An Outline Of The Fossil Record

Radioactive isotopes of rubidium and uranium decay slowly through time, and measurements of very small quantities of these isotopes show that some igneous rocks in western Greenland are about 3,800 million years old (Moorbath, 1975). These are the oldest dated rocks on the Earth's surface. Some inclusions in these very old rocks appear to have been derived from sedimentary rocks, suggesting that water has existed on the Earth's surface for more than 3,800 million years. Other isotopic work suggests that the Earth's crust is about 4,600 million years old. If this estimate is correct, the first 800 million years of the Earth's history have not yet been deduced from the rocks on its surface.

The oldest fossils known occur in rocks of about 3,400 million years old; they appear to be microscopic prokaryotes. Later in these Precambrian times (at about 2,700 million years) layered larger, dome-shaped structures (stromatolites) appear, which were formed by blue-green algae. Apart from a few burrows, possibly made by worms, these are the only fossil groups known prior to about 700 million years before the present. Thus, during the greater part of these Precambrian times, it is stromatolites which dominate the fossil record.

During the later part of the Precambrian (about 700 to 570 million years) the impressions of some soft-bodied coelenterates and annelid worms occur (the chief characteristics of the common animals and plant fossils are set out in the Classification of Organisms on page 30). Thus animals existed for some considerable time before they developed hard parts at the base of the Cambrian.

The Cambrian system was first named by Adam Sedgwick after the rocks of Wales, which he mapped and described during the nineteenth century. Like all systems, Cambrian beds can be recognized by the fossils which they contain. The base of the Cambrian is usually defined by the first appearance of trilobites, hyolithids or archaeocyathids.

The Ordovician system was first defined by Charles Lapworth (1879), who had to clear up a controversy between Sedgwick and Sir Roderick Murchison. The original Cambrian system of Sedgwick contained many beds which were included by Murchison in his Silurian system. To resolve the overlap, Lapworth introduced the Ordovician (like the Silurian, it was named after an old British tribe). He defined it as the beds lying between the base of the Arenig Series and the base of the Llandovery Series; and he thus restricted the Silurian to the younger beds of Murchison's original system, and the Cambrian to the older beds of Sedgwick's system.

To the geologist working in Wales, Lapworth's new Ordovician system was easy to recognize, as unconformities exist above and below it. These mark local episodes of uplift and folding, but, fortunately, there were also faunal changes which can be detected at many sites around the world. Not only did several new trilobite families appear at the base of the Ordovician, but also the first families of bryozoans, the first strophomenide brachiopods and some other new brachiopod families, the first crinoids, the first graptoloids (excluding the dendroids), and the first agnathan fish.

During the Ordovician the brachiopods became steadily more abundant relative to the trilobites, and, except in some carbonate environments (where colonial corals were making their first appearance or where algae were common), the brachiopods dominated most Silurian shelf communities.

Three main types of biological change occur near the base of the Silurian:

1. Many Ordovician shelf animals became extinct; some of this may be attributable directly to the lowering of sea level because of the formation of ice caps.

2. Most Silurian faunas were cosmopolitan. The provincialism seen in the Ordovician disappeared except for a polar fauna in parts in Gondwanaland. When the faunas mixed, competition increased, so that some well adapted animals increased their geographical range, while others died out.

3. In the Lower Silurian, when the shelf seas spread over the continental margins (and in some cases over whole continents), many habitats became available for recolonization. The shallow communities at this time showed many large changes in composition; some habitats were occupied by several different genera in succession.

In eastern Wales and western England (where Murchison did his most important work), the base of the Silurian is marked by an unconformity. It now appears that perhaps all shallow water sequences have a break in sedimentation at this time. Certainly in the better studied sections of Europe (Ziegler, Rickards and McKerrow, 1974) and North America (Berry and Boucot, 1970) there appears to be no continuous sedimentation from the Ordo-vician into the Silurian within a shallow shelf sequence. But these breaks do not mean that there was a widespread folding at the end of the Ordovician, for most deep water sequences show a continuous succession of beds. A likely explanation for these observations is to be found in Africa and South America, where terrestrial glacial deposits are present over a wide area (Sheehan, 1973). A major Ashgill ice age could account for a temporary lowering of sea level sufficient to produce the break seen in the basal Silurian shallow water areas. The lowering of sea level was, in places, accompanied by the erosion of a well marked shelf. Certainly in the British Lower Palaeozoic it is only in the Lower Silurian that a sharp shelf margin can be detected (Hancock et al. 1974).

In England, the marine Silurian beds are followed by the river and brackish water deposits of the Old Red Sandstone. Murchison had doubts as to where to draw the upper boundary of the Silurian, and there has been debate about the matter ever since. It would now appear that non-marine environments developed in several parts of Britain before the end of the Ludlow Series, and the transition into these can certainly not be used to mark a synchronous event. Moreover, work in Poland and Czechoslovakia has recently shown that there is a succession of beds in many areas which are younger than the Ludlow but older than the basal Devonian of the Ardennes (currently the area where the base is defined). We follow Berry and Boucot (1970) in recognizing the Pridoli as the top series in the Silurian system. The Downtonian beds, which contain freshwater fish and have often been mapped as part of the Old Red Sandstone, are of Pridoli age and are thus included in the Silurian.

It was during the latest Silurian that the first land plants (psilophytes) developed, followed almost at once by myriapods and insects. By the Upper Devonian the amphibia had also colonized low-lying terrestrial semi-aquatic environments.

Devonian benthic marine animals were not very different from those in the Silurian, but among the brachiopods spiriferides became more common, and productoids and terebratulides appeared for the first time. A more significant change is seen in the pelagic faunas: Monograptus became extinct just before the end of the Lower Devonian. This extinction was nearly coincident with the rise of the ammonoids. The earliest true ammonoid families developed in the Lower and Middle Devonian; from then until the

Late Carboniferous these ammonoids serve as the best zonal indicators in stratigraphy.

The base of the Carboniferous system, although named on account of its coals, was originally defined by the return of the sea over much of the Old Red Sandstone continent of north-west Europe. Like most of such large marine transgressions, the sea did not spread everywhere at once. The base of the system is now defined by goniatitic ammonoids in marine facies, though in many freshwater successions it is still not possible to place the boundary precisely. In many marine environments, the most common fossils were brachiopods (especially productoids and spiriferides), crinoids and rugose corals. Among the echinoderms, not only did the crinoids flourish, but the blastoids too reached their acme. Marine bivalves became more common.

The Carboniferous saw the spread of deltaic conditions over a large part of northern Europe and North America, which resulted at times in widespread brackish water and freshwater conditions. The non-marine faunas included bivalves (myalinids and anthra-cosiids), branchiopods and fish; and the marginal marine faunas included lingulids, arenaceous foraminifera and worms. Coal-producing forests occurred throughout Carboniferous times, but they were more common in the later parts of the period; the commonest plants were lycopods and horsetails, but some gym-nosperms were also present. In the Late Carboniferous and Permian, fusulinid foraminifera became important in marine environments; they were often a major component of the fauna and they provide good zonal indices for this part of the stratigraphic column.

The Late Carboniferous mountain-building episode (the Hercy-nian orogeny) has been interpreted as the result of southern Europe colliding with northern Europe (McKerrow and Ziegler, 1972). The westward continuation of this fold belt lies in the southern Appalachians which is considered to be the result of a collision between Africa and North America. The Uralian orogeny also took place in the Permian, uniting Siberia with the Russian platform. The consequence was that, by the end of the Permian, the largest continents of the world were fused to form Pangea, a single supercontinent (Briden et al. 1974). This large continent was only sporadically covered by shelf seas. Those seas near the margins of Pangea had normal marine faunas, for example those covering the Perm (later called Molotov) region north-east of Moscow, parts of southern Europe, Mexico and western America, Indonesia and the Himalayas, while inland seas, like the Zechstein Sea of northern Europe, only had euryhaline species which could tolerate the increase in salinity. Bryozoans, bivalves and product-oids were among the commonest of these animals that could tolerate life in the Zechstein Sea. The normal marine forms were mostly descendants of the same groups that were present in the

Carboniferous.

The Late Permian or Early Triassic saw the extinction of the following marine animals:

1. Many families of foraminifera, including the fusulinids.

2. The tabulate and rugose corals.

3. The cystoporate, trepostomate and most of the cryptostomate bryozoans.

4. Orthide brachiopods, and many strophomenides and spiriferi-des.

5. Many ammonoid families.

6. All the trilobites, and many ostracode families.

7. The blastoids and many crinoid families.

8. Many fish families, including acanthodians and primitive cross-opterygians.

Most of these groups gradually decreased in numbers and diversity during the Permian, but some appear to have died out suddenly at the very end of the period (Harland et al. 1967; Kummel and Teichert, 1970). Some groups became very restricted geographically before they became extinct.

The reasons for this massive extinction of faunas was perhaps due to increase in competition. By the end of the Permian, shelf seas only extended over about fifteen per cent of the area covered in the Early Permian. The geographic extent of shallow marine benthic species therefore would have become reduced, and the probability of their extinction increased (Schopf, 1974).

On land much, but not all, of the present northern hemisphere was desert, while, by contrast, India and parts of the present day southern hemisphere (Gondwanaland) were, at least at times, covered by ice. Glaciation started in the Early Carboniferous of South America and Africa, but did not reach India or Australia until the late Carboniferous; and glacial tillites continued through much of the Permian of Australia (King, 1958). Over this large area the glaciation was followed by the Glossopteris flora (Chaloner and Lacy, 1973) again earlier in South America and Africa, and later in India and Australia. The Glossopteris distributions were one of the first pieces of evidence to suggest Gondwanaland existed as a single continent. At the end of the Permian, apparently independently of climate (as it had become warm in South America early in the Permian while remaining cold in Australia), many Pteridospermopsida (the class containing Glossopteris) became extinct. Also on the land, the amphibians decreased in diversity throughout the Permian, while the synapsid reptiles showed extinction of many families at the very end of this period (Harland et al. 1967). The pattern of extinctions on land is thus as complex as that in marine life. The causes of extinction of the land flora and faunas may also be due to competition in the extremes of climate we know existed. But not all life was going through hard times in the Permian; many new insect suborders appeared throughout the period.

The Triassic system was named after the threefold division present in Germany. During this period, the sea spread over the margins of Pangea, and especially along the borders of the Tethys Ocean which extended from Indonesia to Spain between Asia and Gondwanaland. This increase in area allowed some marine animals to diversify, but they did so slowly. The appearance of the new groups was spread out through Triassic time. They included: scleractinian corals, several new ammonoid families, actinoptery-gian fish. Although few new orders appeared, bivalves became more common. On land, the Triassic saw some new amphibians and many varied reptiles. The first mammals appeared in the Late Triassic.

The Jurassic system was named after the Jura Mountains of eastern France. In much of north-west Europe, there was a major transgression of the sea around the Triassic-Jurassic boundary; though, like many such transgressions, it occurred over a moderate length of time. Ammonites dominated the open seas of the Jurassic; while the benthos was characterized by an abundance of bivalves (especially oysters) and burrowing worms and Crustacea (represented more by their burrows than by their skeletons). Echinoids diversified greatly, with the appearance of many new families.

The Cretaceous system is named after the widespread development of chalk deposits seen in much of Europe north of the Alps. This is composed entirely of marine organisms of which the protistid coccoliths make up a large part. In general, the higher Cretaceous taxa were very similar to those present in the Jurassic.

The close of the Cretaceous saw the extinction of many reptiles: some families became reduced gradually during the Upper Cretaceous, but others disappeared suddenly at the very end of the period (Romer, 1966). Most of the reptiles, (like the majority of dinosaurs and the pterosaurs) were land animals, but some (like the ichthyosaurs and the sauropterygians) were marine. The Mesozoic mammals diversified gradually during the Late Cretaceous, but this development was small compared with the great radiation seen in the early Tertiary; both may be linked with the contemporaneous development of angiosperms.

The Late Cretaceous saw the extinction of marine animals, including many foraminifera families, both pelagic (Globotruncata) and benthic. Some bivalves (like the rudists) died out at the end of the Maastrichtian (the top Cretaceous stage), but others did not become extinct until the Eocene. The belemnites showed a similar pattern, with some groups continuing until the Late Eocene. Ammonoids decreased from twenty-two families at the base of the Upper Cretaceous to eleven in the Maastrichtian; all families died out before the Tertiary (Hancock, 1967). Many echinoid families died out, and some groups of bony fish became extinct. As in the Permian, many Upper Cretaceous animals became restricted in their geographical distribution, so that they have different upper limits in different areas.

After these extinctions (or in some cases while the Cretaceous groups were decreasing in numbers and diversity), many other animal groups showed evolutionary radiation. These included the pelagic foraminifera (Globorotalia and its descendants), some benthic foraminifera, gastropods, echinoids and bony fish (teleosts) and, most markedly, the mammals.

Many explanations have been put forward for these changes, and no doubt the critical factors are complex and may vary from one group to another. For the marine animals, the key factor was possibly a reduction in the area of shelf sea after the widespread Cretaceous transgressions; the sea had probably covered more of the continent in the Upper Cretaceous than at any other time in the Earth's history. This reduction in living space would (as is postulated for the Permian) cause increase in competition, and hence extinction of the less successful groups. On land, there are several possible (and not necessarily exclusive) reasons for the reptilian extinctions; the three most popular are:

1 The rise of the angiosperms, which could have necessitated changes in feeding habits for many large herbivorous dinosaurs. If the herbivores could not obtain enough to eat (some of them must have had to consume many hundreds of pounds of foliage each day), then carnivores which preyed on them would also suffer.

2 Uplift of many land areas, reducing the amount of swamp-covered areas where the lush vegetation was situated (Romer, 1966).

3 A change in climate, produced by the same uplift, possibly coupled with the northward movements of all the northern hemisphere continents (Smith and Briden, 1977); this would have resulted in cooler conditions over many land areas, to which the coldblooded reptiles would be unable to adapt.

During the Cenozoic, most of the major changes in faunas and floras are a reflection of climate. The climatic changes that we see in the fossil record of Europe and North America took place in two ways: there was a slow northward drift of these two continents throughout the Tertiary, which resulted in progressively cooler climates; and, especially in the Pleistocene, there were more rapid world-wide fluctuations in temperature. It is still not yet clear why these Pleistocene fluctuations took place (and they are probably still in action), but they provide a very convenient method for dating sediments during the past few million years. Each cooler period is marked by a cooler fauna and flora, and although the composition of these fossil assemblages depends on the latitude of the collecting site the fluctuations in temperature can be determined, so that the hot and cold peaks can be correlated over large areas.

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