Article

Fossil Graptolites

Photo of two pieces of dark rock with thin light-colored fossils. — Middle Ordovician graptolites, Hot Springs National Park.

Introduction

Graptolites are early Paleozoic fossils that are important index fossils, used for correlating stratigraphic units and providing relative dates. They look like scratch marks made by a pencil on rocks, but are actually the remains of tiny animals that lived together in colonies. They have been found in several parks in Alaska and a variety of parks in the contiguous United States.

Graptolithina

Graptolites are marine colonial organisms. They are members of the subclass Graptolithina within the class Pterobranchia of the phylum Hemichordata. Their name comes from the Greek words for “writing on the rocks.”The fossil record of graptolites extends from the Cambrian to the Mississippian. Some graptolites were free-floating, but others, particularly the earliest ones, lived attached to the sea floor. They were long thought to be extinct, but a group of modern deep-sea colonial worm-like filter feeders called pterobranchs have been discovered to be modern graptolites.Graptolite fossils usually appear as long dark carbon films in mudstones. They generally resemble narrow sawblades with “teeth” on one or both sides. The sawblades may occur singly, in groups, or as nets. The teeth on the blades (or branches) are in reality tubes or cups in which the tiny graptolite animals lived. The colonies varied widely in shape: some are shrub-like with numerous slender branches; others had only a few or even a single branch.

Graptolite Fossils in National Parks

^{Upper Ordovician graptolite. Delaware Water Gap National Recreation Area.

NPS photo by Ariana Miranda.}

Photo of a rocks with fossils in a display box. — Graptolite fragments in the Upper Cambrian St. Lawrence Formation, St. Croix National Scenic Riverway. Scale bar is in mm, with 1 cm black and white squares.
Courtesy of the Cincinnati Museum Center, NPS photo by Justin Tweet.

Graptolites have been identified in at least 13 national parks. St. Croix National Scenic Riverway in Wisconsin and Minnesota has a particularly significant graptolite fossil record in Cambrian rocks. All known graptolite fossils from the park are of those that lived attached to the seafloor.

In Delaware Water Gap National Recreation Area in New Jersey and Pennsylvania, graptolite fossils are found in the Upper Ordovician Martinsburg Formation. Graptolites in this region are used to date the members of the formation.

Black shales in Hot Springs National Park in Arkansas also have graptolite fossils.