Graptolite

[grap-tuh-lahyt] /ˈgræp təˌlaɪt/

noun
1.
any colonial animal of the extinct class Graptolithina, most common in the Ordovician and Silurian Periods, thought to be related to the pterobranchs.
/ˈɡræptəˌlaɪt/
noun
1.
any extinct Palaeozoic colonial animal of the class Graptolithina, usually regarded as related to either the hemichordates or the coelenterates: a common fossil, used to determine the age of sedimentary rocks
n.

1838, from Modern Latin graptolithus, literally “written-stone,” from Greek graptos “engraved, written, painted” (verbal adjective of graphein; see -graphy) + lithos “stone” (see litho-). So called because the fossils resemble writing.
graptolite
(grāp’tə-līt’)
Any of numerous hemichordates of the class Graptolithina. Graptolites form colonies consisting of interlocked cuplike chambers arranged in one or more branches and covered by an exoskeleton. They flourished from the late Cambrian to the early Mississippian Period, and were thought to be extinct until 1992 when scientists discovered what is believed to be a living species. Graptolites are important index fossils used to date the rocks of the Silurian and Ordovician Periods.