Graptolites

Graptolites are marine animals that lived from the Middle Cambrian to the Upper Carboniferous, during which period they became completely extinct.

They are stomochordates which, on the evolution scale of life on Earth, are between invertebrates and chordates, therefore organisms with a backbone. In other words, Graptolites can be considered the ancestors of vertebrates. They lived in the clayey sea beds, in small oceanic colonies composed of a chitinous exoskeleton.  The single colony, named Rabdosome, has the shape of a small more or less elongated or branched stick. It was dentated just like a small saw, where each dent, called Hydrotheca, was the home of each polyp.  All graptolites have this general shape, although they have been found with a wide variety of forms. The knowledge of various different species, each of which even had a very vast distribution and a very brief existence, enabled the fossils to be used as “guide fossils” to identify the different stratigraphical levels of the Ordovician and Silurian periods.