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The Verdun Syndrome: Simultaneous Origin of Protective Armour and Infaunal Shelters at the Precambrian–Cambrian Transition

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The Verdun Syndrome: Simultaneous Origin of Protective Armour and Infaunal Shelters at the Precambrian–Cambrian Transition The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian–Cambrian 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 animals 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 Devonian 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 Jurassic 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 animal 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
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