Archean Animals

Evolution Brachiopods


Simple and reef-building algae. Heyday of graptolites. Abundance of trilobites, brachiopods, gastropods, crinoids, corals, echinoids, bryozoans and cephalopods.

First green and red algae. Trilobites abound in shallow seas. Many shelled brachiopods, gastropods, bivalves; also crinoids, graptolites, sponges and segmented worms.

First multicellular, soft-bodied animals, which looked like worms, jellyfish and seapens, appeared toward end of Eon.

First single-celled organisms appeared toward end of Archean Eon, including the first bacteria and blue-green algae

First land plants. Calcareous algae abundant in seas. Profusion of marine invertebrates. Appearance and radiation of eurypterids (sea scorpions). Decline of graptolites.

Plants and invertebrates similar to today's groups.

Burgeoning of flowering plants, including grasses and other non-woody types. Also, formation of vast forests in tropical and temperate zones. Radiator ot modern bivalves and gastropods. Invertebrate groups similar to modern types.

First flowering plants (angiosperms), including beech, fig and magnolia. Pollinating insects diversify. Ammonites and belemnites extinct. Brachiopodos decline.

Abundance of cycads, ferns and conifers. Abundance of ammonites and belemnites. Brachiopods common. Diversification of moliusks and echinoderms.

Horsetails, ferns, ginkgos and conifers dominant land vegetation. Appearance of cycads. Extinction of seed ferns. Heyday of ammonites Appearance of modern corals.

Abundance of ferns, conifers and seed ferns. Decline of horsetails. Glossopteris flora in Gondwanaland. Radiation of insects. Extinction of trilobites and other marine groups.

Giand club mosses and horsetail ferns abound. First gymnosperms (ginkgos, yew, conifers). First flying insects. Decline of trilobites and brachiopods. Graptolites become extinct.

First ferns appear, initially leafless and rootless, eventually treelike (tree ferns). First true seed plants (seed ferns). First ammonites, crabs, spiders, mites and insects (non-flying).

The leaves and débris of land plants produced the first deposits rich in organic remains. When mixed with rainwater, these formed muds, which provided a habitat for the first fresh-water communities of creatures. Many types of fish lived here, and at the end of the Devonian their descendants, the amphibians, first crawled out of the water, and for a time led a largely land-based existence.

By the Late Carboniferous (Pennsyl-vanian), different floras had evolved. A lush, swampy tropical rain forest covered much of North America and Europe, and was inhabited by a variety of fishes, amphibians and the first reptiles.

During the Permian period, reptiles gradually came to dominate the land, and ousted the amphibians. Herbivorous reptiles became common in the Late Permian, and, in turn, provided an opportunity for the evolution of large, carnivorous reptiles that preyed on them.

At this time, and into the Triassic, the continents of the world were united in one huge landmass called Pangaea (see p. 11). Land-living creatures could spread anywhere in this vast area. However, many parts of that landmass were far from the sea and its rain-providing clouds, and these areas became arid. Many older types of plant became extinct during this time, and were replaced by the seed ferns, ginkgos and many new types of conifers.

This was the type of arid scene, with its changing flora, that witnessed the long reign of the ruling reptiles, starting in the Triassic and culminating at the end of the Cretaceous. The mammals arrived in the Late Triassic, but did not take center stage until the Tertiary, when the dinosaurs, and most of their kin, had bowed out. The first birds appeared in the Late Jurassic.

The modern plant world began in the Early Cretaceous, when the first flowering plants (angiosperms) evolved. Hand in hand with this came a great diversification of insect pollinators. So by Early Tertiary times, the world was dominated, as it is today, by flowering plants, birds and mammals on land, and by advanced types of bony and cartilaginous fishes in the waters.

The geography of life

The geography of yesterday's world was very different from that of today. The way in which ancient and modern animals are distributed can be explained by the discovery that the Earth's surface is divided into moving sections, or "plates."

The force that powers these move ments — which are still happening — originates in the heat of the Earth's core. In some areas, upward currents move this heat so that it is just below the crust of the Earth. There, the current spreads out sideways, and cools before returning toward the core. If this upward current lies below a continent, it can split the continent into diverging fragments. Between these, new oceanic crust is formed. Elsewhere, old oceanic crust disappears down a system of great trenches that lies around the edge of the Pacific Ocean.

Sometimes, these movements — known as continental drift or plate tectonics — caused existing continents to split into pieces, or else to become fused together (see opposite). This process, together with encroachments or retreats of the sea, lead to progressive changes in the world's geography.

In the Devonian period, between 408 and 360 million years ago, North America and Europe were part of the single continent "Euramerica," since the North Atlantic Ocean did not exist at that time. Fossils of the first land vertebrates — the early amphibians and reptiles — have been found almost exclusively on that continent, which suggests that they may have evolved there. Only during the Carboniferous period did Euramerica become attached to the great southern continent of Gondwana-land, made up of what is today South America, Africa, Antarctica, Australia and India.

The early amphibians and reptiles did not penetrate into Gondwanaland until it became united to Euramerica in the Late Permian. By the Triassic all the major land areas of the world were united into the single supercontinent of Pangaea.

Toward the end of the Triassic, or at the beginning of the Jurassic, Pangaea started to split up. While oceans developed between the continents, shallow seaways spread across them during the Cretaceous. By the Late Cretaceous, there were 2 landmasses in the northern hemisphere, which contained different faunas of dinosaurs (see p. 93) and mammals, as well as different plants. At the same time, Gondwanaland was breaking up also. By the middle of the Cretaceous, India had already split away and started its long journey northward. Our familiar, present-day geography appeared only after Africa had joined Europe in the Miocene, as recently as 20 million years ago, and the Panama Isthmus had linked North and South America in the Pliocene.

By chance, the separation of the continents took place after modern types of mammal had started to spread through the world. As a result, each continent came to have its own typical fauna of mammals, which had evolved within that landmass.

The fossil record

The evidence of life in these prehistoric eras comes from a unique "history book." This is the record of the rocks, in which is inscribed in fossil form a picture of life in times past.

A fossil is any trace of an ancient form of life. Most animals or plants live and die without leaving any permanent evidence of their existence. But, occasionally, the conditions are suitable for organisms to become fossilized.

When an animal dies, the soft parts of its body normally decay quickly or are eaten by other animals. The hard parts, too, may be gnawed away and broken up. But if the animal's body is buried in soft sediments — such as mud at the bottom of a stream or lake, or fine sand on the seabed — it may be protected from attack by other animals. Though the soft parts will probably decay, the skeleton or the teeth may remain. It is now, potentially, a fossil.

Several events can now turn the skeleton into a true fossil (see pp. 12—13). First, sediments continue to be laid down, and the layers (strata) of mud or sand become consolidated and harder. The water that continues to seep through the sediments contains minerals, which then cement the particles of sand or mud into solid rock. And those minerals also percolate through the skeleton itself, and gradually replace the material of the bones themselves. This often happens in such a way that the detailed microscopic structure of the original bone is still visible, unchanged.

Sometimes, after the skeleton has become entombed, it may be completely dissolved away by the percolating waters, leaving a cavity. If this cavity then becomes filled with a mineral deposit, this may form a natural cast of the original bone (see p. 13).

Possibly the most unchanged and perfect fossil bones are those from Pleistocene mammals and birds entombed in the tar pits of Rancho La Brea in Los Angeles. Here, tar seeped to the surface and formed deep pools. After rainfall, the water would lie on top of the tar, and attracted animals such as mammoths and ground sloths to drink. They would then become mired in the sticky tar and gradually sink into it. Before they disappeared forever, their carcasses attracted predatory direwolves, sabertooth cats and scavenging vultures, all of which also became trapped and died. All of their bones are perfectly preserved in

Animals The Archean Eon

3 Pangaea started to split up so that, by the Mid-Jurassic, seaways spread down the eastern coast of Africa. The fledgling Atlantic appeared as North America started to separate from Europe.


The series of globes shows how the face of the Earth has changed over the millennia. In these maps, areas at the "back" of the globe have been folded out. Dotted lines indicate the coastlines of the modern continents; shallow seas are shaded light gray, deep seas in dark gray.

1 In the Late Carboniferous to Early Permian, there were 2 great continents, Euramerica in the north and Gondwanaland in the south. Three other land masses were the forerunners of eventual Asia. Euramerica was the exclusive home of early amphibians and reptiles.

2 By the Late Permian, all the world's continents were united into one huge landmass called Pangaea. At this time, early amphibians and reptiles penetrated into Gondwanaland, and spread into Asia.

3 Pangaea started to split up so that, by the Mid-Jurassic, seaways spread down the eastern coast of Africa. The fledgling Atlantic appeared as North America started to separate from Europe.

4 In the Early Cretaceous sea had spread around Africa's southern tip. North and South America split apart at this time. Seaways spread northward to separate Europe from Asia. India had split away from Gondwanaland and begun its long journey northward.

5 By the Late Cretaceous, there were 2 landmasses in the northern hemisphere. One, "Asiamerica," included Asia and western North America. The other, "Euramerica," comprised Europe and eastern North America. These 2 continents contained different faunas of dinosaurs and mammals, as well as different plants.

6 By the Eocene, the modern continents had nearly taken shape. India had nearly completed its northward journey and Australia and Antarctica had split from the tip of South America. Each continent came to have its own fauna of mammals.

Pangaea the tar, and need only to be washed in petrol to extract and clean them.

An even more complete fauna and flora, dating from Middle Eocene times, is found at Messel in southern Germany. Dead animals and plants were swept into a deep lake in a subtropical forest. Each year a thick growth of algae sank to the bottom of the lake and covered them. As a result, the bones and hair of a complete fauna of early mammals — including bats, horses, anteaters and rodents — are preserved, together with their stomach contents and even the skeletons of unborn young. Crocodiles, snakes, frogs, insects, fruits, flowers and leaves are also preserved in this 50 million-year-old time capsule.

Traces of the soft parts of animals can very occasionally be seen as markings on the rocks. For example, it is known that ichthyosaurs, preserved in the fine shales of Holzmaden in southern Germany, had a fin on their backs and sharklike tail fins because, although these structures contained no bony skeleton, an outline of the body exists as a stain in the rock (see p. 80).

The fine limestones of Solnhofen in southern Germany have preserved detailed impressions of the plumage of Archaeopteryx, the link between reptiles and birds (see p. 176). Similar impressions show that at least one type of pterosaur may have had a covering of hair (see p. 105).

Fossil footprints are another type of impression. Dinosaur tracks, formed in soft ground which dried and hardened

Any animal is most likely to be fossilized if it is buried in soft sediments, like the ichthyosaur body illustrated here in fine sand on the sea bed. The flesh decays, but the bony parts — skull, teeth and skeleton — do not.

before new layers of sediment covered them up, remain in this permanent, compacted surface. Other traces of the existence of ancient animals are their fossilized droppings, or coprolites.

Which rocks contain fossils?

Because fossils are most commonly formed when an animal's skeleton is entombed within sediments that are being deposited, fossils are thus most likely to be discovered in sedimentary rocks, which are laid down in layers and composed of either mineral or organic matter. Fossils are not usually found in igneous rocks, such as granites and basalts, which pour hot and molten from the earth's interior. Nor are they present in metamorphic rocks, such as marble, which have been transformed by great heat and pressure.

The oldest sedimentary rocks were laid down about 350 million years ago, and vary widely in the size of mineral particles they contain. Fossils are rarely found in rocks in which the "particles" are holders or pebbles, since the bones are crushed or ground up as the rocks are deposited.

Many fossils have been found in finer-grained sandstones. For example, in Silurian and Devonian times, between 440 and 345 million years ago, a thick belt of Old Red Sandstone was deposited across northeastern North America and northwestern Europe, before those areas had moved apart to form the Atlantic Ocean. These sandstones contain a rich fauna of Devonian freshwater and marine fishes.

Layers of sediment pile up on top of the bony remnants. Minerals from the sea water percolate through the skeleton and become deposited in the bones, filling any spaces between them. They gradually replace the material of the bones.

Rocks made up of even finer particles, the clays and shales, are common; they form about 60 percent of all sedimentary rocks. As mud dries, it loses its organic débris, and becomes compacted. It first becomes a soft clay, and then either a hard shale, which readily breaks into flat pieces, or mudstone, which breaks up randomly. Clays laid down in the Eocene, about 50 million years ago, have preserved many fossils around both London and Paris.

In contrast with sandstone rocks, limestones and dolomites are not mineral in origin, but are made up mainly of calcium carbonate, or calcium magnesium carbonate, which has been produced by marine animals or plants. The hard shells of many mollusks and of sea urchins and their relatives, the hard bases secreted by reef-building corals, and the cell walls encasing some algae — all these materials are made of solid calcium carbonate. This becomes broken into minute fragments, and is the major constituent of such organic sedimentary rocks.

Chalk is one such rock, and is composed almost entirely of the shells of tiny, marine planktonic organisms. Thick deposits of chalk, laid down in the Late Cretaceous (between 70 and 65 million years ago) in southeastern England, France and central North America, contain the remains of many marine fishes, reptiles and birds.

Peats and coals also have their origins in organic matter, but are formed from the remains of dead land plants. The plant material originally accumulated in

The fossilized skeleton is compressed and distorted by the addition of more layers of sediment and by movements of the land. Here the rock has been pushed upward so that the strata have become tilted and are exposed as dry land.

Any animal is most likely to be fossilized if it is buried in soft sediments, like the ichthyosaur body illustrated here in fine sand on the sea bed. The flesh decays, but the bony parts — skull, teeth and skeleton — do not.

Layers of sediment pile up on top of the bony remnants. Minerals from the sea water percolate through the skeleton and become deposited in the bones, filling any spaces between them. They gradually replace the material of the bones.

coastal swamps (rather like the Everglades of Florida today), or in inland basins. They first formed peats, some of which later dried and became compressed into solid coal. The Carboniferous coal belts of eastern North America and of Europe are examples of these, and contain the fossils of many fishes, early amphibians and early reptiles.

Finding fossils

Paleontologists must search for fossils in areas where the appropriate rocks are exposed. That is easiest in the deserts, where many hundreds of square miles of rock may be on view.

The rocks can also be seen in the mountains or hills, where they are not covered by sediments, or in cliffs that may be exposed as the result of earthquakes, the action of rivers or streams, or quarrying by humans.

Although it is not difficult to find rocks which may contain fossils, it is much more difficult to locate the fossils themselves. The rocks of the more densely populated areas of North America and Europe are now fairly well explored, although surprising discoveries can still be made (such as Baryonyx, see p. 113). In search of new finds, paleontologists have turned their attention to other parts of the world, particularly to arid regions where rocks are eroding over wide areas.

But paleontologists do not merely go in search of any fossil. Rather they try to answer very specific questions — for example, "What land-living vertebrates

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  • ivana li fonti
    What animals were in the archean eon?
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