The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian– transition

J. DZIK Instytut Paleobiologii PAN, Twarda 51/55, 00–818 Warszawa, and Instytut Zoologii, Uniwersytet Warszawski, Banacha 2, 00–913 Warszawa, Poland (e-mail: [email protected])

Abstract: All of the structurally identifiable latest Ediacaran and earliest Cambrian infaunal trace fossils represent shelters of feeding above the sediment surface. It is the case with the most complete and oldest radiometrically dated Precambrian–Cambrian transition strata along the Khorbusuonka River in northern Siberia, in the basal Cambrian succession at Meishucun in southern China, richest in small shelly fossils, as well as in the type succession of the Vendian in Podolia, Ukraine. The oldest traces of feeding within the mud are known from no earlier than the late Tommotian of Siberia, Mongolia, Sweden, and Poland. This suggests that the inven- tion of hydraulic mechanisms of sediment penetration was enforced by predation, not by trophic needs. Various ways to protect the body by secretion of a mineral skeleton or building tubes by collected mineral grains were developed by other animals at the same time. Predation may thus appear to be the triggering mechanism for the ‘Cambrian explosion’. Subsequent increase in the depth of bioturbation resulted in a profound change of taphonomic conditions, artificially enhancing the effects of evolution.

There are two primary sources of evidence on disappeared during the early Palaeozoic, probably the Precambrian–Cambrian evolution of marine the result of increased bioturbation depth in faunas: the ‘Ediacaran biota’ of imprints of soft- pelagic sediments (Dzik 1994). Fossilization of bodied organisms in sandstone facies and the entirely soft bodies, not easily escaping scavengers ‘small shelly fossils’ assemblages of mostly secon- and bacterial decay, is even less likely. After their darily phosphatized remains of mineral skeletons in Ediacaran abundance, such fossils became extre- limestone facies. Owing to recent progress in under- mely rare. In the Ediacaran White Sea locality standing the taphonomy of the Ediacaran fossils, Zimnie Gory in northern Russia, the Bun- significant anatomical data has rapidly emerged denbach slates of the Hunsru¨ck Mountains in from the long-known fossil collections. Imprints Germany and the Solnhofen lithographic of dorsal surfaces of decaying bodies, fixed by limestone of Bavaria, ‘death tracks’ occur, rep- early diagenetic iron sulphide cementation of soft resented by short (frequently spiral) trace fossils sand (‘Ediacaran death masks’; of Gehling with the carcass of the at the end, apparently (1999)); may offer information on the distribution thrown into a toxic environment. These were thus and mechanical properties of the internal organs extreme environments, in the Ediacaran time (Dzik 2003). Impressions of animal bodies pas- supporting only bizarre, and probably chemoauto- sively or actively settling on the surface of a trophic organisms in place (Dzik 2003). Even if microbial mat (‘Shroud of Ediacara’; of Dzik the stratigraphic ranges of the Ediacaran biota and (2003)) inform about the external morphology of small shelly fossils may partially overlap, there is Ediacaran animals. Among the minute phosphatic a wide gap in our knowledge of Precambrian– and secondarily phosphatized fossils, easy to Cambrian organisms in between these two tapho- extract from limestone samples with organic acids nomic windows. The gap exists not only in their and frequently numerous, there are not only time distribution, but also in a range of less elements of the skeletal armour (scleritomes), but unusual environments. also phosphatized organic tissues or even whole This gap can be filled partially with evidence embryos (e.g. Bengtson & Zhao 1997). offered by traces organisms left in their activities, These highly valuable sources of palaeonto- where their bodies had no chance to fossilize. logical information have their limitations, however. Much data on the latest Precambrian and earliest Early diagenetic phosphatization was a rare pheno- Cambrian trace fossils has accumulated, but the menon, except for the Early Cambrian, and gradually main difficulty in interpreting this data results

From:VICKERS-RICH,P.&KOMAROWER, P. (eds) The Rise and Fall of the Ediacaran Biota. Geological Society, London, Special Publications, 286, 405–414. DOI: 10.1144/SP286.30 0305-8719/07/$15.00 # The Geological Society of London 2007. 406 J. DZIK from their lack of precise dating. The situation has (Manykayan) Kessyusa Formation in exposures improved significantly with the acquisition of near the mouth of the Mattaia Creek in the Khorbu- diverse and unusually informative trace fossil suonka River section of the Olenek region. assemblages from the Mattaia Creek section at Many types of trace fossils occur in the Kessyusa Khorbusuonka in northern Siberia (Dzik 2005). Formation. Virtually all bedding plane trace fossils These occur primarily below the occurrence of the assemblages there are monospecific, and it is difficult abundant Manykodes (¼‘Phycodes’) pedum, the to separate results of evolutionary change from appearance of which defines the base of the Cam- changes controlled by purely ecological and environ- brian (although it probably also occurs somewhat mental factors. Different species seem to be below in the stratotype section of Newfoundland; restricted to specific kinds of sediments. Narrow Gehling et al. 2001), and from above a radiometri- U-shaped, shallow burrows are found in thin layers cally dated volcanic breccia (Bowring et al. 1993). of dark mudstone intercalated with siltstone and All are traces of infaunal activities of animals pene- sandstone. The mud was apparently firm, because trating the sediment but feeding above it. Their the burrows remained open during life of their burrows were, thus, protective shelters. makers, preserving a cylindrical cross-section (Dzik The first skeletal remains of a variety of organ- 2005). Synsedimentary faults offer proof that the isms occur in the same rock unit (Khomentovsky light-grey quartz mud, in which serial cylindrical & Karlova 1993), as well as in coeval strata else- burrows of Manykodes pedum were dug, was firm where. Thus, it is likely that predation forced the in its whole bed volume already before deposition latest Ediacaran animals to either seek shelter of the overlying pure sand layer. Also, the small tri- under the sediment surface or protect themselves lobate burrows of Podolodes were produced in a with mineral skeletons (‘the Verdun Syndrome’; relatively firm, fine-grained mud. In a softer, but Dzik 2005). Such explanation of the sudden emer- coarser, sediment trace makers tended to dig gence of skeletonized animals at the beginning of deeper and make burrows circular in cross-section the Cambrian was first proposed by Evans (1912). (Dzik 2005). The horizontal galleries of Mattaia In the present paper this idea is developed miettensis were excavated in green glauconitic sand. further, with a more specific presentation of the probable anatomy of the earliest producers of infau- Continuous horizontal galleries in sand nal trace fossils. Additional evidence on the succes- sion of specific modes of infaunal penetration, as Probably the most bizarre Early Cambrian traces of reflected by trace fossils from Podolia, Ukraine, infaunal activity are those produced by Psammich- and the Meishucun locality in Yunnan, China is nites. As interpreted by Seilacher (1995) and also presented. McIlroy & Heys (1997; referred to as Plagiogmus), these were made by animals horizontally penetrating sandy sediment at a stable depth. Sediment stripes The late Ediacaran infaunal trace makers were left behind as a result of their movement with retrograde peristaltic wave. An organ exposed to The Olenek River region in polar Siberia is among the surface was cutting the sediment while the worm the few sites in the world where the Precambrian– moved (Seilacher 1995; Jensen 1997) and collecting Cambrian transition is documented by radiometric food together with the sediment from the surface. dating (Bowring et al. 1993), a stable isotope Continuous burrows of Mattaia miettensis are record (Pelechaty et al. 1996; Kouchinsky et al. similar to those of Psammichnites. They occur 2001), Ediacaran biotas (Fedonkin 1985), skeletal within the glauconitic sandstone beds in the remains (Khomentovsky & Karlova 1993), and Mattaia section, as well as in the coeval strata trace fossils (Fedonkin 1985). As in fossiliferous exposed along the Olenek River in the same region successions elsewhere, traces of infaunal activity of northern Siberia (Fedonkin 1985). Owing to a are missing in strata with Ediacaran fossils in clear delimitation of particular rock layers by Siberia. Although Seilacher (1999) suggested that clay-rich laminae (possibly representing remnants most Ediacaran trace fossils makers fed below the of microbial mats), the cross-section of the rock microbial mat, this was falsified by the discovery shows the internal structure of the trail (Fig. 1a). of intact mats in the Zimnie Gory section (Dzik The lack of deformation of layers below the trail 2003). Both body and trace fossils occur there on indicates that it was not produced by action of any the upper surface of the mat. Only shallow surface hydraulic pushing mechanism, but the sediment burrows are known from deposits of such age else- was simply abraded by the organism. The trace where (Jensen 1995, 2003; Jensen et al. 2005). The itself is filled with two bands of a homogenized oldest common traces of deep sediment penetration sediment, laterally raised and depressed in the by worm-like animals have been recovered from centre, where the bands are separated by a vertical the middle part of the Nemakit-Daldynian fissure filled with clay (cf. Young 1972). The VERDUN SYNDROME 407

Fig. 1. Reconstruction of Mattaia miettensis (Young 1972) from the latest Ediacaran Kessyusa formation at Mattaia Creek, inferred from the structure of its horizontal bilobed galleries. (a–c) Cross-section, lower, and upper surface of a glauconitic sandstone slab with two associated galleries. (d–e). Diagram showing the arrangement of sediment layers while the trail was made. The morphology of the inserted priapulid worm is based on the mid-Cambrian Louisella (data from Conway Morris 1977). volume of these bands corresponds roughly with the organization is unlike any living animal and volume of sediment removed from the channel. seems strange at a first glance, there were similar This means that all the material removed mechani- organisms in the Cambrian. The Burgess Shale pria- cally from below was transported behind. The pulid Louisella pedunculata, as restored by Conway smooth surface of the double ventral furrow at the Morris (1977), had a long, eversible proboscis, the lower surface of the gallery suggests that this was prosoma armed with hooks, an annulated body the route of the sediment transport. In well- and, most interestingly, two ventral strips of preserved specimens the outer lateral bevel consists minute appendages. Moreover, its cross-section of series of transverse, slightly oblique folds was significantly depressed (Fig. 1d). Using Loui- suggesting that the animal was propelled by retro- sella as a prototype, not much transformation grade peristaltic wave (Dzik 2005). would be necessary to have the body organization The clay laminae above the trail remained intact, necessary to produce all those trails (Fig. 1e). but are folded and raised evidently as a result of a gentle hydraulic elevation of sand by the organism. Cylindrical branching burrows Sediment above the trail first appears to have been pushed upwards to flow on both sides and then col- The earliest Cambrian clay-rich sediment surface lapsed in the middle behind the animal. In the centre was probably firm enough that empty burrows in of the trail, the laminae dip down to form a vertical it did not collapse without additional impregnation zone where the organ exposed to the surface was (Droser et al. 2002). These most likely permanently apparently cutting the sediment while the organism open shelters of a surface detritus feeder sub- moved, collecting food together with the sediment sequently filled with sand (Jensen 1997). That from the surface (Hofmann & Patel 1989). they were empty during life of the producer is In the glauconitic sandstone of the Kessyusa convincingly demonstrated by an aggregation of Formation such galleries were dug in so sophisti- clay-rich faecal pellets at the bottom of burrows cated a manner that much anatomical information of related Manykodes rectangulare from the latest can be extracted from the observed deformations Cambrian of the Holy Cross Mountains, Poland of the sediment layers. Although the inferred body (Fig. 2; Orłowski & Z˙ ylin´ska 1996). 408 J. DZIK

Fig. 2. Biology of Manykodes rectangulare (Orłowski & Z˙ ylin´ska 1996) from the latest Cambrian Klono´wka Shale at Wis´nio´wka in the Holy Cross Mountains, Poland, inferred from the morphology of its serial burrows. (a, b) Specimens showing scratches left by hooks covering the animal (a) and faecal pellets at the bottom of chamber (b). Such traces indicate that the burrow was originally empty and that the animal was deposit feeding above the sediment surface (originals of Orłowski & Z˙ ylin´ska 1996). (c) The Early Cambrian priapulid Corynetis (data from Huang et al. 2004) situated in the burrow of Manykodes to demonstrate the correspondence in distribution of scratch marks in the burrow and anchoring hooks in the animal’s body.

These Late Cambrian trace fossils provide begin infaunal life. If so, the oldest traces of peri- important information about the anatomy of staltic activities of the metazoans should be con- their makers, which can be extrapolated also to nected with their infaunal lifestyle. The fossil older, earliest Cambrian species. Swollen parts of evidence does not support this. the burrow are marked with a ring of 15–20 The geologically oldest evidence of purposeful longitudinal striae. Digging was apparently per- penetration of sediments for feeding are the hori- formed with hooks surrounding the body, such as zontal ‘rhizocorallium’ spreite structures in a those in the priapulid prosoma. Such swellings marly limestone facies of the late Tommotian (or developed most probably when the animal early Atdabanian) in the Tiktirikteekh section on retreated to the burrow, contracted and, as a the Lena River near Jakutsk (Fig. 3a; Dzik 2005). result, widened its body. Such feeding traces are known also from roughly The recently published information on the coeval sandstones of northern Poland (Paczes´na anatomy of the Early Cambrian priapulid Corynetis 1996) and Mongolia (Goldring & Jensen 1996). from the Chengjiang Fauna (Huang et al. 2004) More sophisticated spreite structures, apparently suggests that it was not necessarily the prosoma, produced by related organisms in the Atdabanian but rather anchoring hooks at the posterior part of of Sweden, show a helicoidal organization the body which were responsible for scratches (Jensen 1997). noted on the burrow wall. The long proboscis of In the late Tommotian marly limestone at the Corynetis and the circle of long setae near its base Zhurinsky Mys locality along the Lena River suggest that this trace maker lived in a shelter section, narrow, linear burrows of probable with only its proboscis and protective setae extend- mud-eaters are restricted to the laminated sediment ing from it, exactly what is suggested by the fill of earlier meandering furrows in the firm burrows of Manykodes, likely excavated by a ground surfaces, 2–3 cm wide and deep. The related priapulid (Fig. 2c). already bioturbated sediment was probably softer and easier to penetrate (Fig. 3b–c). It may also First mud eaters have been richer in organic matter, the actual impetus for digging. It is commonly argued that internal body cavities Even among the infaunal burrows of the Late originated in connection with hydraulic locomotion Cambrian, those produced by organisms feeding and burrowing (Clark 1964) and that it was the above the sediment surface are the most common search for food which forced early animals to (e.g. Orłowski & Z˙ ylin´ska 1996). Not before the VERDUN SYNDROME 409

Fig. 3. The oldest infaunal mud-eaters from the Tommotian to earliest Atdabanian Pestrotsvetnaya Formation of the Lena River section near Yakutsk. (a) Horizontal spreite structure (‘rhizocorallium’) from the Tiktirikteekh Locality, probably produced by a relative of Diplocraterion. Note secondary burrows penetrating the reworked sediment of the spreite. (b, c) Vertical and horizontal section across the sedimentary discontinuity surface at Zhurinsky Mys with minute horizontal burrows preferably penetrating the soft sediment filling the network of older and wider horizontal burrows.

Ordovician do feeding burrows dominate the trace have nearly circular cross-section. Similar burrows fossils assemblages. are known from the Chmielnicki Formation of Podolia, Ukraine, usually exhibiting a medial fissure along their ventral surface and arranged in Transitions between various forms of the linear series (Palij et al. 1983; Dzik 2005). Some- Ediacaran infaunal behaviour times, the fissure splits, giving the ventral surface of the burrow a three-lobate appearance. At first glance, the difference between the short, tri- The similarity of the three-lobed burrows of lobed and depressed shallow burrows in firm clay, Podolodes to a branching series of cylindrical continuous galleries in sand and serial U-shaped burrows of Manykodes is highlighted by the plait- cylindrical burrows in firm clay from the latest like arrangement of alternating series of shallow Ediacaran of the Olenek River region look like a burrows of Podolodes triplex from Podolia (Palij fundamental and abrupt change. However, traces et al. 1983; Dzik 2005) and eastern Poland (Paczes´na indicative of transitional behaviours are known 1996). In both the specimens from Khorbusuonka from other Ediacaran–Cambrian localities. and Podolia there is a gradation from three-lobed shallow to fully submerged burrows with a round Intermediates between cylindrical and cross-section and a smooth or bilobed lower trilobed burrows surface. They may alternate but may also be arranged linearly (Jensen & Mens 2001). About 15 m above the base of a mudstone series There is, thus, a gradation of morphologies of exposed at Khorbusuonka near the mouth of the trace fossils from shallow U-shaped burrows with Mattaia Creek, the lower surface of glauconitic circular cross-section to even shallower burrows sandstone beds is densely covered with casts of gen- with a three-lobed ventral surface. Although the erally shallow burrows in the underlying claystone burrows do not offer much information about the (referred to Podolodes tripleurum in Dzik 2005). detailed anatomy of their producers, some infer- The dominant morphology is three-lobed. Similar ences can be attempted. Indistinct, transverse wrin- traces are known also from Namibia (Geyer & kles in the central belt of the trace may reflect a Uchman 1995; Jensen et al. 2000) and Mongolia peristaltic wave moving along the body. The clear- (Goldring & Jensen 1996). In profile view, the cut course of the lateral ridges suggests that they burrow casts are gently convex, with both ends were produced by longitudinally arranged series smoothly disappearing at the bedding plane. of appendages. This is not unlike the body plan Rarely, they are arranged in linear series. On sand- inferred for Mattaia, which is represented by hori- stone slabs covering a more sand-rich clay, burrows zontal galleries in sand from the same strata. 410 J. DZIK

Intermediates between bilobed horizontal of calcitic, aragonitic, phosphatic, and siliceous galleries and trilobed burrows composition emerged virtually simultaneously, although not necessarily in the same environment; The collapsing galleries of Mattaia miettensis were (2) amongst the first protective skeletal innovations dug horizontally between sand layers. In this were agglutinated tubes built of foreign objects by respect they differ rather markedly from other the animals inhabiting them—exemplified by the trace fossils recognized in the terminal Ediacaran worm Onuphionella, with its collection of mica strata of northern Siberia. However, in the Chmiel- flakes lining its shelter (Signor & McMenamin nicki Formation of Podolia, Ukraine, similar hori- 1988). Such particle collecting behaviour required zontal galleries occur on the sole of sandstone not only anatomical adaptations allowing organisms beds covering claystone. to pick out and glue together skeletal debris with The lower surface of the trails of Mattaia tira- organic secretions, but also an advanced neural sensis from Podolia is bilobed (Fig. 4; Palij et al. system enabling such sophisticated behaviour. 1983). In places the traces are arranged into series In order to explain the origin of skeletonization of separate burrows, but in others the burrows are in early animals, it is necessary to establish the continuous and parallel to the sediment surface. In chronological relationship between first minera- the short burrows with a three-lobed ventral lized armours and the fossil traces of activity of surface, sometimes developed centrally, the gener- these early ‘arms manufacturers’. The first calcar- ally irregular appearance of the traces makes obser- eous tubular fossils occur in strata of similar age vation difficult. These shallow, continuous burrows to those containing the Ediacaran fauna (Bengtson in clay, possibly produced at the clay-sand inter- & Yue 1992), as is also true in the Khorbusuonka face, demonstrate a transition from the temporary section (Khomentovsky & Karlova 1993). However, shelters of Podolodes to more permanent pen- the metazoan nature of the tubes cannot be estab- etration of sand, resulting in continuous galleries lished yet. The oldest taxonomically identifiable below the sediment surface. skeletal fossils of undoubted animal affinity remain the phosphatic small shelly fossils. In Siberia, the Kessyusa Formation has yielded Correlation of earliest infaunal traces the oldest record of phosphatic conchs and tubes and small shelly fossils of unquestionable metazoan affinities. In the section at Mattaia, horizons within the dark, laminated There are two reasons to think that the advent of mudstone unit containing winnowed concretions skeletonization was related to the function of the have yielded numerous calcareous tubes. In acid- external skeleton, rather than to it resulting simply resistant residue, phosphatic internal moulds of as a response to the geochemical environment of their apices with swollen tips demonstrate that these the Precambrian–Cambrian oceans: (1) skeletons are laterally compressed conchs of a hyolith-like

Fig. 4. Burrows of Mattaia tirasensis (Palij 1974) from the lower part of the Chmielnicki Formation at the village Subocz on the right bank of the Dniester River, Podolia, Ukraine, dug in clay and morphologically transitional between horizontal bilobed galleries of Mattaia and three-lobed series of Podolodes (originals of Palij et al. 1983 housed at the Geological Institute, Kiev). (a) Sole face of sandstone slab with deep, bilobed burrows tending to develop a separation between ventral furrows, similar to partially three-lobed linearly short burrows of Podolodes sp. from the same formation (cf. Dzik 2005, Fig. 3a–b). (b) Slab with shallow burrows more similar to Mattaia miettensis; note than in both cases the animal tended to produce short burrows instead of truly continuous galleries. VERDUN SYNDROME 411 mollusc—Ladatheca annae. Other tubular fossils Poorly preserved alternating serial burrows (enigmatic Conotheca and probable anabaritid similar in size to M. pedum occur in Bed 5 of Spinulitheca) are associated (Dzik 2005). In the same rock unit (Fig. 5d), thus it is definitely coeval strata exposed along the Olenek River, Cambrian in age (Tommotian according to planispiral conchs of the bellerophontid mollusc Jenkins et al. 2002). Co-occurring horizontal gal- Latouchella, the probable sinistral gastropod Bars- leries resemble Mattaia in showing a convex kovia, and the low-conical possible monoplaco- ventral surface, but the ventral furrow is not recog- phoran Purella occur (Khomentovsky & Karlova nizable on the specimen (Fig. 5c). Undoubted 1993). The first infaunal traces of activity in the Mattaia (Didymaulichnus in Crimes & Zhiwen area are thus coeval with the oldest occurrences of 1986) occurs in Bed 7 of the Zhongyicun small shelly fossils. Member, in which small shelly fossils with a Tom- The earliest Cambrian succession at the Meishu- motian aspect occur. In the same bed, the first cun section in Yunnan, China, is the richest in phos- arthropod resting traces occur (Crimes & Zhiwen phatic fossils. However, its correlation with the 1986), associated with cylindrical U-shaped Cambrian units of Siberia, is highly controversial. burrows (Fig. 6a). Although the trilobite nature The lowest unit containing skeletal fossils in has not been established, an Atdabanian age of the China is Bed 3 at the base of the Zhongyicun bed has been suggested. Large horizontal galleries Member of the Dengying Formation (Qian & with collapsing roof, representing Mattaia or Psam- Bengtson 1989). Qian & Bengtson (1989) recov- michnites (Taphrhelminthopsis in Crimes & ered small shelly fossils in Beds 3–6, insufficient Zhiwen 1986), emerge in Bed 9 at the base of the to assign a precise age correlation with strata else- Badaowan Member and continue throughout the where, but probably about the same age as the succession upwards to the Canglangpu Formation Nemakit-Daldynian (Manykayan) strata in Siberia. (Fig. 6b, c), which is located above the mudstone This is consistent with the occurrence of the first unit with the Chengjiang fauna, containing fossils infaunal trace fossils which occur in a phosphate- of soft-bodied animals. According to Qian & Bengt- rich oolite of Bed 3—serially branching circular son (1989), the Badaowan Member small shelly tubes with openings at the surface (Fig. 5a, b). fossils suggest an age not older than Atdabanian, Most tubes are enveloped in phosphorite, which possibly Botomian. obliterates details of their morphology. The mode In general, the Meishucun assemblages of trace of branching and circular cross-section is similar fossils are similar to those from northern Siberia, to Manykodes, although diameters (5–7 mm) are although the specimens are not as informative. larger than in M. pedum from Siberia. Their advent is roughly synchronous with the

Fig. 5. Trace fossils from the oldest unit containing small shelly fossils (Zhongyicun Member of the Dengying Formation) at Meishucun, Yunnan (specimens collected by Wang Zhongzhi, housed at the Kunyang Phosphorite Mine museum at Kunyang). (a, b) Horizontal cylindrical burrows from Bed 3 remotely resembling Manykodes, with serial apertures at the sediment surface; partially weathered out phosphoritic envelopes of the burrows with linearly arranged apertures (a) and branching on an upper bedding plane cut by erosion (b). (c) Horizontal galleries from Bed 3 with convex, possibly bilobed lower surfaces, resembling small-size Mattaia.(d) Alternating serial burrows of Manykodes from Bed 5. 412 J. DZIK

Fig. 6. Trace fossils from the post-Tommotian Early Cambrian of the Meishucun section (specimens collected by Shishan Zhang, housed at the Kunyang Phosphorite Mine museum at Kunyang). (a) Cylindrical U-shaped burrows and marks of arthropod appendages from Bed 7 of the Zhongyicun Member; sole of the bed. (b) Wide horizontal galleries with collapsing roof of Mattaia or Psammichnites from Bed 12 of the Badaowan Member. (c) Similar but narrower gallery from the Canglangpu Formation.

appearance of phosphatized skeletal remains of (Dzik 2003). Ironically, the profound change in con- unquestionable metazoan affinities. Also in this ditions of sedimentation that so strongly biased the succession, infaunal feeding traces are unknown fossil record of the animal evolution near the Pre- in the earliest Cambrian strata. All of the animals cambrian–Cambrian transition was at least partially producing elaborate chambers and galleries in the caused by the evolving animals themselves. sediment seem to have searched for food at the surface. I am thankful to A. Y. Ivantsov (Paleontological Institute, Moscow) for providing logistics during the expedition to the Khorbusuonka River section in 2001 (Research Conclusion Grant 6 PO4D 010 13 from the Polish Committee of Scien- tific Research); S. Weiguo (Nanjing Institute of Geology All interpretable trace fossils from the Ediacaran– and Palaeontology) guided me to the Meishucun section; Cambrian transition strata of northern Siberia, S. Zhang kindly allowed me to examine and photograph South China, Ukraine and elsewhere represent shel- specimens of trace fossils collected by him in the Meishu- cun Quarry, housed at the Kunyang Phosphorite Mine ters of infaunal animals which appear to have been Museum at Kunyang; many thanks are due to collecting food from above. McIlroy & Heys P. Vickers-Rich for invitation to the Prato meeting and (1997) found it paradoxical that these animals inspiration. Two anonymous reviewers significantly wasted so much energy in burrowing, while improved the text. seeking for the organic detritus on the sediment surface. True mud-eaters appeared later (McIlroy & Logan 1999). The synchronicity of the develop- ment of various sorts of protective armour with the References origin of the infaunal mode of life suggests a similar cause—probably the development of preda- BENGTSON,S.&YUE, Z. 1992. Predatorial borings in tory organisms targeting large metazoans. This was Late Precambrian mineralized exoskeletons. Science, 257, 367–369. the beginning of an ‘arms race’, giving the choice to BENGTSON,S.&ZHAO, Y. 1997. Fossilized metazoan either shelter behind a strong mineralized skeleton embryos from the earliest Cambrian. Science, 277, or to hide in the sediment. Eventually, the expansion 1645–1648. of infaunal life destroyed the widespread and vast BOWRING, S. A., GROTZINGER, J. P., ISACHSEN, C. E., cyanobacterial mats in shallow regions of the sea KNOLL, A. H., PELECHATY,S.M.&KOLOSOV,P. VERDUN SYNDROME 413

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