Fossil

I

Introduction

Fossil, the physical evidence of a prehistoric organism, often comprising a shell, bone, or other durable skeletal part, which in the majority of cases belongs to an extinct species. Fossils also include the imprints of organisms that have dissolved or eroded away; preserved footprints and tracks; remarkably unaltered remains preserved in peat bogs, asphalt lakes or tar pits, frozen ground, or amber; and fossilized excrement called coprolites, which may contain remains of eaten animals and plants.

The term was originally applied to almost anything dug up from the ground (from the Latin, fodere, “to dig”) but in its scientific usage is often restricted to the preserved remains of organisms dating from prehistoric times. Remains of animals and plants associated with archaeological sites are usually regarded as more the province of the historian than the palaeontologist, but there is a grey area in between, and specimens dating from medieval times may be preserved in the same manner as material millions of years old.

II

Early Understanding of Fossils

Fossil bones of mammals, including elephants, are frequently found in areas around the Mediterranean, and on the Greek islands, where they may erode out of the cliffs or be turned up by the plough. In classical times they were popularly considered to provide evidence of the gigantomachy—the battle between the gods and the giants. It has been claimed that the legend of the griffin may have been inspired by the common preservation of small dinosaurs such as Protoceratops in areas surrounding the Gobi Desert. The major Greek philosophers touched briefly on geological matters, and Herodotus (c. 484-425 BC) even speculated that the deposits of the Nile delta were the consequence of repeated floods. However, attempts to explain fossils rationally only slowly attracted serious scholars. Leonardo da Vinci made pertinent remarks on fossils in his notebooks, regarding them as more than mere “accidents of nature” and recognizing their biological origin. Such explanations did not achieve wide currency, in spite of observations by the Danish naturalist Niels Stensen (Steno) that clearly identified fossil sharks’ teeth for what they were, and outlined the principles of stratigraphic succession of strata. During the 17th century the majority of published works stressed the inorganic nature of fossils, such as the description given by Robert Plot in The Natural History of Oxfordshire of fossils as “naturally produced by some extraordinary plastic virtue latent in the earth or quarries where they are found”. The view that fossils directly recorded the universal Flood described in the Bible—a view that, while based on Scripture, nevertheless recognized that fossils were indeed organic remains testifying to early historic times—had as long a tradition, and achieved wide currency in the 17th and 18th centuries. However, by 1824, when William Buckland published Reliquiae Diluvianae advocating the diluvian explanation, such biblical catastrophism was already obsolete. The notion that it was possible to understand the formation of fossils by studying processes still acting on Earth had been proposed by the Scottish scientist James Hutton at the end of the 18th century. It took several decades for his ideas to become widely established, a period which coincided broadly with the recognition that fossils changed systematically in harmony with the order of the strata. An extended compass of geological time was required to allow this succession. Even so, it was not until the establishment of the modern geological timescale that the chronology of Earth history was confirmed, with fossils playing a central part in populating the narrative of the temporal succession of organisms.

III

The Fossilization Process

Fossils are entombed in sedimentary rocks. Their preservation depends upon being enclosed in a suitable soft sediment, which, as layer accumulates upon layer, ultimately becomes a rock incorporated in the geological succession. Most sediments are marine, and hence the majority of fossils are of former marine animals. Freshwater (rivers and lakes) and terrestrial habitats preferentially preserve the remains of animals and plants that lived in their vicinity, but there are some erosive habitats—high mountains, for example—the former inhabitants of which will be virtually unknown. Most animals decay rapidly upon death, and so the materials most likely to be fossilized are resistant: mineralized bones, teeth, and shells, or woody tissue. Such remains are left behind by scavengers, and are usually not fully decomposed by bacteria. They may be redistributed by currents, with the result that not every collection of fossils necessarily represents a natural community. Fossils can be of any size: from the thigh bone of a sauropod dinosaur several metres long and weighing several tonnes, to minute coccoliths—protective plates of marine algae—a few microns (µm) long, which require study under the electron microscope. Smaller fossils are often abundant, but they are the more easily redistributed. Fossils smaller than 1 mm or so in diameter are termed microfossils. Much of the plant fossil record comprises microfossils of spores and pollen, often in the absence of remains of the plants that produced them.

When they are first incorporated in sediment fossils may differ little from recent material, but through time (varying from tens to millions of years) the small pores that are present in bone, shell, or wood may become filled with mineral material which serves to harden and increase the weight of the specimen. The fossil is not “turned to stone” as is a common misconception; rather, the stone infiltrates the fossil and makes it part of the enclosing rock. When a fossil coral is polished, for example, the original shelly material is usually clearly visible, although “filled in” with limestone. By contrast, in porous rocks like sandstones the fossil shell may be dissolved away by water leaving a cavity where it once was. From such moulds the original can still be reconstructed by taking appropriate casts. Rarely, the fossil may be replaced almost molecule by molecule by another mineral—silica (SiO₂) is perhaps the commonest—allowing its extraction by chemical means. Silicified fossils may be dissolved out of limestones in this fashion. The sporopollenin walls of plant spores and pollen are extremely resistant and hence these fossils can be extracted by digesting shaly (shale-containing) rocks in hydrofluoric acid, which removes virtually everything else. Despite such drastic treatment, these fossils can preserve details on a scale less than 1 µm for hundreds of millions of years. The fossilization process operates on varying timescales: certain examples of tropical “beach rock” feature objects cemented with calcium carbonate after a few years. On the other hand, some Cambrian (540 Mya—that is, 540 million years before the present) clays are known with perfectly preserved shells that can still be washed out of the matrix. As a general rule, however, older fossils tend to be more mineralized and are more likely to have undergone tectonic distortion, where the rocks in which they are found are folded or caught up in earth movements (see Plate Tectonics).

IV

Special Preservation of Fossils

The more durable parts of animals and plants are usually preserved as fossils; soft tissues are rarely preserved. Small and delicate animals and plants, and those lacking mineralized tissues, such as fungi or nematode worms, are therefore unlikely to be found as fossils. We will never have a complete inventory of the biosphere. However, in special geological circumstances we are afforded privileged glimpses of this “missing” biota, and though these examples are rare they are vital to our understanding of evolutionary history (see Evolution).

Amber is the fossilized resin of trees. In its raw state such sticky resin trapped delicate insects, spiders, and even mites which then became entombed; eventually, the resin balls hardened and became incorporated into sediments, still carrying their cargo of perfectly preserved fossils. Amber specimens are now well known from early Cretaceous rocks (125 Mya). Preservation of body features is superb, down to the last fine hair. Thanks to amber we have a fossil record of such delicate animals as mushroom flies and pseudoscorpions. Despite the hubris of the movie Jurassic Park, however, no convincing DNA has yet been extracted from ancient amber.

Freshwater sedimentary rocks have, rarely, preserved very fine details of animals as delicate as bats. In the famous locality of Messel, Germany, the bottom waters of an Eocene lake (60 Mya) were deficient in oxygen, and probably rich in anaerobic bacteria, and there were no scavengers to destroy the carcasses of animals that fell into the lake and died. They are preserved in exquisite detail. Lower in the geological column, the Solnhofen Limestone was laid down in a Jurassic (180 Mya) lagoon exposing flats of sticky mud. Animals that became entrapped died in this inhospitable environment, and the fine mud preserved the fossils with great faithfulness. Most famous of these is the early (not now quite the earliest) bird Archaeopteryx, but fragile dragonflies, sea spiders, and horseshoe crabs are as important in their own way. Devonian (c. 400 Mya) petrifactions around a hot siliceous spring—the Rhynie Chert beds—in Scotland preserved the stems and shoots of some of the earliest land plants, and also some of the small joint-legged animals that accompanied this pioneer vegetation in what was to be the most important new colonization on our planet.

Cambrian (c. 540 Mya) special preservation has supplied extraordinary insights into how the animal kingdom diversified. The Burgess Shale (British Columbia) is possibly the most famous of a dozen or more localities of this age. Numerous kinds of soft-bodied animals (and the limbs of trilobites) are preserved in this locality as silvery films. It is thought that these animals were rapidly and catastrophically buried, before the soft parts could decay. This fauna includes a variety of phyla of worm-like animals, as well as some odd-looking creatures, the classification of which has stimulated much argument. Although once interpreted as unknown “phyla”, most of the latter are now seen as interesting intermediate stages in the development of groups of animals (arthropods, onychophora, etc.) that are still present in the living fauna. Within this scenario, some of the Burgess animals had peculiar features which are not readily matched on any of their living descendants, but which presumably adapted them to life in the Cambrian seas. The curious predator Anomalocaris is probably the most striking of these; it was the size of a large lobster and carried a large pair of head limbs.

Much of our knowledge of life in the Precambrian (Proterozoic c. 2500-545 Mya) depends on special preservation of single-celled organisms, including algae and cyanobacteria. Cellular preservation is usually in cherts (a fine grained deposit of silica) and can only be detected by making thin sections through the rock and then viewing them under high magnification. Famous examples are the Gunflint Chert in Canada and the Figtree Chert in South Africa, but new discoveries are still regularly coming to light. Some of these occurrences are in association with stromatolites, cushion-like structures made up of fine concentric layers, which were the product of successive accumulation by bacterial mats. Stromatolites were very widespread in the Precambrian, and can be recognized even in the absence of evidence of their associated fossils. These special circumstances give us a glimpse, but no more, of the early biosphere.