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Index fossils contain incredible amounts of information about the past because they are fossils of organisms that were widespread but had a short time span on Earth, making them useful in determining the relative age of rock layers. Mold, amber, and trace fossils may contain some information about the past, but they are not as useful in determining relative age as index fossils. Mold fossils only contain the impression of an organism and do not provide as much information as a complete fossil. Amber fossils are preserved tree sap and typically only preserve small organisms, limiting the amount of information they can provide about the larger ecosystem of the past. Trace fossils, such as footprints or burrows, can give clues about an organism's behavior, but they do not provide as much information about their biological traits or the overall environment of the past as index fossils do.