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New Data on Kimberella, the Vendian Mollusc-Like Organism (White Sea Region, Russia): Palaeoecological and Evolutionary Implications

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New Data on Kimberella, the Vendian Mollusc-Like Organism (White Sea Region, Russia): Palaeoecological and Evolutionary Implications New data on Kimberella, the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications M. A. FEDONKIN1,3, A. SIMONETTA2 & A. Y. IVANTSOV1 1Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul., 123, Moscow, 117997 Russia (e-mail: [email protected]) 2Dipartimento di Biologia Animale e Genetica ‘Leo Pardi’, Universita degli studi di Firenze, via Romana 17, 50125 Florence, Italy 3Honorary Research Fellow, School of Geosciences, Monash University, Melbourne, Victoria 3800, Australia Abstract: The taphonomic varieties of over 800 specimens of Kimberella (collected from the Vendian rocks of the White Sea region) provide new evidence of the animal’s anatomy such as: shell morphology, proboscis, mantle, possibly respiratory folds and possibly musculature, stomach and glands. Feeding tracks, crawling trails and, presumably, escape structures preserved along with the body imprint provide insights on the mode of locomotion and feeding of this animal. The shield-like dorsal shell reached up to 15 cm in length, 5–7 cm in width, and 3–4 cm in height. The shell was stiff but flexible. Evidence of dorso-ventral musculature and fine transverse ventral musculature suggests arrangement in a metameric pattern. Locomotion may have been by means of peristaltic waves, both within the sediment and over the surface of the sea floor, by means of a foot resembling that of monoplacophorans. Respiration may have been through a circumpedal folded strip (possibly an extension of the mantle). Feeding was accomplished by a retractable pro- boscis bearing terminal hook-like organs and provided with a pair of structures interpreted here as glands. Whilst feeding, Kimberella moved backwards. The structural complexity of Kimberella poses questions about the time of origin of the triploblastic metazoans. The Late Precambrian Kimberella quadrata with powerful venom, and Kimberella thus has also Glaessner & Wade, 1966 was originally described been reconstructed by some as a coelenterate preda- from the late Precambrian Pound Quartzite of tor (Jenkins 1992). Ediacara Hills, South Australia. Originally consi- New fossil material from the Vendian of the dered as a problematic fossil, possibly belonging to White Sea region forced reinterpretation of the Siphonophora (Glaessner, in Glaessner & Daily Kimberella as a mollusc-like animal (Fedonkin & 1959), Kimberella was then reconstructed as a Waggoner 1997) with a high dorsal, non- medusa of uncertain affinities (Glaessner & Wade mineralized shell and an animal capable of active 1966). Known from a few specimens, it was later locomotion. The radioisotope U–Pb age interpreted as a cnidarian pelagic medusa, closely 555.3 + 0.3 Ma of a zircon from a volcanic ash related to extant cubozoans or box jellies (Wade bed in the Vendian marine siliciclastic rocks 1972). Later authors have reconstructed Kimberella inside the stratigraphic range of Kimberella in the as an animal similar to extant chirodropid cubozoans Winter Coast of the White Sea region was, there- or sea wasps (Glaessner 1984; Jenkins 1984, 1992; fore, assumed as the minimum age for the oldest tri- Gehling 1991). Thus reconstructed, Kimberella has ploblastic invertebrates (Martin et al. 2000). This is been used as one of the best examples of a metazoan the oldest calibration point of molecular clocks for lineage crossing the Precambrian-Cambrian bound- the Precambrian bilaterians. Co-occurrence of the ary with essentially no morphological change up to body fossils and trace fossils produced by the present day. It has been cited as one of the most Kimberella revealed the mode of feeding and loco- convincing counterexamples against hypotheses motion of this animal (Ivantsov & Fedonkin 2001; grouping all Vendian macrofossils in non-metazoan Fedonkin 2001, 2003; Gehling at al. 2005). higher taxa (sensu Seilacher 1992). The acceptance In this paper we describe a new fossil collection of Kimberella as an essentially modern cubozoan of unique preservation, which sheds light on the also had major palaeoecological implications: previously unknown aspects of morphology, living chirodropids are fast-swimming predators anatomy and lifestyle of Kimberella. From:VICKERS-RICH,P.&KOMAROWER, P. (eds) The Rise and Fall of the Ediacaran Biota. Geological Society, London, Special Publications, 286, 157–179. DOI: 10.1144/SP286.12 0305-8719/07/$15.00 # The Geological Society of London 2007. 158 M. A. FEDONKIN ET AL. Stratigraphic setting positions, and often associated with traces of their activity on the vast bedding planes, appears to rep- Over 800 specimens of Kimberella have been exca- resent actual community structure. vated from the Vendian siliciclastic rocks in the We conclude that the depth of Kimberella’s White Sea region, northern Russia during the last habitats ranged from the uppermost subtidal zone few years. Major fossil localities were discovered down to a depth of tens of metres. That most of in the deposits exposed in the valleys of rivers of the fossiliferous sediments seem to be formed the Onega Peninsula (Suzma, Karakhta, and Solza below the storm wave base does not necessarily rivers) and on Zimny Bereg (Winter Coast) of the reflect a great depth in a broad, shallow marine White Sea. Most of the collection described here basin. The habitat would probably have provided has been systematically excavated by the field a reasonably high primary production due to the teams led by A. Ivantsov. Some specimens were availability of dissolved mineral nutrients eroded found in the core of boreholes. All the fossils are from a nearby subaerial landmass. This conclusion preserved on the soles of the fine-grain sandstones is supported by the fact that the Vendian metazoan or mudstone beds intercalated with the clay fossil assemblages had a very high density, compar- lamina within the Verkhovka, Zimnie Gory and able to that of the recent populations living in Yorga formations of the Vendian. Stratigraphy shallow marine environments. Sedentary benthic and the sedimentology of the fossiliferous deposits species such as Nemiana and the burrowing organ- have been described by Grazhdankin (2003). isms that produced the traces Skolithos, may reach These sedimentary successions were formed in off- 100 and 400 individuals per square decimetre. In shore shallow water marine conditions under influ- the Solza River basin, A. Ivantsov was able to ence of a vast deltaic system, which prograded trace one fossiliferous layer whose surface was towards the SW from the Kanin–Timan completely processed by grazing Kimberella for a fold-and-thrust belt. The stratigraphic range of few hundred metres laterally. This fossiliferous Kimberella exceeds the radiometrically dated time bedding plane bears both the trace fossils and interval 555–558 Ma based on ages determined numerous body imprints of small individuals on zircons from the volcanic ash beds (Martin (,1 cm long), perhaps reflecting initial coloniza- et al. 2000; Grazhdankin 2003). tion of an open habitat by newly hatched Kimberella. The taphonomic variability of Kimberella is Taphonomy and ecology very high. Preservational diversity allows detailed reconstruction of the external morphology and Environmental conditions of the deltaic platform of even the internal anatomy, as well as locomotion the Vendian Basin were characterized by sporadic and feeding styles. Several preservation modes of turbid flows initiated by seasonal precipitation, the soft body imprints of Kimberella occur in snow melt or storm events. These ‘density currents’ addition to the imprints of separate shells, feeding produced deep erosion that modified bottom relief tracks, crawling trails and perhaps escape struc- and sporadically brought about catastrophic sedi- tures. This rich taphonomic spectrum for a single mentation, burying entire benthic populations species of metazoan seems to have formed under in situ. Preservation of Kimberella on the sand-clay influence of a variety of factors: (1) Kimberella’s interface is often associated with the imprints of anatomical and morphological complexity pre- bacterial mats (‘elephant skin structure’ and other served at different stages of development, from kinds of shagreen surfaces) and abundant benthic juveniles to adults; (2) a relatively high diversity algae such as the filamentous Striatella and the of the habitat occupied by the animal; and (3) vari- net-like Orbisiana (Grazhdankin & Ivantsov ations in the properties of the fossiliferous rocks. 1996). This indicates that Kimberella preferred The case of Kimberella demonstrates the immense calm, well-aerated conditions of shallow marine value of large fossil collections, systematically habitats, well inside the photic zone where benthic sampled from different localities and a variety of photosynthesis could be quite effective. These con- sedimentary facies that represent a wide spectrum ditions seem to have existed long enough to allow of palaeoenvironments. epibenthic communities to form and reach maturity. Most specimens of Kimberella seem to preserve This assumption can be substantiated by the usual the relative positions that they occupied in life on association of Kimberella with other benthic organ- the sea bottom. These organisms were unable to isms, such as the vagile bilaterians Yorgia, Dickin- move quickly enough to escape rapid burial in sonia and Andiva, discoidal Tribrachidium and sand. The initial disturbance of their surroundings frond-like Charniodiscus. The co-occurrence of caused these animals to retract
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