The Ecology of the Cambrian Radiation - Andrey Zhuravlev - Chapter 13 doc

Megascopic life evolved in the Archean with the buildup of stromatolitic mounds in shallow-water environments. By the Proterozoic, stromatolites had already extended down to well below fair-weather wave base. During the late Vendian there was an increase in megascopic biota in shallow water, with both soft-bodied fossils and trace fossils becoming relatively abundant. Some of the soft-bodied forms, such as Pteri- dinium, were large and preserved three-dimensionally, with remarkable detail, in high-energy medium-to-coarse-grained sandstones. This style of preservation re- sembles that of trace fossils, which were produced within similar sequences during the Phanerozoic, and may suggest that some of these early life-forms grew through already deposited sediment as a unicellular protoplasmic mass. Some Ediacaran body fossils (e.g., Charniodiscus, Ediacaria, Pteridinium) may have survived into the Cambrian by migrating into deeper water, where many of the reported body fossils were exceptionally preserved soft-bodied forms. There was also a slight increase in trace fossil diversity in deep water during the Cambrian, and this too may reflect the activity of a dominantly soft-bodied fauna. There was a major progressive colonization by hard-bodied forms of the outer shelf by the Early Ordovician, and of the slope toward the end of the Middle Ordovician. In contrast, there is a significant increase in trace fossil abundance and diversity in deep-water flysch sequences as early as the Early Ordovician. It appears that soft-bodied animals, including those which produced trace fossils, were involved first in the onshore-offshore migration and were generally well established in deeper-water niches before the arrival of faunas rich in skeletal forms. INTRODUCTION The colonization of deep-sea environments appears to have been a slow process (Crimes 1974; Sepkoski and Miller 1985; Bottjer et al. 1988), and a high percentage CHAPTER THIRTEEN T. Peter Crimes Evolution of the Deep-Water Benthic Community 13-C1099 8/10/00 2:12 PM Page 275 276 T. Peter Crimes of Precambrian and Cambrian megascopic body and trace fossils occur in sediments considered to have been deposited in shallow water, mostly above storm wave base. There are, however, several abiological factors that might emphasize this apparent distribution. First, deep-water sediments, by the nature of their tectonic setting, are more prone to deformation and metamorphism, and these processes will eliminate some forms and make recovery of others difficult. Second, shallow-water shelf seas were dominant late in the Precambrian and early in the Cambrian. Consequently, the exposed area of shallow-water strata representing the period when life was evolving rapidly far exceeds that of deep water, and third, it is easier to find definitive sedimentological evidence for shallow-water environments than for deep-water ones. Nevertheless, it is generally accepted that many animals evolved in shallow water during the late Precambrian and early Cambrian and then gradually spread into the deep oceans (Crimes 1974; Sepkoski and Miller 1985; Sepkoski 1990). Indeed, it has been claimed that there is something unique about shallow-water environments that promotes the origin of evolutionary novelties or the assembly of novel community types (Sepkoski and Miller 1985). The most distinctive ecological features of shallow- water environments are the frequent disturbances and the high-energy, stressful, am- bient conditions, and these factors may be conducive to the evolution of novel taxa and communities (Steele-Petrovic 1979; Jablonski and Bottjer 1983; Sepkoski and Shee- han 1983; Valentine and Jablonski 1983). The evolution of a deep-water fauna requires adaptation to certain extreme conditions, such as permanent darkness, high pressure, and low temperature (except in the case of hydrotherms). In addition, deep seas show low fertility. In the absence of ter- restrial plant debris influencing community structure, early deep benthos would probably suffer from very limited food (Bambach 1977). The late Precambrian and Cambrian circumstance of high diversity in shallow water and decreasing diversity in progressively deeper water is in marked contrast to that in modern oceans, where unusually high diversity has been found in deep water (Hessler and Sanders 1967). For example, the diversity of polychaetes and bivalves increases with depth below the continental shelf and, at bathyal depths, reaches levels equivalent to those in tropical soft-bottomed communities at subtidal depths (Sanders 1968). Similarly, when considered for a single type of substrate, the diversity of gas- tropods and several other groups increases from the shelf to bathyal depths (Rex 1973, 1976, 1981). Trace fossil evidence suggests that significant colonization of the deep sea may have been delayed until the Ordovician (Crimes 1974; Crimes et al. 1992), while analysis of body fossil diversity data implies that a shallow-water “Cambrian fauna” became progressively restricted to deeper-water environments from the Ordovician onward (Sepkoski 1990:38; Sepkoski 1991). Recent investigations (e.g., Narbonne and Aitken 1990), however, suggest that 13-C1099 8/10/00 2:12 PM Page 276 EVOLUTION OF THE DEEP-WATER BENTHIC COMMUNITY 277 even during the Precambrian, animals were penetrating at least into intermediate water depths, and by the Cambrian there was a limited colonization of even bathyal depths (e.g., Crimes et al. 1992; Hofmann et al. 1994). The purpose of this chapter is to review the progressive colonization of the deep sea from the Precambrian to the Ordovician, that is, through the period of Cambrian radiation. THE ENVIRONMENTAL SETTING OF THE EARLIEST LIFE It has become fashionable to regard hydrothermal systems as likely sites for organic synthesis and the origin of life (see Chang 1994 and references therein). Indeed, it has been claimed that present-day microorganisms with the oldest lineages based on molecular phylogenies are anaerobic, thermophilic, sulfur-dependent chemolitho- autotrophic archaebacteria (Woese 1987). It has been suggested that deep marine communities had formed around black smokers and white smokers already in the Precambrian (Kuznetsov et al. 1994). Fossil examples of such communities have been reported in Silurian, Devonian, and Carboniferous sulphur-rich, hydrothermal strata in the ophiolitic suites of the Urals and northeastern Russia, where they are accompanied by vestimentiferans (Pogonophora) and calyptogenid pelecypods similar to the inhabitants of present-day smokers (Kuznetsov 1989; Kuznetsov et al. 1994). Recog- nition of such sites in early Proterozoic sequences is, however, likely to prove difficult, and although it might be argued that they were more common during early Earth history, they must nevertheless have occupied a small percentage of available ecospace. Therefore, unless they were almost uniquely favorable locations, it is sta- tistically unlikely that they would be the “chosen” sites. The earliest well-documented signs of life come from ~3–3.5 Ga, in early Archean strata in the Swaziland Supergroup of South Africa and the Pilbara Supergroup in western Australia (Schopf 1994). These units contain stromatolites and microfossils, and it is considered that the former, at least, grew in narrow, shallow-water zones along shorelines of volcanic platforms subject to periodic agitation by waves or cur- rents (Groves et al. 1981; Byerly et al. 1986). The similarity between these stromatolites and much more recent ones suggests strongly that they exhibited bacterial or cyanobacterial photosynthesis (Schopf 1994) and were therefore restricted to shallow water. The first sediments considered to have been deposited on a stable carbonate plat- form occur in the Middle Archean Nsuze Group, which includes stromatolitic dolo- mites in a tidally influenced environment (Walter 1983; Grotzinger 1994). By the Late Archean, stromatolite-bearing carbonates were being deposited in cratonic and non- cratonic settings (Grotzinger 1994), but the growth of large cratonic masses of continental lithosphere during the Archean-Proterozoic transition (Veizer and Compston 13-C1099 8/10/00 2:12 PM Page 277 278 T. Peter Crimes 1976) gave rise to a dramatic increase in carbonate platforms (Grotzinger 1994), and this provided ecospace for a significant increase in abundance and diversity of stromatolites (figure 13.1A), which peaked in the Middle Proterozoic (Awramik 1971; Walter and Heys 1985). This increase was accompanied by the occupation of more- varied niches extending down to well below fair-weather wave base but still presum- ably within the photic zone (Grotzinger 1990; Walter 1994: figure 4). The colonization of “deeper” water seems to have already commenced. The decline of stromatolites is commonly ascribed to the advent of soft-bodied Metazoa, as evidenced by the Ediacara fauna and its associated trace fossils (Garrett 1970; Awramik 1971). Some of these forms may have been able to destroy stromatolites by grazing and burrowing, but there has been no significant documentation of stromatolites affected in this way. Competitive exclusion by higher algae may also have contributed tothe decline (Hofmann 1985; Butterfield et al. 1988; andsee Droser and Li, Pratt et al., Riding, this volume). Many later organisms may have responded to competitive pressures by migrating into deep water (Crimes 1974; Sepkoski 1990), but stromatolites, being limited to the photic zone, had probably occupied much of the available ecospace by the late Proterozoic and, consequently having “nowhere to go and nowhere to hide,” might have suffered badly from increased competition with an expanding trophic web. One of the earliest records of probable metazoan life is Bergaueria-like trace fossils (see Crimes 1994:114) from the 800–1100 Ma Little Dal Group of the Macken- zie Mountains, Canada (Hofmann and Aitken 1979). These occur in a carbonate- dominated sequence of varied lithology, considered to be of basinal aspect and deposited in water several tens to 200 m deep (Hofmann and Aitken 1979:153). These fossils may therefore also mark an early colonization of slightly deeper water. THE COLONIZATION OF DEEPER WATER DURING THE VENDIAN The Vendian era, extending from ~610–545 Ma (Grotzinger et al. 1995), commences with the Varanger tillites and their equivalents and is the first to yield relatively common and diverse undisputed body fossils and trace fossils. The oldest Vendian biota, consisting of Nimbia, Vendella?, and Irridinitus?, was found in the intertillite Twitya Formation of the Mackenzie Mountains, Canada (Hof- mann et al. 1990). This sequence comprises siliclastic turbidites associated with major channel-fill conglomerates and is considered to be relatively deep-water (Hofmann et al. 1990). The majority of post-tillite Vendian biotas have been found in shallow-water sequences, apparently deposited above fair-weather wave base, and in some regions (e.g., Australia, Namibia, Russia, Ukraine), remarkably abundant, diverse, and well- preserved faunas have been found (see reviews in Glaessner 1984; Sokolov and Iwa- nowski 1985; Fedonkin 1992; Jenkins 1992). Indeed, in some sequences deposited 13-C1099 8/10/00 2:12 PM Page 278 EVOLUTION OF THE DEEP-WATER BENTHIC COMMUNITY 279 Figure 13.1 “Snapshots” of the ocean floor faunas for Middle Proterozoic, Vendian, and Cambrian, showing the progressive colonization of deeper water based on body fossils. 13-C1099 8/10/00 2:12 PM Page 279 280 T. Peter Crimes under varied depths of water, fossils occur only in the shallower-water lithologies. For example, in the Tanafjorden area of Norway, the Vendian Innerelv Member con- sists of two shallowing-upward sequences, each representing a transition from offshore marine (quiet basin, below wave base) to wave-influenced, shallow, subtidal and intertidal deposition (Banks 1973), but a biota consisting of Cyclomedusa, Ediacaria?, Beltanella, Hiemalora, and Nimbia? occurs only in sediments interpreted as representing a current-swept, wave-influenced environment (Farmer et al. 1992). There are, however, a few well-documented examples in which body and/or trace fossils do occur in deeper-water deposits (figures 13.1B and 13.2A). In the case of body fossils, it might be possible to claim that they have been transported from shallow water, but such an argument cannot be applied to trace fossils, which reflect life activity at the precise location where they are now found. In the Wernecke Mountains, Canada, Narbonne and Hofmann (1987) record a fairly extensive Ediacara fauna, most of which comes from Siltstone Units 1 and 2, deposited under shallow-water conditions. This includes the body fossils Beltanella, Beltanelliformis, Charniodiscus, Cyclomedusa, Kullingia?, Medusinites, Nadalia, Spriggia, and Tirasiana, as well as the trace fossils Gordia, Neonereites?, and Planolites. However, Charniodiscus was also recorded from the Goz Siltstone, which includes slump and load structures and was deposited on a slope in a deeper-water setting. A more extensive deeper-water biota has been described by Narbonne and Aitken (1990) from the Sekwi Brook area of northwestern Canada, where the Sheepbed and Blueflower formations include turbidity current–deposited sandstones and common slump deposits and are interpreted as representing a deep-water basin slope setting, below storm wave base. The biota includes the body fossils Beltanella, Charniodiscus?, Cyclomedusa, Ediacaria, Eoporpita, Inkrylovia, Kullingia, Pteridinium, and Sekwia andthe trace fossils Aulichnites, Helminthoida, Helminthoidichnites, Helminthopsis, Lockeia, Neo- nereites, Palaeophycus, Planolites, and Torrowangea. More recently, Hiemalora and Win- dermeria have been reported from the same sequence (Narbonne 1994). Pteridinium has also been recorded from the South Carolina Slate Belt in deep- water, thinly bedded to finely laminated pelites and siltstones of the Albermarle Group, which may have been deposited between 586 and 550 Ma (Gibson et al. 1984). This sequence has also yielded the trace fossils Gordia, Neonereites, Planolites, and Syringomorpha (Gibson 1989). Surfaces covered with numerous predominantly frondlike and bushlike Ediacaran body fossils, including Charnia and Charniodiscus, occur within volcaniclastic turbidite sequences interpreted as deep-water submarine fan and slope deposits (Myrow 1995) within the Conception Group on the Avalon Peninsula, Newfoundland, Can- ada (see Anderson and Misra 1968; Misra 1969; Anderson and Conway Morris 1982; Conway Morris 1989a; Jenkins 1992). Taphonomic and sedimentological data indi- cate that this is an in situ life assemblage that suffered rapid burial by volcanic ash at 13-C1099 8/10/00 2:12 PM Page 280 EVOLUTION OF THE DEEP-WATER BENTHIC COMMUNITY 281 Figure 13.2 “Snapshots” of the ocean floor faunas for Vendian, Cambrian, and Ordovician, showing the progressive colonization of deeper water based on ichnofossils. 13-C1099 8/10/00 2:12 PM Page 281 282 T. Peter Crimes some horizons ( Jenkins 1992; Seilacher 1992; Myrow 1995). The turbidites may not have formed at truly oceanic depths but perhaps on a continental terrace (Benus 1988; Jenkins 1992). A broadly similar setting has been postulated for the occurrence of Charnia, Charniodiscus, and Pseudovendia within a Vendian sequence at Charnwood Forest, Leicestershire, England, where Jenkins (1992) suggests that the frequency of slumps, together with some current rippling and an absence of oscillation ripples, implies deposition on a slope environment below storm wave base. Boynton and Ford (1995) record three new genera from this sequence (Ivesia, Shepshedia, and Black- brookia), but conclude that, despite the presence of graded bedding and absence of shallow-water indicators, water depth may be little more than wave base. The classic sequence at Ediacara, Australia, which has yielded an abundant and diverse nonskeletal fauna, has been interpreted by Gehling (1991) as deposited in an outer shelf setting below fair-weather wave base, with burial ofthe organisms by storm surge sands. Seilacher (in Jenkins 1992:152) considers that the common occurrence of wave oscillation and interference ripples suggests deposition on the shoreface, al- beit perhaps by storm events, and a shallow-water tidal environment also seems in- dicated by the large polygonal desiccation cracks in the highly fossiliferous parts of the section ( Jenkins 1992:153). Evidence of life at truly bathyal depths is largely absent during the Vendian, although records of the trace fossil Planolites within the deep-sea turbidite sequence of the South Stack Formation of the Mona Complex on Anglesey, Wales, by Greenly (1919) have been substantiated during recent fieldwork. The age of these rocks is debatable, but radiometric dates on intrusive granites suggest that it is greater than 600 Ma (Shackleton 1969). The conclusion appears to be that while most Ediacarian body andtrace fossils from the prolific localities in Australia, Namibia, Russia, and Ukraine occurred in shallow- water environments at or above wave base, other localities, including Charnwood Forest, Newfoundland, Sekwi Brook, and Wernecke Mountains, show features sug- gestive of a slightly deeper-water environment below storm wave base, mostly on the continental slope. There is not, however, any evidence of significant colonization of truly oceanic depths during the Vendian. Such colonization as took place in intermediate water depths was dominated by sessile body fossils (e.g., Cyclomedusa, Ediacaria) and detritus-feeding animals that produced traces either on muddy substrates (e.g., Helminthoida, Helminthopsis) or at very shallow depths (e.g., Paleodictyon). Significant bioturbation did not occur until the Early Cambrian (Crimes and Droser 1992). In present-day oceans, faunas inhab- iting muddy substrates are more abundant and diverse than those of sandy areas (Menzies et al. 1973), whereas in these ancient seas, the absence of algae and the scar- city of large animals increased the survival possibilities of the detritivorous trophic group (Sanders and Hessler 1969; Sokolova 1989). 13-C1099 8/10/00 2:12 PM Page 282 EVOLUTION OF THE DEEP-WATER BENTHIC COMMUNITY 283 BIOTIC CHANGES ACROSS THE PRECAMBRIAN-CAMBRIAN BOUNDARY Diversity curves of metazoan genera show a fall at the Vendian-Tommotian boundary (Sepkoski 1992: figure 11.4.2). This data set includes genera from all depositional environments, and the fall has been interpreted as reflecting a mass extinction. Seilacher (1984) suggested that Vendian biota mark not simply a nonskeletal start to metazoan evolution but a distinct episode to the history of life, terminated by a major extinction. He later suggested that they were quilted constructions that represented an evolutionary experiment that failed with the incoming of macrophagous predators (Sei- lacher 1989). There is, however, also a remarkable change in the style of preservation of many of the body fossils in passing across the Precambrian-Cambrian boundary (cf. Seilacher 1984). The Vendian shallow-water sequences are dominated by relatively large forms, commonly exceedingly well preserved in three dimensions and found within fine- to-coarse-grained, well-washed, matrix-poor sandstones (figure 13.3). Such three- dimensional preservation is almost unknown in the Phanerozoic (cf. Seilacher 1984, 1989). By that time, these high-energy sandstones commonly lack body fossils and are dominated by trace fossils, many of which are produced within or between beds. Explanations for the three-dimensional preservation of Vendian body fossils include early mineral precipitation within the matrix ( Jenkins 1992), low rates of microbial decomposition (Runnegar 1992), absence of scavengers (Conway Morris 1993), and the supposed existence of mineral crusts formed by cyanobacterial mats (Gehling 1991). The parallels between the three-dimensional preservation of these body fossils in the Vendian and the trace fossils produced within similar sandstones in the Phanero- zoic is remarkable but is consistent with the conclusions of Crimes and Fedonkin (1996) that many of these three-dimensionally preserved Vendian body fossils ac- tually formed by growth within the sediment by a process of plasmic permeation. Such animals would then undoubtedly suffer from the incoming of macrophagous predators in the Phanerozoic as envisaged by Seilacher (1989). They seem to have responded by onshore-offshore migration, and a few appear in deeper-water environments during the Cambrian (Conway Morris 1993; Crimes and Fedonkin 1996). Crimes (1994) has argued that Vendian trace fossils also include many unusual and short-ranging forms. The trace fossil diversity data (Crimes 1992: figure 2; Crimes 1994: figure 4.1) do not, however, support a mass extinction, nor indeed is this indi- cated by the body fossil data set of Sepkoski (1992: figure 11.4.1) when considered in terms of families, orders, or classes. Evidence for such an event is perhaps best shown when the fauna is divided into “Ediacaran, “Tommotian,” and “Cambrian sensu stricto” (Sepkoski 1992: figure 11.4.2). There is, however, increasing evidence that some, or perhaps many, elements of the Ediacara fauna continue through the Nemakit- Daldynian (see Brasier 1989) and into later Cambrian strata (Conway Morris 1992; 13-C1099 8/10/00 2:12 PM Page 283 284 T. Peter Crimes Figure 13.3 Three-dimensional nature of Pteridinium from the Kliphoek Member of the Neopro- terozoic Nama Group (South Namibia). A, Field photograph at Plateau Farm, near Aus; B–F, speci- mens lodged in a small museum at Aar Farm, by permission of Mr. H. Erni. All scale bars 2 cm. Crimes et al. 1995; Crimes and Fedonkin 1996). Additionally, the data are imprecise because of correlation problems at this level. Although an overall reduction in diversity cannot be discounted, the picture is far from clear, and, interestingly, Jablonski (1995) places the first of his “Big Five” mass extinctions at the end of the Ordovician. One might also anticipate that any extinction event could have greater consequences in shallow water than in the more constant slope environments considered here. In contrast, the dramatic increase in diversity of both body and trace fossils in the earliest Cambrian strata is obvious (Sepkoski 1992; 13-C1099 8/10/00 2:12 PM Page 284 [...]... It is possible therefore that the Ediacara-type fauna may have expanded into deep water during the Cambrian, rather than retreated FAUNAL CHANGES ACROSS THE CAMBRO-ORDOVICIAN BOUNDARY AND MAJOR COLONIZATION OF THE DEEP SEA The Cambrian evolutionary fauna started a long, gradual decline as the end of the Cambrian approached (Sepkoski 1990) This was accompanied by the migration of many of its component... Crimes, T P 1994 The period of early evolutionary failure and the dawn of evolutionary success: The record of biotic changes across the Precambrian -Cambrian boundary In S K Donovan, ed., The Palaeobiology of Trace Fossils, pp 105 133 England: John Wiley and Sons Conway Morris, S 1989a South-eastern Newfoundland and adjacent areas (Avalon Zone) In J W Cowie and M D Brasier, eds., The Precambrian -Cambrian Boundary,... Proto- 1 3- C1099 8/10/00 2:12 PM Page 287 EVOLUTION OF THE DEEP-WATER BENTHIC COMMUNITY 287 virgularia, Tasmanadia, and Torrowangea Deep-water Cambrian sediments in North Greenland have also yielded Helminthopsis, Gordia, Planolites, and Protopaleodictyon (Pickerill et al 1982) In the Early Cambrian sequence of the Holy Cross Mountains of Central Poland, there is a diverse ichnofauna in the shallow-water.. .1 3- C1099 8/10/00 2:12 PM Page 285 EVOLUTION OF THE DEEP-WATER BENTHIC COMMUNITY 285 Crimes 1994) and has led to the concept of “explosive evolution.” There are a significant number of short-ranging forms in the late Precambrian, but the Cambrian is dominated by much longer-ranging forms of Phanerozoic type, and this has prompted Crimes (1994) to suggest that the major change is a... figure 2) as a time-environment diagram show that there was only minimal colonization of outer-shelf and slope environments even by the Late Cambrian, but that there was major progressive colonization of the outer shelf by the Early Ordovician, and of the slope toward the end of the Middle Ordovician Even trilobiterich communities, which dominated Cambrian shelf seas, penetrated into deep-water environments... trilobites from the Hell’s Mouth Grits of St Tudwal’s Pen- 1 3- C1099 8/10/00 2:12 PM Page 286 286 T Peter Crimes insula (Bassett and Walton 1960) and Green Slates of northern Gwynedd (Wood 1969) There may also be doubt as to whether even these meager faunas are in situ The earliest well-documented subthermocline fauna is the Botoman Elliptocephala asaphoides fauna from the Taconics of New York and Vermont,... S 1989b The persistence of Burgess Shale–type faunas: Implications for the evolution of deeper-water faunas Transactions of the Royal Society of Edinburgh (Earth Sciences) 80 : 271–283 Conway Morris, S 1992 Burgess Shale–type faunas in the context of the Cambrian explosion”: A review Journal of the Geological Society of London 149 : 631– 636 Conway Morris, S 1993 Ediacaran-like fossils in Cambrian. .. Pteridinium These possibilities may suggest that the Ediacara fauna survived beyond the PrecambrianCambrian boundary in part by moving into deeper water The discovery of a rigidbodied but nonskeletal biota of Ediacaran affinities in a thick (thousands of meters), deep-water, turbidite sequence of Late Cambrian age in Eire suggests colonization not just of “deeper water” but also of the deep ocean The biota... and G R Heys 1985 Links between the rise of the Metazoa and the decline of stromatolites Precambrian Research 29 : 149–174 Woese, C R 1987 Bacterial evolution Microbiological Reviews 51 : 221–271 Wood, D 1969 The base and correlation of the Cambrian rocks of North Wales In A Wood, ed., The Pre -Cambrian and Lower Palaeozoic Rocks of Wales, pp 47– 66 Cardiff: University of Wales Press Zezina, O H 1989... Colonization of the early ocean floor Nature 248 : 328–330 Conway Morris, S 1985 Cambrian Lagerstätten: Their distribution and significance Philosophical Transactions of the Royal Society of London B 311 : 49– 65 Crimes, T P 1992 Changes in trace fossil biota across the Proterozoic-Phanerozoic boundary Journal of the Geological Society 149 : 637– 646 Conway Morris, S 1986 The community structure of the Middle Cambrian . 1992; Hofmann et al. 1994). The purpose of this chapter is to review the progressive colonization of the deep sea from the Precambrian to the Ordovician, that is, through the period of Cambrian radiation. THE. water during the Cambrian, and this too may reflect the activity of a dominantly soft-bodied fauna. There was a major progressive colonization by hard-bodied forms of the outer shelf by the Early. macrophagous predators (Sei- lacher 1989). There is, however, also a remarkable change in the style of preservation of many of the body fossils in passing across the Precambrian -Cambrian boundary (cf.

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