Middle Ordovician graptolites, Hot Springs National Park. Image from the NPS page on fossil graptolites
Graptolites look like pencil scratches on rock. They are really the remains of tiny animals that lived together in colonies — and they are important index fossils for the early Paleozoic, used to match rock layers from place to place and give them relative ages.
What they were
| Classification | subclass Graptolithina, class Pterobranchia, phylum Hemichordata |
| Name | from the Greek for "writing on the rocks" |
| Time range | Cambrian to Mississippian |
| Way of life | marine colonies; some floated freely, others — especially the earliest — lived attached to the sea floor |
They were long thought to be extinct. Then a group of living deep-sea, worm-like, colonial filter feeders, the pterobranchs, turned out to be modern graptolites.
What a fossil looks like
Graptolites usually appear as long, dark carbon films in mudstone, shaped like narrow saw blades with "teeth" on one or both sides. The blades can be single, in groups, or joined into nets. Each "tooth" was actually a tube or cup that housed one of the tiny animals. Colonies ranged from shrub-like, with many slender branches, to just a few branches, or only one.
In the national parks
An Upper Ordovician graptolite, Delaware Water Gap National Recreation Area. NPS photo by Ariana Miranda
Graptolites are known from at least 13 national parks, in Alaska and across the contiguous United States.
| Park | Graptolites |
|---|---|
| St. Croix National Scenic Riverway, Wisconsin and Minnesota | a particularly important record in Cambrian rock, including the Upper Cambrian St. Lawrence Formation — every known specimen from the park lived attached to the sea floor |
| Delaware Water Gap National Recreation Area, New Jersey and Pennsylvania | in the Upper Ordovician Martinsburg Formation, where they are used to date the formation's members |
| Hot Springs National Park, Arkansas | in black shales |
More: Delaware Water Gap Geodiversity Atlas · Hot Springs Geodiversity Atlas · St. Croix Geodiversity Atlas · Body fossils · previous in the series: Fossil echinoderms
Sources
Based on "Fossil Graptolites," part of the series "Invertebrate Fossils in National Parks," National Park Service; a work of the United States government in the public domain. A photograph of specimens credited jointly to the Cincinnati Museum Center is not reproduced.
In these publicationsInvertebrate Fossils in National Parks
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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