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Paleoecology is the branch of paleontology that studies ancient organisms and their environments. Paleoecologists study the physical structure and biological functions of organisms, their interactions with each other, and their role in ancient ecosystems. In addition to the basic principles of paleontology, paleoecology research depends on concepts from biology, sedimentology, and geochemistry. The goals of paleoecology are to understand the details of ancient environments and the functioning of ancient ecosystems and their evolution.

The primary database for paleoecological studies is the fossil record. For example, by studying fossils and comparing them with similar living organisms, paleoecologists attempt to understand how the fossil organism lived and its ecological duties. If no living organism is analogous to a fossil, an engineering approach may be taken. This involves constructing models of the fossil and testing how it behaves under various environmental conditions. In this way, the researcher can determine why the organism evolved that particular combination of body shape, density, size, etc., and what type of environment it probably inhabited.

An organism's fossilized hard parts (for example, the shell of a clam), through their composition, provide clues to the environment as well. A clam shell's trace element content relates to the concentration of trace elements in the aquatic environment. The concentration in the environment is a product of environmental variables such as salinity and water temperature. So, by studying the trace element chemistry of a fossil, it may be possible to determine the approximate climatic conditions where the organism lived, as well as its latitude.

In addition to body fossils, paleoecologists study trace fossils. These are things like footprints, gopher burrows, or worm trails preserved in sediments. Trace fossils indicate the behavior of the organism that made the trace and physical factors about the environment. For example, a trail of dinosaur footprints preserved in stream bed sediments provides evidence of how fast the dinosaur was moving, based on spacing of the prints. The depth of the footprints suggests how soft the sediments were, and hence, whether the stream bed was dry at the time the dinosaur traversed it.

Unlike ecologists, paleoecologists must cope with a very incomplete database, because many organisms do not easily fossilize. The evidence that is available to paleoecologists is also misleading at times. For example, a dinosaur nest discovered in the 1923—the first ever found—contained several broken dinosaur eggs. The remains of a small, previously unknown dinosaur, later named Oviraptor, were also found nearby. Certain skeletal characteristics of this dinosaur lead researchers to conclude that it was in the act of stealing eggs when it died, hence the name Oviraptor (egg plunderer). In 1993, evidence from another nest illustrated that rather than stealing some other dinosaur's eggs, Oviraptor was brooding on its own nest.

As this example illustrates, perhaps more so than in any other science, old interpretations sometimes must be revised to accommodate new data or theories. However, overall, paleoecology is crucial to our understanding of the long and diverse history of life on Earth.

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