Although these categories seem well-defined, a given fossil may fall into several categories or may elude them all. Consequently, these categories should be thought of as broad modes of preservation rather than shoe boxes into which all fossils must go . When thinking about types of fossils and modes of preservation, it is more important to consider what types of biologically interesting information is or is not present than to fret over strict classifications. Each type of plant fossil carries different types of anatomical and biological information. Consequently, to piece together the most complete picture of an ancient plant, paleobotanists hope for the same type of plant or plant part will be preserved in several different styles.
Compressions retain organic matter, usually more or less coalified.
Peat, lignite, and coal are essentially compressions of thick accumulations of plant debris relatively free of encasing mineral sediment. Compressions are excellent records of external form, especially for planar structures like leaves. The cuticle retains the imprint of epidermal cells, but other than this, cellular information can seldom be recovered from compressions. In addition, because compressions preserve organic material, carbon isotopic studies can be performed on compressions. From these studies, paleobotanists can sometimes recognize the biochemical signature of C3, C4, and CAM photosynthetic physiology in extinct species.