Journal of the Geological Society. London, Vol. 149, 1992, pp. 631-646, 3 figs, 1 table. Printed in Northern Ireland

Changes in the trace biota across the Proterozoic- boundary

T. P. CRIMES Department of Earth Sciences, University of Liverpool, L49 3BX, UK

Abstract: Tracefossils became relatively diverse in shallow-water clastic seas in the late Proterozoic (Vendian), with a further significant increase in abundance, diversity and complexity in the Tommotian and Atdabanian. Little change followed in the remainder of the Lower Palaeozoic. Traces typical of deeper- water facies evolved in shallow water during the Vendian and early , and may have slowly migrated into the deep ocean during the remainder of the Lower Palaeozoic. There are a limited number of short-ranging ichnogenera which may be useful for correlation but the first appearance of ichnogenera may be a more satisfactory method. Three zones covering the ranges Redkino and Kotlin, Rovno and Tommotian-lower Atdabanian are recognized and may be useful for world-wide correlation.

‘Explosive evolution’ was how Seilacher (1956) described the Platformare partly time-equivalent to the -bearing changes in thetrace fossil biotaacross the Proterozoic- Talsy Horizon in the . Phanerozoic boundary. He compared the dearth andsimplicity The Rovno Horizon has yielded a relatively diverse ich- of trace in upper strata with the abundance nofauna,including Gyrolithes,Phycodes, Teichichnus and and complexityof those from the Cambrian. These rapid . It appears, therefore, that part of the ‘explosive changes have led to suggestions that tracefossils may be useful evolution’took place duringthe Vendian (sensulato). De- forcorrelation at theboundary level (Crimes 1975,1987; velopment of more advanced forms such as trilobite furrows Alpert 1977; Narbonne et al. 1987). (Cruziana) and complexspreite (Diplocraterion, Rhizocoral- The evolution of behaviouraldiversity was investigated in- lium,Zoophycos) occurredlater, in theearly Cambrian dependently by Crimes (1974) and Seilacher (1974) who both (Tommotian and Atdabanian). concluded that, while there was a rapid increase in trace fossil The purpose of this paperis briefly to review the evolution abundanceand diversity in shallow-waterCambrian seas, and diversification of trace fossils acrossthe Proterozoic- significant colonization of the deep oceans did not take place Phanerozoic boundary and into the Lower Palaeozoic. until the Ordovician and then blossomed in the Cretaceous. In the absence of formal acceptanceof a type section for the Seilacher (1977,1978, 1986) suggested that the evolutionary Precambrian-Cambrian boundary,the Russian subdivisions changes in trace fossils, particularly those from the deep sea, will be used, since at present they are most widely known. The involved optimization of feeding behaviour, increase in com- base of the Cambrian will therefore informally be taken as the plexity and reduction in size. It has, however, been recognized base of the Tommotian. The tracefossil zones were, however, recently that some trace fossils which are typical of deep seas erected,as far as possible, independently of body fossil throughout much of the Phanerozoic, actually evolved and correlations and on a world-wide basis (Crimes 1987). Selec- reached a high level of complexity,and optimizationof feeding tion of a type section outside the USSR may therefore result in behaviour, in Tommotian to Toyonian andeven ‘late Precam- a different position for the Precambrian-Cambrian boundary brian’ shallow-water (Crimes & Anderson 1985; Crimes 1987, within the Russian sequence, but should notsignificantly affect 1991; Hofmann & Patel 1989) andperhaps upper-slope the utility of trace fossils in future correlations. sequences (Narbonne& Aitken 1990). The timingof these early evolutionary changes is important, both in relation to the de- Late Proterozoic and Early Phanerozoic trace fossils velopment of the metazoa and in the use of the rapid diversifi- cation of trace fossils in correlations at this level. Thelast 20 yearshave seen numerousinvestigations of Recent investigations have led to a reappraisal of corre- sequences of late Proterozoic and/or early Cambrian age in lations of some of the trace fossil-bearing upper Precambrian many parts of the world, principally: and Tommotian to Toyonian strata across the USSR and into (Young 1972; Fritz 1980; Fritz & Crimes eastern Europe. The traditionalview was that the Rovno Hori- 1985; Crimes & Anderson 1985; Nowlan et zon of the and the Nemakit-Daldyn Horizon of al. 1985; Narbonne & Hofmann 1987; Nar- were the lowest units of the Cambrian (Savitsky 1959, bonne et al. 1987; Hofmann & Patel 1989; 1975). Later work by Shishkin (1974) showed an assemblage of Narbonne & Aitken 1990) fossils in the top 11 m of the Nemakit-Daldyn Horizon which USA(Alpert 1976; Mount et al. 1983; Gibson also suggested an early Cambrian (Tommotian) age. In con- 1989) trast,Sokolov & Fedonkin (1984) equatedthe Rovno and Argentina (Acefiolaza & Durand 1973; Acefiolaza Nemakit-Daldyn horizons and assigned them to the Vendian, 1978; Acefiolaza & Toselli 1981; Regalia & generally regarded as Precambrian,and thisview is apparently Herrera 1981; Poire et al. 1984) becomingaccepted inthe USSR. However,Moczydiowska Greenland (Cowie & Spencer 1970; Pickerill et al. 1982) (1991) considered the evidence to demonstrate that Scandinavia(Banks 1970; Bergstrom 1981) the Nemakit-Daldyn may belong to the Cambrian, and that United Kingdom (Crimes 1970a, b; Brasier et al. 1978; Brasier depositsreferred tothe Tommotian Stage in the Siberian & Hewitt 1979; Cope 1977, 1982) 637

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Spain (Crimes et al. 1977; Brasier et al. 1979; Legg and simple butwinding and meanderingtraces such asHelmin- 1980; Perejon 1982; Fedonkin et al. 1983; thoida and Helminthopsis are interesting in that such habits are Liiian 1984; Liiian & Palacios 1987) uncommonin post-Cambrian shallow-water deposits. They Poland (OrIowski et al. 1970; Paczeina 1985a, b, appear to provide further examples of the evolution of deep- 1986,1989; Orlowski 1989) water habits in early shallow-water seas. USSR (Fedonkin 1976, 1977, 1978, 1979, 1980a, b, Only a limited number of traces appear in the Kotlin Hori- 1981,1988; Palij 1976; Palij et al. 1983; zon but they include the screw-like burrow Harlaniella which Keller & Rozanov 1979; Urbanek & may be restrictedto that horizon. In contrast, the Rovno Hori- Rozanov 1983; Sokolov & Fedonkin 1984; zon has four traces so far only recorded from that level and Sokolov & Ivanovskii 1985) alsoincludes the appearance of morecomplex, Palaeozoic- Namibia (Germs 1972; Crimes & Germs 1982) type traces including Phycodes, Teichichnus and Treptichnus. India (Seilacher 1955; Bhargava & Srikantia 1982, Almostall Vendian traces have been recordedfrom 1985; Kumar et al. 1983; Raina et al. 1983; shallow-water sequences andthey show a wide variety of types, Shah & Sudan 1983; Singh & Rai 1983; Bhar- including simple burrows (e.g. Planolites), winding and mean- gava etal. 1984; Bhargava & Bassi1988; deringtraces (e.g. Gordia,Nereites, Helminthoida), circling Kumar et al. 1984; Brasier 1989) traces (e.g. Circulichnis, Gyrolithes), spirals (Planispiralichnus, China (Jiang Zhiwen et al. 1982; Yang Zunyi et al. Protospiralichnus), branching burrow systems (e.g. Phycodes, 1982; Luo Huilin et al. 1984; Crimes & Jiang Treptichnus), simple spreite burrows (e.g. Teichichnus), resting Zhiwen 1986; Luo Huilin & Zhang Shi-shan traces (e.g. Asterichnus, Bergaueria, Intrites, Lockeia) and 1986) scratch marks (Monomorphichnus). The main Phan- (Glaessner 1969; Webby 1969,1970, 1982, erozoictrace fossil lineages therefore evolved inlate Pro- 1984; Daily 1972, 1973; Jenkins et al. 1983; terozoic shelf seas. Recently. however, Narbonne & Aitken Walter et al. 1984, 1989) (1990) havedescribed a limited ichnofauna of Aulichnites, Helminthoida,Helminthoidichnites, Helminthopsis, Lockeia, Correlations between these world-wide sections are at best Palaeophycus, Planolites and Torrowangea from the late Pre- difficult. Nevertheless, it is becoming evident that, within the cambrian Blueflower Formation of NW Canada, which they Vendian, the Redkino, Kotlin and Rovno horizons and their interpret as being depositedbelow storm wavebase on the slope suggested correlatives have distinctive ichnofaunas. butpassing upwards into shallow-water dolomites. There The lowest occurrences of tracefossils in these horizons are may, therefore, have been some colonization of intermediate shown below, with asterisks indicating traces which may be water depths as early as the late Precambrian. restricted to that particular horizon. In drawing up such lists, Trace fossils from the overlying Tommotian to pre-trilobite only the most obvioussynonymies have been omitted and some Atdabanian strata include: more complex forms of trilobite forms are included which may not even be trace fossils. traces (Cruziana, Dimorphichnus, Diplichnites, Rusophycus), Redkino Asterichnus,Aulichnites, Bergaueria, Bilinichnus, spreitetraces (Diplocraterion,Rhizocorallium), winding and Buchholzbrunnichnus*, Bunyerichnus*, Cochlichnus, meanderingtraces (Belorhaphe, Cosmorhaphe, Taphrhelmin- G ordia, Helminthoida,Gordia, Helminthoidichnites, thopsis), resting traces (Astropolichnus, Asteriacites, Mammil- Hehinthopsis, Hormosiroidea, Intrites,Lockeia, lichnis), branchingburrows (Chondrites), sediment-filled Medvezhichnus*, Monornorphichnus, Nenoxites*, burrows (Muensteria, Plagiogmus) and networkstructures Neonereites, Palaeopascichnus, Palaeophycus, (Paleodictyon,Squamodictyon). Of these, Cruziana,Dimor- Planolites,Scolicia, Skolithos, Stelloglyphus, Syr- phichnus and Plagiogmus appear in mostsections not farbelow ingomorpha, Torroulangea, Vendichnus*,Vimenites*, the firsttrilobites and probably indicate a low Atdabanian Yeloviochmus*. level. More complex spreite structures, including Dictyodora Kotlin Archaeichnium*,Brooksella, Chomatichnus, Circu- and Zoophycos, perhaps appeared slightly later. lichnis, Harlaniella*, Monocraterion, Nereites. Rovno Arenicolites,Buthotrephis, Conichnus, Curvolithus, The use of trace fossils in correlating boundary strata Didymaulichnus,Gyrolithes, Olenichnus*, Phycodes, Planispiralichnus*, Protospiralichnus*, Sokolovich- The possibilities for tracefossil correlation at this Proterozoic- nites*, Teichichnus, Treptichnus. Phanerozoicboundary interval were discussed by Crimes (1987) and Narbonne et al. (1987) following Alpert (1977) and Some of the tracesfirst appearing in strata from the Redkino Palij et al. (1983). Crimes (1987) was able to recognize three Horizon to the pre-trilobite lower Cambrian are illustrated in zones with respect to the incoming of trace fossils, but accept- Fig. 1. ance of the inclusion of the Nemakit-Daldyn and, more parti- The Redkino Horizon has some unusual traceswhich are of cularly,the Rovno horizons in the Vendian(Sokolov & limited range andappear to represent early evolutionary Fedonkin 1984) would require some changesto the age ranges, failures, such as Nenoxites, Palaeopascichnus, Vendichnus and and the zones are now considered to be as follows. Vimenites. They are generally small and relatively simple. A Zone I. Age: Vendian(Valdai Superseries = Redkinoand few of those listed may not even be trace fossils (e.g. Bunyer- Kotlin) ichnus) but a detailed discussion of such matters would be out Arenicolites, Asterichnus, Aulichnites, Bergaueria, Bilinichnus, of context here. There are, however, also traces which range Chomatichnus, Circulichnis, Cochlichnus, Didymaulichnus, Gor- from the Vendian through most or all of the Phanerozoic in- dia, Harlaniella*, Helminthoida, Helminthoidichnites, Helmin- cluding: Asterichnus, Bergaueria, Cochlichnus, Gordia, Helmin- thopsis,Intrites*, Lockeia, Medvezhichnus*,Monocraterion, thoida, Helminthopsis and Monomorphichnus. Most abundant, Monomorphichnus, Nero.uites*, Neonereites, Nereites, Palaeo- however, are simple sediment-filled horizontalburrows pascichnus*, Palaeophycus, Planolites, Skolithos, Stelloglyphus, (Palaeophycus, Planolites). These traces are commonly small Torrowangea, Vendichnus*,Vimenites*, Yeloviochmus*.

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Zone 11. Age: Latest Vendian (= Rovno = Nemakit-Daldyn) Gyrolithes,Olenichnus*, Phycodes, Planispiralichnus', Proto- spiralichnus', Sokolovichnites*, Teichichnus, Treptichnus. Zone 111. Age: Tommotian-arly Atdabanian. Arthrophycus, Asteriacites, Astropolichnus', Belorhaphe, Chon- drites;Conichnus, Cosmorhaphe, Cruziana, Cylindrichnus, Dim- orphichnus, Diplichnites, Diplocraterion, Halopoa, Laevicyclus, Mammillichnis, Muensteria,Paleodictyon, Plagiogmus*, Proto- paleodictyon,Rhizocorallium, Rosselia, Rusophycus, Sagit- tichnus, Squarnodictyon, Taphrhelminthopsis. * Probably restricted to this zone. Three sections are under active consideration for the Pre- cambrian-Cambrianboundary stratotype: Aldan River, / / / / Siberia (USSR), BurinPeninsula, Newfoundland (Canada) 0" and Meishucun, YunnanProvince (China). Neither thesection, Redkino Kotlin Rovno Tommotian nor the boundary pointwithin that section,has yet been agreed, although the Newfoundland sequence has been recommended Fig. 2. Trace fossil diversity across the Proterozoic-Phanerozoic for adoption. Trace fossils have advantages for correlation in boundary. that they arecommonly abundant in shallow-water clastic sequenceswhich predominate at this level. If an horizon is chosen which corresponds to the base of the Rovno and its equivalentNemakit-Daldyn, then willit probably be using mainly the references already cited, but also from Lower correlatable on the incoming of more complex Phanerozoic Palaeozoic occurrencesworld-wide. Noattempt has been traces such as Phycodes (especially P. pedum),Teichichnus and made to eliminate other than the most obvious synonymies, Treptichnus. A higher boundary horizon, at the base of the since this would be a major task and, in the case of some old Tommotian, would be indicated by theappearance of an descriptions, exceedingly time consuming. assemblage containingtraces such as Diplocraterion, The results relating to the Precambrian-Cambrian bound- Rhizocorallium and Rusophycus, all of which occur relatively ary strata arebased largely on data from thelast 30 , since low in Zone 111. There is little point in detailed discussion of trace fossils were previously almost unknown in rocks of this thecorrelation possibilities until the stratotype section and age. These descriptions probably includefew synonymous ich- boundary point have been selected. nogenera but there are likely to be more in data from middle Ordovician andyounger strata, which incorporate many forms Trace fossil diversity from the Proterozoic into the Lower described andillustrated rather inadequately in thelast century. Palaeozoic The increase in diversity across the Precambrian-Cambrian Trace fossil diversity should not be confused with animal or boundary can be appreciated from Fig. 2. The Redkino Hori- body fossil diversity; it represents behavioural diversity of soft zon is the lowest so far tohave yielded a significant ichnofauna and hard bodied forms and is dependent on many factors, in- and some 29 ichnogenera are present. There is little change in cluding preservation potential. the Kotlin Horizon but a further burst in diversity has been The increase in diversity of trace fossils from the Precam- recorded from the Rovno Horizon, withthe appearance of brian through the Phanerozoic hasbeen considered by numer- more complex traces such as Phycodes, Teichichnus and Trep- ous authors (e.g. Crimes 1974; Seilacher 1974; Pickerill 1980; tichnus. There is a further markedincrease in diversity in pass- Crimes & Crossley 1991). Any diversity studies, whether with ing from the Vendian into the Tommotian, and this is mainly fossils or tracefossils, suffer from variations in the area of due to the appearance of many complex Phanerozoic traces exposed strata,quality of outcrop, differentlength of geo- such as Chondrites, Cosmorhaphe, Diplocraterion and logical periodsand stratigraphically unevendistribution of Rhizocorallium. research. Trace fossildiversity hasthe added problem that Thereare eightichnogenera apparently restricted tothe identifications, particularly at the species level, can be influ- Redkino Horizon, two to the Kotlin and four to the Rovno. In enced by personal preference and some synonymy is inevitable. contrast,almost all Phanerozoic tracesshow great time Identifications are more reliable at the ichnogeneric level but stabilitywith long ranges (Crimes 1975; Frey & Seilacher even here the inclusion of some synonymous ichnogenera is 1980). Theseearly, short-ranging forms probably represent likely. In the first studies, Crimes (1974) attempted to alleviate failed evolutionary behaviouralexperiments. TheEdiacara these problems by using only well known and recently used fauna may represent a similar situation with regard to body ichnogenera, whereasSeilacher (1974) worked at theich- fossils. nospecific level but employedonly his own field experience The ranges of trace fossils from theupper Proterozoic to the and identifications. Nevertheless, they reached identical con- lower Silurian are shown in Table 1. Data have been taken clusions, notably that trace fossils evolved in early Cambrian from Hantzschel(l962, 1975), updated from recent studies too shallow-water seas and onlyslowly colonized the deep numerous tocite here. The ageof some deep-water Ordovician oceans, with the first significant dispersal in the Ordovician. and Silurian successions is not precisely known, and ranges In the 17 years since those investigations, there have been such as upper Ordovician-lower Silurian are quoted(e.g. Pick- numerous papers, not only on Precambrian-Cambrian traces erill et al. 1987a, b). In these cases, the trace fossils have been butalso on ichnofaunas throughout the Phanerozoic. The recorded as occurring throughout the suggested time period. author has recorded the stratigraphical ranges of ichnogenera, Trace fossil diversity in upper Proterozoic tolower Silurian not only fromthe Precambrian-Cambrian boundary strata strata is illustrated in Fig. 3. Nearly all studies of deep-water

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Table 1. The ranges of trace fossils in upper Proterozoic ( Vendian V) to lower Silurian (LS) strata Table 1. cont.


Aglaspidichnus X Fucusopsis x**xxxx Alcyonidiopsis xxx Furculosus xxx** Allocotichnus X Fustiglyphus X* Amphorichnus X Gordia xxxxxxxx Anconichnus X Glockerichnus x**xxxx Angulichnus X Granularia xx**xxx Arachnomorphichnus X Gyrochorte x**xx*x Archaeichnium X Gyrolithes xx*xx*** Archaeonassa X Halopoa xx Arcuatichnus X Harlaniella X Arenicolites xxxxxxxx Heimdallia X Arthraria xxx*xx Helicolithus X Arthrophycus xx*x*xx Helminthoida xx**xx*x Asaphoidichnus xx Helminthoidichnite 'S X X Asteriacites x***xxx Helminthopsis XX*XXXXX Asterichnus Homopodichnus X Asterosoma xx** Hormosiroidea xx*****x Astropolichnus X Imbrichnus Aulichnites xx*xx**x Imponoglyphus Belorhaphe x**xx*x Intrites X Bergaueria xxxxxxxx Isopodichnus x**xx** Bifasciculus xx Ixalichnus X Bifungites xx* Kouphichnium X** Bilinichnus xx Luevicyclus Brooksella xx X Laminites Buchholzbrunnichnus X Lingulichnites X Bunyerichnus X Lockeia x**xx*x* Buthotrephis xx***x*x Lophoctenium X* Calycraterion xxx** Lorenzinia x**x Caprionichnus X Magaritichnus Caridoidichnus X Mammillichnis Catenichnus xx Medvezhichnus X Chomatichnus Megagrapton xxx Chondrites xxxxxxx Merostomichnites xx**x*x Circulichnis x**xxxxx Monocraterion xxxxxxxx Clematischnia xxxx Monomorphichnus xxxxx*xx Climatichnites X Multilamella X Cochlichnus xxxxxxxx Nenoxites X Compaginatichnus X Neonereites xx*xxxxx Conostichus xxx Nereites xx***xxx Conichnus xx***xx* Oldhamia xx Corophioides xxx* Olenichnus X Corpusculichnus X Ormathichnus X Cosmorhaphe x**xxxx Palaeopascichnus X Crossochorda xx*x*** Palaeophycus xxxxxxxx Crossopodia x****xx Paleodictyon xxxxxxx Cruziana xxxxxxx Paleohelcura X** Curvolithus xx*xx**x Petalichnus X Cylindrichnus Phycodes xxxxxxxx Dactyloidites X Phycosiphon xxx Dactylophycus xx*x Plagiogmus X Daedalus xxxxx Planispiralichnus X Desmograpton X Planolites xxxxxxxx Dictyodora x**xxxx Protichnites xxx**** Didymaulichnus xxxxx*** Protopaleodictyon x**xxxx Dolopichnus X Protospiralichnus X Dimorphichnus xxxxxx* Protovirgularia xx***** Diplichnites xxxxxxx Psammichnites Diplocraterion xxxxxxx Quebecichnus X Elingua X Rauffella X Enigmatichnus X Rhabdoglyphus X* Fascifodina X Rhizocorallium xxxxxxx

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Table 1. contd

v LC MC UC L0 MO U0 LS 120 - 100 Rosselia X * ..* 4 80 Rusophycus X X xxxxx- Sabellarifex X * * 60 Saerichnites X 40 Sagittichnus X * ***** Scalarituba xxX 20 Scolicia xx X xxxx X 0 Sellaulichnus X V LC MCUC L0 MO U0 LS Skolithos xx X xxxx X Total 49 86 76 80 94 113 115 104 Sokolovichnite S X m 4 27 27 27 29 Spirodesmos xx * Deep Water 35 44 45 Spirophycus xx Spirorhaphe xx * Fig. 3. Trace fossil diversity from the upper Proterozoic (Vendian) Squamodictyon X **** to the lower Silurian. There are also eight ichnogenera present in Stellascolites X intermediate water depths in the Vendian. Stelloglyphus X* ***** Strobilorhaphe X** Stipsellus few body fossils and their age is imprecisely known. For example, Sublorenzinia X** * in the Cambrian almost all deep-water trace fossils come from Subphyllochorda xxX sequences dated only as ‘Lower to MiddleCambrian’. These Syncoprulus X X records have therefore been used for the bars for both lower and Syringomorpha xx middle Cambrian,and, as they are alllong-ranging ichno- Taenidium X xxx* **** genera, the same total has been applied to the upper Cambrian, Taphrhelminthoida X since no additional ichnogenera have been recorded at that level. Taphrhelminthopsis X **** The graphs can therefore onlybe used in a general sense but Tasmanadia X they reveal such a definite picture that major interpretational Teichichnus xx xxxx errors are unlikely. Teratichnus X The total diversityis relatively high in the Vendian but there Tigillites X xx** X * is a significant increase in the lower Cambrian (Tommotian to Tomaculum X *xxx Toyonian), most of which occurs in the pre-trilobite strata, Torrowangea xx especially the Tommotian. The apparentslight decrease in the Trachomatichnus X middle and upper Cambrianis probably at least partly dueto a Treptichnus xx * Trichichnus xxx* reduction in suitable facies and fewer investigations. There is Trichophycus x*x then a small increase in the Ordovician, but the high figure for Tuberculichnus xx the upper Ordovician includes many ichnogenera described Tylichnus X somewhat inadequately over 75 years ago and probably con- Uchirites X tains a significant number of synonyms. Clearly the diversity Vendichnus X ‘explosion’ took placein the Vendian and early Cambrian, and Vimenites X later variations were relatively minor. Volkichnium X In contrast, therewas a virtual absenceof traces from the deep Walcottia X sea during theVendian and then onlya slow build-up through Yakutatia xxX the Lower Palaeozoic. Narbonne & Aitken (1990) claim that Yeloviochmus X some traces had at least reached the slope during the Vendian Zoophycos X * *xxxX but the topof their sequence includes shallow-water limestones 49 86 76 80 94 113 115 1 04 and the traces have not been included as ‘deep-water’. This updated information tends to confirm the conclusions X Recorded at this horizon. of Crimes (1974) and Seilacher (1974) that behavioural pat- * Recorded above and below and therefore inferred to be present at this terns evolved inshallow water and that the appearance of horizon. comparatively diverse ichnofaunas in the deep oceans came much later. Lower Palaeozoic tracefossils have been undertaken in the last 20 years and data for the front bar-graphcomes almost entirely The trace fossil colonization of the deep oceans from: Crimes 1970~;Aceiiolaza & Durand 1973; Crimes et al. 1974; Pickerill et al. 1977, 1987a, b, 1988; Aceiiolaza 1978; Hof- The deep-sea ichnospectrum is dominated by tightly meander- mann 1979; Pickerill & Forbes 1979; Pickerill 1980, 1981; Hof- ing, spiral and patterned traces produced by animals eating mann & Cecile 1981; Pickerill & Keppie 1981; Benton 1982; their way through mud, mainly at or close to the surface. It has Fillion & Pickerill 1984; Durand 1985; Pickerill & Harland commonly been assumed that these behavioural responses, and 1988; Pickerill & Williams 1989; McCann 1990; Crimes & Cross- theresultant traces, evolved in thedeep sea. Nereites and ley 1968, 1991; Crimes, Garcia Hidalgo & Poire, unpublished Helminthoida were analysed behaviourally by Seilacher (1967) data. Most of the sequences described in these papers contain andRaup & Seilacher (1969), andinterpreted in terms of

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evolutionary optimization of available food (Papentin 1973; near-shoreto deep-sea environments, whereas in post- Seilacher 1974). Seilacher (1974) suggested that complex Palaeozoicstrata it isgenerally only reportedfrom rocks meanders did not appear before thelate , and he deposited in deep water. Similarly,Ophiomorpha first appeared showed examples, such as Nereites, with only simple meanders in Permian shallow-water environments and was common in in the Cambrian, tighter meanders in the Devonian and more deep-sea fan environments by the Cretaceous. Bottjer et al. regular and tighter onesby the Cretaceous.Seilacher (1986, fig. (1988) point out that ichnogenera producedby a wide variety of 3-5) claimed that the meanderingof Nereites is rather irregular higher taxa might be least likely to show an onshore-offshore in shallow-marine, shelf environments, as in the Cambrian, pattern, whereas ichnogenera produced by members of only and denser and more systematic in later deep-sea flysch depos- one or two higher taxa might be more likely to have an on- its. Similarly, he inferred (Seilacher 1986, fig. 3-6) that - shore-offshore trend as partof their time-environment history. atic meandering behaviourin Scolicia also evolved in Mesozoic Theytherefore conclude that both Ophiomorpha and Zoo- to Tertiary deep oceans. There is, however, a paradox in the phycos may be made by only one, or a few, higher taxa. It is argumentfor such evolutionary development in thedeep therefore also implicit in their argument that the animals mi- ocean. Organic-rich mudcovers extensive areas in the deep sea grated offshore and the behaviour patterns producing these and it is difficult to understand why such a closelyprogrammed tracefossils went with them. Inthe case ofthe Lower response would be necessary or advantageous in such anenvir- Palaeozoic forms, the appearanceof an ichnogenus in the deep onment. An animal could well acquire all its food by travel- sea is accompanied by its absence or near absence in shallow ling in a straight line for its entire . Performing complex water. This also would seem to imply a migration of the ani- meanders would probably be energy-inefficient. Equally, it is mals, unless there were changes in the nature of the substrate difficult to see why remaining in one limited area would, statis- for which we have no firm evidence.The alternative hypothesis, tically, make it any less subject to predators. in which the behavioural programmes evolved independently Recent investigationsshowing the development of deep- in the deep ocean (Seilacher1986), does not explain why those water traces in quiet, muddy,shallow-water Vendian and early animals producing ‘deep-water traces’ in shallow water would Cambrian (Tommotian-Toyonian) niches (Crimes& Anderson not continue to do so, unless it be argued that there was a 1985; Narbonne et al. 1987; Fedonkin 1988; Hofmann & Patel depletion in organic production in muddy areasof the shallow- 1989) could provide a clue to this dilemma. It may be that the water seas. great abundance of trace-making animals in these early seas The occurrence of complex, carefully guided meanders in forced some awayfrom the more favoured andextensive sandy shallow-waterVendian and Cambrian sequences(Glaessner Substrates and into the quiet-water, areally restricted, muddy 1969; Crimes & Anderson 1985; Hofmann & Patel 1989) also niches. Here, there would be a distinct advantage for the ani- arguesagainst an evolutionary optimization of behaviour mals to graze their habitatefficiently and hence produce close, through the Phanerozoic,with more carefully guided meanders regular meanders.In contrast, proceeding in astraight line being produced in younger seas (Seilacher 1974, 1986). This is would rapidly take them into adifferent environment. Efficient also contradicted by the complex meanders recently reported grazing is shown by the trail illustrated by Glaessner (1969, fig. from deep-water Lower Palaeozoic strata (e.g. Pickerill 1980; 5c, d) from theVendian Pound Quartziteof South Australia, or Crimes & Crossley 1991). by the examples of Helminthoida crassa figured by Crimes & Anderson (1985, fig.7) fromthe lower CambrianChapel Island Formation of Newfoundland, and Taphrhelminthoida dailyi described from thelower Cambrian Radford Brook For- Conclusions mation of New Brunswick, Canada (Hofmann & Patel, 1989, figs 6,7). As animal diversity increased rapidlyduring the Cam- (1) Trace fossils first show significant diversity in the Vend- brian and dispersal pressures built up, animalsbehaving in this ian shallow-water seas. way and adapted to living on or within mud would be advan- (2) A higher degree of trace fossil complexity is shown in tageouslyplaced tomigrate to deeper-water environments the Rovno with the development of forms such as Phycodes, where extensive areas of muddy substrateexist. This migration Teichichnus and Treptichnus. appears, however, to have been very limited, at least until the (3) Overallichnogeneric diversity increased duringthe Ordovician (Fig. 3), because many of the traces recorded from Tommotian and achieved very high levels by the Atdabanian; Cambrian deep-water deposits are in fact shallow-water forms thereafter there was little change in the Lower Palaeozoic. (Crimes et al. 1991). Crimes (1974) suggested that the delay in (4) The relative abundance and diversity of trace fossils in penetration of the deep oceans may have been the resultof low clastic sequences spanning thePrecambrian-Cambrian bound- oxygen concentration in the early deepseas, or it might reflect ary may make them useful for correlation and three zones can inadequate supplies of organic detrituswithin the muds of the be recognised. Zone I (Redkino and Kotlin) includes Bergau- deep ocean floor. Seilacher (1974) similarly suggested that a eria, Cochlichnus, Didymaulichnus, Gordia, Harlaniella, Intrites, later increase in deep sea diversity during the Cretaceousmight Neroxites, Nereites, Palaeopascichnus and Vimenites, whereas reflect an increase in foraminiferal and calcareousoozes or the Zone I1 (Rovno) has forms such as Olenichnus and Planispira- growing amount of terrigenous cellulose debris reaching the lichnus, as well as the first appearanceof Phycodes, Teichichnus ocean floor. The arrival of adequate supplies of organic-rich and Treptichnus. Zone 111 (Tommotian-lower Atdabanian) has mud intoprogressively deeper water might therefore havecon- manymore complex Phanerozoic types such as Cruziana, trolled such a migration. Trace fossils would therefore paral- Dimorphichnus, Rhizocorallium, Rusophycus and Squamodict- lel the migrationsof body fossils, which reacheda high level of yon, as well as some forms with restricted ranges including diversity in shallow Cambrian seas (Raup 1976a, b; Sepkoski Astropolichnus and Plagiogmus. 1978, 1979) and then penetrated slowly into the deep oceans. (5) Ichnogeneric diversity in deep ocean sediments was low Recently,Bottjer et al. (1988) showed that Zoophycos is in the Cambrian andthen increased gradually during the Ordo- commonlyreported from Palaeozoic strata deposited from vician and early Silurian.

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Received 25 March 1991; revised typescript accepted 27 November 1991

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