Graptolites are an extinct group of hemichordates that are most commonly preserved as carbonized compressions on shale bedding planes. They are typically not glamorous fossils, but they are critically important guide fossils and are widely used in biostratigraphy and for international correlation.
The most abundant group of graptolites in the fossil record is the graptoloids. Graptoloid graptolites typically resemble small hacksaw blades. Each “tooth” of the hacksaw blades housed a tentaculate, filter-feeding organism. The entire hacksaw blade is the graptolite skeleton, known as a rhabdosome - a nonmineralized colonial skeleton. Most graptolites were planktonic.
Shown below is a basinal black shale from Scotland with Climacograptus wilsoni Lapworth, 1876 fossils on it (Animalia, Hemichordata, Graptolithina, Graptoloidea, Diplograptidae). It comes from the lower part of the Lower Hartfell Shale (Moffat Shale Group, Climacograptus wilsoni Zone, Soudleyan Stage, ~mid-Caradocian, upper Middle Ordovician).
Locality: Main Cliff at Dob’s Linn (~36.1-43.1 meters below the Ordovician-Silurian boundary GSSP), north side of A708 Road, just west of the small village of Burkhill, ~20 km northeast of Moffat, far-northern Dumfries County, Southern Uplands, southern Scotland, UK.
Climacograptus wilsoni Lapworth, 1876 rhabdosomes on black shale (2.6 x 2.5 cm) from the Lower Hartfell Shale (upper Middle Ordovician) at Dob’s Linn, southern Scotland.