9.3: Describing Fossils
Before we can start to identify and interpret fossils, we need to know what features to look for and what words to use to describe these features. Some of the most important characteristics are discussed below.
Holes
With certain types of invertebrate fossils, holes in the organism represent cavities where parts of an organism, or sometimes the whole organism lived. The number (single versus multiple), size (large versus small), and internal structure (simple versus complex) of these holes can all provide important clues about the type of organism that the fossil represents.
Symmetry
Symmetry in a fossil occurs when you can draw a line through the organism in a way that splits it into two halves that are mirror images of each other. Radial symmetry happens with circular objects that have an almost infinite number of symmetry planes, penta-radial symmetry is when there are five symmetry planes, bilateral symmetry is when there is one symmetry plane that splits the organism into two halves, and some organisms have no symmetry at all.
Symmetry in organisms with valves
With organisms that live inside of a shell with two halves ( valves ), some may have shell halves that are mirror images of each other (bilateral symmetry with similar valves) and others might have symmetry that cuts through the shell halves (bilateral symmetry with dissimilar valves).
Shells
For organisms that have shells we describe the shape and internal structure. Shells can spiral upwards ( conisprial ), spiral along a single plane ( planispiral ), be conical to flat (patellate), or be straight coned ( orthocone ). Some shells will have chambers separated by walls ( septa ). The place where those septa meet the outside of the shell are called sutures.