Classes of the Phylum Hemichordata (currently in Atlas)
What is a graptolite?
Graptolites are colonial fossil organisms which lived from the Cambrian to the Carboniferous. Graptolites were mostly pelagic, however, there are some forms that have been found with a stem, indicating they attached themselves to the substrate. Graptolites are found in a variety of forms, ranging from leaf-shaped colonies to structures which resemble a floating upside-down mesh cup. A graptolite is composed of several parts. The entire graptolite colony is called a rhabdosome, which has several branches called stipes. Each stipe houses several individuals in zooids, which are housed in a cup-like structure called a theca. Other graptolites are considered dendritic, or branching. These forms are considered to have been attached to the sea floor. The graptolites found in the Late Ordovician strata within and around Cincinnati are branched pelagic forms, which were derived from their early dendritic benthic counterparts.
The name “graptolite” is derived from the Greek words graptos, meaning “written”, and lithos, meaning “rock”, as the organisms are often thought of resembling hieroglyphs written on rocks. Graptolites are commonly found as compressed carbonized forms on the bedding planes of shales. When the organism died, it floated to the bottom of the seafloor, which in some cases was anoxic, and became buried in sediment. Because the environment in which the graptolite died was devoid of bacteria that breaks down organic matter, the graptolite was fossilized. In the Cincinnati region, graptolites are commonly preserved as 3D fossils, which enables paleontologists to determine several aspects about how the organisms lived.
Why are graptolites important?
Ordovician graptolites were pelagic, meaning they were able to disperse throughout the oceans. Because of the ability to disperse far distances, one species can be found across continents as well as oceans. Therefore, graptolites are useful in biostratigraphy to date and correlate rock units. Biostratigraphers have used easily identifiable graptolite species to create graptolite biozones. These biozones are another tool paleontologists, stratigraphers, and biostratigraphers can employ to help identify where they are in the rock record, and the approximate ages of rock units they may be working with.