Originally regarded as being related to the hydrozoans, graptolites are now considered to be related to the pterobranchs, a rare group of modern marine animals.
The graptolites are classed as hemichordates (phylum Hemichordata), a phylum of marine deuterostome animals and come in a variety of weird and wonderful designs. They were a major component of the zooplankton in our early Paleozoic ecosystems, most likely living as suspension feeders, drifting freely on the surface of ancient seas or attached to floating seaweed by means of a slender thread. Some forms of graptolite lived attached to the sea-floor by a root-like base. The deceased planktonic graptolites would sink down to and settle on the seafloor, eventually becoming entombed in the sediment and are thus well-preserved.
Graptolite fossils are often found in shales and slates and can be mistaken for scratches on the rock. The name graptolite comes from the Greek graptos meaning "written", and lithos meaning "rock." It is a very suitable name as many graptolites look very much like hieroglyphs written on rock and not the impressions of animals.
We find graptolite fossils flattened along the bedding planes of the rocks in which they occur. My first graptolite finds were from roadcuts up near Cranbrook, British Columbia, Canada. I was on a fossil field trip looking for Cambrian trilobites. It was a thrilling experience and completely unexpected when the first graptolite met my eyes.
They vary in shape, but are most commonly dendritic or branching (such as Dictoyonema), saw-blade like, or "tuning fork" shaped, such as Didymograptus murchisoni. The lovely specimen of Arienigraptus sp. you see here is from the Lower Darriwilian of Bolivia and in the collections of the deeply awesome Gilberto Juárez Huarachi. The second photo is from the Encyclopaedia Brittanica - From the Encyclopaedia Brittanica, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6886878
