Fossil Mollusks in the National Parks: Clams, Snails, Ammonites and More

Mollusks may be the most common kind of fossil in the national parks. Their record reaches back to the early Paleozoic, they are still abundant today, and Mollusca is the second-largest animal phylum after the arthropods. Most fossil-bearing marine rocks contain mollusks, and they have even turned up alongside dinosaur bones in land deposits.

The phylum

Mollusks are among the most varied and successful invertebrates. They first appeared in the early Cambrian, and thousands of living species occupy every marine habitat from shallow shores to the deepest abyss, as well as fresh water and land. The name means "soft-bodied," and the earliest mollusks had no shells, but most have hard parts that fossilize readily. The main fossil groups are:

• bivalves, or pelecypods (clams), in fresh and salt water;
• cephalopods (nautiloids, ammonites and belemnites), only in the sea; and
• gastropods (snails), in a wide range of environments, including on land.

Gastropods have been found in more park areas than any other kind of invertebrate fossil, with bivalves close behind; cephalopods are known from about half as many parks.

Bivalves

Fossil mollusks, mostly bivalves, from Colonial National Historical Park. NPS photo by Mackenzie Chriscoe

Bivalve shells are documented in at least 117 park units. Common since the Paleozoic, they have lived in marine, brackish and fresh water.

• Paleozoic. Bivalves occur in many parks' Paleozoic marine fossil communities, though less abundantly than the similar-looking brachiopods. Grand Canyon National Park and Grand Canyon-Parashant National Monument have unusually ornamented Paleozoic scallops, and the unusually large Silurian bivalve Pycinodesma giganteum was named from Glacier Bay National Park and Preserve. Carlsbad Caverns and Guadalupe Mountains (Permian) and Mississippi National River and Recreation Area (Ordovician) also have notable assemblages.
• Mesozoic. Katmai and Lake Clark in Alaska have significant Jurassic and Cretaceous bivalves. Marine incursions left abundant bivalves at Devils Tower and Yellowstone (Jurassic) and Chaco Culture and Mesa Verde (Cretaceous), while freshwater river mussels of the family Unionidae are common at Petrified Forest (Upper Triassic) and Dinosaur National Monument (Upper Jurassic).
• Cenozoic. The Pliocene scallop Chesapecten jeffersonius, from Colonial National Historical Park — the site of the first permanent English settlement in North America — is the first published fossil from a national park unit, illustrated in 1687. Castillo de San Marcos in Florida, the oldest masonry fort in the continental United States, is built mainly of coquina, a rock made of small fragments of fossil bivalves. Channel Islands National Park has an excellent Miocene and Quaternary bivalve record on all five islands, and Theodore Roosevelt National Park has abundant freshwater bivalves from the Paleocene, a few million years after the extinction at the end of the Cretaceous.

Paleozoic nautiloid. Mississippi National River and Recreation Area. NPS photo by Justin Tweet.

Miocene bivalves from Santa Rosa Island, Channel Islands National Park. NPS photo by Justin Tweet

Permian bivalves, Grand Canyon-Parashant National Monument. NPS 2020 field inventory

Cephalopods

Cephalopods are marine predators with tentacles and large eyes. Today's octopuses and squids have soft bodies and leave few fossils, but many ancient cephalopods had shells or large internal structures that fossilized easily. Their fossils are known from at least 65 parks.

• Nautiloids were the dominant marine predators of the early Paleozoic, and two species survive. Their chambered shells are coiled in the living species, but many Paleozoic ones were straight or gently curved, some several meters long. Nautiloid Canyon in Grand Canyon National Park is a favorite stop on Colorado River raft trips, and Chesapeake & Ohio Canal National Historical Park and Mississippi National River and Recreation Area have some of the best records. Nautiloids became much rarer after the Paleozoic.
• Ammonites (more properly ammonoids) lived in the Paleozoic and Mesozoic and dominated in the Mesozoic. Though their chambered shells resemble nautiloids', they were more closely related to squids, and complex suture lines between their chambers usually tell them apart. Beyond the classic flat spiral, some grew in open spirals, straight shells or "squiggles." Cliffs along the Rio Grande in Big Bend are studded with Cretaceous ammonites, Fossil Point in Lake Clark has a great diversity including large ones, and the Pierre Shale at Badlands yields ammonites with mother-of-pearl coatings.
• Belemnites were squid-like Mesozoic animals whose bullet-shaped internal guard is what usually fossilizes. They are abundant at Devils Tower and occur at Katmai and Lake Clark.

Late Cretaceous ammonites from the Cliff House Sandstone, Chaco Culture National Historical Park. NPS photo by Phil Varela

Middle Jurassic belemnite guards, Lake Clark National Park and Preserve. NPS photo

Gastropods

Gastropods include snails and slugs. Slugs have no shells or only tiny ones and seldom fossilize, but snail shells are among the most common invertebrate fossils in the parks, documented in at least 118 units. Most parks with Paleozoic marine rocks have a few easily recognized species. Some snails in the Kaibab Formation at Grand Canyon-Parashant have been replaced by red silica, making striking fossils. Chaco Culture's Cretaceous Cliff House Sandstone holds a diverse assemblage, snails are the second most common fossil at Colonial, and Vicksburg National Military Park has land snails in Quaternary loess as well as significant Oligocene marine species.

Dentalium, an Oligocene scaphopod. Vicksburg National Military Park. NPS photo by Charles Beightol.

Other mollusks

• Scaphopods, or tusk shells, live buried in the sea floor and have hollow, tapering tube-shaped shells. They are recorded in at least 31 park areas, with good examples at Vicksburg (Oligocene) and Colonial (Pliocene).
• Chitons, small mollusks covered by eight plates, have been found in seven parks, by far best in the Quaternary deposits of Channel Islands National Park.

Sources

• National Park Service, "Fossil Mollusks," from the series Invertebrate Fossils in National Parks: https://www.nps.gov/articles/000/fossil-mollusks.htm
• Photographs on the source page credited to William & Mary and to the Arkansas Geological Survey are not reproduced. Pictures an earlier version of this page left out were restored on 2026-09-26.