The significance of the fauna of the Burgess Shale, Middle Cambrian, British Columbia

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The significance of the fauna of the Burgess Shale, Middle Cambrian, British Columbiat H. B. Whittington WHIITINGTON, H. B., 1980. The significance of the fauna of the Burgess Shale, Middle Cambrian, British Columbia. Proc. Geol. Ass., 91 (3), 127-148. The fauna includes about 150 species of 119 genera, one-third had mineralised hard parts, the remaining two-thirds embraces at least 30 genera of arthropods, together with fewer kinds of worms, coelenterates, lophophorates, hemichordates, the oldest known chordate, and animals of unknown affinities. The stratigraphy shows that this community lived at a depth of at least 100 m, on the edge of a continent, and was not peculiar to an isolated habitat; it may be more representative of Cambrian marine communities than assemblages consisting only of hard parts. Genera rarely include more than one species, and are separated one from one another by wide morphological gaps; many are difficult to fit into an existing order, class or phylum. The fauna shows that the early evolutionary diversification of Metazoa in the marine environment was along many parallel, distinct lines, in the absence of the more intensive later competition. The major taxa of today were not defined, they were clarified by subsequent evolution and extinction. If the evolution of metazoans was taking place independently on separate Cambrian continents, then parallelism may have been great and extended back to the beginnings of metazoans.

Department of Geology, Sedgwick Museum, Downing Street, Cambridge CB2 3EQ.

1. INTRODUCTION

The discoverer of the unique locality in this shale was Charles D. Walcott, then Secretary of the Smithsonian Institution, Washington, D.C., U.S.A. After a distinguished career for almost 30 years as a palaeontologist of the United States Geological Survey, Walcott was elected in 1907 Secretary of the Smithsonian Institution, and so was able to indulge his curiosity about the thick Cambrian rock sections in the Canadian Rocky Mountains. In 1909 Walcott was riding on horse back on the trail that runs along the west side of the ridge joining Wapta Mountain and Mount Field (Fig. 1), and continues through the pass between Mounts Burgess and Field, and down to the town of Field. His horse stumbled, and lying on the trail was a block of shale which contained a remarkably preserved fossil. Next season he came back with his two sons (c. D.Walcott, 1911a, p. 18; S. S. Walcott, 1971) and they examined every layer in the section above the trail, eventually finding the fossil-bearing strata. That season, and in four subsequent summers he quarried out the 7 ft 7 ins thick layer of his 'Phyllopod bed' bringing blocks down to his camp to split and trim. In this way a collection of tens of thousands of specimens was amassed between 1910 and 1917. In 1930 Professor Percy E. Raymond, of Harvard University, obtained a small collectionirom the quarry, and also from a less profitable level some 70 feet higher up the slope (one which Walcott had also sampled). In 1966 and 1967 a new investigation was launched by t Special Invitation Lecture

parties organised by the Geological Survey of Canada, of which I was privileged to be a member. Collections were made at carefully measured levels in Walcott's original quarry and at the level explored by Raymond (see Whittington, 1971 a, for details of this and the earlier work). In 1975 a party from the Royal Ontario Museum (Collins, 1978) was permitted to collect from the talus in the quarry, and obtained much valuable additional material, some specimens being unique and one the counterpart of one collected by Raymond. Between 1911 and 1931 (this last paper posthumous) Walcott published a remarkable series of short descriptions and photographs, which he himself called preliminary, of the fauna and flora ofthe shale. The majority of the approximately 150 species, placed in some 119 genera, are unique to the locality. They include many softbodied animals, 'worms' and coelenterates, as well as trilobites and a wide variety of other arthropods with appendages, hitherto unknown and still unparalleled in Cambrian or younger rocks. Small wonder that they have greatly interested zoologists, while to the palaeontologist they remain puzzling, rare curiosities quite outside the span of ordinary experience. Are-assessment and redescription of the faunas was long overdue, particularly since much of Walcott's work, and almost all subsequent studies, were based on the early part of his collection only. In the collection in the U.S. National Museum (now the National Museum of Natural History) the best material of each species had been segregated by Walcott, and manuscript names placed on undescribed species. Walcott was 59 years of age when he made his remarkable discovery, and did not live long enough to

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make the detailed studies that he intended. Almost all the subsequent assessments of the nature and evolutionary position of Walcott's species has been based on his illustrations, or on a study of his figured specimens, and the hidden treasure in his great collection has been ignored. First to dip into it was A. M. Simonetta (l962, 1963, 1964, 1970, 1976; Simonetta and Delle Cave, 1975, 1978a, b; Delle Cave and Simonetta, 1975) in his work on the non-trilobite arthropods. He redescribed earlier species, and described the new forms that Walcott had picked out as well as some additional species. The photographs illustrating his work are indifferent, and the descriptions brief, so that it is difficult to understand the bases for particular portions of the reconstructions. The Geological Survey of Canada invited me to be chairman of a group which would undertake a comprehensive re-study of the fauna and flora. Drs. D. E. G. Briggs, S. Conway Morris, and C. P. Hughes have worked with me in Cambridge, and Dr. D. L. Bruton, University of Oslo, was a member of the 1967 collecting party and is studying certain of the arthropods. We have all spent several short periods in Washington, D.C.,

working on the Walcott collection. Mrs. D. F. Satterthwait completed a Ph.D. thesis in 1976 on the algae, supervised by Professor J. W. Schopf, University of California, but this work has not yet been published. Professor J. K. Rigby, University of Utah, has begun studies of the extensive sponge fauna. Thus none of us has completed his studies, so that the present work can only be a preliminary assessment of the significance of part of the fauna. The animal fossils in the Burgess Shale appear only slightly darker grey than the shale itself (Plate 2, fig. 1), and the exact boundary between fossil and matrix may be obscure. In light which is reflected from the fossil, however, most specimens, or parts of them, are shiny (Plate 1, fig. 1). All Walcott's photographs are taken in a manner that exploited this shininess. It is said that they were made by Dr. R. S. Bassler, using the oblique rays of the sun late in the day. Walcott (1911c, p. 111) knew that all the characters of a specimen could not be brought out in a single photograph, so that he retouched the prints. I experimented with various methods of lighting, types of film, and filters, and concluded that photographs taken in ultra-violet radiation (after focusing in

FAUNA OF THE BURGESS SHALE

ordinary light) on panchromatic film gave the clearest and most detailed prints. Photographs in which the radiation is directed at a low angle across the specimen serve to bring out the changes in level between parts of the animal (Plate 2, figs. 2-5), so important to interpretation of morphology. The direction from which the radiation came is given with reference to north as the top of the plate. Photographs taken with the radiation at a high angle and the specimen tilted slightly to give maximum reflection, obscure the changes in level but bring out other details of the specimens (Plate 1, fig. 1). Particular details may also be revealed by a photograph taken with the specimen submerged in liquid, or with it covered by a film of distilled water held by a cover glass (Plate 3, fig. 5). These techniques have been used by all the investigators, together with making camera lucida drawings, placed opposite appropriate photographs, to explain the photograph and how the specimen has been interpreted. The making of such drawings is valuable in that it requires a detailed study of all the features of a particular specimen, and a decision as to what changes of level are shown and the meaning of the shapes of particular portions. 2. PRESERVATION OF THE FAUNA

Study of the arthropod Marrella splendens (Whittington, 1971a, b) raised the problem of how the fossils were preserved, of why some specimens appeared bilaterally symmetrical (Plate 2, fig. 4), others not. When split along the bedding planes some specimens appear in top view, others side or front view (Plate 2, fig. 5), and others obliquely disposed at intermediate angles (Plate 1, fig 3). Thin layers of rock intervene between successive appendages imbricated like slates on a roof, between appendages and exoskeleton, and between branches of an individual appendage (Plate 2, fig. 4). All the parts of a specimen do not lie on a single bedding plane, but on slightly different levels. Thus the original split of the rock which revealed the specimen passed in part through the specimen, and different portions are left on one side or the other. Hence the importance of having part and counterpart of an individual specimen, because one branch of an appendage may be in the part, the other in the counterpart. The camera-lucida drawings could thus be used to bring together in one drawing features seen in one or other half of a specimen. Every effort was made by the Geological Survey of Canada parties to keep part and counterpart, and such specimens have proved invaluable in unravelling the complicated morphology. Unfortunately Walcott made no such effort, and in at least one case the counterpart of one of his figured specimens has been found in an exchange collection which he sent abroad. The value of some of his rare specimens, for example of Aysheaia, would have been greatly enhanced by the counterpart. A few have

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been recognised in his collection, and more may yet be found. Walcott did little or no preparation of his material, using only a small chisel. Modern binocular microscopes, and the use of a needle suitably sharpened, held in a vibrating attachment in a dental drill, has made possible preparation of delicate appendages. The realisation that appendages may lie beneath the exoskeleton, or that one branch may be present below the one revealed by an original split, has enabled preparations to reveal a wealth of new morphological information, as for example by Briggs (Plate 2, figs. 2, 3). The way in which the specimens are oriented in the rock led me to conclude (1971a) that the animals must have been caught up in a turbulent cloud of sediment in suspension that was moving down-slope, and the bodies buried as the current slowed down and the suspension settled out (Fig. 2). This conclusion has been amply confirmed by subsequent work, and understanding of the mode of preservation has slowly grown. A dark, oily-appearing stain in the rock is associated almost universally with specimens of Marrella splendens (Plate 2, fig. 4; Whittington, 1971a, pp. 1188-90; 1971 b, pp. 16-17), and is the result of a concentration of organic matter. This material probably seeped out of the decaying carcass after burial (Conway Morris, 1977b, pp. 625-{'), and suggests that the animals were alive when caught in the suspension of sediment, and killed during transport and burial. There are arguments, including the presence of a dark stain and of gut contents (Plate 3, fig. 4), for considering that many other species were alive when trapped. On the other hand, over half the specimens of Olenoides serratus with appendages (Whittington, 1975b, pp. 102-5) are extended parallel to the bedding (Plate 1, fig. 1). In this typical example the hypostome, flattened beneath the glabella, is displaced, free cheeks are in place, the last three segments of the thorax are partially telescoped, and the appendages are neither symmetrically arranged nor in a natural relation to the exoskeleton. Other specimens have the body bent or lie at various angles to the bedding, the example in Plate 1, fig. 4, having the left and right appendages lying close together in the bedding planes, while fragments of the exoskeleton lie across these planes. A small dark stain occurs at the posterior end, but no other specimen shows a stain, and traces of the gut or other soft parts are unknown. When picked up in the mud slide (Fig. 2) these specimens may have been carcasses rather than moults, since hypostome and free cheeks were not detached. They may have suffered some decay, but insufficient to break up the ventral cuticle or that of the limbs. Partial decay of muscles and ligaments would have left the limbs free to be rotated into an imbricated series, or swept to point back (left and right sides respectively of Plate 1, fig. 1), or swung down on burial as suggested in Fig. 3. Thus for particular species or individuals death and some decay may have preceded transportation (Conway Morris,

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FAUNA OF THE BURGESS S HALE

1979a, p. 238) and after burial further decay may have occurred, resulting for exampl e in the dark stain . Conwa y Morris (1977e, PP' 30-3 3) has described the varying de gree s of decay shown by the worm Ottoia. The association with the fossils of minut e pyrite spheres, and occasionally of patches of fine-grained pyrite on the surface of the fossil, suggests that burial was in an oxygen-poor environment. There is no sign of bioturbation or scave nging in the evenly -bedded, fine-grained layer s of the Phyllopod bed . I agre e with Conwa y Morris (197 9a, p. 236) that while the animals lived in an oxygenated environment (Fig. 2A ; possibly within the photic zone because of the association with apparently benthonic algae), they were carried by the mud-flow some distance down-slope into an an aerobic environment where they were buried (Fig. 2B, C). D ecay after burial was slow, and must have been halted before it had progressed far , by some unknown factor which resulted in the exquisite preservation. The extremely thin , dark film in which soft parts are preserved appears to be a calcium aluminosilicate (Conway Morris, 1977e, p. 5) , and the exo skeleton of the trilobite Olenoides has been shown to be pre served as a layer of illite group minerals (Whittington, 1980 , p. 173). Thi s may be the result of a mineralising solution influxing the Burgess Shale shortly after deposition and impregnating the soft tissues (Conw ay Morri s, 1979a, p. 237). The wet mud in which the bodies were buried mu st have undergone considerable compaction. I have argued (Wh ittington , 1971a, fig. 24) that it was this compaction that distorted the symme trical four- spined exoskeleton of Marrella splendens when it was bur ied at an oblique angle , or brought the cephalic spines of a specimen buried on its side to lie one belo w the other, separated by thin layers of rock (Plate 1, fig. 3) . Figure 3 suggests how compaction brought the flattened limbs more nearl y into the bedding planes, and crushed the exo skeleton , in the original of Plate 1, fig. 4. Likewise, compression modified the original symmetrical outline of the exoskeleton of Naraoia compacta (Plate 1, fig. 5; d. Whittington, 1977, figs. 1, 3) . Zanger! (1971, pp. 1218-21) has pointed out that exceptionally preserved complete fossil fish were not 'crushed flat' by compaction, but that decay shortly after burial brought the skeleton into a plane long before compaction began. Such bacterial decay (before it was inhib ited) must have played its part in reducing the vertical dimen sions of the Burgess Shale animals (Conway Morri s, 1979a, pp. 237-8), particularly soft-bodied worms. In the fossils we see the results of a short period of decay and a relat ively long period of comp action , the form er having affected the soft pans and the latter having affected particularly the mor e he avily sclerotised (mineralised in the case of Olenoide s ) exoskeleton.

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Fig.3. The effects of compaction. The original of Plate 1, figA, a carcass of the trilobite Olenoides serratus (Ro minger, 1887), was buried (upper block) lying obl iquely on its side. Compaction (centre and lower blocks) brought the coxae and leg branches of the right side, and the posterior 3 leg branc hes of the left side, to lie in an imbricated series with the cerci. The exoskeleton was fragmented and lies across the bedd ing plane s. In th e original (Plate I , fig. 4) the cephalon and its append ages are not visible, exoskeletal fragments lie on the vertical ed ge of the btock, shown at th e upper edge of the figure.

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3. ENVIRONMENT OF DEPOSITION Studies of the stra tigra phy and sedimentology of the Burgess Shale have suggested an en vironm ent of deposition (Fig. 4) that accords with deductions made from fossils. Fritz (1971) gives th e evidence that th e earlyformed portion of the 'thick' Ste phen Formation , which ove rlies the dark grey lime stone of the 'thin' Cath edral Format ion , includes stra ta of lat est Glossopleura zone age, and passes upwards into sha les which lie within th e earliest part of the succee ding Bathyuriscus-s-Elrathina zone . The topm ost portion of the dolomites of the 'thick' Cathedral Form ation are also of Glossopleura zone age, and the subma rine escarpment made by the se dolomites was in places as much as 300 m high at the end of Gl ossopleura zone time (F ig. 4). Subsequently the siliceous, slightly calcareous shales ofthe 'thick ' Stephen Formation , together with the Boundary Limestone Member, wer e laid down against the escarpment , filling the basin until in mid-Bathy uriscus-Elrathina zo ne time the sha les ove rto ppe d th e ban k and were laid down upon the 'thick' Ca thedral Format ion as well as across the basin. Th e Phyllopod bed of the quar ry is slightly olde r, bel onging within rocks cont aining the und erl ying Pagetia bootes faunule. It was deposited close to the edge of the esca rpment at the north end of the embayment adjacent to Mt s. Field and Stephen (F ig. 1; Mcllreath, 1977), the escarpment was about 100 m high at this point (F ig. 4). Th er e is no evidence (Aitken, 1971, p. 568; Mcllreath, 1977, p. 116) that the escarpment wall was ever breached by canyons or channels. Mcllreath considers that the Boundary Limeston e was derived from lime sand and mud brought from the shelf a bove the escarpment , swept over the edge to form a wedge diminishing in thickness westwards. He portrays it (Mcllreath, 1977, figs. 2 , 9) as having accumul at ed to a thickness of 100 m again st the wall, the upp er surface as cusped . Fig. 4 sugges ts that th e succee ding mud s had a similarly cusped surface, on which slides took place close beneath the wall, down the sides of the cusps, and 011 the

slopes away from th e wall. Piper ( 1972) describ ed the muds of the Phyllop od bed , and co ncluded that these lam inated calcar eou s siltstones were probably turbidites. His model ( 1972 , fig. 5, pp. 174- 5) suggested the possibility of deep sea fan-like dep osits originat ing at gaps in the escarpment. Because such gaps have not been observed, McIlr eath consid ers (p ers . comm .) that the siliclastic muds cam e from the north and were tr anspor ted par allel to the escarpment. Piper (1 972 , p. 175) suggested that an an aerobic environment might have been present in stag na nt waters betw een two coal escen t fans. Fig. 4 suggests that such an enviro nment may have occurre d again st the escarpment, bet ween the cusps of mud bank ed against it, and that the flows mo ved from higher, aero bic environme nts down int o it.

4. THE FAUNA Most localitie s in fossiliferous Cam brian rock s on any continent yield abundant remains of trilob ites, togeth er with spo nge spic ules, ina rticula te and articul at e br achiopods , perhap s isolated echinoderm plates, con e-shaped hyolith id she lls, and small, coni cal she lls referred to Monoplacoph ora , Faun as o f a similar general composition com e fro m a wide variety of rock types, prob ably dep osited in a con siderable ran ge of en vironment s. T he Burgess Shale fauna (Conway Morris 1979b ; Conway Morris and Whitt ington, 1979; Fig. 12) includes the remains of these anim als with a mineralised (calcium carbonat e, calcium phosphate, or silica) ske leton , but two-thirds of the speci es were soft bodi ed (coelenter ates, pr iapul ids, polychaet es and miscellaneous worm s) or like the non -tril obite arthropod s had an organic exoskeleton, albeit thi ckened and tanned . Thi s dominance of soft-bodied form s is the mo st strik ing feature, but remarkabl e also is the variety and perfection of the sponge rem ains. A brief account of the groups sho wn in Fig. 12 will show what the fauna is like and the port ions that have bee n re-studi ed in detail.

PLATE 1 Fig. 1. Th e trilobite Olenoides serratus (Ro minger, 1887), U.S.N.M. 58589, reflect ed radiat ion , x1·7. (Whitt ingto n, 197 5b, pI. 5, fig. 2). Impr ession of hypost ome (h) benea th glabe lla, left cerc us (lc) preserved betw een ext ended left series of limbs , and right series swept back one below the other. Fig. 2. Th e priapulid worm Selkirk ia columbia Conway Mor ris, 1977 , G .S.c. 4533 0, rad iat ion from west, x2. (Conway Morri s 1976 a, pI. 18, fig. 5) . Proboscis proj ects out from tub e , and shows spinules at tip; trace of gut run s inside tub e. Fig. 3. The arthro pod Marrella splendens Walcott, 1912, G. S.c. 26605, radiation from west, )(4.(Whittingto n, 1971 b, pI. 21 , fig. 1). Th e two med ian (m) and right late ral (I) exoskeletal spines, and the second (2) head app endage, lie successively one below the othe r in this obliq uely compressed spec imen. Fig. 4. Th e trilobite Olenoides serratus (R ominger, 1887), U.S.N.M. 188573 , radiat ion from east, xl . (Wh ittington, 1975b, pI. 16, fig. 1). Right appendages and cerci of an oblique right laterally compressed specim en , compare Fig. 3. Fig. 5. The trilobite Naraoia compacta Walcolt, 1912, U.S.N.M. 576 87, lectotype, radi ation from north, x2' 7. (Whittington, 1977 , pI. I , fig. 5). A dor sal oblique compression, antenn ae (a ) and biram ous appendages pr oject out from below the dorsal shield s, overlapping where they are swept back posteriorly.

FAUNA OF THE BURGESS SHALE

PLATE I

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FAUNA OF THE BURGESS SHALE

(a) Arthropoda; Trilobita The appendages of Olenoides serratus (Fig. 5A , B, C; Whittington, 1975b; 1980) are better preserved than those of any other trilobite known. The uniramous, multijointed antenna and posterior cercus (unique to this species) were long, and there were between 14 and 16 pairs of biramous appendages, those with more pairs being larger animals . The basal joint of the appendage was large and spiny, the leg branch also spiny, the outer branch of the biramous limb appears to have been a gill. I have discussed (1980) how the animal may have walked or ploughed shallowly in the sediment, and the Rusophycus type of excavation it may have made in hunting or for concealment. Only a single specimen of Kootenia burgessensis (Whittington, 1975b), shows some of the biramous appendages, and this chance survival of decay is indifferently preserved. An unexpected result of the recent studies is that Naraoia compacta (Fig. 5 F, G , H) is a trilobite (Whittington, 1977). The dorsal exoskeleton is divided into two shields, the posterior more elongate than the anterior. The tips of the appendages project out from below the shields, and in the specimen Walcott first described (Plate 1, fig. 5) they are swept together at the back and overlie one another. A subtriangular, posterior feature, formed by the overlapping appendages, was mistaken for a segmented abdomen or telson, so that the creature was regarded as merostome-like (Stermer, in Moore, 1959). New preparations have shown up to 19 pairs of biramous, trilobite-like appendages, following the uniramous antenna. Naraoia is a unique trilobite, the exoskeleton not mineralized, divided into cephalon and pygidium , without a segmented thorax, representing a separate order. That shrewd palaeontologist P. E. Raymond (1920), arrived at this conclusion on far less evidence than is now available, and regarded Naraoia as an example of what we now call paedomorphosis (e.g. McNamara, 1978). The limb is remarkably spiny and like Ole no ides it was probably a predator and scavenger. The three species in which appendages are known

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must have lived in or near the environment in which the mud slides originated (Fig. 2A), and the partially decayed carcasses buried in the post-slide environment (Fig. 2c). However, the Burgess Shale also yields many fine specimens of extended, articulated exoskeletons, some having free cheeks and hypostome in place , of 11 additional genera of trilobites (Fritz, 1971 , fig. 5; Rasetti, 1951 , pp. 103-4 , pis. 25-30). These mineralised exoskeletons may be the remnants of a carcass after decay , or of a moulted cuticle after the unmineralised ventral cuticle, including that of the limbs, had either been det ached or removed by decay. It is thought that eodiscoids and agnostoids were pelagic trilobites (Robison, 1972 ; Jell, 1975), so that their remains could have accumulated in either the pre- or the post-slide environment. In the latter such accumulation would have been in fine muds settling out between slides. Depending on the depth of water they inhabited, remains of the other species could have been deposited in either environment. In the case of those which may have been deposited in the pre-slide environment, presumably the low specific gravity of the remains would have made it possible for them to be carried down -slope without being broken or disarticulated. Some of the trilobites without appendages occur in the same bedding planes as fossils with soft parts, and so were derived presumably from the pre -slide environment; whether all such specimens were is uncertain. (b) Arthropoda other than trilobites

The thirty or so genera of these animals outnumber trilobites more than two to one, the thousands of individuals of Marrella splendens , Canadaspis perfecta and Burgessia bella in Walcott's collection show that certain species were far more common than trilobites in this particular environment. This unique character of the arthropod fauna is approached in the Palaeozoic only by the exceptionally preserved Lower Devonian Hunsriick Shale of Germany. The number of arthropod genera in the Hunsriick is much lower than in the Burgess Shale , but kinds of

PLATE 2

Fig. 1. The arthropod Burgessia bella Walcott, 1912, U.S.N.M. 57677, radiation from west,x5. (Hughes, 1975, pI. 1. fig. 1). The antennae (a) and caudalspine (s)projectfrombelowthecarapace,which showsthe impression of the branching digestive system. Figs. 2, 3. The crustaceanCanadaspis perfecta (Walcott, 1912), U.S.N.M . 189017, radiation from north, respectively before and after removal of left appendages to reveal all the right appendagesbelow, xI·5. (Briggs, 1978, pI. 5, figs. 67, 69). Fig. 4. The arthropod Marrella splendens Walcott, 1912,G.S.C. 25430, radiation from west, x4. (Whittington, 1971b, pI.22, fig. 2). A dorsally compressed specimen showing antenna (1) and second(2) head appendage, walking legs (w) lying beneath the left exoskeletal spine, gill branches (g), and the dark stain (ds) surroundingthe body posteriorly. Fig. 5. The arthropod Marrella splendens Walcott, 1912, U.S.N.M. 165847, radiation from north, x4. (Whittington, 1971a, figs. 18, 19). Preservedinobliqueanterior aspect, showing lateral spines(1) of cephalonand second(2) head appendage, belowwhich the long walking legs legs curve downward.

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Fig. 4. Block diagram showing the environment of dep osition of the Burgess Shale, at the northern end of the emba ymen t in the submarine escarpment of the 'thick' , dolomitic Cathedral Formation. Th e basin ad jacent to the escarpment is being filled by the siliclastic mud of the 'thick' Steph en Formati on. The scars of slips at which flows were initiated are shown as having occurred in the muds banked against the escarpm ent. Calcareous mud and sand from the platform , swept earlier over the escarpment edge, has accumulated to form the Boundary Limestone memb er of the 'thick' Stephen Form ation. Based on Fritz, 1971 ; McIlreath, 1977 and personal commun ication.

genera of non-trilobites again outnumber tr ilobites. Because of the age and vari et y of the Burgess Shale non-trilobites they have loom ed large in all con sideration s of phylogen y. It is astonishing that since Walcott (1912,1931 ) made them known , they have rem ained so

long unstudied. Simonetta (1962 -76) has given reconstructions of old and new genera, but the new investigation is showing these to be erroneous in many resp ects. The examples discussed below are based on thes e new investigations, either published or in progress.

Fig. 5. Recon structions of Burgess Shale species. A , B, C, Olenoides serratus (Rominger, 1887) , dorsal and left lateral views, posterior view of first thoracic segment (from Whittington, 1975, figs. 25, 26b , e) . D, E , Burgessia bella , Walcott , 1912, cross section throu gh body at 3rd app enda ge, beneath carapace are diverticula (d) and caeca (c) ; dorsal view with carapace and underl ying soft parts remo ved to show appendages (from Hughes, 1975 , figs. 6, lOa). F, G, H, Naraoia compacta Walcott , 1912, dorsal and left lateral views, cross section through body showing appendage pair V in posterior view (from Whitcington, 1977, figs. 96, 97). In A and F, right and left half, respectively, of exoskeleton and ventral cuticle have been removed to show appendages and hypostome; certain gill branches have been removed, where cut body is diagonally shaded. Appendages inA, E and F shown in 'still' position of a particular gait, solid circles in transverse line with tips of leg branches which are in contact with sea floor.

137

FAUNA OF THE BURGESS SHALE IOmm

'-------'

~~';"~"";;";;;;."' ....

/1- ,-' - C

B

IOmm

E

F

H

lOmm

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H. B. WHITIlNGTON

By far the most abundant is Marrella splendens (Whittington, 1971b), almost alI specimens are entire animals , and the dark stain referred to above in discussing preservation is characteristic (Plate 2, fig. 4) . These animals appear to have been benthonic in habit, and caught alive and buried by the mud flows. Marrella splendens (Fig. 6) had a four-spined head , which lacked Fig. 7. Canadaspis perfecta (Walcott, 1912) , recon struction, left valve removed, right valve (r v) outlined. Left appendages are first (an 1) and second (an 2) antenna, mandible (rna), first (mx 1) and second (mx 2) maxilla, segmented ramii of thoracic appendages (se), eighth labeUed.a, anus; ab, abdominal somite; aI, alimentary canal; cs, cephalic spine ; e, eye ; m, mouth; 0, labrum ; 1 pr, ventral projection of somite in front of telson (te) ; tI, first thoracic somite (from Briggs, 1978, fig. 27).

Fig. 6. Marrellasplendens Walcott , 1912, restoration in oblique view, standing on anterior three walking legs. Legs of right side omitted, gill branches 1-4 and 10-26 cut off (from Whittington , 1971b, text -fig. 5).

eyes and carried one pair of antennae and a second, jointed, setiferous appendage. The cylindrical trunk bore some 26 pairs of biramous limbs, the jointed inner branch of the 8 or so anterior pairs being walking legs, the outer branch tapering and bearing many fine filaments. The latter may have been primarily gills, but also have assisted in swimming. If this animal was a particle feeder, it is not clear how such particles were trapped and brought into the mid-line . Manton (1977, pp. 8, 53) has suggested that the close approximation of the basal limb joints was helpful in moving food forward in the mid-line, and that the setiferous second appendage may have been used to sweep food towards the mouth. The long series of similar limbs on the trunk recal1s those of the trilobite, but the head, with its second, different appendage, does not. Marrella cannot be classed as a trilobite, and bears only a superficial resemblance to members of that group. The second most common non-trilobite, Canadaspis perfecta , is regarded by Briggs (1978) as the earliest wel1-preserved crustacean, markedly like living leptostracans. His preparations (Plate 2, figs. 2, 3; Fig. 7) have revealed the two pairs of antennae, the presence of a mandible, and the fol1owing 10 pairs of biramous appendages, the first two cephalic , the fol1owing 8 thoracic. The abdomen lacked appendages and projected behind the bivalved carapace, which attained a

length of 4 ern or more. Briggs suggests that Canadaspis was a benthonic animal, feeding on large particles scraped up from the bottom by the spinose tips of the jointed branch of the limb. Isolated valves or entire carapaces occur as commonly as whole animals , and their occurrence in clusters of a few to a hundred or so individuals is a phenomenon shown by this particular species. Whether this clustering was post-mortem, by sedimentary processes, or represents a life habit preserved by catastrophic burial, is unknown. Species of some ten additional genera have been based on bivalved carapaces of differing outline and sculpture (Rolfe, 1962), and in certain of them details of body and appendages are known, e.g. Branchiocaris Briggs, 1976; Perspicaris Briggs, 1977, and Plenocaris Whittington, 1974. These forms are al1 very rare compared to the abundant Canadaspis . M. splendens and C. perfecta are represented by many thousands of specimens in the Walcott colIection, and of other species only Burgessia bella (Hughes, 1975) is known from over 1,000 specimens. There are a few hundred of Yohoia (Whittington, 1974), Waptia (Hughes, 1977), and Sidneyia (Bruton, 1977), while the remaining 25 genera are represented at most by a few tens of specimens, and some are great rarities known from 1-3 specimens. Burgessia (Fig. 5D, E) was a smalI, benthonic animal, the post-antennal series of limbs not uniform, the walking legs bearing ventralIy-directed projections that suggest that these limbs were also used to grasp food. A branching digestive system lay beneath the subcircular carapace (Plate 2, fig. 1). Burgessia was neither a trilobite nor a crustacean, but displays a mixture of characters seen in Recent arthropod groups . From levels in the Phyl1opod bed which yield abundant Marrella splendens and Burgessia bella come also the 400 odd specimens of Yohoia tenuis (Whittington, 1974). This peculiar benthonic animal (Plate 3, fig. 1; Fig. 8) had an anterior pair of jointed, great appendages

FAUNA OF THE BURGESS SHALE

cap '"

apl

Fig. 8. Yoh oia tenuis Walcott, 1912, reconstruction in left lateral view, showing left (lga) and right (rga) great appendages, three left cephalic appendages (cap), and left trunk appendages (ap) 1-10. a, anus; al, alimentary canal; cs, cephalic shield; 1, lateral area of cephalon; m, median frontal lobe; t, telson (from Whittington, 1974, text-fig. 2a).

139

(Plate 3, fig. 5) has an anterior, lateral pair of branched appendages, and 10 pairs of stubby, unbranched limbs with terminal claws. The anterior mouth was surrounded by papillae. This soft-bodied animal may have been protected by living amid clumps of branching sponges (Fig. 9), on which it fed . It is the kind of marine animal from which the various groups of arthropods with unbranched limbs-e-On ychophora, Tardigrada, the late Palaeozoic Arthropleurida, Myriapoda and Hexapoda-were derived. Of these groups, the last two have become particularly important on the continents today.

terminating in four movable spines, presumably used to grasp food and bring it to the mouth. There may have been 3 pairs of jointed walking cephalic legs, the following somites bore lobate appendages fringed with setae; these latt er were possibly used in swimming. This animal bore only a most superficial resemblance to a crustacean. Sidneyia (Plate 3, fig. 4) is the most abundant large arthropod from the Burgess Shale, and the first described by Walcott (1911a) . Understanding of it has been hampered because Walcott (1911a, pl. 4) attributed to it certain large, branched appendages only known isolated. Simonetta (1963) accepted this view, but new preparations of complete specimens show that it is erroneous, and have revealed new details of the coxa and gill (Bruton, 1977). The supposed trilobite-like appendages, and the superficially eurypterid-like features of the exoskeleton, were used by Stermer (1944) to support the view that merostomes and trilobites were related. Bruton's new reconstruction of Sidneyia shows that this benthonic predator had limbs which in certain respects were like those of Limulus; chelicera were absent. In the rare animal L eanchoilia (Walcott, 1912, 1931; Ra ymond, 1935; Bruton, 1977) jointed walking legs are not conspicuous, but the large , branched anterior appendages (Plate 3, fig. 3) are unique. In Em eraldella (Walcott, 1912 ; Bruton, 1977) on the other Fig. 9. Aysheaia pedunculata Walcott, 1911, reconstruction suggesting that the animals (up to 6 em in length) preyed on hand , there are prominent long jointed legs (Plate 4 , fig. 4). One of the larger fossils in the Burgess Shale is sponges (from Whittington, 1978, fig. 90). Anomalocaris , long believed to be the body of a phyllocarid, i.e. a Canadaspis type of animal. Briggs (1979) The non-trilobite arthropods of the Burgess Shale has shown , however, that it is the isolated limb, probably include animals of a wide range in size, morphology and habit, the numbers preserved of certain species suggestof an elongate, many-segmented animal in which each segment bore a pair of such limbs . A length of about a ing that they dominated this particular environment. metre would be rea sonable for such an animal , which With the exception of Aysheaia, they were grouped by would have been far the largest known from Cambrian Stermer (in Moore, 1959) into Trilobitoidea, suprock s. posedly linked to trilobites by having biramous limbs, First described as a worm by Walcott (1911c), but the outer branch bearing filaments, but exhibiting disimmediately recognised by zoologists as resembling tinctive, non-trilobite like features. As the morphology modem, land-dwelling onychophorans is Aysheaia of each species becomes known in detail, an unsuspected (Whittington, 1978). The elonga te, annulated body variety and of combinations of characters is exhibited

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by these animals. How they may be classified awaits further study, but it is clear that they cannot be brought under a single umbrella, and the use of the term 'trilobitoid' to express a supposed underlying unity is misleading.

arranged in two loops beside the mouth. He thus interprets this fossil as the remains of a soft-bodied, bilaterally symmetrical conodontophorid, a conodont-bearing animal. (e) Priapulida

(c) Porifera Over half the known Cambrian genera of sponges in western North America occur in the Burgess Shale, and 10 of these 18 genera are based on species unique to the Phyllopod bed (Rigby, 1976) . Since Walcott's (1920) original descriptions, no detailed new work has been published, but a study of all available material by Prof. J. K. Rigby is now in progress. As Walcott remarked, the spicules may be replaced by pyrite, and his photographs show the excellent preservation of entire specimens, which of some species are common. The fauna includes calcareous sponges (Heteractinida), Hexactinellida and Demospongea, and presumably lived in the mud-slide environment where they were caught up and subsequently buried. (d) Lophophorata

This superphylum is characterised (Conway Morris, 1976a) by having a tentacular lophophore, and includes brachiopods, phoronids and bryozoans. Brachiopods from the Burgess Shale include four genera of inarticulates (Plate 4, fig. 2) and one articulate, an orthid (Rasetti, 1951, p. 103); none is common. Conway Morris (1976a) described a unique specimen, of length 7 em, a thin film poorly preserved on a weathered surface of the shale, vaguely outlined but showing traces of annulation. At one end is a If-shaped area about 4 mm in maximum dimension, on which lie minute, sharplypointed, tooth-like objects. These objects are like contemporaneous conodonts, and Conway Morris argues that they were internal supports of tentacles which were

Recent species of this phylum of burrowing marine worms are characterised by a retractable spiny proboscis and an annulated trunk. Priapulids are insignificant in benthonic faunas today, inhabiting cold waters and often anaerobic muds. The five species in the Burgess Shale (two quite common, the others much rarer) all have the characteristic priapulid proboscis, but are so different from one another that Conway Morris (1977e) places each in a separate family. He suggests that there was a Cambrian radiation of these mainly carnivorous worms but that they were later replaced as infaunal predators by the polychaete worms, and are reduced today in kinds and to inhabiting less favourable environments. The commonest (well over 1000 specimens) species of these worms in the Burgess Shale has the annulated body, proboscis, muscles and even nerve chord preserved, and the gut in some examples shows the content, which may include hyolithid and brachiopod shells. Selkirkia (Plate 1, fig. 2) was a less common species, a burrower that occupied a tube. The organic-walled tube was resistant to decay, so that empty ones are far more common than those showing soft parts. The empty tubes have been found with brachiopods and sponges attached. These priapulid worms, alive in their burrows in the mud which originated a slide (Fig. 2A), and the empty tubes from dead Selkirkia, were carried down-slope and buried. (I) Annelida (Polychaeta)

Examples of these beautifully preserved worms are far less numerous than those of priapulids, but their diversity is striking, there being 6 monospecific families re-

PLATE 3 Fig. 1. The arthropod Yohoia tenuis Walcott, 1912, U.S.N.M. 179012, radiation from west, x5. (Whittington, 1974, pl. 7, fig. 6). Right lateral compression, showing great appendages and traces of trunk appendages. Fig. 2. The medusoid Peytoia nathorsti Walcott, 1911, holotype, U.S.N.M . 57538, radiation from northwest, xO·8. (Conway Morris, 1978, pl. 1, fig. 6). The body had four large lobes, at right angles to each other, separated from each other by seven narrower lobes, arranged around a central opening. Fig. 3. The arthropod Leanchoi/ia superlata Walcott, 1912, U.S .N.M. 83943 , reflected, x1.5. (Original of Walcott, 1931, pl. 12, fig. 3) . Anterior portion of body, oblique compression, showing branched anterior appendages and following lobate appendages, having a fringe of setae. Fig. 4. The arthropod Sidneyia inexpectans Walcott, 1911, U.S.N.M. 139676, north, xl. Specimen showing form of body, a patch (g) broken away posteriorly showing gut contents. Fig. 5. The arthropodAysheaia pedunculata Walcott, 1911, U.S .N.M. 235880, covered by a thin film of distilled water held under a glass slip, reflected radiation, x4. (Whittington, 1978, pl. 8, fig. 48). Body is compressed obliquely laterally, papillae (p) surround mouth , lateral appendage (I, top left) with branches, claws (c) at tips of posterior limbs. The associated sponge spicules appear as black lines on the right side.

FAUNA OF THE BURGESS SHALE

PLATE 3

141

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H. D. WHITTINGTON

PLATE 4

FAUNA OF THE BURGESS SHALE

143

presented (Conway Morris, 1979a). Evidently considerable evolution had already occurred, though forms with jaws (scolecodonts) are not present. Polychaetes have increased in diversity since the Cambrian, and today occupy a wide range of marine habitats, burrowing, crawling and swimming, as well as dwelling in a tube. Burgessochaeta (Plate 4, fig. 1) had the small anterior portion of the body bearing tentacles, the trunk of 24 segments, the parapodia on each (except the first) bearing two bunches of setae of similar length. This worm may have inhabited a burrow, propelling itself with the setae.

The small, conical shells of the monoplacophoran Scenella are common near the base of coarser siltstones in the Phyllopod bed (Piper, 1972). Also relatively abundant are the elongate conical shells of Hyolithes, and these shells have been recognised in the gut contents of the priapulid worm Ottoia (Conway Morris, 1977e, pI. 11, figs. 3,6) and the arthropod Sidneyia (Bruton, pers. comm .). Rare specimens are complete with the operculum and lateral supports (Yochelson, 1961).

(g) Chordata and Hemichordata

(k) Miscellaneous

Walcott (1911c) attributed the various worms he described to Annelida, but as shown above some are Priapulida and one, Aysheaia , is an arthropod. Others are still under investigation by Conway Morris, who considers (1977a ; 1979b) that Pikaia graci/ens may be a primitive chordate. Ruedemann (1931) described the graptolite Chaunograptus scan dens from the Burgess Shale and thi s and other form s ma y be hemichordates.

In this category are species known from 1 or 2, rarely more than 10, specimens that exhibit novel morphological characters and combinations of such characters. Consequently they are difficult or impossible to place in any recognised class or even phylum . The five that have been described so far illustrate these points. Opabinia (Plate 4 , fig. 3) has long been thought to be a crustacean, but its strange appearance has provoked other interpretations of its nature and relationships which are fantastic. Preparation of old and new material (Whittington, 1975a) has shown (Fig . 10) the 5 eyes on the head and the long, flexible frontal process, presumably used to gather food and convey it to the backwardfacing mouth. The flexible trunk had a pair of lateral lobes and gills on each segment, the terminal portion having 3 pairs of upwardly-directed lobes. This segmented, benthonic animal presumably swam slowly by movements of the lateral lobes, seeking its food in the mud . It lacked jointed limbs , so was not an arthropod, but other features are worm-like. It may represent the kind of animal from which annelid worms or early arthropods may have been derived. Amiskwia was considered by Walcott to be an arrow-worm , but in more recent years arguments have been advanced that it was a deep water marine species, a nemertean, Conway Morris (1977d) concludes that it was not an arrow-worm, and that the critical nemertean characters are absent, so that the affinities of this worm are problematical.

(h) Echinoderma Rare are specimens of the eocrinoid Gogia (?) radiata , the crinoid(?) Echmatocrinus , and the pinnule bearing arms belonging to an unknown echinoderm (Sprinkle, 1973). The edrioasteroid Walcottidiscus Bassler, 1935, is equally rare. The only relatively abundant echinoderm is Eldonia ludwigi , which Durham (1974) considers to be a holothurian . This form occurs in a single thin layer in the Phyllopod bed (Walcott, 1912 , pp. 152-3), and is the only fossil from the Burgess Shale now widely accepted as a holothurian (Conway Morris, 1978). (i) Coelenterata Rare specimens of only two genera are known . Mackenzia Walcott, 1911 b, is considered to be an actinarian (Wells and Hill, in Moore, 1956), and Peytoia (Plate 3, fig. 2) a medusoid (Conway Morris, 1978).

(j) Mollusca

PLATE 4 Fig. 1. The polychaete worm Burgessochaeta setigera (Walcott, 1911), U.S.N.M. 83 930b, reflected radiation, x3·2. (Conway Morris, 1979a, pI. 5, fig. 71). Dorsoventral compression showing parapodia and setae, reflective gut along body . Fig. 2. The inarticulate brachiopod Micromitra pannula (White, 1874) attached to the spherical sponge Choia carteri Walcott, 1920, G.S.c. collection, radiation from north , x4. Traces of the fine setae of the mantle are preserved around the margin of the brachiopods, and the stout , radi ally-arranged spicules of the sponge are prominent. Fig. 3. The enigmatic animal Opabinia regalis Walcott, 1912, lectotype, U.S.N.M. 57683, radiation from west, xl ·7. (Whittington, 1975, pI. I , fig. 5). Oblique lateral compression, showing the frontal process (fp) with spines at the tip, eyes (e), the lateral lobes of the body and an upwardly directed blade (b) of the posterior fan. Fig. 4. The arthropod Emeraldella brocki Walcott , 1912, U.S.N.M . 144923b, north, x3. Dorsoventral compression in ventral aspect, showing long, jointed antennae (a), long walking legs and posterior spine (s) .

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H. B. WHITTINGTON

5mm

Fig. 11. Hallucigenia sparsa (Walcott, 1911), reconstruction in anterolateral view. An, anus ; Hd, head ; S, spine; St. Tt., short tentacle ; Tt , tentacle (from Conway Morris, 1977. text-fig . 2).

head, is uncertain. As the author remarks, this animal cannot readily be compared to any living or fossil form. The single specimen on which Nectocaris (Conway Morris, 1976b) is based provides another puzzle. The head is partly covered by an oval shield, the portion in front bearing eyes and 1 or 2 pairs of appendages. Behind the shield is a long, slim trunk having dorsal and ventral fins. This animal may have been a swimmer, and a predator. An example of a sessile animal is Dinomischus Conway Morris, 1977c, a calyx with a fringe of arms , attached to the sea floor by a long stem . It shows some resemblances to the small, living group of entoprocts, but the affinities arc uncertain. 5. NATURE AND SIGNIFICANCE OF THE FAUNA

Fig. 10. Opabinia regalis Walcott, 1912, reconstruction in dorsal and left lateral view. Eye lobes are cross hatched, one lateral lobe and gill removed to show succeeding lobe and gill (from Whittington, 1975, fig. 82).

Bizarre is an appropriate description for the animal, Hallucigenia (Fig. 11) reconstructed by Conway Morris (1977b) as a result of his re-study of what Walcott regarded as probably an annelid worm. Apparently it perched on the muddy bottom on 7 pairs of long, pointed spines, food being grasped by the tentacles along the back. Whether each tentacle had a separate mouth, how or if food was passed forward to the bulbous

The Burgess Shale fauna inhabited relatively deep waters (over 100 m), more than half the animals preserved lacked mineralised hard parts and the environment (Fig. 4) appears to have been on a continental margin facing an ocean, and thus open to migration. It is not the fauna of some peculiar, physically isolated environment, and the exceptional preservation has given a soft bodied component unmatched in the marine fossil record. The implication is that while the preservation is a one-in-amillion chance, the fauna may well be much more representative of Cambrian faunas of the time than those usually collected, composed only of animals with hard parts (d. Conway Morris and Whittington, 1979). The unique glimpse it gives us of the stage reached in invertebrate evolution by mid-Cambrian time is of first importance. The composition (Fig. 12) is peculiar in the high proportion of soft-bodied animals, but also differs

FAUNA OF THE BURG ESS SHALE

o

lO

20

40

50%

ARTHROPODA 44 PORIFERA 18 LOPHOPHORATA 8 PRIAPULIDA 7 ANNELIDA, POLYCHAETA 6 CHOROATA,HEMICHORDATA 5 ECHINOOERMA 5 COELENTERATA 4 MOLLUSCA 3 MISCELLANEOUS 19

Fig. 12. Histogram of number of genera given in each major group as percentages of total fauna of 109 genera . Figures compiled by S. Conway Morris.

from that of post-Cambrian marine fauna s in that the hard parts of diverse bryozoans, brachiopods, gastropods, bivalves, cephalopods, corals or echinoderms do not form a high proportion of the fauna. The genera of arthropods, echinoderms or priapulid and polychaete worms recognised in the Burgess fauna seem to be separated one from another by a wide morphological gap, and so each is referred to a separate family or taxon of higher rank . In most cases a genus is represented by a single species, rarely two. Little or nothing is known of the subsequent evolution of these genera, for example the arthropods (Whittington, 1979), but certain forms such as Canadaspis and the ostracod are the first known representatives of important later groups of Crustacea. In other major taxa represented by 6 or less genera there are early representatives of important later groups, such as the presumed crinoid and the chordate. In addition are a high proportion of miscellanea, those that have been studied difficult to place in any higher taxon, each widely different from any other. This strange assemblage of unfamiliar animals is far more diverse than the late Precambrian Ediacara fauna, or the earliest Cambrian faunas (Durham, 1978). This diversity apparently arose during the first major occupation of the marine environment, especially of the epicontinental seas, by Metazoa. This occupation may well have lacked the intense competition for food and space that characterised subsequent radiations of particular .groups with mineral ised exoskeletons, for example those of articulate brachiopods, or of classes of molluscs, in the succeeding Ordovician period. These new Cam-

145

brian animals were evolving new structures related to novel modes of life and ways of behaviour , for living below, at, or above the sediment-water interface. These new structures would have facilitated moving about in, and sensing, the environment, as well as ways of capturing food, macerating it and conveying it to the mouth. Not all the se early animals were small, their size in the Burgess Shale ranged from 1 or 2 ems up to a metre or more . This early major radiation of invertebrates may well have been along many parallel lines der ived from different ancestors, the independent evolut ion of similar-looking structures. An example may have been 'arthropodisation' , the independent evolution of jointed legs to support an elongate, segmented body. In such circumstances may have evolved the animals Hallucigenia and Opabinia, misfits in our scheme of classes and phyla, 'experiments' in new ways of life. This early evolution is .thus viewed as different from subsequent evolutionary radiations of the classic type, as having taken place along distinct, parallel lines from the earliest metazoans, in conditions of less severe competition between predators and prey, and between carnivore and herbivore, than obtained in later Phanerozoic time. Acceptance of this view emphasises that Phylum and Class in current classification are derived from the results of Phanerozoic evolution, as seen in Recent fauna s, and shows why it is difficult to use these major taxa in relation to the Burgess Shale fauna . The fauna does not display, either, a fossil which 'appears to be ancestral to two or more modern phyla' (Conwa y Morris in Valentine, 1977, p. 32) . It does contain fossils difficult or impossible to classify in modern taxa , and such fossils are known from other exceptional preservations in the Palaeozoic, e.g. Ainiktozoon (Scourfield 1937) from the Silurian of Scotland, or such enigmas from the Car boniferous as the supposed conodont-bearing animal (Melton and Scott , 1973) , or those from the Mazon Creek fauna (Richardson and Johnson, 1971; Nitecki and Solem 1973; Nitecki and Schram , 1976; Nitecki, 1979). If we knew more of the soft-bodied faunas of the Phanerozoic, our major subdivisions of invertebrates might well be viewed differently. There clearly were many different phylogenetic lines at different times during the Palaeozoic (for example the arthropods, Whittington, 1979) which may have been short or longer lived, but of which we know nothing subsequently. The major taxa of Recent invertebrates are those animals which have survived many vicissitudes. Looking forward from the Burgess Shale fauna it would have been difficult to predict which they would have been (Conway Morris & Whittington, 1979). Aysheaia , slow-moving amid sponge colonies, hardl y would have looked to be one of the ancestors of those formidable conquerors of the land , myriapod s and insects. The general pattern of early Palaeozoic evolution of metazoans thus appears to be one of many parallel lines

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H . B. WHITII NGTON

which emerged from the obscurity of the Precambrian into the Cambrian. The elimination of many of these lines left animals constituting the phyla of today to diversify and become dominant (at least among an imals with mineralised hard parts). This pattern is the opposite of the upwardly branching pattern shown in the muchdebated problem of the origin and relationships of major animals group s, as for example by Valentine (1977, figs. 1 and 2) . In the latter diagram Valentine shows phyla and classes as parallel lines, joined basally as superp hyla (or what are regarded as basic body plans in metazoans) , and these in turn stem from a common ancestral form. The Burge ss Shale fauna hints at no such pattern, showing no 'common ancestor' but rather a spectrum of apparently not closely related animals , even among those we group as structurally similar, such as worms or arthropods. Perhaps this is because early Middle Cambrian is not far enough back in time to reveal these supposed basic early forms. Such an assumption implies that in some geographical region in the late Precambrian there was an interbreeding population of a 'common ancestor' , from which evolved the stem forms of our Recent phyla. I que stion whether the early evolution of metazoans was in this manner. The Burgess Shale fauna is from a single localit y on the margin of one continent. What were the soft bodied fauna s like on other margins of that continent or around other Cambrian continents? If the evolution of invertebrate animals was proceeding in geographically isolated areas around separate continents, were coelomate animals, animals with jointed legs and worm-like animals, arising independently in parallel? The sub sequ ent eliminations among such a plethora of metazoans, and the radiations of the forms that were best adapted, may have resulted in the emergence of

what we recognise in retrospect as phyla. The argum ent s about whether or not a phylum is polyphyletic (e.g the arthropods, Manton , 1977 ; Patterson , 1978), may be hollow. That is, there may not have been a single, ancestral population in one area from which a particular phylum arose, no single ancestral species to which it may be trac ed back . What we know of late Precambrian metazoans does not shed much light on these speculations, but did lead Cloud (1968, p. 47) to make a similar proposal, as a working hypothesis, that the Metazoa are broadly polyphyletic in origin. Certain genera of the Ediacara fauna appear to be widely distributed (e.g. Durham, 1978 , table 2), but the peculiar mode of preservation of these faun as, and our ignorance of the geography of the times , leads to doubts that these early metazoans were world-wide.

ACKNOWLEDGMENTS Work on the Burgess Shale fauna has been made possible by Natural En vironment Research Council grant GR3 /285 . Stimulating and critical discussions with Dr s. D. E. G. Briggs , D . L. Bruton, S. Conway Morris and C. P. Hughes are gratefully acknowledged, but I accept responsibility for the ideas expressed here. Dr. L A. McIlreath kindly critic ised drafts of Fig. 4, which grew out of his work. Drs . Briggs, Conway Morris and Hughes have allowed me to use photographs and drawings taken from their published papers; I am indebted to the Royal Society, the Palaeontological Association, the Geologica! Survey of Can ada , and the edit or of Fossils and Strata for permission to reproduce figures.

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