Palaeontology

I

Introduction

Palaeontology, the study of fossils, the remains of once-living organisms which are preserved in the sedimentary rock record.

II

The Nature of Palaeontology

Fossils can be found in almost any type of sedimentary rock, and, rarely, can even be preserved in igneous lava flows and ash fall deposits. Providing that deformation has not been too intense and the original shell was robust, fossils may still be preserved in metamorphosed sedimentary rocks, such as slates, even though deformed from their original shape.

Fossils fall into two broad groups, termed body fossils and trace fossils. A body fossil is a preserved, mineralized skeleton or part of a skeleton, such as bones, teeth, and shells. Body fossils are almost invariably comprised of the hard parts of an organism and it is only under rare geochemical conditions that soft tissues are preserved, most commonly involving replacement by minerals. In contrast, a trace fossil is preserved evidence of the activities of an organism, rather than the skeleton. Examples of trace fossils include footprints, borings, and burrows.

Palaeontology encompasses a number of interrelated areas of study. One of the most important fields is biostratigraphy and biostratigraphic correlation, which utilizes the property of the fossil record that each species of organism occurs over a limited range—equivalent to a rock depth or thickness. This is because each species evolved once and (with the exception of those still alive today) eventually became extinct. Thus, the fossil record preserves life in a set order. Biostratigraphers have unravelled this order for many fossil groups and can determine the stratigraphic position (analogous to age) of any fossiliferous deposit by comparison with other rocks containing similar organisms. Biostratigraphy has many applications within geology, notably petroleum exploration.

The “ordering” of the fossil record, in which the sequence of fossil organisms shows a pattern that is reproducible through multiple samplings of the rock record, preserves the only direct record of the evolution of life. Notably, the fossil record includes evidence of major biotic changes, such as mass extinctions and faunal radiations, which are rare events in Phanerozoic history.

III

Early History of Palaeontology

Fossils have been recognized as curious objects since prehistory, by both our own species and, apparently, the Neanderthals. Palaeolithic (see Stone Age) and earlier sites of human occupation have yielded diverse fossil shells, such as mollusc shells, corals, and sharks’ teeth, which were pierced to allow them to be worn as jewellery such as necklaces. Geometrically distinctive fossils, such as belemnite guards, have been found buried with Bronze Age human remains.

Greek philosophers interpreted fossils on the basis of their own observations. For example, Xenophanes of Colophon, among others, accepted that fossil shells found in rocks on mountains indicated a marine origin. Similarly, Chinese philosophers, such as the 6th-century philosopher Li Tao-Yuan, correctly identified fossilized “stone fishes”. However, such “modern” ideas about fossils did not persist and they were replaced by mystical interpretations lacking an observational foundation. Most commonly, fossils were considered to be the product of some latent plastic character of the Earth itself (the vis plastica or virtus formativa) or the product of some influence from the heavenly bodies.

The legend of the Biblical Flood did not gain general acceptance until the 18th century. However, acceptance of a universal deluge led to fossils being considered antediluvian relicts, transported by the flood, most notably the articulated fossil skeleton Homo diluvii testis (Latin, “Man who witnessed the Flood”), so named by the Swiss Johann Scheucher who died in 1733. Scheucher interpreted this specimen as the remains of a sinner drowned by the flood; it was only in the 19th century that the Frenchman Baron Georges Cuvier correctly identified it as a Miocene salamander. The universal deluge persisted as an idea until truly scientific geology and palaeontology developed in the 19th century.

During the 17th and early 18th centuries many European thinkers continued to support the common view that fossils were inorganic sports of nature, although such ideas were largely discarded during the later 18th century. Theological considerations supported such an interpretation; the English theologian and naturalist John Ray did not support the organic origin of fossils, an idea that would “…put a weapon into the Atheists’ hands.” The antithesis of this view was taken by Ray’s contemporary and fellow Englishman Robert Hooke, who made detailed comparisons of fossils with living organisms, and developed many modern palaeontological ideas for the first time. At the same time, similar ideas were being formulated by the Dane Niels Stensen (Nicolaus Steno). For example, Stensen argued that tonguestones, or Glossopetrae, generally considered products of the vis plastica and thought to take the shape of snakes’ tongues, were the teeth of fossilized sharks. Like Hooke, Stensen’s interpretation was made by direct comparison with an extant organism following his dissection of a stranded shark. However, Hooke and Stensen failed to influence contemporary geological thought and their achievements remained essentially unrecognized until the 19th century. In contrast, the French naturalist Georges Louis Leclerc, Comte de Buffon, published notable speculations on the succession of biotas, evolution, “lost species” (extinction), and the great antiquity of the Earth, despite opposition from the Church.

IV

The 19th Century and the Roots of Modern Palaeontology

Palaeontology became established as a science during the late 18th and early 19th centuries. Three of the most revered natural scientists of the past 200 years, among the founders of scientific palaeontology, were pursuing major research programmes, encompassing biostratigraphy, invertebrate and vertebrate palaeontology, evolution, extinction and functional morphology, that were important in establishing this field of study.

The Englishman William “Strata” Smith was the first modern biostratigrapher. He determined the succession of strata in Britain and their included fossils, a truly monumental achievement. Biostratigraphic correlation was not a new idea, having been suggested by Hooke, but it was untried. Smith, an engineer, travelled widely in Britain surveying mines and canals. Successive strata exposed during excavations demonstrated that characteristic fossil species and assemblages (guide or index fossils) were limited to particular parts of the rock record. The utility of this methodology was demonstrated when Smith’s biostratigraphic schemes were proved to be applicable elsewhere in Europe.

The Frenchman Jean-Baptiste Lamarck, an invertebrate palaeontologist, is best recognized for the formulation of the first widely discussed theory of evolution, which involved the retention of acquired characteristics by ancestors (that is, descent with modification). This theory lacked a demonstrable mechanism whereby acquired characteristics could be retained. It was vigorously attacked by Cuvier, who regarded it as a rival to his own catastrophist theories based on the immutability of species.

Cuvier developed and applied rigorous techniques of comparative anatomy to fossil vertebrates. His law of correlation enabled the interpretation of the whole structure of a skeleton, even if it were only incompletely known. Further, Cuvier’s acceptance of extinction was a conspicuous breakthrough at a time when many naturalists still asserted the fixity of species. Extinction formed an important component of Cuvier’s catastrophist theory, in which he postulated that the Earth had been devastated by a succession of major extinction events, each of which was followed by a new creation of life. This concept received more general acceptance than Lamarckian evolutionary theory.