1 - OROGENY AND CONTINENTAL DRIFT (showcase 1)
Numerous geographical changes have affected our planet over the course of millions of years. Many of these, such as the formation of mountain ranges (orogeny), the expansion or enclosure of oceans, earthquakes, the formation of volcanoes and their various levels of activity can be explained by plate tectonics, the model of the earth’s dynamics. The lithosphere, consisting of the earth’s crust and the part of the mantle in contact with it, is in fact divided into plates that “float” on the underlying asthenosphere, which behaves like a very viscous fluid capable of producing limited movements over infinitely long periods. These movements are reflected in the positions of the plates, which drift apart, collide or slide past each other, moving about 1-8 cm each year and producing large displacements over what we can call geological eras. The model of plate tectonics has replaced the celebrated theory of continental drift, while sharing its basic concepts and presenting them in a more comprehensive and detailed way.
Formation of Mountain Ranges
Along the converging edges of the plates, processes take place that lead to the formation of mountain ranges. A trough which is gradually filled up with successive beds of sediment that compose the "building blocks" for the construction of mountains is called a geosyncline.
The models represent schematically the processes of formation of a mountain range.
- Two plates converge. The magma ascends through fractures, so forming a volcano.
- A subduction process (meaning the sliding of one plate under the other) in the open ocean causes the formation of a volcanic island arc.
- Other troughs and areas of consumption are formed.
- Due to the collision between the continent and island arcs the ocean closes up and the continents come into contact.
- The mechanism of overriding results in overlaps between plates. The erosive action of atmospheric agents then models the landscape, forming mountain ranges and river valleys.
When two continental plates collide, part of the oceanic crust is also involved in the orogenic process leading to the formation of great sheeted dikes consisting of ophiolitic rocks (see box), like the Alps and the Apennines and those found in the Himalayas. About 40 million years ago, the collision of the African plate with the Eurasian caused the uplifting of a series of mountain ranges, extending from the Alps to the Taurus Mountains in southern Turkey.
Ophiolites are allochthonous fragments of oceanic crust. The ophiolitic series is generally characterized by a vertical succession consisting of peridotites partially metamorphosed (serpentines), pillow lava and radiolarites representing the primary sedimentary deposits. Ophiolitic complexes in both the Apennines and the Himalayas reveal a characteristic ovoid or domal formation. These great rounded-looking rocks, embedded in the side of a mountain, are called pillow lava. The lava that spills out from an underwater rift has become a circular mass due to its rapid cooling by seawater.
2 - THE HISTORY OF PLANET EARTH AND CHANGES IN TIME
To make it easier to grasp the immensely long time scale involved, palaeontologists and geologists conventionally divide the past into major time intervals, eras, subdivided in their turn into periods, epochs and ages.
This time scale makes it simpler to understand the information gleaned from the study of fossils, which enables us to reconstruct the various stages in the history of life on Earth and the long process of changes – evolution and extinction – that have characterized it.
3 – MINERALS: FORMATION AND PROPERTY (showcase 2)
Minerals are natural organic substances made up of one or more elements. They are mostly solid, homogeneous and characterized by a definable chemical composition that can be expressed with a precise formula. Colloidal substances such as opal and allophane are also considered minerals, but not the various kinds of coal and petroleum.
Formation of crystals
Crystals can be formed by:
- solidification of a molten mass;
- sublimation (direct transition from the gaseous to the solid state without passing through the fluid);
- recrystallization in the solid state.
A fall in temperature causes a substance to change from the fluid to the solid state. During this change, the atoms, initially distributed more or less randomly in the molten mass, cluster together according to well-defined rules in a crystalline structure, giving rise to a crystalline germ. This germ grows as more atoms are deposited on the structure in parallel layers. During growth, the crystals sometimes trap foreign materials, which may be solid, liquid or gaseous. These materials are called inclusions (the most common ones are hematite, chlorite, tourmaline and rutile). If the crystalline germs are not numerous they may form crystals of considerable size and with a regular external shape.
Properties of minerals
Colour – Minerals are termed idiochromatic when their colouring is their own, due to their chemical composition. These include sulphur (yellow), malachite (green), turquoise (blue) and cinnabar (red). They are termed allochromatic when their colour is due to different causes, such as the presence of foreign substances incorporated by chance or traces of elements partially replacing their principal components, or defects in their lattice, meaning structural imperfections. This is the case of quartz, which is sometimes a greenish colour because it incorporates small amounts of chlorite. Allochromatic minerals also include calcite, topaz, zircon and fluorite.
Hardness – A mineral’s resistance to being scratched. Hardness is measured on the Mohs scale, which consists of a series of ten minerals suitably selected and arranged in order of increasing hardness. In this way, each mineral scratches the one below it and is scratched by the one above it on the scale.
1 talc 2 gypsum 3 calcite 4 fluorite 5 apatite 6 orthoclase 7 quartz 8 topaz 9 corundum 10 diamond
Magnetic properties – Many minerals that contain iron, including magnetite, pyrite and hematite are attracted more or less easily by a magnet. These are known as the paramagnetic minerals.
Electrical properties – The ability to conduct an electric current depends on the mobility of the electrons within the crystal structures. Among metals, those possessing a simple lattice (like copper and silver) have high conductivity.
Optical Properties – The study of the optical characters of minerals is very important for their identification. When a beam of light strikes a transparent body two important phenomena occur: the reflection and refraction of light.
Septarie are argillaceous limestone concretions, generally spherical, which contain numerous fissures (“septa”), resulting in a series of cavities often filled with crystals (calcite, barite, quartz, gypsum, aragonite and pyrite). The most likely explanation of their origin is that they are formed from clay cores that in their first (wet) phase can be shaped and in their second (dry) phase shrink and flake, forming septa and cavities. These are then filled by minerals present in the soil transported by water
4 – ROCKS (showcase 3)
A rock is an aggregate of particles that forms part of the lithosphere. On close examination it will be seen to be composed of various minerals usually present as distinct crystals. The rocks of the earth's surface can be divided into three basic types: igneous or magmatic, metamorphic and sedimentary.
Igneous or Magmatic
Igneous rocks are formed by the cooling of a molten magma. The magma that rises from the earth’s interior can cool and solidify in two ways: inside the crust, forming rocks such as granite, or emerging from volcanoes and being solidified on the surface, forming rocks such as basalt and obsidian.
Metamorphic rocks are derived from the heating and compression of pre-existing rocks. In the lower part of the earth’s crust, rocks are subjected to very high pressures and temperatures, with the result that they are recrystallized (metamorphism) into rocks of a different kind. Examples of metamorphic rocks are gneiss, marble and slate.
Sedimentary rocks are composed of fragments of pre-existing rocks altered, eroded and transported by atmospheric agents, as well as the remains of living organisms.
Wind, water and ice erode the earth’s surface. Fragments of crumbled rocks end up in rivers, which carry them along and eventually deposit them on their banks or on the sea bed. Here they accumulate together with the remains of seaweed, shells and animals, and over thousands of years they are turned into rocks. This is how limestones, conglomerates and sandstones are formed.
The rock cycle
The rock cycle shows how surface rocks (igneous and sedimentary) may gradually be buried at greater depths, where, by a progressive increase in pressure and temperature, they are changed into metamorphic rocks and then, having reached melting temperature, into plutonic rocks. These rocks, together with the magma coming directly from the mantle, may be brought back to the surface by tectonic movements and eruptions: in this way the endless evolutionary cycle of the earth's crust continues.
The recognition and study of rocks is carried out by two different types of analysis: macroscopic observations which can be performed in nature with the naked eye, and microscopic observations. In the second case we examine a “thin section”, that is, a slice of rock taken from the original block and made thin, almost transparent, by cutting and grinding. The sample, placed on a special slide, is observed under the microscope by exploiting the special qualities of polarized light, revealing the nature and form of the crystals of the minerals composing the rock and their chemical and mineralogical properties.
5 - FOSSILS IN THE SERVICE OF INDUSTRY (showcase 4)
Fossils in the Service of Industry
Fossils are widely used in searching for minerals and petroleum. Foraminifera are the index fossils of modern geologists. The information they provide enables hydrocarbon deposits to be located. Petroleum itself is a fossilized organic substance. The existence of hydrocarbons near the ancient city of Velleia, in Val Chero, and close to Montechino, in Val Riglio, was already known in antiquity. Early searches were conducted by drilling boreholes in about 1866 and expanded rapidly, with some success, so as to create a profitable industry for over half a century.
Hydrocarbons are organic compounds that originate from thermal decomposition (thermogenesis) and/or microbial decay caused by vegetable and animal microorganisms (plankton) together with clastic or organogenic sediments deposited on the beds of the seas, lagoons and lakes. The maximum concentrations of living matter occur in shallow, calm, warm water, generally found along the continental margin, a short distance from the coast. In these conditions the clastic sediments that are deposited are largely clays. As a result the clays are the potential mother-rocks for hydrocarbons. The drawing shows schematically the formation of oil and gas deposits:
- initial phase of migration
- advanced stage of formation and build-up
A flysch is composed of clastic sedimentary rocks, of synorogenic origin, deposited in a marine environment by mechanisms of deposition that follow the force of gravity, such as so-called submarine landslides and turbidity currents.
Flysch deposits often contain imprints of the sea bed due to mechanical action, generally turbiditic. The flute marks are produced by turbulent eddies in a current that erodes around an object lying on the sea bed. They appear in the form of elongated semi-cones with their vertices in the direction from which the current flows.
Pillori (Val Perino): the base of this verticalized calcarenite still displays, clearly visible, the ripples created by the current on the seabed.
6 – SAND (showcase 5)
Sand is a clastic sedimentary rock, made up of granules whose dimensions are between 2 and 0.062 millimetres across. Particles smaller than sand fall into the category of silt, mud or shale, while larger particles are termed gravels or conglomerates.
Sand forms in three principal ways:
- by erosion of existing rocks, principally by wind, water and ice; hence its composition depends largely on the source rocks found in the surrounding areas. The commonest minerals are quartz and feldspar in light-coloured sands, magnetite, hematite and garnet in dark sands. Sand created by erosion is found on beaches in many non-tropical seas (for tropical regions see the next point). Carried down to the sea as a result of being eroded and transported by rivers and wind, they are then deposited on the coast again by wave action.
- by chemical precipitation from hypersaline waters due to the extremely high quantity of ions in solution, often the result of strong evaporation and poor circulation. This is the case of the ooliths or of some brines that are precipitated onto the shallow sea beds in tropical regions. The white sand beaches characteristic of the Caribbean and Seychelles, or in seas where the circulation of water is limited, like the Red Sea, consist entirely of a precipitate of calcium carbonate.
- by the accumulation of organic remains (skeletons and shells) of organisms such as shellfish, green algae, corals, crinoids and foraminifera. During their life cycles these organisms develop supporting and protective structures that entrap the calcium carbonate or silica present in solution in water. After their death, these structures are transported by water and accumulate on beaches.
The various kinds of sand presented here are part of a collection of 180 samples gathered around the world, donated to the museum by Mrs. Giovanna Magnanini of Reggio Emilia.
7 - FOSSILS
Fossils are the remains of organisms that lived in past times, as well as the products of their life cycle and any other evidence of their existence.
Fossils are preserved until the present inside sedimentary rocks, meaning those that have arisen from the slow accumulation of layers of organic and inorganic materials known as sediment. They are not found, however, in other types of rocks formed under conditions of heat and pressure, which would affect the preservation of their remains.
The word “fossil” comes from the Latin fodere, which means “to dig”. Fossils are, in fact, hewn out of rocks and brought to light, by human labour or nature itself by erosion.
8 - PALAEONTOLOGY
Palaeontology is the science that seeks to reconstruct the history of life on Earth and the long process of change, or evolution, which characterized it. This discipline has its roots in antiquity and throughout the centuries has involved philosophers, scientists and the religious in a ceaseless debate. But it was not until the second half of the nineteenth century that it was recognized as a real science and fossils – from which Palaeontologists develop the interpretation of the past – are considered as unique documents, authentic ancient organisms that tell us about lost worlds.
Palaeontologists are scientists who seek to recompose a complete picture of the history of life on the basis of the fragmentary clues presented by fossils, reconstructing the anatomy, appearance, biology, ecology and the relations between prehistoric creatures as well as the characteristics of the environments, including the vegetation, in which they lived.
9 - FOSSILIZATION
Every fossil is a priceless treasure and tells us about only a fragment of our distant past: in fact, despite the sensational palaeontological discoveries may lead us to think, fossilization is a very rare phenomenon that occurs only with the concurrence of many favourable circumstances. Which? Essentially, for the remains of an organism to preserved as a fossil, the basic factor is that they have be quickly removed from the action of destructive agents: this happens in particular situations, as when the remains are deposited in aquatic environments on sea beds where the water is poorly oxygenated and quickly buried by very fine sediments, or when they are in a desert and the sand covers them quickly.
With a leap back in time to the Jurassic, 145 million years ago, the story of a dinosaur whose skeleton was fossilized will exemplify the complicated processes – called "taphonomic" in the jargon of palaeontologists – that take place between the death of an organism and its discovery in the fossil state.
- On the shore of a lake, in the country we now call Portugal, a hungry Torvosaurus caught a Dacentrurus intently grazing.
- After a rapid, furious struggle, the Dacentrurus was killed and the Torvosaurus devoured its flesh, while other smaller dinosaurs and reptiles prowled around, eager to clean up the carcass.
- The skeleton and remains of the carcass were covered by water; while the remaining soft parts of the skin, muscles and internal organs were attacked by microorganisms that decomposed it.
- The hardest parts that remained intact, such as the bones and teeth, were covered by a layer of sediment – mud, sand and mineral granules – transported by water currents and wind; this protective layer prevented them from decomposing.
- With the passage of time further layers accumulated one above the other, while the lower ones were compacted and cemented, gradually turning into rock by a process called “lithification”.
- Water seeped down and deposited the minerals present in the sediment in the pores and cavities of the bones, reinforcing them and making them heavier. The minerals that made up the bones themselves may also have been gradually replaced, until the bone itself was turned into stone.
- Because of the movements of the earth's crust, the rocks containing the bones, now fossilized, were raised, and instead of the ancient lake a mountain was formed. Rocks that are not hidden under other rocks or vegetation, soil, water, glaciers and buildings are said to be “exposed”.
- The exposed rocks are eroded, that is “eaten away” by the rain and wind and crumbled by the action of the roots of trees, until they exposed the layer containing the fossil. In this way the fossil has been brought back to light and is only waiting for someone to find it!
10 - WHAT FOSSILIZES (showcase 6)
Bones and teeth, egg shells, sea shells, but also eggs, nests and burrows, footprints, stomach contents and excrements – called coprolites – are examples of fossils of animal origin. Of plant origin are fossils such as logs, leaves, seeds, pollen grains and resin: the last of these has come down to us in the form of hardened amber.
In rare and exceptional cases, even the most delicate parts of an organism can ne fossilized: Scipionix samniticus, the little Italian dinosaur discovered in Campania and better known by its nickname Ciro, even preserves parts of muscles and traces of blood in the fossil state, as well as some internal organs such as the intestines!
In special, extraordinary cases the whole body of an animal is sometimes found, just as it was in its lifetime: this is the case of the mammoths found in the Siberian ice or insects enclosed in amber.
11 - THE ORIGIN OF LIFE AND THE PRECAMBRIAN
The term Precambrian has no formal status in the geological time scale, but is commonly used to indicate the phase of Earth's history that stretches from the formation of the planet Earth, about 4.6 billion years ago, until the beginning of the Palaeozoic Era.
Little is known of this period, because the Precambrian rocks are buried beneath younger rocks or have been destroyed by erosion and the movement of the continental masses. In addition, the first forms of life had not yet developed mineralized parts (such as skeletons or shells) and so they were rarely fossilized.
Life appeared about 3.5 billion years ago on a planet very different from the present: the primordial atmosphere was devoid of oxygen: it consisted of carbon monoxide and carbon dioxide, water vapour, nitrogen and hydrogen (gases produced by volcanic eruptions), and did not act as a screen against ultraviolet radiation. In these conditions life could only exist in the hydrosphere, meaning the seas and oceans.
The first single-celled organisms, chemosynthetic and later photosynthetic, were formed in a complex chemical environment, a solution of organic substances such as amino acids and sugars formed as a result of non-organic processes.
Some 2 billion years ago, single-celled organisms with a more complex architecture appeared (e.g. with chromosomes and nuclei), and the oxygen produced by photosynthesis also began to be released into the atmosphere. Towards the end of the Precambrian, around 650 million to 600 million years ago, this oxygen had created a new kind of atmosphere, where ozone (O3) acted as a shield against lethal UV rays. The new environmental conditions also allowed for the evolution of more complex organisms, for example, the famous Ediacara fauna consisting of multicellular creatures with medusoid, frondiform and ovoid forms, endowed with segments and appendages.
12 - LIFE IN THE SEAS AND THE CONQUEST OF THE LAND (showcase 7)
The Palaeozoic Era goes from 540 million to 250 million years ago and consists of six periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian (for the discussion of the last two see the next showcase).
Geography and climate
At the start of the Palaeozoic most of the continents were clustered around the equator. The weather was warm, humid and mild, favoured by the free circulation of the ocean. Towards the end of the Ordovician, a glaciation, its causes still unknown, triggered a mass extinction. In the Silurian and Devonian there followed a period of climatic stability, with Gondwana situated in the southern hemisphere and the northern continental masses clustered together near the equator. Low shallow seas covered most of the continents.
Many exceptionally well preserved rock deposits, including the famous Burgess Shale in Canada, bear witness to the so-called “Cambrian explosion”, the biggest recorded event of the appearance and diversification of living creatures. All the principal groups (phyla) known to us appeared, including, for the first time, organisms with shells and exoskeletons, such as brachiopods, molluscs and the arthropod trilobites. Trilobites were particularly common in the first half of the era and today they are considered the most distinctive of all Palaeozoic fossils.
Other creatures well represented in the Cambrian were sponges, echinoderms, priapulida, chordates (the ancestors of all vertebrates) and foraminifera.
The enigmatic Anomalocaris was the most fearsome predator, being at times to over a metre long. We should also mention the archaeocyathids, sponge-like organisms capable of building reefs. Among the plants only algae are documented.
In the Ordovician, following the increase in the quantities of microplankton, the molluscs and filtering organisms in general underwent rapid development. Corals, bivalves and bryozoans appeared, and there was a boom of nautiloid cephalopods, intelligent predators with shells up to 10 metres long. Among echinoderms the crinoids (sessile filter feeders) diversified, while plants ventured timidly out of the water.
In the Silurian, after the late Ordovician extinction, there was an immediate resumption of life with a boom in brachiopods and crinoids. The trilobites were already in decline while the fishes become more widespread and larger. However they still lacked jaws, a weapon that would only appear towards the end of the period. At the same time, the plants conquered the land and formed the first terrestrial ecosystems, with herbivorous myriapods, centipedes and carnivorous arachnids, scavenger worms and decomposer fungi.
In the Devonian there was a great radiation of fish in the sea, so that this period is often called the “age of fishes”. Many forms appeared, including the first sharks, the sarcopterygians, the actinopterygians, parallel to the further diversification of the placoderms, which disappeared, however, towards the end of the period. Others that appeared included the ammonoids and thriving reefs were formed, inhabited by algae, sponges and corals. On the land, wingless insects and tetrapods first appeared, such as the famous Ichthyostega, which had turned the fins of their sarcopterygian ancestors into limbs that enabled them to drag themselves across the land. The major advances in this period, however, were made by plants, which in this period “invented” leaves (giving them larger surfaces for performing photosynthesis), wood (capable of offering stronger support) and, with the gymnosperms, seeds (capable of freeing the plant from water for reproduction and facilitating the colonization of much larger and more arid areas of the land).
A massive extinction towards the end of the period, perhaps caused by glaciations or global cooling as the result of an extra-terrestrial impact, principally affected the tropical marine forms of life.
13 - CARBONIFEROUS AND PERMIAN (showcase 8)
Geography and Climate
At the beginning of the Carboniferous, the greenhouse effect was reduced by a temporary peak in the concentration of oxygen in the atmosphere, released in abundance by the many lush forests and associated with a decline in the amounts of carbon dioxide. Mitigating the greenhouse effect destabilized the climate and caused a series of severe glaciations, but without greatly affecting the development of life.
During the Permian, the continental masses clustered together to form a single supercontinent called Pangaea, surrounded by a single great ocean, Panthalassa. Pangaea had a sort of gigantic inlet at the equator, facing east, which experts have called the Sea of Tethys. The grouping of the continents gave rise to a vast, arid inland region, characterized by extreme temperatures unmitigated by the sea, with alternating periods of drought and rain. Towards the end of the period, the fall in sea levels and deterioration of the climate led to the most devastating biological crisis known, the late Permian mass extinction, which is estimated to have eliminated about 90% of the species present.
In the seas of the Carboniferous sharks replaced the placoderms and other types of predators developed, in some cases with amphibious habits, such as the giant eurypterids. Ammonites, bryozoans, brachiopods and echinoderms were abundant and highly diversified, while ammonoids and trilobites were declining and had almost disappeared. On the mainland, in the northern part of Pangea, the first marshes formed and soon became vast and luxuriant. In these marshes developed the pteridosperms and other plants, such as club mosses, Lepidodendron and Sigillaria, the sphenopsid Calamites and the cordaitales, which might have grown tens of meters in height. These were the plants that would give rise to the great coal deposits from which the period takes its name. The cooler south was dominated by other pteridosperms (Glossopteris). Perhaps favoured by the higher concentration of atmospheric oxygen, the arthropods gave rise to giant forms such as the arthropleurids, and an immense variety of insects. The insects were the first creatures to learn to fly, including some of huge dimensions such as the dragonfly Meganeura. In the second half of the period, the arid climate and droughts created difficulties for the lycopods among the plants and the arthropods and amphibians among the animals. Among the tetrapods, the first amniotes appeared, creatures capable of spawning out of the water. Though insignificant at first, thanks to the invention of what is called the amniotic egg, they diverged and grew in size. Among them, the pelycosaurs, the first representatives of the evolutionary line of mammals, soon became dominant in the mainland ecosystems and wetlands. In the semi-aquatic ecosystems, stereospondyl amphibians were still the commonest predators, with forms similar to crocodiles, eels and salamanders, but at the top of the food chain were the rhizodonts, 7 meters in length, making them the largest freshwater fish to have ever existed.
In the Permian seas some ammonites diversified by developing a more complex patterning, hybodonts replaced the xenacanth sharks and the bony fishes (Actinopterygii) became widespread.
On land, plants related to water, such as lycopods and sphenopsids, which formed the coal forests, were reduced in size to little more than shrubs, and the ferns and pteridosperms became the dominant forms of vegetation. Conifers and ginkgos spread, while in Gondwana Glossopteris flora was replaced by Dicroidium flora (seed ferns). The decline of the giant arthropods was accompanied by the appearance of new insects, such as flies and cockroaches, with a more complex life cycle. The amniotes, including pelycosaurs became widely established: these were predominantly tropical forms over three meters long and often equipped with structures capable of rendering their control of body temperature more efficient (for example, the dorsal sail of the predator Dimetrodon and the herbivore Edaphosaurus). In the mid-Permian appeared the therapsids, the most advanced of the early pelycosaurs and often improperly called “mammal-like reptiles” because of their characteristics, which were not yet, however, fully mammalian. The most primitive are classified in the group of dinocephalids, also gigantic creatures, up to 6 meters long, which comprised herbivores and carnivores. As they declined, they were replaced by the gorgonopsians, therocephalians and dicynodonts. Alongside them lived other types of reptiles, including the pareiasaurians (armoured herbivores up to 3 meters long), procolophonids (perhaps related to the turtles) and small diapsid reptiles that look like lizards.
14 - TRIASSIC
Geography and climate
The Triassic is the first of the three periods of the Mesozoic Era, also known as the “Age of Reptiles”.
The land masses were reunited in the supercontinent Pangaea, though a rift valley was beginning to open between the two land masses that would become South America and Africa. The weather was hot but generally dry, with the central areas of Pangaea being markedly arid.
On the land, the arid climate favoured plants that reproduced by seeds and anemochorous (wind-borne) pollination. Some of these groups developed forms with many adaptations to survive in the event of drought. In spite of the united continental masses, the flora retained a high degree of provinciality, with marked differences between northern and southern species. The northern flora (Laurasian/Pangean) was lusher, especially in the extreme north, where forests of tree ferns and araucarias, cycads and gingkoales grew, and the ground was carpeted with ferns. The forests were sparser towards the more arid equator. The southern (Gondwanian) flora, as in the Permian was dominated by Dicroidium.
In the first half of the Triassic, the herbivores were the dicynodonts dominant in the south, while to the north they were the arcosauromorphs, such as the armoured aetosaurs. Among the predators were the cynodonts (which gave rise to the mammals) and other arcosauromorphs, semi-aquatic and similar in appearance to crocodiles (phytosaurs) or terrestrial like the great carnivorous rauisuchids. The anapsid procolophonids were also widespread.
Towards the end of the period, after a mass extinction, there appeared further archosaurs such as primitive crocodiles, dinosaurs and pterosaurs, but also some lepidosaurs (for example, the sphenodonts and the first lizards) and the first turtles. Among the amphibians the frogs evolved, while the stereospondyls remained widespread and included some giant forms.
The seas were dominated by bivalves, brachiopods and ammonoids (especially the ceratites). Among the cephalopods appeared also the belemnites, and among the corals the scleractinians, which were similar to modern forms but not yet builders of immense barrier reefs. Some reptiles returned to the water and diversified into many forms: ichthyosaurs (similar in appearance to tunas and dolphins), notosaurs, placodonts and protorosaurs (like the bizarre, long-necked tanystropheids).
The dinosaurs, with the exception of the birds, were land-dwellers. Depending on the structure of the pelvis, dinosaurs are classified into saurischians, in which each pubic bone consists of a single branch, and ornithischians, in which each pubic bone consists of two branches. The ornithischians had another distinctive skeletal feature: an additional bone, called a predental bone, which formed the tip of the mandible. The diet of the Mesozoic dinosaurs was varied: the ornithischians were mostly herbivorous, while the saurischians comprised herbivorous forms as well as fearsome predators.
Though they laid eggs, the dinosaurs were very different from most existing reptiles – lizards, varanids, iguanas, snakes and turtles, to name only a few – to which they are not closely related. The principal difference immediately perceptible is that the dinosaurs’ legs remained nearly vertical beneath their bodies and their tails were held well above the ground. Some dinosaurs had thick skins covered with scales, like crocodiles, the only existing reptiles that are their distant cousins, while others were covered with feathers like birds.
In the mid-Triassic, a group of reptiles called cynodonts gave rise to mammals. They were distinguished by having a varied dentition and a perfect occlusion, the ability to maintain a constant body temperature thanks to their high metabolism and the fur covering their bodies, the ability to nourish their young through lactation, specific changes in the anatomy of the ear and the mandible (consisting of the single dental bone) and the undulating motion of the spinal column from top to bottom, instead of from right to left as in reptiles.
In the last century it was thought that Mesozoic mammals were small unspecialized insectivores or omnivores, similar in appearance to the shrews, and that they lived in the shadow of the dinosaurs.
Today, the number of known fossil species has more than doubled and the new finds reveal a great variety of alimentary and locomotor adaptations, showing that though mammals were small they complemented the dinosaurs in creating complete ecosystems.
Geography and climate
During the Jurassic the fragmentation of Pangaea continued with the opening up of the Gulf of Mexico and the North Atlantic. Meanwhile, the continental masses of Gondwana began to break up. The climate was tropical, with a sort of cosmopolitan greenhouse effect. The oceans were very extensive, with large seas that even came to cover part of the continents (epicontinental seas).
The flora was cosmopolitan, with gymnosperms spreading widely. The cycadophytes, palm-like in appearance, were very widespread. Among the most common shrubs were the bennettitales, while the big trees belonged to the group of the conifers: Araucariaceae, Cephalotaxaceae, Pinaceae, Podocarpaceae, Taxaceae, Taxodiaceae and the extinct Cheirolepidiaceae. The ferns, still widespread, dominated the undergrowth.
The insects, little-known but certainly very numerous, included some herbivorous forms (e.g. the orthopterans, hemipterans, cimicids and coleopters). The dinosaurs dominated the land. Among the herbivores there were the sauropods (diplodocids and macronarians), the thyreophores (mainly stegosaurs), and ornithopods (hypsilophodons and camptosaurs). All the major groups of theropods also appeared: coelophysoids, dilophosaurids, allosauroids, megalosauroids, compsognathids and the first birds. The latter shared the skies with the reptile pterosaurs. The mammals, small in size, became diversified. The amphibians continued to decline, while among the reptiles the sphenodonts occupied the ecological niches of today’s lizards. The crocodiles, numerous and very diversified on land, gave rise to semi-aquatic and marine forms. The rulers of the seas, however, were other reptiles, such as plesiosaurs and ichthyosaurs. With them lived the first forms of modern sharks and holostean fish (at that time the commonest of the bony fish). Sponges, corals, gastropods, bivalves and bryozoans were common, but the true rulers among the invertebrates were the nektonic cephalopods (ammonoids and belemnites). In the Jurassic, there was also an immense development of plankton, for example the dinoflagellates, diatoms, planktonic foraminifera and ostracods, with photosynthetic forms that adopted the c-type chlorophyll usually contained in the chloroplasts of red algae.
15 – AMMONITES (showcase 9)
Ammonites are among the best known and commonest marine fossils from the Mesozoic. They belong to the group of cephalopod molluscs, with a variously patterned external shell which usually takes the form of a flat spiral. As in today's nautilus, a distant relative of the ammonites, the shell was internally divided into successive chambers, separated by septa, with the animal living in the most recent and largest. Their numerical abundance in rocks, worldwide occurrence and rapid evolution over time produced many different forms and make fossil ammonites a very important instrument for the stratigraphic identification of Mesozoic marine sediments.
HOLZMADEN SHALES (GERMANY)
The fossils in these beds of rock have enabled us to reconstruct life in the tropical seas that in the Early Jurassic (about 180 million years ago) covered the central-western part of Europe.
In this sea there lived ichthyosaurs, plesiosaurs, fishes, cephalopods (such as those fossilized on the plate shown here) and other invertebrates. All these bodies have been preserved in the fossil state in grey bituminous shale deposited on the shallow seabed.
Even amid the highest mountains on the planet the remains of marine animals can be found, bearing testimony to the orogenic processes that led to the formation of mountain ranges where there was once an ancient ocean bed.
Peculiar dark pebbles containing the fossils of ammonites are found In the Kathmandu Valley, Nepal, along a bend in the Kali Gandaki River which flows southward to the west of the Annapurna Group. These fossils are much sought after in the region as venerable objects, manifestations of Vishnu, the Hindu god of preservation.
The local hunters are so expert they can tell from the outside whether a stone contains a fossil. They skilfully deal it a sharp blow, splitting it open as two halves to reveal the ammonite.
Their fossilization is due to the decomposition of the original creature which died in the silt that then became a rock: it released organic substances around it which compacted the sediment for a limited radius, favouring the formation of a nodule distinct from the surrounding detritus.
16 - BIRDS
The origin of birds
With over ten thousand species still living and a long evolutionary history behind them, birds are one of the most successful groups of vertebrates.
They evolved from small theropod dinosaurs with feathers, closely related to the deinonicosaurs, through the progressive strengthening of the front legs, chest and rib cage, and the shortening and remodelling of the tail, increasingly involved in flight and freed from its former role for the attachment muscles useful in walking (to swing the leg back). However, it is still open to debate just how, in the transition from dinosaurs to birds, these creatures began to use their arms and feathers to attain flight. Was it to sustain themselves in the air while jumping from one branch to another, as part of their arboreal lifestyle? To stay in the air longer after leaping, in mostly earth-dwelling life forms? Or even to maintain balance and block the escape routes of their prey after immobilizing it with their talons, as raptors do today? The discoveries of recent years seem to suggest that, at the dawn of their evolutionary history, the ancestors of birds exploited all these possibilities.
Archaeopteryx lithographica is a fossil bird of the Upper Jurassic in Germany that was discovered in 1861. It had characteristics common to both non-avian dinosaurs and birds. This and other discoveries have shown that the fossils provide solid proof of the evolutionary theory published in 1859 by Charles Darwin (1809-1882), who saw species as natural entities that can change over thousands of years in response to the environment and its alterations.
Pterosaurs were distant relatives of dinosaurs and crocodiles, and not the ancestors of modern birds. The bones of the arm and fourth finger of the hand (corresponding to our ring finger) appear disproportionately large compared to the rest of the body because they supported the wing, consisting of a membrane of skin, like the patagium of bats. This membrane was sustained by collagen or keratin fibres that made it both flexible and load-bearing.
17 - THE LIMESTONES OF SOLNHOFEN (showcase 10)
The town of Solnhofen in Bavaria is known to palaeontologists because of the presence of a fine-grained limestone formation from the Jurassic period that forms an outstanding stratum for the preservation of fossil remains, in which even traces of soft body parts can be found. The rocks are all that remain of an ancient lagoon, characterized by calm waters, poorly oxygenated at the bottom and therefore devoid of destructive biological agents, such as bacteria and predators. This meant that bodies brought by waves inside the lagoon could fossilize completely after death.
Alois Senefelder used blocks of limestone from Solnhofen in lithographic printing process which he invented in 1798. The quarries that yielded the limestone used in lithography also produced spectacular finds, including the famous Archaepoteryx lithographica.
The material presented here was donated by the Bürgermeister Müller Museum of Solnhofen, which has been affiliated with the Geological Museum of Castell’Arquato since spring 2012
18 – CRETACEOUS (showcase 11)
Geography and Climate
Numerous areas of the continental masses were covered by shallow continental seas, encouraging the regionalization of the flora and fauna. As Pangaea continued to break up, the continents moved towards their present locations: the Atlantic Ocean was expanding and Africa, now separated from South America, travelled to Europe so that it closed Tethys; in Europe the Alps were formed; India had separated from Gondwana and become an island-continent moving towards Asia.
Besides ferns, conifers and cycads the Cretaceous saw the appearance of the angiosperms or flowering plants. Towards the end of the period many types of modern plants had evolved.
Together with the flowering plants insects also evolved, with the emergence of contemporary forms such as the ants, and with a boom in the pollinators, including the first butterflies.
By this time the amphibians were relegated to their present forms (frogs and salamanders). Among the crocodiles the modern groups appeared, while other groups gave rise to gigantic forms that lived in the tropical ecosystems by preying even on the dinosaurs. These were further diversified. The sauropods remained widespread, especially in the southern continents. Other herbivores were: the armoured ankylosaurus and ornithopods (hypsilophodons, iguanodons and the duckbeaked hadrosaurs), with cosmopolitan distributions; the ceratopsians (horned dinosaurs) and pachicephalosaurs, found mainly in Asia and North America; some theropods like the therizinosaurids, with long arms terminating in claws, and the ornithomimosaurs or “ostrich dinosaurs”, which probably supplemented their diets with small prey. The predatory dinosaurs were represented by a wide range of theropods of all shapes and sizes: examples are the carcharodontosaurs, the tyrannosaurs (commonest in the north), the deinonicosaurs, the abelisaurids (very widespread in the south), and the spinosaurs (the last-named being specialists in preying on fish). In the sky the birds diversified into many forms (some of them modern), while the pterosaurs began to decline slowly in about the middle of the period. Among the mammals, still small, the three existing groups became clearly distinguished (monotremes, marsupials and placentals). The sphenodonts were replaced by the squamates or scaled reptiles (lizards, monitors and snakes), which also gave rise to giant marine predators, the mosasaurs. Even the turtles, at the height of their diversity, gave rise to giant marine forms. The ichthyosaurs, by contrast, began to decline, while among the plesiosaurs new forms still appeared.
Among the fish, the teleosteans supplanted the holosteans, while the evolution of modern forms continued among the sharks. New types of gastropods (neogastropods) appeared, mainly predators. The corals were similar to those of today but the reefs were built mostly by the rudists, bivalves with one large conical valve and the other reduced to a thin lid.
Among the ammonites bizarre shapes also developed, some being convoluted while others were not, some with sutures and simple ornamentation and others complex.
THE ARARIPE NODULAR CONCRETIONS
In some deposits, the fossils are found in nodular concretions.
A well-known example is the Cretaceous deposit in the Araripe Basin (Brazil). A feature of the fossils of this deposit, especially the fish, is their conservation in three dimensions: this is due to the outward pressure exerted by the gas that formed inside the decomposing carcasses, which proved capable of resisting the weight of the sediments and preserving the volume of the organisms.
19 - ITALY IN THE MESOZOIC: Italian dinosaurs
Geologists long believed that Italy was submerged by the sea in the Mesozoic, and that it would therefore be impossible to find the fossil remains of dinosaurs here. In the nineties this idea was refuted by a long series of discoveries.
The skeletal remains found to date belong to four types of dinosaurs: a large theropod of the Early Jurassic, which was found at Saltrio (Lombardy) and resembled the dilophosaurs; the celebrated baby compsognathid Scipionyx samniticus, nicknamed “Ciro” and found in Cretaceous rocks at Pietraroja (Campania); six examples, of which at least one is almost complete, of the hadrosauroid Tethyshadros insularis from the Late Cretaceous found at Villaggio del Pescatore (Friuli Venezia Giulia); and finally the tibia of an indeterminate theropod found at Capaci (Sicily), dating from the Late Cretaceous.
Finds of footprints are much more numerous, with sites identified in many localities. Some of the notable finds include: the Central-Eastern Alps, for example in the Dolomia Principale formation of the Late Triassic and in the Calacari Grigi formation of the Early Jurassic; Esperia and Sezze in Lazio, respectively dating from the Early Cretaceous and Late Cretaceous; the town of Mattinata in Puglia (Late Jurassic), Borgo Celano and Bisceglie (Early Cretaceous), and the rich and extensive site of Altamura (Late Cretaceous).
The distribution of finds in coastal deposits at different locations across a broad time span suggests that all through the Mesozoic the carbonate platforms of Central-Eastern Tethys must have repeatedly formed quite extensive land masses. Only in this way could they sustain ecosystems on land inhabited by dinosaurs and be useful as zones of passage between different geographical areas.
20 - THE CLOCK OF TIME
The planet Earth was formed 4.6 billion years ago and the first forms of life date back to about 3.5 billion years ago. The dinosaurs appeared “only” 230 million years ago, early humans less than 2 million years ago.
It is impossible for humanity to grasp, or even imagine, the abyss of time that lies behind us. To try to realize the overwhelming length of geological time we have to resort to devices like the “Clock of Time”.
In this “prehistoric clock”, 4.6 billion years of life on Earth have been compared with 24 hours and the millions of years covered by the principal events in the history of life on this planet have been translated proportionately into hours and minutes.
0:00 formation of planet Earth
5:46 appearance of the first single-celled life forms in the seas
12:00 (noon) appearance of more complex unicellular life forms
8:40 PM appearance of the first multicellular life forms in the seas
9:18 PM appearance of the first vertebrates in the seas
9:56 PM plants and arthropods colonize the land
10:07 PM vertebrates venture onto the mainland
10:16 PM appearance of the amniotes, vertebrates capable of colonizing the land
10:47 PM among the amniotes, the first mammals and the first dinosaurs appear
11:23 PM appearance of flowering plants
11:39:12 PM all the dinosaurs except for birds become extinct
11:59:22 PM appearance of the species Homo sapiens
0:00 of the following day: the age we now live in
21 - MASS EXTINCTIONS
Life on our planet is a story of continual changes over time, which have seen the emergence, evolution and extinction of countless species.
When they occur due to natural causes, extinctions are not a negative phenomenon. In the past, the extinctions of dominant species left new paths open to evolution, favouring the appearance and spread of new species.
On at least five occasions in the history of life whole groups of animals have disappeared more or less suddenly from the face of the Earth: these are known as mass extinctions.
Ordovician/Silurian (443.7 million years ago): disappearance of many trilobites, brachiopods, corals and cephalopod molluscs.
Causes: it appears that, due to continental drift, the Gondwana supercontinent passed near the South Pole, causing a prolonged glaciation and a drastic lowering of ocean levels.
Late Devonian (?360 million years ago): disappearance of many marine organisms including the placoderms, the ancient armoured fish represented here by Dunkleosteus.
Causes: unknown. Suggestions have included asteroids and glaciations.
Permian/Triassic (251 million years ago): in the most catastrophic mass extinction 96% of living beings disappeared. Those permanently effaced from the face of the planet included fusulinids, trilobites and Palaeozoic ammonites.
Causes: Sudden climatic variations, triggered by intense volcanism or by the impact of one or more asteroids.
Triassic/Jurassic period (199.6 million years ago): extinction of the therapsids (ancestors of the mammals) on land and many freshwater and marine organisms.
Causes: global warming, perhaps due to the release of large amounts of methane from the ocean floor.
Cretaceous/Tertiary (65.5 million years ago):
The Cretaceous ended with the most famous mass extinction in the history of life on Earth, which wiped out the non-avian dinosaurs, some lines of birds and mammals, many marine reptiles, the ammonites, many species of belemnites, rudists and numerous microorganisms.
Causes: intense volcanism and the impact of a huge asteroid, which temporarily disrupted the planet’s climate.
The sixth mass extinction?
Over the past 350 years, hundreds of species have become extinct because of humanity’s widespread and rapid destruction of the environment!
22 - ITALY IN THE MESOZOIC: BESANO-MONTE SAN GIORGIO
The deposits of Besano-Monte San Giorgio are located in Lombardy near the Swiss border. The area is well known to Palaeontologists around the world as one of the most important fossil deposits from the Triassic Period. With its wealth of wildlife, in 2010 it was named a UNESCO World Heritage Site, so joining the Swiss side of the site, which had already been recognized in 2003.
It contains the fossilized remains of many sea creatures, including invertebrates, sharks and primitive bony fish, but is celebrated above all for finds of reptiles.
The reptilian fauna of the area includes:
Tanystropheus, a bizarre aquatic predator with a neck so elongated that it amounts to more than half its overall length;
the ichthyosaurs, swimming reptiles similar in appearance to dolphins or fish, represented by the Mixosaurus and the large Besanosaurus;
the placodonts, such as Cyamodus, with a flattened and armoured body, and large flattened teeth suitable for crushing its prey.
Less striking marine reptiles, but of great interest to palaeontologists, the notosaurs were medium-small in size. The Besano formation also comprises terrestrial reptiles such as Macrocnemus, a relative of the tanystropheus, and Ticinosuchus, a quadruped form similar in appearance to a long-legged crocodile.
The Besano fossils are preserved in oil shales, a sedimentary rock containing a high percentage of organic matter formed about 235 million years ago (Middle Triassic) by the accumulation of the remains of animals and plants on the bed of a poorly oxygenated marine basin with calm shallow waters. Until the 1960s the deposit was exploited industrially: ichthyol, an ointment used in pharmacology against inflammations of the skin, was extracted from the oil shales. Palaeontological excavations conducted since the seventies by the Museo di Storia Naturale in Milan and other research institutions have yielded hundreds of new fossils. Judging by the number of specimens that continue to be brought to light, the deposit still seems far from exhausted.
23 - PALEOGENE (showcase 12)
Geography and Climate
One of the most important geological events of the Palaeogene was the continuous widening of the Atlantic Ocean, which split Greenland definitively from North America and Europe, destroying the land bridge between these continents. Meanwhile, the northward drift of the four Gondwanian continental masses (South America, Africa, India and Australia) continued, and was destined to have far-reaching consequences in the Neogene.
During the Palaeogene the climate that had persisted for 200 million years began to deteriorate. Nevertheless it can be said that the palaeocenic world was still climatically quite similar to that of the Mesozoic, with hot, humid weather, no clearly marked seasons, and widely distributed tropical forests.
The extensive tropical forests of the Palaeogene were the ideal environment for the diversification of many groups of birds and mammals that had survived the extinction at the end of the Cretaceous. At the beginning of the period, the continents were isolated by shallow seas that favoured the evolution of endemic species. Among the mammals there were giants with small brains similar in appearance to rhinoceroses, such as the brontotheres and uintatheres of Asia and North America, or the arsinotheres of Africa. Among the birds there existed huge wingless predators, over 2 m tall, with sharp, sturdy beaks: diatrymids in the northern continents and phororacids in South America. Other predators were the last of the campsosaurs, creatures related to the crocodiles, which died out during this period, and giant snakes such as Titanoboa, which was over 12 m long. The insects were similar to those of the present day, with the ants being even more widespread and diversified than now.
In the seas the cetaceans began to evolve, being destined to replace the great marine reptiles of the Mesozoic, all of which were extinct except for some turtles. The niches left vacant by the extinction of the ammonites and belemnites were occupied by the teleosts or bony fish, which became the most widespread and diversified into neritic and pelagic forms.
Bivalves, gastropod molluscs, echinoderms, corals and bryozoans were already represented by species almost identical to those of the present day. In the mid-Palaeogene there appeared ancestral forms of the modern cephalopods, and among the protozoa, some exceptionally large foraminifera that were the size of lentils.
THE EOCENE DEPOSITS OF BOLCA
The fossil beds of Bolca, in the province of Verona, is one of the world’s most important Cenozoic deposits. It consists of yellowish limestone deposited during the Early Eocene (50 million years ago) in a shallow lagoon separated from the open sea by a coral reef.
The abundance of fossils in the beds (hundreds of species of fish, sea and land plants, crustaceans, insects, some reptiles and a few bird feathers), suggests that the death of the organisms was caused periodically by particular environmental phenomena, such as underwater eruptions.
24 - THE SELACHII (sharks and rays) (showcase 13)
The group of the Selachii comprises todays fishes with cartilaginous skeletons.
Their form is related to the way they swim and their mode of life: they are fusiform or siluriform fish (sharks) adapted to a pelagic lifestyle, or fish with flattened bodies (rays) adapted to a benthic lifestyle.
The parts of the Selachii usually preserved as fossils are only the teeth and cutaneous denticles.
These fish lack a swim bladder and have to swim continuously to maintain a constant position in the water column.
Most Selachii are predators. They locate their prey by relying on vision and a highly developed sense of smell.
The reception of chemical stimuli is concentrated in the nostrils and the sensory cells spread across the surface of the body.
Sharks are widespread in all seas. In size they range from the 20 cm of the dwarf shark all the way up to the 13 m recorded of the whale shark (but some are believed to reach 20 m).
The fossil remains show that this already group existed 300 million years ago.
The oldest fossil representatives of the rays date from the early Cretaceous, 140 million years ago. In the course of their evolutionary history they have adapted to living on the seabed; their bodies are flattened and the pectoral fins extended to the head and hips, giving them their familiar rhomboid form. The tail is whip-like, and bears a very dangerous venomous sting at the end.
25 - NEOGENE (showcase 14)
Geography and Climate
The Neogene saw a drastic cooling of the global climate. Possible causes included the rise of the Tibetan plateau caused by the immense collision between the Indian subcontinent and the rest of Asia. The collision of the African plate with the European was the driving force for the elevation of the Alpine range.
Towards the end of the period there was also the climax of the collision between North and South America, the effects of which were less extensive, thanks to the presence of small plates in the Caribbean that formed a buffer mitigating the impact.
Other notable events were the crisis of salinity in the Mediterranean (see below) and the movement of the Arabian plate at the end of that period, which opened up the Afar Rift Valley, the cradle of human evolution.
Towards the end of the period, Antarctica, by this time separated from the other land masses, was surrounded by a circumpolar current. At the same time the barrier formed by Eurasia, Greenland and North America led to the isolation of the Arctic waters. In this way both poles were insulated from the equator and began to build up ice cover, so reflecting more sunlight (albedo) and entrapping water, leading to a fall in the sea level. The climate became colder and more arid worldwide, and the deserts of Central Asia and North Africa formed.
During the Neogene mammals and flowering plants reached the pinnacle of their evolution. The event that had the strongest impact on ecosystems was the appearance and rapid spread of the grasses. The aridity of the climate shrank the forests, which gave way to grass-dominated savannahs, prairies, steppes and tundras. The response of the herbivores was the co-evolution of long-legged running mammals, such as the equids (horses), well-adapted to living in open spaces, with high-crowned teeth capable of chewing grass, which is poor in nutrients and rich in abrasive silicates. Carnivores also gave rise to new forms, capable of stalking and pursuing their prey over longer distances.
In South America, a peculiar fauna made up of strange mammals – litopterns, notungulates, giant ground-dwelling sloths and borhyaenids – continued to evolve, until many of these animals became extinct following the devastating effects of the great faunal interchange with North America: the formation of a land bridge (the Isthmus of Panama) between the two continental masses introduced new predators and competitors.
The mastodons were widespread on every continent except Australia. With them appeared a host of small and medium-sized creatures, generalists, capable of counting on high reproductive rates and in some cases of hibernating in the cold seasons. They included the rodents, rabbits, opossums, raccoons, foxes, dogs, cats and snakes.
Towards the end of the Neogene, in the African savannahs appeared the australopithecines, apes that eventually gave rise, among others, to the human evolutionary line.
In the oceans, modern cetaceans such as those found in the fossil sites of Castell’Arquato replaced the archaic forms and lived alongside the giant carnivorous shark Carcharocles megalodon. Among the giants of the seas should also be remembered the desmostylian mammals, in appearance midway between the seals and the proboscidates.
THE MEDITERRANEAN AND PARATETHYS
About 20 million years ago, the collision between the African plate and the Eurasian caused the uplifting of a series of mountain ranges, extending from the Alps to the Taurus Mountains (in southern Turkey). In addition to closing communications between the Indo-Pacific region and Tethys, the latter was divided into two basins, one further northern and “East European” (Paratethys) and the other more southern (the Mediterranean).
Paratethys originally appeared as a sort of huge salt lake that extended from Hungary to the Aral Sea and the Persian Gulf. Because it was hemmed in between the continental masses, and also due to a number of geodynamic processes that altered the structure of the region, it was soon fragmented into a series of basins with brackish waters, where numerous endemic forms developed. Following its partial filling and under the constant pressure of tectonic compression, Paratethys was reduced until it only occupied the depressions corresponding to what are now the Black Sea, Caspian Sea and Aral Sea.
Some 5.3 million years ago the Mediterranean was affected to the west by a minor geological event with far-reaching consequences: a small submarine plate collided with Spain and Africa, closing the outlet to the Atlantic Ocean for 600,000 years. Since it was no longer replenished by water from the ocean, evaporation was no longer offset by the quantity of water brought by rivers and falling by rain. The result was that the sea began to evaporate, becoming increasingly salty and dry. This event is known as the Messinian Salinity Crisis, and evidence of it is found in the immense deposits of evaporites.
26 - LUCY WAS TWENTY YEARS OLD... AND AN UPRIGHT CHARACTER!
The skeleton exhibited here is a copy of the celebrated “Lucy”, a female of Australopithecus afarensis discovered in 1974 at Hadar (Ethiopia) in sediments from between 3.4 and 2.9 million years old.
Lucy was little more than 1.10 meters tall and weighed about 60 pounds. The extraordinary nature of the find lies in the bones of the pelvis and legs: they are quite similar to those of a modern human and suggest an erect bipedal gait.
Based on these data, scholars place A. afarensis in the evolutionary line leading to modern humans.
The long arms and curved fingers are characters indicative of arboreal habits, perhaps inherited from more archaic forms.
The copy was made with the support of the Lions Club Valdarda and the Società Piacentina di Scienze Naturali.
27 - THE PATH OF MAN (showcase 15)
For many years human evolution was compared by anthropologists to a ladder on which each rung represented a gradually more highly evolved form until it reached the apex, our own species. Today, because of the complexity revealed by the fossils, the long path of human evolution is instead compared to the ramifications of a bush. At the base of this bush is a common ancestor to humans (Homo) and chimpanzees (Pan), as confirmed by examination the genome and comparison of the soft tissues.
At the beginning of the Pliocene, in the Rift Valley of north-eastern Africa, the savannah encroached on the forest, so that this common ancestor found itself living in two types of environment, one forest and one with extensive open spaces. The populations that gradually gave rise to the modern chimpanzees (the line of the Panins or primates) remained in the forests. Some populations (the line of the Hominins) instead began to colonize the newly formed open spaces. In this savannah environment, a more upright posture had the advantage of allowing the hominins to rise above the grass and survey their surroundings, keeping watch for predators, at the same time leaving their hands free to use tools (stones, sticks).
These facultative bipeds underwent an evolutionary radiation with archaic forms (australopithecines) widespread initially in Ethiopia and Tanzania, and then across the whole of Africa. From various australopithecines at least two independent lines branched out, the megadont or paranthropus hominins (Paranthropus boisei), with a very strong chewing apparatus, and some transitional forms such as Homo habilis (or Australopithecus habilis, according to some researchers, who believe that the morphology is not sufficiently distinct from that of australopithecines). At the beginning of the Pleistocene there appeared the first true representatives of the genus Homo (for example H. erectus), termed pre-modern men, spreading to Asia and Europe. Towards the end of the Pleistocene two species with distinctive morphological traits can be identified: the Neanderthals (Homo neanderthalensis) and anatomically modern humans (Homo sapiens).
Africa, Cradle of our Species
There are three types of evidence to support the hypothesis that the species Homo sapiens originated in Africa. The first is that, on the basis of current data, the oldest fossils of anatomically modern humans actually come from Africa. The second is that the oldest stone tools, by their complexity associated with modern humans, also appear in the archaeological record in Africa before any other part of the world. The third is genetic evidence, based on the study of mitochondrial DNA (mtDNA). Differences at the level of mtDNA do not cause variations in the appearance of individuals or benefits from the metabolic point of view, therefore they are considered neutral and not subject to natural selection. This means that the quantity of variations accumulated at the level of mtDNA can be correlated with the length of time over which this DNA has been subject to mutation, hence the time elapsed since a given population has differentiated and evolved independently. The variations observed in sub-Saharan Africa are greater than in the whole of the rest of the world put together, showing that the populations of H. sapiens in this area were the earliest and subsequently colonized other parts of the globe.
Morphological and Behavioural Evolution
The complex evolutionary path that has led to anatomically modern humans can be briefly summarized in a list of morphological transformations that took place gradually over time: from facultative to obligate bipedalism; upper limbs gradually less adapted to brachiation and the hand better suited to manipulation; variation in the size of the teeth (canines smaller and premolars/molars larger); a bigger mandible and thicker tooth enamel; less prognathous face; increase in the size of the skull in proportion to the size of head and body; pelvis with the pelvic canal gradually larger to give birth to infants with bigger skulls; an increasing trend in bodily size and decrease in the degree of sexual dimorphism.
Paralleling anatomical changes, there was also a progressive behavioural and cultural evolution, which became very significant and rapid with the appearance of pre-modern and above all anatomically modern humans. The salient achievements of our ancestors included: the ability to develop language; the cognitive skills and dexterity necessary to design and fashion sophisticated tools in stone and bone, such as needles and fish hooks, as well as symbolic objects and artistic representations; the introduction of rituals and the custom of burying the dead.
28 - PLEISTOCENE (QUATERNARY)
Geography and climate
During the Pleistocene the continuous climatic cooling led to a series of ice ages alternating with warmer interglacials.
At the beginning of the Pleistocene, the flora and invertebrates were almost identical to those at the present day, while the mammals included forms now extinct, some of them huge (known as megafauna), which included the famous mammoth (Mammuthus), the giant deer (Megaloceros), several species of rhinoceroses and the Australian diprotodons. Most of this megafauna became extinct as a result of profound climatic changes and hunting by modern humans, who in Europe evolved at the time of the glaciations.
Numerous remains of Pleistocene mammals are also found in the Piacenza area, in the Po valley.
Nowadays anthropization continues incessantly on all continents, an activity that is radically changing the appearance of the planet, often to the detriment of the natural environment.
In 2009, the ICS (International Commission on Stratigraphy) extended the duration of the Pleistocene (also known as the Quaternary) by transferring the Age known by the name of Gelesian from the Pliocene to the Pleistocene, so as to make the beginning of the Pleistocene Epoch (and therefore the Pleistogene Period) coincide with the onset of the glaciations on a global scale.