File:Dictyonema crassibasale fossil graptolite (Lockport Dolomite, Middle Silurian; Hamilton area, southeastern Ontario, Canada) (15126818037).jpg

Dictyonema crassibasale Gurley in Bassler, 1909 fossil graptolites, preserved as carbonized compressions in dolostone from the Silurian of Ontario, Canada (public display, Colorado School of Mines Geology Museum, Golden, Colorado, USA).

Classification: Animalia, Hemichordata, Graptolithina, Dendroidea, Dendrograptidae

Stratigraphy: Lockport Dolomite, upper Middle Silurian

Locality: Hamilton area, southeastern Ontario, southeastern Canada

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.

The second most abundant group of graptolites is the dendroids (seen here). Dendroid graptolites attached to substrates and had colonial skeletons (rhabdosomes) that are generally broadly branching (conical to fan-shaped to shrub-like to flat spirals).

Other grapolite groups are very rare: the crustoids, tuboids, camaroids, and stolonoids.