The Earth is the only planet we know to support life. Its long history shows that life and the planet it inhabits have a complicated relationship. Free oxygen in the atmosphere, the ozone shield, the movement of carbon into long-term reservoirs in the deep oceans, and the rapid weathering of rocks on the land surface are obvious examples of this relationship.
The evolutionary history of life on Earth points to the development of a series of faunas that occupied the changing surfaces of the land and sea. Through the extraordinary medium of lagerstatten, or sites of exceptional preservation, it is possible to visualize these vanished communities and to restore some of their behaviors and interactions.
In addition, the process of evolution, via Darwinian natural selection, is recorded in the fossil record. Though incomplete and tantalizing in places, fossils are the only direct information source about the nature of our ancestors, and the ancestors of any life on modern Earth.
The study of fossils offers a view of the past at all scales of space and time. From a single moment, for example the single act of making a footprint, to the study of the evolution of tetrapods, or from the study of a single locality to an analysis of the effect of the break-up of Pangea on the evolution of dinosaurs, the fossil record is the primary source of data. Paleontologists build detailed interpretations and analysis from the study of individual fossils; most are invertebrate animals, preserved in great abundance in the shallow marine record.
In this book, we provide an introduction to the methods by which fossils are studied. We discuss the biases that follow from the process of fossilization, and explain how this can be analyzed for a particular fossil locality. We provide an introduction to evolutionary theory, which is the basis for explaining the consistent changes of shape seen in fossils over time.
We describe the major groups of invertebrate fossils that form the bedrock of the discipline, and also of most introductory courses in the subject. We discuss microfossils, plants, and vertebrates, which, while less commonly encountered, are of such importance to understanding life on Earth. Finally, we briefly narrate the evolution of life on Earth as it is currently understood, including episodes of huge diversification and mass extinction. Throughout the text, we discuss the many ways in which fossils contribute to an improved understanding of the Earth's system, for example through allowing accurate relative dating of rocks, or as proxies for particular environmental settings.
By the end of this book, you should be able to identify the most common fossils, discuss their ecology and life habits based on an analysis of their detailed shape, understand how each group contributes to the wider studies of paleontology and earth systems science, and appreciate their importance at particular points in Earth's history. You should have a broad understanding of how life has both evolved on Earth and must be factored into any analysis of the evolution of the planet. You can read the book in sequence, or dip into it at will. You will find that some sections follow on from a previous chapter, but in most cases information is presented in self-contained pieces that fall on a couple of facing pages. We have used diagrams and tables wherever possible to summarize information and we have used as few technical terms as possible, to try to lay bare the ways in which fossils matter.
Types of fossils (Fig. 1.1)
Trace fossils are the preserved impressions of biological activity. They provide indirect evidence for the existence of past life. They are direct indicators of fossil behavior. As trace fossils are usually preserved where they were made, they are very good indicators of past sedimentary environments. Trace fossils made by trilobites have provided an insight into trilobite life habits, in particular walking, feeding, burrowing, and mating behavior.
Coprolites are fossilized animal feces. They may be considered as a form of trace fossil recording the activity of an organism. In some coprolites recognizable parts of plants and animals are preserved, providing information about feeding habits and the interaction of coexisting organisms.
When some organisms decompose they leave a characteristic chemical signature. Such chemical traces provide indirect evidence for the existence of past life. For example, when plants decompose their chlorophyll breaks down into distinctive, stable, organic molecules. Such molecules are known from rocks more than 2 billion years old and indicate the presence of very early plants.
Body fossils are the remains of living organisms and are direct evidence of past life. Usually only hard tissues are preserved, for example shells, bones, or carapaces. In particular environmental conditions the soft tissues may fossilize but this is generally a rare occurrence. Most body fossils are the remains of animals that have died, but death is not a prerequisite, since some body fossils represent parts of an animal that were shed during its lifetime. For example, trilobites shed their exoskeleton as they grew and these molts may be preserved in the fossil record.
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