Fragments of the mineralized parts of animals and plants make up a large proportion of grains in limestones. Even the greater volume of carbonate mud is believed to be constituted primarily of disaggregated and comminuted biogenic carbonate. Skeletal grain assemblages provide important information on depositional environment but these assemblages have also varied through geologic time as a consequence of evolution. The age of the rock is a vital piece of information in terms of the predictions it allows you to make about the skeletal allochems that are likely to be present. You won't be seeing trilobites in a Cretaceous rock nor rudists in the Ordovician! This section of the tutorial gives a few examples of common fossil fragments and their key attributes, but guides such as Horowitz and Potter (1971), Scholle and Ulmer (2003), and Flugel (2005) are vital resources to support your quest to gain expertise in grain identification.
A key element in the identification of skeletal fragments hinges on mineralogy: among carbonate allochems the ones composed initially of calcite or Mg-calcite tend to display a higher-fidelity preservation of fabric in contrast to those composed of aragonite, which tend to dissolve during diagenesis resulting in the total destruction of the primary fabric. Aragonitic allochems are typically preserved as either an open mold of the shell or a mold that is partially or completely filled with sparry calcite. This is not to say that calcitic allochems are not altered to some degree by diagenetic processes, only that sufficient information on fabric generally survives to be of some use in identification.
For allochems composed initially of calcite, the shell's microstructure, which is the size and shape of the calcite crystals and their orientations with respect to the outer wall of the allochem, are important clues to identification. For allochems composed initially of aragonite the petrographer must rely on the overall shape of the fragment and often little else. Much information on the particular taxonomic affinity of such fragments is destroyed by diagenesis, and in many cases the best one can say is "mollusk" or "possible algae"?
Click on the thumbnails below to navigate to sections containing examples of various fossil groups. Within these individual sections you can click the thumbnails to open a larger view of the image; once they are opened, some of the enlarged images can be toggled between plane-light and cross-polarized views with a mouse-over. Image windows can be re-sized or scrolled, depending on your preferences and the capabilities of your computer monitor.