TheVendian geological period, discovered by Academician Boris Sokolov back in the 1950s, has long remained puzzling and mysterious for modern researchers. It was by bits and pieces that scientists were gleaning data about microorganisms, algae and animals which inhabited this planet some 620 to 545 million years ago. Today this period with its quite peculiar "biological features" has firmly taken a place of its own in the history of the Earth. Of decisive importance for this "discovery" were uniquely rich deposits of fossilized Vendian fauna on the White Sea. These sediments of greenish-blue clays and sandstones have been listed by UNESCO among the first candidates to the title of natural monuments of world importance.

PUZZLING ORGANISMS

On the cold shores of the northern White Sea there is a place called Summer Coast (Letny Bereg) which is situated on the north- eastern edge of the Onezhsky Peninsula. It was there near the village of Syuzma at the estuary of a river by the same name that geologists discovered fossilized remains of ancient animals (age of more than 550 mln years) in the early 1970s. Clearly visible ribbed imprints of ancient organisms of an uncertain nature were found in argillo-arenaceous deposits washed out by the river. The nature of deformation of the fossils attested to the fact that the animals possessed soft and elastic bodies. The expedition which made the find was led by Prof. Boris Keller of the Geological Institute of the USSR Academy of Sciences and he was able to identity one of the species which had been found before in deposits of similar age in Namibia (South-West Africa).

Some similar fossil remains had also been found by that time in the pre-Cambrian deposits in Britain, Australia and Canada. But they were quite a rarity in Russia coming mainly from rock samples extracted from deep wells. The first such finds were made back in the 1930s on the Onezhsky Peninsula, but at that time geologists were still in the dark concerning the antiquity of their finds. This transpired later when researchers established their pre-Cambrian age (of more than 545 mln years). An imprint of an organism called Vendia sokolovi Keller extracted from a borehole of 1,552 m (Yarensk) produced quite a sensation and its photos with all sorts of explanations appeared time and again in journals in this and other countries. The very fact that the borehole (and not the only one) "struck" the ancient imprint seemed to indicate that these fossils were many in number. But they had never been found before in any of the natural outcrops of ancient rocks either in European part of Russia or Siberia. A formal description of the first such representative of the Vendian fauna was published only in 1968.

The subsequent excavations at Syuzma, conducted under my guidance, revealed a wealth of new data, including the discovery of several species found earlier in the pre-Cambrian deposits of Southern Australia. This served as an encouragement, but also raised problems: in order to use the fossils for stratigraphic purposes it was necessary to establish the length of the existence of the related fauna. And the paleobiogeographic features of these organisms also remained obscure with the problem being further complicated by the absence of any reliable reconstructions of the position of continental plates in the Vendian period.

The first discovery of fossil fauna at Syuzma was followed by others. The most important was on the Zimny (Winter) Coast of the White Sea - a belt of high cliffs stretching along the coast for more than 40 km where we discovered the first imprints in 1977. This was the main "source" of the fossils. The 30 years of systematic studies and excavations revealed that in the vast zone of the South-Eastern Belo-morye region - from the midflow of the Onega in the west to the Zimny Coast and the Mouth (Gorlo) of the White Sea in the east (a distance of some 400 km) - nearly all of the outcrops of bedrock contained the remains of ancient animals. In this area our lab staff traced scores of new deposits of ancient fauna and amassed a vast collection of their imprints unprecedented in the world. And we have long-established links with some of the leading geological and paleontological research centers in different countries and we sponsor regular international expeditions to the Vendian geological sections on the White Sea and other regions in this and other countries, participating in major national and international scientific programs.

MOST REPRESENTATIVE IN THE WORLD

Thanks to the natural course of events, and, naturally, our own per-

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sistent efforts in expeditions and our labs, Vendian deposits of the White Sea opened up a "window" on the early history of the organic world of this planet. Identified there was the world's most representative fauna province of the Vendian. The many types of sediments piled up in different circumstances of that period reveals a wide range of habitats. The great time range of the propagation of the ancient fauna, the fine granularity of the ancient deposits without subsequent transformations, frequent episodes of catastrophically rapid accumulation of sediments - all of these factors combined to ensure a unique state of preservation of fossils in both quality and numbers.

By comparing geological sections at small intervals along the sea coast and river valleys (sometimes at intervals of 10-15 m) with the full sequence of Vendian sediments as revealed by wells - this covers nearly one kilometer of sediments - we reconstruct events of earliest period of evolution of the animal world of the planet. And although boreholes reveal an uninterrupted "Vendian chronicle", chances are slim to find an imprint upon a core sample 5 to 10 cm in diameter. And in natural outcrops or bank cuttings we conduct systematic excavations, accumulating every year more data on the diversity of excavated forms and on the life and environment of the period of basin formation we study Later on we compare the data with borehole samples and draw a general picture of the period.

FOSSIL CHRONICLE OF LIFE

Preserved from the pre-Vendian geological period of some 3 bin years are mainly traces of vital activities of microbes (first only bacteria) which were later joined by single-cell algae and other protozoa. Therefore petrified and mummified traces of single-cell organisms are quite common in early sediments. Much more impressive are the traces of microbic biogeochemical activity in them - layers of carbonate and carboniferous rock and many kinds of sedimentary ores which accumulated with the appearance of oxygen in the atmosphere of the planet. Nearly all of this gas in the ancient biosphere was "generated" by cianobacteria algae - it was a "by-product" of their vital activities. Following a long and monotonous history of microbic life in the Archean and Proterozoic, the Vendian period offered an example of explosive evolution of high organisms. Investigations of the causes and mechanisms of this major biospheric event are now top of the list for many researchers.

Chronicle of excavations is no simple directory The history of paleontology makes it necessary for us to read its pages "back- wards" - from the known to the unknown. And the further we sink into the past, the less we understand the language of this chronicle and the more blanks are revealed. The chronicle of the Vendian is no bed of roses. The remains of the fauna are not always preserved where it originally dwelled and mass burials are often found in sediments with conditions unfavourable for the existence of the organism. Different communities existed in different conditions and some types of sediments are just empty and nothing is left in them. Finally, organisms of one species, especially those without a skeleton, are preserved in different forms which can be hard to compare.

SEDIMENTARY ROCK-HABITAT DATA

Dealing with fossil corals of ancient reefs we can assume that, like the corals today, they existed in clear seawater with normal salinity within the limit of the photic zone of the ocean (zone of photosynthesis) at temperatures not below 18 ⁰ C. But as regards Vendi an fauna such analogies hold no water. What we need is a "reversed" logic of studies-from the vestages to the reconstruction of the habitat and from there to the physiology and morphology of an organism.

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Thus it was necessary to study in detail the structure, texture and composition of the sediments as the source of information on the habitat and the conditions of burial of animal remains. This task was undertaken by member of our staff, postgraduate Dmitry Grazhdankin. In the South-Eastern Belomorye Region he amassed a unique collection of sedimentary rocks of the Vendian which reflects not only the dynamics of the aqueous medium, depth of basin or the degree of aeration of sediment, but also the role of microbic, algae communities in the marine ecosystems of the period under investigation. He established the following regularities in the distribution of bacterial communities and bigger algae along the whole basin. Quiet and relatively deep-water coastal undersea planes were covered with meadows of algae. In the more shallow, but also relatively quiet conditions of the prodelta there predominated what we call bacterial-algae membranes. An area of propagation of bacterial communities, or mats, was formed on near-estuary sand shoals. Finally, on the delta plane with anomalous salinity there appeared stromatolites - sandy warts glued with bacterial mucus. This picture is very important for more general paleoecological recostructions, for understanding where and how the ancient communities existed whose remains have been preserved.

"READING OFF DATA" FROM FOSSILS

As it became clear with time, the sediments richest in fossil remains were formed in the conditions of a catastrophic accumulation of precipitation. Under the effect of storms, reverse currents, episodic mud torrents containing sand and other suspensions, living organisms were buried so quickly that they could not escape and simply perished. Then the bacteria took over, which disintegrated the flesh and formed a medium which promoted a rapid fossilization of sediments around the remains of animals. The volume of a disintegrating body was gradually filled in with fine silt which moulded all of the external and internal features of an organism. This moulding is a real gift for a paleontologist of this day. Most of the fossils of the earlier geological periods are solid mineral parts (shells, carapaces, bones and teeth) and it is they which are the main object of classification of classical paleontology.

But the possibility of seeing the imprints of the soft body and internal organs of ancient animals also has a

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negative side. The thing is that such a "mould" can provide a lot of different data - about the body exterior, the system of its internal organs, degree of decomposition, different deformities accompanying the complex process of living flesh becoming a fossil. And some such imprints also carry external morphological traits and the picture of inner structure. Decoding such puzzles is no simple matter which is further complicated by the absence of any organic substances in most cases. Such problems were solved with greatest success by our senior researcher Andrei Ivanov who was able to reconstruct details of the complex anatomy of a number of new Vendian invertebrate species. This became possible years later when a vast store of fossils was amassed together with the experience of its interpretation. We realized that practically for every species it was necessary to reconstruct its taphonomic history - from the place of habitat and the mode of life down to its burial and the subsequent processes of degradation and fossilization.

ECOLOGY OF VENDIAN BIOTA

All of the remains of ancient fauna come from marine sediments. We do not know of a single pre-Cambrian fossil from sediments accumulated in fresh-water basins. As soon as we come across instances of water basin losing its salinity (which can be established by geochemical and lithologomineralogical methods) we fail to find there any remains of animals and traces of their vital activities. One such episode - what is known as the Kotlinskoye desalination on the Russian Platform dates back to the end of the Vendian. It was reflected in one kind of sediments found in the Zolotitsa River valley and its tributaries and in the upper part of the bank cliffs of the Zimniye Gory (Zimny Coast). Studies conducted by Grazhdankin proved that the desalination there occurred

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gradually and was accompanied by brief marine transgression (sea Hooding dry land) on the delta plains. At the early stages of shoaling of the marine basin such transgressions were accompanied by a return of the bacteria and algae communities and of the invertebrates. But no remains of molluscs can be traced in marine sediments of the late stages of this process. This could point to a major ecological crisis reaching out far beyond the limits of the Russian platform. Analogues of the Kotlinsk desalination can also be observed in quite different climatic conditions of the Vendian - in the carbonate (which means warmer) basin in the north of the Siberian platform which is also an object of our systematic studies.

Our experience of paleoecological studies of Vendian biota shows that the volumes and diversity of the fauna, its physical activity (which can be judged by the intensity of sediments processing by benthic organisms) and even the size of animals was dwindling in proportion with the decreasing depth of the water. This was due to the deteriorating conditions there, such as lower influx of food particles, lower temperature and, probably, reduced aeration of the fine sediments. But in making such assumptions one has to bear in mind that certain types of relatively deep-water sediments are less favourable for a rapid burial of organisms.

VALUE OF VENDIAN FOSSILS

In the early years of our studies of the ancient fauna on the White Sea we observed that the composition of fossil communities changes from bottom upwards along the rock profile, that is with time. This being so, have we been so lucky as to trace the evolution of the animal world of which no one knew anything until recently. And that was not impossible! The thickness of Vendian sediments here is up to 1,000 m, more than was accumulated in similar conditions on the Russian Platform in any other period. According to scarce isotopic dating the duration of the Vendian was 80 mln years. And the profile of Belomorye sediments, according to the remains of the fauna and thickness of deposits, made up its main share.

It became clear back in the 1970s that preserved in the lower strata of these sediments were fossils which had been earlier described in the pre-Cambian deposits of Namibia.

Discovered in the middle layers were remains like those in the rock mass of England and Newfoundland, and in the upper ones - many varieties of what is known as the Ediacaran fauna of the late pre- Cambrian found in Southern Australia. This indicates that the White Sea region is

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so far the only one in the world where faunist communities of all of the Vendian epochs have been preserved in an unbroken succession (only scattered fragments were found elsewhere). That means that it is only here that one can reconstruct the whole "film" of this very important natural process which is not fully understood in many respects - a relatively rapid and "revolutionary" transition from archaic ancient biosphere to that of the present-day type.

Almost three decades of studies on the White Sea helped us to establish four facts of fundamental value: the increasing diversity of fauna with time; division into stages of the growing biodiversity; modification of faunist links in the paleobasin with time; disappearance of remains of animals in sediments of desalinated paleobasins at the end of the Vendian. Over these years we have discovered new classes and types of animals, described tens of organisms hitherto unknown to science and shedding light on the early evolution of the organic world and have established what we call the stratigraphic bands of propagation of different fauna species.

The ancient Vendian rock masses opened up in the natural outcroppings in the White Sea region have become a geological and paleontological site of international importance. Working there practically every year are expeditions including experts from leading universities of different countries. Thus with the assistance of our American colleagues from the Massachusetts Institute of Technology and the California Institute of Technology we have identified the minimal age (555.3 mln years) of the Triploblastica including all the Metazoa with the exception of the Coelenterata (medusas, corals and Ctenophora}. We have also proved that the absence in the Vendian of simple links between fauna diversity and the isotope composition of carbon in sea-water (our joint article on the subject appeared in the year 2000 in the Science magazine in the United States). We have now on the agenda what we call the calibration of the biostratigraphic scale by methods of isotope chronology, that is determining the length of existence of separate species and the rate of the early evolution of animals.

Studies of the Vendian in the White Sea region arouse great interest not only on the part of paleontologists. Geologists feel that remains of Vendian fauna can be used for the differentiation and global correlation of the ancient rock masses which is the basis of all the geohistorical reconstructions and geological mapping. Geophysicists and specialists in chemostratigraphy, investigating the succession of rock strata by chemical methods, see the possibility of independent paleontological control of the data of magnetostratigraphy and isotope chemostratigraphy respectively. Zoologists regard Vendian fauna as giving them a chance to understand the sources of animal life and of the establishment of complex biological systems. Specialists in molecular biology, building phylogenetic "trees of life" on the basis of a comparative analysis of the INA and RNA of contemporary organisms, include into it data about Vendian fauna in order to understand the peculiarities of the biological evolution at the most hidden molecular level.

One example of the importance of such information was an article which I wrote together with my American colleague Benjamin Waggoner ( Nature magazine, 1997). In this article we reconstructed an ancient mollusc-like organism Kimberella (described on the basis of four samples from the Upper pre- Cambrian of Southern Australia and interpreted as cubomedusa). The new material from the White Sea region (more than 40 samples) indicated that this organism had a soft body, high shield- shaped shell, a large foot with scalloped folds on the edges which served for movement and breathing. And Kimberella was a predator. The article aroused great international response with commentaries on the reconstruction of the mollusc with its color pictures appearings in the magazines Earth, Discover, Science News, American Scientist and in a number of newspapers. The discovery of the most ancient mollusc-like organism of the gender Kimberella was noted as

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an outstanding achievement of paleobiology in the British Yearbook of 1998. The reason for this agitated and widespread response was the fact that we have touched upon a broad range of problems connected with the early evolution of the Metazoa, the origin of molluscs, molecular phylogenetics and calibration of "molecular clock" and the problem of the origin of the skeleton of the invertebrates.

GREATER THAN EXOTIC IDEAS

Studies of the Vendian fauna is a young branch of paleontology, but even in its short history there is the drama of conflicting ideas and collisions of different approaches. Pioneers of the science, like the Australian researchers Reg Sprigg, Martin Glaessner and Mary Wade who made the Ediacaran fauna of Southern Australia a kind of a standard of the Vendian period, and also their followers refer the fossils to the known types and classes of the invertebrates, including those which exist today. Other paleontologists attribute a number of groups of the Vendian fauna to some hitherto unknown types and classes of the invertebrates. Today the overwhelming majority of specialists interprete these fossils as nothing but the remains of ancient animals, although there remain certain disagreements caused above all by the unusual state of their preservation and the complexity of interpretation.

The thing is that they almost do not differ from the sediments in which they occur; most often these are flat imprints, and more seldom-moulds filled with sandstone and aleurolite.

It could have been the thickness of the imprints which played the tragic role in the debates on the nature of the Vendian fauna, giving them a new turn. In 1984 Dr. Adolf Seilacher of Tubingen University (Germany) suggested that these fossils are a dead kingdom of Vendosoa organisms which he later renamed Vendobionta. They used to be flat and giant unicellular protozoans clad in a strong and elastic membrane which also divided the viscera of the organisms into many chambers. Such multinuclear protozoans fed on organic substances dissolved in water and performed gas exchange in the same way (through a tough membrane?!). And let me note that this hypothesis about a very special life form was widely propagated in the 1980s by journalists and popular science writers. A similar suggestion was to regard certain Ediacaran fossils as the remains of some giant protozoans, close to the presentday Xenophiophoria. This was suggested at that time by Russian zoologists Yaroslav Starobogatov and Andrei Zhuravlev. In 1994 American pale- ontologist Greg Retallack suggested a hypothesis claiming that Ediacaran organisms could have been lichens. But do we know many varieties thereof dwelling in a marine environment? And why is it that neither the protozoans, Vendobiontas, lichens or algae of such complex morphology are known neither before or after the Vendian?

All of these different concepts rest on simplified, or just erroneous ideas about the complex processes of fossilization of invertebrate molluscs. Finds of the past few years made on the White Sea have finally ascribed these hypothesis to the ranks of mere historical curiosities.

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Attempts to interpret all of the Venetian fossils within the framework of just one structural model are most likely unrealistic and nonproductive because in the following - Cambrean period - there already appeared all types of animals which exist today And among the Vendian organisms there must have been relicts of some even more ancient epochs of the Proterozoic which had been formed in the conditions of a different archaic biosphere. But there must have existed the ancestors of the latter groups of invertebrates. I am confident that the real drama of discoveries - from the find of a fossil to its interpretation and reconstruction of the conditions of the environment and fossilization, and the whole process of discovering the early history of the organic world of the Vendian is much more colourful than any exotic hypothesis.

"ONE GOOD FOSSIL BETTER THAN TEN HYPOTHESES"

This phrase belongs to an old friend of mine Dr. Ellis Yochelson of the Smithsonian Institute (Washington). We have been studying together over many years puzzling objects like, for example, meters-long prints of a giant mollusc upon the littoral zone of the ancient Cambrian sea (520 mln years ago) in the United States and Canada, or the most ancient fossils of me-tazoans from the sediments of the middle Proterozoic of Montana (USA) and Western Australia - their age being 1.4-1.5 bln years! And we do have more than enough of hypotheses which are often contradictory and call for verification on a concrete sample or at an ancient rocks outcrop.

Let me give just two examples which make it quite clear that Ediacaran fossils are really animal remains. Recently A. Ivantsov discovered fossil traces of movement of two such species: Yorgia (described by him in 1999) and the aforementioned Kimberella. The traces of crawling of Vendian animals in and on top of sediments are encountered quite often, but we were never able to link them with some known imprints of the body of an animal. The unique nature of the new finds consists in the fact that we have been able to do that. The remains of Yorgia together with the traces of its movement were preserved upon the bottom of a phacoidal, or lenticular, layer of fine-grain sandstone 10-15 cm thick. The history of the burial can be reconstructed as follows: during a catastrophically rapid inflow of sand a whole community of benthic organisms was buried in the place of their dwelling. It is interesting that the imprints of the body and tracks have been preserved in opposite relief: the body - as a concave imprint and the track as a convex imprint on the bottom of sandstone layer. The tracks are chains or groups of oval imprints, reflecting moments of the animal's temporary immobility. They have been preserved because the organism was abundantly excreting mucus, and the longer it remained on

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the same spot, the more the sediment under it was glued together.

The imprints of Kimberella together with the tracks are located on the bottom of a thin sandstone layer and they unfold not only the story of its death, but many facts about the organism itself. Sand brought in by current, was piling up so quickly that the animal was unable to climb out and was moving inside it. The traces of its movements in the sediment mass and the body imprint indicate that the organism had developed muscles and supporting skeleton (spinal shell). And in the year 2000 I received an unexpected present from my Italian colleague - a rare sample from the White Sea upon which a Kimberella imprint was preserved together with the traces of the animal's food. This sheds completely new light upon the morphology and physiology of the organism.

NATIONAL PROPERTY OF UNIVERSAL IMPORTANCE

In October 2000 the Landau Foundation invited me to read a lecture about early evolution of the biosphere at the UNESCO Autumn School on global climate changes and their impact upon the biosphere which was meeting at the Bicocco University in Milan. There an old friend of mine, Prof. Georgio Teruzzi, Director of the Natural History Museum and a great enthusiast of his studies, invited me to take a look at the latest acquisitions of the paleontology section, including a collection, bought in Germany, of Vendian fossil animals from the White Sea. By the nature of the species and the state of fossils' preservation I was able to easily pinpoint their place of origin. Since the description lacked any exact geographical attribution, and the names of the animals were often simply wrong, I got the impression that the collection had not been submitted to the obligatory expert examination required for its export from Russia. And no one at our Institute had seen the samples and we alone have experts on the Vendian fauna.

Most of the fossils belonged in the category of mass material and had no particular scientific or museum value. But it turned out to be really unique because it bore the traces of the diet of Kimberella together with its body imprint. I could not hide my joy and also regret over the fact that a collection of great scientific value was lost for Russia. And then Prof. Teruzzi gave it to me as a present...

A few years ago we came across some people on the Zimny Coast of the White Sea hunting for ancient fossils in order to sell them later. Such fossils sell abroad at rather high prices and some such objects, we were told, are ordered by foreign dealers and some researchers. And when the extraction and selling of fossils breaks the legal norms this inflicts some irreparable damage to the national property of our country, its science and culture.

After this unpleasant encounter on the Zimny Coast we, together with geologists from Arkhangelsk and the Northern Committee for Natural Resources applied to the administration of the Arkhangelsk Region with a request for putting a temporary ban on commercial collection of fossils on its territory. Fortunately, this request was supported by the head of the local Administration, A. Efremov

Studies of the Vendian fauna of the White Sea in 1999-2001 were conducted with the support of the Russian Fund of Fundamental Studies (Grants 00-15-98610 and 99-05-64547) and the US National Geographic Society (Grant 6015-97).

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