The first fossil ever described from what is now Dinosaur National Monument wasn't a dinosaur. It was a horn coral, collected by one of John Wesley Powell's expeditions in Utah. Corals are the best-known members of the phylum Cnidaria, and at least 74 national parks preserve their fossils.
The tabulate coral Eofletcheria utahia in Great Basin National Park. Credit: Gordon Bell, National Park Service.
Stinging animals
Cnidarians — corals, jellyfish and sea anemones — have tissues, a stomach, and a mouth ringed with tentacles. Their name comes from the Greek cnidos, stinging nettle: all have specialized cells that inject poison into prey, or unlucky passers-by. Soft-bodied jellyfish have a sparse fossil record and none are known from Park Service units; something once reported as a Precambrian jellyfish in Grand Canyon National Park turned out to be an inorganic structure.
Corals are essentially sea anemones that build skeletons of calcium carbonate, and those hard skeletons left a rich record. Hard corals first appear in tropical Ordovician seas. The Paleozoic groups — including the distinctive horn corals — are extinct, and most weren't reef builders: Paleozoic reefs were often built by microbes and sponges. Modern corals first appear in the Triassic, and today's stony corals build reefs in warm, shallow seas.
Three kinds of fossil coral
| Group | Age | What to look for | Examples |
|---|---|---|---|
| Rugose | Paleozoic | Solitary or colonial, with complex vertical walls inside the corallites (each animal's cup); cups a centimeter or more across. Solitary ones, horn corals, grew anchored at the narrow end | Petoskey stones are polished pieces of the colonial rugose coral Hexagonaria |
| Tabulate | Paleozoic | Colonial, with few or no vertical walls but well-developed horizontal "tabulae"; cups a few millimeters across at most | Favosites ("honeycomb coral"), Halysites ("chain coral"), Syringopora ("organ pipe coral") |
| Stony | Mesozoic and Cenozoic | Reef-forming corals | Common in the Pliocene and Pleistocene bedrock of Everglades National Park |
Horn coral in the Upper Devonian–Lower Mississippian Englewood Limestone at Wind Cave National Park: side profile (A) and cross-section (B). Credit: Theodore Herring, National Park Service.
Where to see them
- Paleozoic rugose and tabulate corals turn up in parks with Paleozoic marine rocks, with outstanding examples at Death Valley, Delaware Water Gap, Grand Canyon and its neighbors, Great Basin, Mammoth Cave (in the cave walls) and Yellowstone. Horn corals are the largest fossils in the building stone of Fort Scott National Historic Site in Kansas, and colonial rugose corals appear in the Pakoon Limestone of Grand Canyon-Parashant National Monument.
- Stony corals appear at Colonial National Historical Park in Virginia, Dry Tortugas and Everglades national parks in Florida, and Vicksburg National Military Park in Mississippi and Louisiana. Almost all are Cenozoic, because Mesozoic rocks from the right marine settings are rare in the park system.
Fossil stony coral is common in the Pliocene and Pleistocene bedrock of Everglades National Park. NPS Photo by Denise Diaz.
The puzzling conulariids
The conulariid Conularia kaibabensis in the Kaibab Formation at Grand Canyon National Park, named by the legendary Grand Canyon geologist and former park naturalist Eddie McKee. Image from the National Park Service's fossil cnidarians page.
Conulariids, which lived through the Paleozoic and Triassic, are classified as cnidarians, though much about their anatomy is uncertain. They were probably something like horn corals: a tentacled predator living in an elongated, pyramid-shaped shell anchored at its tip, built of tiny mineralized rods stacked in a herringbone pattern. They are known from at least 8 national parks, including Grand Canyon.
Sources
Based on "Fossil Cnidarians," National Park Service Paleontology Program; a work of the United States government in the public domain. Pictures an earlier version of this page left out were restored on 2026-09-26.
In these publicationsInvertebrate Fossils in National Parks
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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