Boreskov

PP‐8The fourth most significant step of skeletonization occurs in the earliest CambrianTommotian Stage. One can observe the mass appearance of numerous and diverse groups ofskeletal animals. Archaeocyaths with carbonate cups, brachiopods with carbonate andphosphate shells, phosphate problematic tommotiids, radiolarians with siliceous skeletonhave appeared along with already existed various pre‐Tommotian groups.The fifth step in skeletonization starts in the Early Atdabanian and marked by‘arthropodization’ – origin and diversification of arthropods with organic and carbonatecarapaces (trilobites, phyllocarids (Isoxys), anomalocarids, bradoriids, etc.) and lobopodswith phosphatic skeleton (Microdictyon). In addition, during the Middle ‐ Upper Atdabaniantime the first echinoderms with the carbonate skeleton appeared.Thus we can trace five main steps in skeletal mineralization lasting from the LateRiphean to the Early Cambrian. By the end of this time all main compositional types ofanimal skeletons have been formed, and further evolution went on in the structural way, butnot in the compositional. Still we are far from precise determination of actual causes thattriggered the process of skeletonization, guessing only that it was a complicatedcombination of a variety of biotic and abiotic factors. However, the study of ancient skeletalorganisms can shed light on this mysterious phenomena of the biota evolution.References[1]. Gnilovskaya M.B. The oldest annelidomorphs from the Upper Riphean of Timan // Dokl. RAN. 1998. V.359. P. 369–372.[2]. Gnilovskaya M.B., Becker Yu.R., Weiss. A.F. et al. Pre‐Ediacaran fauna of Timan (Upper Ripheanannelidomorphs) // Stratigraphy. Geological Correlation. 2000. V. 8. № 4. P. 11–39.[3]. Demidenko Yu.E. New Cambrian Lobopods and Chaetognaths of the Siberian Platform // Paleontol. J.2006. V. 40. № 3. P. 234–243.[4]. Khomentovsky V.V., Karlova G.A. Biostratigraphy of the Vendian–Cambrian beds and lower Cambrianboundary in Siberia // Geol. Magaz. 1993. V. 130. № 1. P. 25–45.[5]. Kontorovich A.E., Varlamov A.I., Grazhdankin D.V. et al. A section of Vendian in the east of West SiberianPlate (based on data from the Borehole Vostok 3) // Russ. Geol. Geophys. 2008. V. 49. № 12. P. 932–939.[6]. Lin Donga, Shuhai Xiaoa, Bing Shen et al. Restudy of the worm‐like carbonaceous compression fossilsProtoarenicola, Pararenicola, and Sinosabellidites from early Neoproterozoic successions in North China //Palaeogeogr. Palaeoclimatol. Palaeoecol. 2008. V. 258. № 3. P. 138–161.[7]. McChaffrey M.A., Moldowna J.M., Lipton P.A. et al. Paleoenvironmental Implications of novel C30 steranesin Precambrian to Cenozoic age petroleum and bitumen // GCA. 1994. V. 58. P. 529–532.[8]. McMenamin M.A.S., McMenamin D.L.S. The Emergence of Animals: the Cambrian Breakthrough. NewYork: Columbia University Press, 1990. 217 p.[9]. Parkhaev P.Yu. The Early Cambrian Radiation of Mollusca // Phylogeny and Evolution of Molluscs / Eds.W. Ponder and D. Lindberg. Univ. California Press, 2008. P. 33–69.[10]. Rozanov A.Yu. What has happened 600 millions years ago? Moscow: Nauka, 1986. 95 p.[11]. Rozanov A.Yu., Zhuravlev A.Yu. The Lower Cambrian Fossil Record of the Soviet Union // Origin and EarlyEvolution of the Metazoa / Eds. J.Lipps and P. Signor. New York: Plenum Press, 1992. P. 205–282.144