well as o ther mol luscs

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similar to those known from the Andrarum Lst. (MacKinnon, 1982, 1985; Berg-Madsen, 1985a).

Bivalves

In Denmark, bivalves have been identified previously only from the Lower Cambrian of Bornholm (Fig. 2). Two species were reported from the middle part of Broens Odde Member: Fordilla troyensis and a poorly preserved unnamed species (C. Poulsen, 1967). The Bornholmian F. troyensis is one of the few well documented specimens recorded from outside North America (Pojeta, 1975). The apparent presence of six poorly developed teeth in the unnamed species suggested it might belong to one of the taxodont groups of bivalves. Re-examination has shown it to be the praenuculid genus Pojetaia Jell (John Pojeta, Jr., pers. comm., 1986). This genus has been described from the Early Cambrian of South Australia and western New South Wales, where it ranges from late Tommotian to earliest Middle Cambrian (Runnegar & Bentley, 1983). The Fordilla and Pojetaia-bearing strata on Bornholm were tentatively correlated with the earliest Atdabanian of the Siberian Platform (V. Poulsen, 1978) and southeastern Newfoundland (Bengtson & Fletcher, 1983). Recently, K/Ar-dating of glauconite from the lower part of Broens Odde Member of Bornholm has shown a minimum age of 536 Ma (Paul Martin Holm, Institute of Petrology, Copenhagen, pers. comm., 1985; Berg-Madsen, 1986b).

In the lower part of the Middle Cambrian of Bornholm, bivalves have not been found. However, an initial investigation of the late Middle Cambrian Andrarum Limestone (Fig. 3) has led to the discovery of Tuarangia gravgaerdensis sp. nov. in the microfauna. The organism is regarded as a bivalve, representing one of several higher taxa of molluscs found in the Andrarum Lst. (Berg- Madsen, 1981, 1985a, b, c).

Localities and lithology Because of faulting, the Middle Cambrian on

Bornholm is only exposed at two localities along the rivulets La~s~ and Ole~ (Fig. 2). The lowermost Middle Cambrian is missing, as in most of SW Baltoscandia, and the total thickness of the remaining part is on average 3.1 m. It consists of alternating grey limestone and black shale (Fig. 3), the shale continuing throughout the Late Cambrian and Early Ordovician. Lenses of black bituminous limestone occur in the Late Cambrian.

The Andrarum Limestone is widely recognized in southern Baltoscandia, the type locality being at Andrarum in Scania, Sweden (Fig. 2). Even where the limestone is absent its characteristic trilobite fauna occurs in the black shales, making correlation possible. The dipping exposure of Andrarum Lst. at Laes~ can be followed over a distance of about 20 m both in the valley and stream bed. At Ole~ the strata are almost horizontal and well exposed for about 25 m. The average thickness is 0.6 m.

The clay content of the Andrarum Limestone is high (> 25%). The clay minerals (mostly iUite) and bituminous matter cover small (> 10 #m) calcite grains in the upper dark and fine-grained part of the bed. Quartz and glauconite grains are rare and found only in the lower, lighter grey and more coarsely- crystalline parts. Concentrations of trilobite exuviae occur in certain parts of the bed. Phosphorite, in the form of diffuse bodies and weak aggregates, is most common in the upper part. Pyrite occurs throughout the bed (Hadding, 1958; Berg-Madsen, 1985c). The Andrarum Lst. is a primary marine deposit as opposed to the diagenetically formed Hyolithes limestone and anthraconite (Fig. 3) (Buchardt & Thorshej Nielsen, 1985).

The diagenetic history of Bornholm is well known from studies of stable isotopes. During a post-Middle Silurian to pre-Lower Jurassic burial the temperature reached approx. 90 ° C corresponding to a depth of at least 1500-2000 m (Buchardt & Thorshej Nielsen, 1985). This, apparently, did not affect the properties of the Lower and Middle Cambrian glauconites: radiometric dating of material from the Exsulans Limestone (Fig. 3) gave an age of 513 _+ 5 Ma (Berg-Madsen, 1986b).

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Methods, material, and preservation In this study, 7 kg of Andrarum Limestone were dissolved in 10% acetic acid, using methods similar to those described by Jell (1980); addi t ional i n fo rma t ion on the preparat ion of samples is given in Berg- Madsen (1985c). After dissolution, insoluble clay minerals fo rmed a spongy mass, occasionally phosphatized, which had to be broken with extreme care. Due to this careful t r e a t m e n t examples of excep t iona l preservation occur (Berg-Madsen, 1986a). Remains of replicated calcite are found on moulds of molluscs and trilobites in both the Andrarum Lst. and the underlying Hyolithes limestone. The processed rock yielded a total of 17 specimens of Tuarangia gravgaerdensis, including two imprints of the exterior in phosphatized matrix. T. gravgaerdensis has not been identified in thin section.

The most abundant fossils were polymerid trilobites, inarticulate brachiopods, and hyo l i th ids . In a d d i t i o n , e c h i n o d e r m fragments, ostracodes, conodonts, sponge spicules and problematic forms were found (Berg-Madsen, 1985c, 1986a). Pelagiellids and other monoplacophorans are very similar although not identical to species described f rom the late Middle Cambrian Tasman Format ion of New Zealand by MacKinnon (1985). The genera include Costipelagiella Horn~ , Protowenetla and Mellopegma Runnegar & Jell.

Increasingly, Middle Cambr ian shelly faunas - - including those of Bornholm - - are found to contain a variety of taxa of remark- ably small size by comparison with faunas of post-Cambrian age. Monoplacophorans are about 2 ram, bivalves such as Pojetaia also 2 mm. Most inarticulate brachiopods are 2-5 mm in Bornho lm (Berg-Madsen & Bassett, unpublished), as well as in the Tas- man Fro. (Henderson & MacKinnon, 1981). In Bornholm, trilobites and hyolithids are

smaller than those known from the type localities in Scania; also smaller than most similar species in Australia.

Fifteen complete phosphatized internal moulds, several fragments, and two external moulds of Tuarangia gravgaerdensis were recovered. At least seven specimens were articulated. Additional articulated specimens may be present as there are several partially embedded within the matrix, with only one valve exposed. The holotype (Fig. 4) shows a thin and smooth outer phosphatized layer with an inner filling of porous phosphatized clay. It is incomplete posteriorly. In some specimens, parts of the innermost replicated shell layer are preserved (Figs 5D, 6E-F). In this respect, the material differs from that of Australasia, in which only imprints of the layer in the phosphatic internal mould have been described. One of my external moulds replicates the presumed innermost layer (Fig. 6C-D, micrograph reversed), the other the outer shell ornament (Fig. 6A-B).

Systematic description Class BIVALVIA Linn6 1758

Subclass P T E R I O M O R P H I A ? Beurlen 1944

Order T U A R A N G I I D A MacKinnon 1982

Superfamily T U A R A N G I A C E A MacKinnon 1982

Family T U A R A N G I I D A E MacKinnon 1982

Tuaraugia MacKinnon 1982

Type species. Tuarangia paparua MacKinnon 1982

Tuarangia gravgaerdensis sp. nov. (Figs 4A- G, 5A-J, 6A-F)

Name. Named after the geological field- station Gravgaerde on Bornholm which has been the base for my fieldwork for more than 15 years.

Fig. 4. Tuarangia gravgaerdensis sp. nov., holotype MGUH17.451. A, left valve, showing the faintly raised umbo. The posterior part slightly more crushed after a second turning over on the stub, x 190. B, dorsal view showing the hinge line and the ligamental area, x 125. C, postero-dorsal view showing the inflation, the crushed posterior part and the filling of the mould, x ! 50. D, slightly oblique view of the right valve, x 125. E, posterior teeth, enlarged from G, x 170. F, anterior teeth, enlarged from G, x 250. G, slightly oblique view of the left valve, x 130.

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Material. Holotype M G U H 17.451, paratypes MGUH17.452-17.459 inclusive, Geological M u s e u m , Un ive r s i ty o f C o p e n h a g e n , Denmark.

Stratigraphic distribution. T. gravgaerdensis is presently known f rom the Solenopleura brachymetopa Zone, Andrarum Limestone, late Middle Cambrian, Bornholm, Denmark.

Diagnosis. Laterally compressed, equilateral bivalved shell with faintly raised umbo; slightly arched to straight dorsal margin with strong peg-like teeth symmetrically disposed on each side of a slot interpreted as the at tachment site of a ligament.

Description. Shell equivalved, anteriorly- p o s t e r i o r l y e longa te , l ength a p p r o x . 1.5-2.0 mm, height aprox. 0.3-0.5 mm, not strongly inflated. Equilateral to only slightly inequilateral. Umbones slightly raised above dorsal margin of shell; beaks unknown. Maximum valve height at posterior valve margin. Maximum valve length below mid- height. In internal moulds, the anterior, posterior, and ventral margins form flanges, which are not seen exteriorly. Dorsal margin long and slightly a rched to s t ra ight , a c c o m m o d a t i n g p r e s u m e d l i g a m e n t attachment area and strong peg-like taxodont teeth. Presumed ligament area located slightly posterior to mid-length, with the remaining dorsal margin occupied by teeth. The anterior teeth consist of four to five long, obliquely disposed, peg-like teeth diminishing in size towards the umbos. The posterior three to four teeth are shorter and smaller than the anterior. All teeth converge ventrally to a point below the presumed ligament area. Seen dorsally the dentition occurs as a wavy suture line interrupted by the ligament at tachment area. Muscle scars are unknown. The valve margins are apparently in close contact. Shell sculpture consists of comarginal growth lines and rugae and posterior radial threads.

Discussion. The out l ine of valves of Tuarangia paparua is variable but the arcoid outline seems most characteristic. This is emphasized in the choice of holotype (MacKinnon, 1982, pl. 1, figs 1-2) although not particularly specified in the diagnosis. In T. gravgaerdensis the valves are equilateral to only slightly inequilateral and the dorsolateral wing less prominent. Arcoid forms have not been found. The dorsal margin of T. gravgaerdensis appears to be more arched than in T. paparua; the umbones are better developed, the teeth seem stronger, and the presumed ligament area is slightly wider in T. gravgaerdensis. The valves seem to have been totally in contact along the commissure but for a slit-like gape that may be present posteriorly.

During SEM examination, a complete layer of replicated foliated calcite peeled off one of the moulds (Fig. 5H) and later disintegrated. The uncovered surface was lying upwards, and the micrographs failed to show any particular details o f the presumably foliated ultrastructure (Fig. 6F). The external mould (Fig. 6C-D) shows the growth lines of some part of an inner layer. The imprint of the outer shell surface, however, shows not only comarginal growth lines but also radial threads, an ornament typical of many bivalves and other molluscs.

There is wide variability in morphology in Tuarangia paparua, and I agree that T. gravgaerdensis may be similar to some of these variants, as believed by MacKinnon (pers. comm., 1986). The discrepancy between the diagnosis and the morphology of the T. paparua holotype is one of the reasons why I prefer to regard T. gravgaerdensis a discrete species rather than including it in a redescribed T. paparua. F u r t h e r m o r e , B o r n h o l m (Baltoscandia) and New Zealand (Australasia) must be regarded as two different faunal provinces, and the similarities as examples of isocommunities. Communicat ion across the equator, over 120 degrees longitude and up

Fig. 5. Tuarangia gravgaerdensis sp. nov. A, dorsal, slightly oblique view of conjoined valves of B, x 125. B, MGUH17.452, left valve x 130. C, MGUH17.453, right valve, x 130. D, MUGHI7.454 left valve with partly preserved layer of replicated foliated calcite, x 150. E, dorsal view of single valve (F), x 140. F, MGUH17.455, single right valve showing almost complete dentition in both sides, x 135. G, MGUHI7.456, right valve, x 150. H, MGUHI7.457, right valve from which the thin layer of replicated foliated calcite (Fig. 6F) peeled off. The surface is uncovered, x65. I, single right? valve in phosphate, specimen lost, x90. J, lateral view of I showing the inflation, x 135.

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to 30,000 km distance is difficult to visualize (Fig. 1). A continuous exchange of the gene pool seems unlikely.

Tuarangia sp. (Fig. 7A-B)

Material. A single specimen is known from an erratic boulder of black biosparitic arenaceous wackestone from Mi~dzyzdroje, Western Pomerania, Poland (Fig. 2). The specimen is in the collections of the Zak/ad Paleobiologii PAN, Warszawa, Poland (Jerzy Dzik, pers. comm., 1986).

Stratigraphic distribution. From the presence of the c o n o d o n t Westergaardodina tricuspidata Mfiller the age of the boulder E-278 is estimated to be early Late Cambrian.

Discussion. Although the specimen appears less well preserved than those from Bornholm it can readily be assigned to the genus Tuarangia. The boulder contains a rich fauna of inart iculate brachiopods , hyolithids (Circotheca), pelagiellaceans, ostracodes, and eocr inoids , as well as the c o n o d o n t Westergaardodina tricuspidata (Dzik, 1980). The origin of the boulder is uncertain, but the lithology and presence of IV. tricuspidata readily exclude late Middle Cambrian strata in southern Baltoscandia.

Bivalvia or bivalved monoplacophoran? MacKinnon (1982) assigned Tuarangia to the Subclass Pteriomorphia of the Bivalvia, and discussed its relationship to other Cambrian bivalves (Fordilla sibirica Krasilova and Pojetaia runnegari Jell).

Runnegar & Bentley (1983) and Runnegar (1983) rejected MacKinnon's assignment of Tuarangia to the Bivalvia. They regarded it as a bivalved monoplacophoran similar to Pseudomyona queenslandica (Runnegar & Jell), which also had a foliated calcite

Fig. 7. A, dorsal view of Tuarangia sp., x 50, from the erratic E-278 of northern Poland (Late Cambrian). B, same showing the right valve, x 50. Photograph J. Dzik.

microstructure. They interpreted the structure in Tuarangia, thought to be a ligament by MacKinnon (1982), as the place from which a protoconch similar to that found in Pseudomyona had been broken off.

MacKinnon (1985) admitted that there are striking gross morphological similarities between Tuarangia and Pseudomyona, that the shell structure in both taxa consists of foliated calcite, and that they possibly are closely related phylogenetically. However, stressing the fact that Tuarangia is known from over 100 conjoined shells, not one of which has any 'protoconch' structure, he claimed that there was no valid reason for revising his t axonomic p lacement o f Tuarangia in the Class Bivalvia.

I agree with MacKinnon that there are gross similarities between Pseudomyona and

Fig. 6. Tuarangia gravgaerdensis sp. nov. A, enlargement of the lower left part of B showing the exterior shell ornament, z 500. B, MGUHI7.458, an almost complete imprint of the exterior shell (right valve?), the inside covered with phosphatic matter, x 90. C, MGUHI7.459, imprint of the foliated calcite layer (micrograph reversed) of an almost complete left valve, x 125. D, enlargement of the umbonal part of C, x 500. E, replicated foliated calcite from the surface of the mould fig. 4D, x 625. F, foliated calcite (perpendicular to the foliation) from the layer peeled off the mould Fig. 4H, x 550.

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256 VIVIANNE BERG-MADSEN ALCHERINGA

Tuarangia. They have the same general outline although T. paparua is more arcoid and T. gravgaerdensis more equilateral in shape than the D-shaped Pseudomyona. The presence of hinge teeth in Pseudomyona may be disputable but the dentition is far better developed in Tuarangia than in any other Early Cambrian bivalved mollusc. Also the exterior shell sculpture of Tuarangia is very similar to that of bivalves. It should also be no ted that a l though Pseudomyona is monomyarian, the adductor is not placed like that of a bivalve. Furthermore foliated calcite ultrastructure is not restricted to bivalves or bivalved monoplacophorans but also occurs in patelloid gastropods and bellerophonts (B~tggild, 1930; MacClintock, 1967; Bathurst, 1971). On basis of the present evidence, I agree with MacKinnon that, for the time being, Tuarangia is best interpreted as a bivalve.

Assignment to Subclass Pteriomorphia MacKinnon (1982, 1985) discussed the shell structure as a means of classifying Tuarangia to a certain subclass. The Palaeotaxodonta seemed the most likely stock with close affinity to Tuarangia. Tuarangia differs from all Nuculoida in having an arcoid outline, a long and straight hinge plate, amphidetic ligament, and foliated calcite ultrastructure. However, the Cyrtodontidae, regarded as the ancestral pteriomorphs, have a segregation of teeth into anterior and posterior components with a l igamenta l area in between. Furthermore, foliated calcite ultrastructure is found among the pteriomorphs. To sum up, MacKinnon (1982) tentat ively assigned Tuarangia to the Subclass Pteriomorphia; la ter , he d r o p p e d the rese rva t ions (MacKinnon, 1985).

At the present stage of our knowledge of the evolution of Early Palaeozoic molluscs, the usefulness of the shell microstructure is limited. A classification based on the morphology of one superfamily and the microstructure of others, even within the same subclass, is highly unsatisfactory. However, as noted by MacKinnon (1982), the subclass

Palaeotaxodonta offers no better alternative, and the assignment to Pteriomorphia is mainta ined here, a l though with grave reservations.

Ecology Among the fordilloids, only F. sibirica has been found in situ in its original environment (Krasilova, 1977). The life habit was not discussed in detail but the size (4-5 mm) and morphology indicate life in the sediment rather than on top of it, and suspension rather than deposit feeding.

Runnegar & Bentley (1983) discussed the size and life habit of Pojetaia. It was not regarded as miniaturized because of the normal size of its shell prisms and posterior adductor muscle. For comparison they noted the extant pelecypod Nucula calcicola which is mature at a size of <2 mm (Moore, 1977). Almost all 345 specimens of P. runnegari were recovered as internal moulds of articulated shells, and thin sections showed these to be more common than disarticulated valves. Furthermore, the specimens were flee of borings by endolithic algae, in contrast to the shells of associated epifauna. From this, it was concluded that Pojetaia lived below the sediment surface as an inhalant deposit feeder.

MacKinnon (1982) stressed that although most O rd o v i c i an bivalves exhibi t morphological characteristics related to an infaunal mode of life, they were 20-50 times the size of the Middle Cambrian Tuarangia. He suggested that Tuarangia lived interstitially rather than as a fully adaptive element of the burrowing infauna. Later, he described several monoplacophorans from the same limestone lens, reconstructing the inferred life positions primarily from the shell morphology (MacKinnon, 1985).

Interstitial means between grains or within existing pores. The pore space large enough to contain a 2 mm-sized Tuarangia can only be produced by the sediment grain size of very coarse sand. In a deposit rich in trilobite exuviae an interstitial mode of life is a possibility. The fossiliferous part of the Tasman Formation is a micritic limestone with trilobite exuviae in its upper part. However,

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MacKinnon (1985) did not state from which part of the lens Tuarangia was obtained. The Andrarum Limestone was deposited close to the centre of a shallow basin (shelf sea). After a short break in sedimentation, quartz and glauconite were deposited, with more and more clay added and continuing into the overlying black shale facies. The rate of sedimentation was slow; slumping structures suggest sudden burial of the microfauna in life positions. Although parts of the limestone consist almost entirely of trilobite exuviae, none of the Tuarangia specimens has been found here. They are restricted to the upper and most clayey (micritic) part of the limestone bed.

In Tuarangia paparua the orientation was deduced from the asymmetrical form. The more equilateral T. gravgaerdensis is less i n fo rm a t ive if not fo r the s l ight ly asymmetrical dentition. The more strongly developed teeth are regarded as placed anteriorly, the posterior end appearing more truncated. The posterior end may have been lying close to or even partly exposed above the sediment surface, a position found in many recent burrowing bivalves. On the other hand, some of the articulated specimens embedded in larger phosphatic lumps definitely lie on the side. This can be deduced from the position of other molluscs (Mellopegma types) with valves lying perpendicular to those of T. gravgaerdensis.

The environment may be compared with that of modern deep-burrowing bivalve assemblages, but a deep-burrowing mode of the Cambrian bivalves seems unlikely since there is no evidence of a siphon. No pallial sinus, or even pallial line for that matter, has been recognized in Tuarangia. It is inferred, t he r e fo r e , tha t Tuarangia l ived semi-infaunally.

Distribution The three Cambrian bivalve genera, Pojetaia, Fordilla, and Tuarangia each represent a separate distribution pattern.

Pojetaia is at present reported only from one faunal province: Australasia. Despite its long stratigraphical range, diversification seems restricted. From the Early Cambrian of

China, P. ovata Chen& Wang (1985), and a new genus, Oryzoconcha prisca He & Pei (1985) have been recorded. Considering the general faunal similarities, and the close proximity to Australia even in Late Cambrian time (Fig. 1), China cannot easily be regarded as a separate fauna province. In this case the almost identical morphology suggests the genus Oryzoconcha to be a junior synonym of Pojetaia (B. Runnegar, pers. comm., 1986).

Fordilla has been found in five faunal provinces. Of these, Laurentia, Avalonia, the Anglo-Welsh area and Baltica were lying closer together in the Early Cambrian than shown in Fig. 1. This made exchange of the gene pool easy before the continents drifted far apart. Siberia may have been in a more isolated position, thus constituting a separate faunal province. It has its own fordilloid species: F. sibirica. The wide geographic range of the genus indicates migration, probably at an early stage of development.

Tuarangia is at present known only from two identical environments in two separate fauna provinces , located in oppos i te hemispheres: Australasia and Baltica. It seems improbable that a free-swimming veliger stage could surmount the physical barriers between these widely separated provinces. Gene exchange would need a series of related populations along the way (Valentine, 1973), but for the time being, no such occurrences are known . A cco rd in g to avai lable information, it is difficult to see T. paparua and T. gravgaerdensis as anything hut separate species.

Summary and conclusions A new bivalve species, Tuarangia gravgaerdensis, very similar to T. paparua described from the late Middle Cambrian of New Zealand, is here reported from an equivalent stratum in Bornholm, Denmark. Its t axonomic assignment to Subclass Pteriomorphia, based on the foliated calcitic microfabric of the shell is questioned but tentatively maintained.

In the case of Tuarangia neither foliated calcite ultrastructure nor morphology are considered unequivocable guides to high-level

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258 VIVIANNE BERG-MADSEN A L C H E R I N G A

taxonomic affinity. However, the consistent lack of a Pseudomyona-like protoconch in Tuarangia paparua as well as in T. gravgaerdensis (altogether more than 100 specimens) supports a bivalve affinity. The assignment of the genus Tuarangia to the bivalved monoplacophorans (Runnegar, 1983) is not supported.

Tuarangia probably lived in clayey sediments, possibly lying on the side, or partly buried with its posterior margin at or above the sediment surface. An interstitial mode of life (meaning life in existing intergranular pores) is difficult to visualize in a originally mud-sized sediment.

Acknowledgements I thank my friends and colleagues Torbj6rn Alexandersson, Jilt Kft~, David I. MacKinnon, John Pojeta Jr and Bruce Runnegar for critical reading and valuable suggestions and advice. Many thanks also go to Jerzy Dzik, for permission to publish his photographs. Tommy Westberg, Palaeontological Institute, Uppsala, and Georg G~defors, Visby, provided photographic assistance.

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