by Sergei NAUGOLNYKH, Dr. Sc. (Geol. & Mineral.), RAS Geological Institute

About 60 mln years, such is supposed duration of one of the six, the fourth geological period of the Palaeozoic, namely, Devonian (419-358 mln years ago). That was the time marked by literally explosive flourishing of living organisms on the Earth. The life colonized more and more ecological niches at that time. The different terrestrial higher plants, the primofilices ("prae-ferns"), progymnosperms, including arborescent forms, first terrestrial vertebrates, in the seas--armored fishes or placoderms, and crossopterygians or lobe-finned fishes, all of them were progressing to their evolutionary rise. Fossil evidences proved existence of these creatures in the Devonian. The fossils, witnesses of the past preserved up to the present days, and now are allowing us to reconstruct the general picture of processes, which took place in the Devonian biosphere. For proper study of these fossilized evidences, the scientists use different approaches of many disciplines, including palaeopedology, a relatively new scientific trend, which studies ancient soils of the geological past, their history and evolution.

FROM THEORY TO PRACTICE

At the very beginning, I would like to mention one remarkable book, which was written by the famous German palaeobotanist Hans-Joachim Schweitzer. The title of this book is very eloquent: Plants Colonize the Land (Pflanzen erobern das Land). Any reader will look through the book with sincere wonder and excitement, traveling through the miracles of the fascinating ancient plant world, hidden inside the pages devoted to the landscape reconstructions of the Devonian period. Some pictures depict small bushes of Zosterophyllum and Sawdonia, no taller than several dozen centimeters, and

The paleosoil profile FPS-1, northern part of the Andoma section, lower part of the Klimovskaya Formation. 1--vertical section of the profile; one can see the vertical channels, which were left by the soil infauna and root traces. 2--structure of the profile; the initially regularly stratificated sediment is disturbed by influence of activity of the soil infauna and root systems. 3--division of the profile into separate samples for geochemical studies; numbers of samples correspond to numbers in the Table with the data of composition of the microelements. 4--cross-section through the root traces; light coverings on the roots are well seen; the coverings originated as a result of recovery processes. Scale--1 cm.

Geochemical characteristics of the paleosoil profile FPS-1. Content of the microelements (n^o 10^-4, %). Numbers of the samples correspond to the numbers on illustration 2, fig. 3.

the candle-like large trunk of the lycopods Duisbergia, and communities of the other lycopod Cyclostigma, now also known as Jurinodendron, with the branched stems reaching 8 m.

I visited Professor Schweitzer in his country house in Bonn, on the left bank of the Rein, and I am very proud of this honor. We had long talks with him about the Devonian vegetation, and while paying meticulous attention to the plant treasures of the Middle Palaeozoic, we inevitably discussed peculiarities of the Devonian paleosoils.

In fact, magnetizing variety of the Devonian plants left no doubts that well-developed and highly specialized

Distribution of the chemical elements (color pictures on the left) and micromorphology of the fossil paleosoil profile FPS-1 (black-and-white pictures on the right); sample AN-2 in SEM Organic carbon corresponds to the position of the root traces. Scales are on the pictures.

soils already existed in the Middle Palaeozoic* world. But where can we find these paleosoils, and how they should look like?

Before we try to answer this intricate question, let us look into the subject of our search. Specialists-paleope-dologists, as a rule, use one and the same set of methodological approaches to studies of the Pleistocene** paleosoils, identical to those, which are used for studies of the recent soils. In contrast to them, some very important corrections should be made for adequate studies of the

* Palaeozoic era (540-250 mln years ago)--the geological era, also known as the era of Ancient life; it is divided into six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.--Ed.

** Pleistocene (2.58 mln years ago--11.7 thous. years ago)--the epoch of the Quaternary period (=Antropogene), characterized by numerous glaciations.--Ed.

fossil soils from the Mesozoic* and Palaeozoic deposits, as ancient soils of such age normally are found in considerably lithificated and geochemically changed sedimentary layers. But nonetheless it is important to mark the fact that these fossilized structures of pedogenetic nature also could be studied by methods of classical genetic pedology. Its basic principles were founded by the great and outstanding Russian naturalist Vasily Dokuchaev at the end of the 19th century.

Sometimes the term "paleosoil" is applied for ancient soil of the Pleistocene or Holocene** age), ini-

* Mesozoic (250-66 mln years ago)--the geological era, also known as the era of Middle life; it is divided into three periods: Triassic, Jurassic, and Cretaceous.--Ed.

** Holocene--the geological epoch with duration of approximately last 12 thousand years up to the present days.--Ed.

Fossil remains of fishes from the Upper Devonian deposits of the Andoma Mountain section. 1--scale of the crossopterygian HOLOPTYCHIUS; 2-4--remains of placoderms BOTHRIOLEPIS spp.; 2--medio-dorsal anterior armor plate; 3--ventral side of the juvenile individual; 4-separate fragments of armor plates. Scale-1 cm.

tially buried, but later re-exposed on the so-called "day surface" again, and again started to function as a normal soil. Thus, I use the term "fossil paleosoil" for fossilized paleosoils, and its abbreviation "FPS" or "FPS-profile".

General logic of investigation of any FPS-profile is determined by three main aspects. First, any paleosoil should be regarded as a causal function of landscape; secondly, function of time, and thirdly, function of activity of living organisms, or even the direct result of it.

Hopefully, I do not tire the reader by this absolutely necessary preamble, but these terminological tools will be needed in our further research, no less needed than the geological hammer or shovel.

DEVONIAN MOUNTAIN

There are no many places in Russia, where one can observe the red-colored deposits of the Devonian age in all their beauty. One of such rare places is the Andoma geological section, or Andoma Mountain, which is disposed in the Vologda region on the northern coast of the Onega Lake. This nice place is a favorite recreation area for many people from the neighboring cities of Vytegra, Petrozavodsk, and St. Petersburg, but lately the Andoma Mountain has also become a place quite frequently visited by tourists from Moscow, looking for a good rest after fussy days and months in the megalopolis, for fresh air and successful fishing. The Andoma Mountain is also very popular among geologists, simply because of its cliffs, which allow to study

peculiarities of the geological structure of this area in many respects.

I was guided to the Andoma Mountain by a search for paleosoils. I learned from my good friend Sergey Sni-girevsky, the palaeobotanist from St.Petersburg, that the mineralized woods of the genus Archaeopteris (formal genus for the woods--Callixylon) frequently are found in the Andoma coastal cliffs, and they are well preserved.

The genus Archaeopteris was a typically terrestrial plant, which evidently grew relatively near the places where its woods were buried. The type of sediments, where the woods of Archaeopteris are found, points to the very shallow water environments. A part of them can be regarded as continental, deltaic or even eolic (wind) deposits. Therefore, the possibility to find fossil paleosoils here in the Andoma Mountain seemed very high.

After careful studies of the Andoma section I found at least three fossil paleosoil profiles (FPS-profiles). The term "profile" in pedology means a pedogenic structure, which is characterized by lateral continuation and distinct vertical anisotropy (genetic horizons A, B, and C) formed by action of different factors, i.e. climate, living organisms, precipitation, and so on. Two of the most well-represented FPS-profiles from the Andoma section are described below. For consideration they were named and numbered as FPS-1 (lower paleosoil profile, more ancient), and FPS-2 (upper paleosoil profile, younger). According to geological tradition we will move in our discussion from older to younger profile.

The older paleosoil profile stratigraphically was found in the lower part of the Klimovskaya Formation, Upper Devonian, Frasnian Stage (circa 385 mln years ago). To characterize this profile in strict terms of genetic horizons is difficult yet, but some of its peculiarities should be specially marked. The FPS-1 was formed on clastic sedimentary layer. The initial stratification of the sediment is clearly seen on the cross-section, but this stratification is disturbed by action of soil-forming factors. Upper surface of the FPS-1 profile is eroded by atmospheric precipitation effects. The profile itself contains small vertical tubes, which can be interpreted as root traces and traces of soil "infauna", i.e. organisms living inside the soil. Coal microtubes belonging to the root systems of higher terrestrial plants were found inside the FPS-profile as a result of SEM studies.

High amount of some microelements, especially barium and zirconium, is typical of this paleosoil, and this fact can be explained by the moving of the clastic terrigen-ic particles from the Khibiny Mountains, the mountains of the Kola peninsula, i.e. geologically from the Baltic shield, from the ancient continent Fennoscandia. Zircon, the mineral built by zirconium silicate is abundant there in the Precambrian nefeline syenites and alkali granites. As a matter of fact, Academician Alexander Fersman, a famous geochemist and mineralogist, described the Khibiny natrolite in 1922 and extracted barium from it, therefore their co-occurrence in the Devonian paleosoils of the Andoma Mountain is not accidental.

Anatomical structure of the wood CALUXYLON TRIFILIEVII Zalessky (the specimen, which was used as a source for preparations is shown up right, scale--1 cm). One can see conducting tissues (tracheids) with pore pairs. Scale for SEM pictures: 50 ìm (1, 3, 4); 10 ìm (2).

Upper part of the Klimovskaya Formation, the Andoma Mountain section. 1--dark-red (cherry color) alevrolites of subaerial origin, which lie above the yellow deltic sands. 2-carbonate pedonodule. 3-fossil paleosoil profile FPS-2 with the supposed cocoon of a lungfish.

The paleosoils of second type (FPS-2) are found in the upper part of the Klimovskaya Formation. They developed much better than the FPS-1 profile, "fuller" as paleopedologists use to say. Let us start description of the FPS-2 by the covering layer, which is represented by sands. The allochtonous mineralized woods of pro-gymnosperms (archaeopteridophytes) Callixylon tri-filievii Zalessky up to 30 cm in diameter and more than half a meter in length are present here, in the sand. The humus horizon (genetic horizon A) is absent in the paleosoil profile. The horizon B is represented by alevrolites with weakly consolidated sandy and clayey-carbonate nodules of 1 cm in diameter, on average. There are vertical and oblique traces presumably belonging to the soil infauna. These traces are filled with alevropelite of bright claret color. Underlying layer (horizon C) of 2-3 cm thickness is formed by yellow-red alevrolite with the numerous clayey-carbonate pedonodules. The rock matrix of the FPS-2 is a sandy packet. The most similar recent soils, resembling the fossil paleosoils FPS-2, are the present day ferruginous and chromic cambisoles*, which are typical of semi-humid subtropics.

RIDDLE OF DIPNOI

One very uncommon and strange structure was found in the FPS-2 paleosoil profile. The solution of this riddle gave basis for a new viewpoint on the fossil paleosoils of the Andoma Mountain and helped to reconstruct the general landscape, which took place in this area by the end of the Devonian period.

The obliquely oriented body of pear-like shape was buried inside the paleosoil profile. This body shows strong similarity with the fossil cocoons of lung-fishes or dipnoi. Such cocoons were described before on the basis of Carboniferous and Permian deposits by the American paleontologist James McAllister in 1992. However, the

* Cambisoles--relatively weakly developed soils, which are characterized by weak changings of color and structure of genetic horizons.--Ed.

** Dipnoi, or Iungfishes--ancient group of fresh-water fishes which have both gill and lung respiration; the group is represented by three relict species in modern fauna.--Ed.

Landscape reconstruction of forming of the fossil soils of the Andoma Mountain section. 1--denudated areas, source of clastic terrigenous material (Fennoscandia); 2--hills, which are not reached by water during seasonal fluctuations of water level; 3-accumulative lowland, near shallow-water estuary or fresh-water lagoon; 4--progymnosperms ARCHAEOPTERIS sp. (stems-CALLIXYLON TRIFILIEVII Zalessky); 5-NEMATOPHYTON (or PROTOTAXITES); 6-the cocoon of a lungfish.

lungfishes existed long before the Carboniferous period, already in Devonian, and played a very significant role in the ecosystems of ancient lagoons, estuaries, and flooded river deltas. Moreover, the teeth of dipnoi were found in the Devonian deposits of the Andoma Mountain, near FPS-2.

The cocoons helped both fossil and recent lungfishes to live through the periods of temporal drying of shallow-water lagoons and other basins. In such critical life circumstances other inhabitants of these basins produced similar adaptations for surviving on land. For instance, we can remember the lobe-finned fishes or crossopteryg-ians, creeping across from one pond to another to save themselves from drying out and, thus step by step adapting for life in aerial conditions. By the way, fossil remains of the crossopterygians also occur in the Devonian sands and clays of the Andoma Mountain. In this respect, the find of the cocoons of dipnoi in the FPS-2 profile is in full agreement with the general landscape situation, which took place here in the Late Devonian epoch.

RECONSTRUCTION OF THE LANDSCAPE

And now we will try to look at the Late Devonian landscape, which existed in the north-west of the Russian platform about 360 mln years ago.

...We can see a high mountain range far off at the horizon. There are tops of the Fennoscandia, formed by mag-matic, intrusive and metamorphic rocks of the Archean* and Proterozoic**. Products of erosion of these mountains were transported to the neighboring plain by waters of rivers, rivulets, and temporal streams. The Archaeop-teris trees with branchy crowns grew on the plain near the broad river estuaries and lagoons. Mineralized woods of these plants are quite often found in the Devonian sands of the Andoma Mountain. The gigantic column-like bodies of ancient mushrooms of the Nematophyton genus could have attracted our attention in the coastal lowlands. Nearby we can see the small low-conic rises with the centra] depression--a miniature crater-like structure on its top. Beneath these mini-barrows of evolution, in the fossil paleosoil profile, lungfishes were hidden, awaiting a new seasonal rise of the lagoon water.

As an epilogue it should be noted that there is a strong possibility to discover new well-developed fossil paleosoils in the Devonian deposits of the Leningrad, Novgorod, Voronezh, and Orel regions. Besides marine, lagoon, and alluvial deposits, various continental sediments are present here, including sediments with the higher plant megafossils. Search for fossil paleosoils in these deposits seems to be very promising.

* Archean--geological era of a duration from 4 to 2.5 bln years ago.--Ed.

** Proterozoic--geological era, which took place after the Archean; of a duration of about 2 bln years.--Ed.