The study of how to identify and name organisms is taxonomy. Taxonomic classification of fossils follows the same principles used for the classification of living organisms and has a hierarchy of divisions. These principles were first proposed in the 18th century by Carolus Linnaeus, a Swedish biologist.
First, all life is divided into three domains, named Archaea, Bacteria, and Eukarya. The domains differ from one another based on fundamental characteristics of their genes. Archaea include a vast array of tiny single-celled microorganisms that occur not only in the mild environments of oceans, soils, and wetlands but also in the harsh environments of hot springs, black smokers, salt lakes, very acidic sediments, and deep subsurface water. Those organisms that can survive in harsh environments are referred to as “extremophiles;” they do not need light, but rather live off the energy stored in the chemical bonds of minerals. Bacteria are also tiny single-celled organisms, species of which inhabit almost all livable environments on Earth. Visually, it may be difficult to distinguish Bacteria from Archaea, but on a genetic level they are profoundly different. Nevertheless, both Archaea and Bacteria have prokaryotic cells, meaning that their cells do not contain nuclei or other organelles surrounded by membranes. In this regard, Archaea and Bacteria differ from Eukarya, for the eukaryotic cells of organisms of the latter domain do contain nuclei and other organelles. Taxonomists divide Eukarya into kingdoms:
- Protista: various unicellular and simple multicellular organisms such as diatoms and forams, two of the major plankton types in the oceans;
- Fungi: mushrooms and yeast;
- Plantae: trees, grasses, mosses, and ferns; and
- Animalia: sponges, corals, snails, dinosaurs, ants, lizards, birds, tigers, fish, and people.
Each kingdom consists of one or more phyla. A phylum, in turn, consists of several classes; a class, of several orders; an order, of several families; a family, of several genera; and a genus, of one or more species. Kingdoms, therefore, are the broadest category of Eukarya and species are the narrowest.
There’s nothing magical about identifying fossils. You can often recognize common fossils in the field by examining their morphology (form or shape). If the fossil is well preserved and has distinctive features, the process can be straightforward, but if fossils are broken into fragments and parts are missing, or if the fossil shares many characteristics with others, identification can be challenging.
Many fossil organisms resemble modern organisms, so it is relatively easy to figure out how to classify them taxonomically. For example, a fossil clam (class Bivalvia) looks like a clam and doesn’t look like, say, a snail (class Gastropoda). At the taxonomic levels below class, identification may involve recognizing such details as the number of ridges on the surface of the shell. Not all fossils, however, resemble living organisms, and figuring out their taxonomic relation to other organisms may be a challenge. Some interpretations remain controversial.
 |
| Common types of invertebrate fossils. |
Figure above shows examples of some of the major types of invertebrate fossils. With this figure, you should be able to identify many of the fossils you’ll find in a bed of sedimentary rock. Some of the notable characterisitics of these fossils include:
- Trilobites: These have a segmented shell that is divided lengthwise into three parts. They are a type of arthropod.
- Gastropods (snails): Most fossil specimens of gastropods have a shell that does not contain internal chambers.
- Bivalves (clams and oysters): These have a shell that can be divided into two similar halves.
- Brachiopods (lamp shells): The top and bottom parts of these shells have different shapes, and the plane of symmetry is perpendicular to the plane of the shell. Examples typically have ridges radiating out from the hinge.
- Bryozoans: These are colonial invertebrates. Their fossils commonly resemble a screen-like grid. Each opening in the grid is the shell of a single animal.
- Crinoids (sea lilies): These organisms look like flowers but actually are animals. Their shells have a stalk consisting of numerous circular plates stacked one on top of the other.
- Graptolites: These look like tiny carbon-saw blades in a rock. They are remnants of colonial animals that floated in the sea.
- Cephalopods: These include ammonites, with a spiral shell, and nautiloids, with a straight shell. Their shells contain internal chambers and have ridged surfaces. These organisms had a squid-like head.
- Corals: These include colonial organisms that form distinctive mounds or columns as well as solitary, cone-shaped species.
Credits: Stephen Marshak (Essentials of Geology)
