File:Graptolites in pyritic black shale (Jigunsan Formation, Middle Ordovician; Seokgaejae section, Gangwon Province, South Korea).jpg

English: Pyritized graptolites in shale from the Ordovician of South Korea.

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

Seen here are pyritized biserial graptoloid graptolites from the Ordovician-aged Jigunsan Formation in South Korea. Other reported fossils in the unit include several species of trilobites: Dolerobasilicus yokusensis, Basiliella kawasakii, Basiliella typicalis, and Ptychopyge dongjeomensis.

Classification: Animalia, Hemichordata, Graptolithina, Graptoloidea

Stratigraphy: float from the lower Jigunsan Formation, upper Taebaek Group, Joseon Supergroup (Choson Supergroup; Chosen Supergroup), Llandeilo Stage, lower Middle Ordovician (sensu traditio)

Locality: loose piece from the eastern side of Route 910 (= stop 6 of the Seokgaejae section for the 9th International Conference of the Cambrian Stage Subdivision Working Group; see Choi et al., 2004, Paleontological Society of Korea Special Publication 8, p. 31), ~16 air-kilometers east-southeast of the town of Taebaek, southeastern Gangwon Province, eastern South Korea (37° 05.234' North latitude, 129° 08.548' East longitude)