Fig. 2.9. Small-sized forms in sand-pelite system (Báldi 2003)

Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones. Normally graded beds generally represent depositional environments which decrease in transport energy as time passes, but also form during rapid depositional events. They are perhaps best represented in turbidite strata, where they indicate a sudden strong current that deposits heavy, coarse sediments first, with finer ones following as the current weakens. They can also form in terrestrial stream deposits. In reverse or inverse grading the bed coarsens upwards. This type of grading is relatively uncommon but is characteristic of sediments deposited by grain flow and debris flow. It is also observed in eolian ripples. These deposition processes are examples of granular convection.

2.2. 2.2. Palaeoecology

Paleoecology is the study of the life and times of fossil organisms, the lifestyles of individual animals and plants together with their relationships to each other and their surrounding environment. We know a great deal about the evolution of life on our planet but relatively little about the ways organisms behaved and interacted. Paleoecology is undoubtedly one of the more exciting disciplines in paleontology; reconstructing past ecosystems and their inhabitants can be great fun. Fossil assemblages suffer from this information loss, paleoecological studies must, nevertheless, have a reliable and sound taxonomic basis – fossils must be properly identifi ed. And although much paleoecological deduction is based on actualismor uniformitarianism, direct comparisons with living analogs, some environments have changed through geological time as have the lifestyles and habitats of many organisms.

There are two main areas of paleoecological research: paleoautecologyis the study of the ecology of a single organism whereas paleosynecologylooks at communities or associations of organisms. For example, aut ecology covers the detailed functions and life of a coral species, and synecology might be concerned with the growth and structure of an entire coral reef, including the mutual relationships between species and their relationship to the surrounding environment. The autecology of individual groups is discussed in the taxonomic chapters. In most studies the functions of fossil animal or plants are established through analogies or homologies with living organisms or structures or by a series of experimental and modeling techniques. Geological evidence, however, remains the main test of these comparisons and models. In this chapter we focus on the community aspects of paleoecology (synecology), reviewing the tools available to reconstruct past ecosystems and see how their organisms socialized.

Within an ecosystem – all the populations of species living in association – there shape the ecosystem and that can trigger large-scale changes if they disappear. A classic keystone species is the elephant: it forms the landscape in large parts of Africa by knocking down trees and feeding on certain plants, and the whole scene looks different when it disappears. Incumbent speciescan occupy the same ecological niche for many millions of years, adding stability to many ecosystems.

In addition, the photic zone is the depth of water penetrated by light; this can vary according to water purity and salinity but in optimum conditions it can extend down to about 100 m. Terrestrial environments are mainly governed by humidity and temperature, and organisms inhabit a wide range of continental environments, ranging from the Arctic tundras to the lush forests of the tropics. Marine environments host a variety of lifestyles. The upper surface waters are rich in floating plankton, and nektonicorganisms swim at various levels in the water column. Within the benthos – the beasts that live in or on the seabed – mobile nektobenthos scuttle across the seafl oor and the fixed or sessile benthos are fixed by a variety of structures. Infaunalorganisms live beneath the sediment–water interface, while epifaunalive above it (Fig. 2.11.).