Fossil vertebrates lived in communities in which some animals ate others, some specialized in eating particular plants, and others suffered from particular parasites. Some fossil vertebrates lived in damp tropical forests, whereas others preferred to burrow in temperate soils, or to swim in deep cold seas. Just as today, organisms have always interacted in different ways with other organisms, and with the physical environment. The study of ancient modes of life and interactions is palaeoecology, and the focus of study may be a single animal or a whole community.

Unlike work on modern ecology, the palaeontologist has to work with one hand tied behind the back. It is obvious that specimens of any particular species will be incomplete, and palaeontologists can never see the animal in action. Also, the collection of fossil plants and animals from any particular site is likely to be incomplete, and biased: the relative numbers of fossil specimens of different species are unlikely to reflect their true abundances in life.

Nevertheless, much can be done. The modes of life of individual species of fossil vertebrate can be deduced from their bones and teeth. If there are enough specimens of some of the species, detailed measurements may show sexual dimorphism, that is, two sets of adult individuals, one presumably female, and the other male. Sometimes, juveniles are found, and these can show how the animal grew up. If several different species are found together, it may be possible to work out which ate what, and to draw up a food web (see Box 4.4). The food web should include plants, insects, and other animals, as well as the vertebrates. The whole assemblage of organisms that lived together in one place at one time, the community, can be compared in detail with communities from other localities of the same age, and with similar communities through time. Some communities remain fairly constant, although different species may take the key roles at different times. In other cases, new communities arise, or communities can become more complex, for example, with the invention of new modes of life such as tree-climbing, flight,burrow-ing, or mollusc-eating.

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