Fossil Mollusks

Chesapecten jeffersonius, a marine bivalve that is the Virginia state fossil. Colonial National Historical Park.

Photo by Rowan Lockwood (William & Mary).

Introduction

Altogether mollusks must be one of the most common types of fossil in the national parks. They have a long and diverse fossil record extending back to the early Paleozoic, and they are still common today. Mollusca is the second largest phyla of animals after arthropods. Most fossiliferous marine sedimentary rocks contain mollusk fossils, and mollusks have also been found in the same terrestrial deposits as dinosaur bones.

Phylum Mollusca

Mollusks are one of the most varied and successful groups of invertebrates. They first appeared in the early Cambrian. Today thousands of living species occupy every marine habitat from the shallowest waters near the shoreline to the deepest abyss. They also live on land (e.g., gastropods) and inhabit freshwater environments (gastropods and bivalves).The term mollusk means “soft-bodied.” The earliest mollusks lacked shells; however, most mollusks have well-developed hard parts that readily fossilize. Major groups of mollusks in the fossil record include:

• Bivalves or pelecypods (clams)

• Cephalopods (nautiloids, ammonites, & belemnites)

• Gastropods (snails)

Cephalopods are exclusively marine, while bivalves inhabit both fresh and salt water, and gastropods live in a wide variety of environments, including terrestrial.

Mollusks Fossils in National Parks

Gastropods have been found in more national park areas than any type of invertebrate fossils, followed closely by bivalves. Cephalopods, the third major type of mollusks, have been identified in about half the number of parks as gastropods and bivalves.

Bivalves

^{A variety of fossil mollusks, mostly of bivalves. Colonial National Historical Park.
NPS photo by Mackenzie Chriscoe.}

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

^{Bivalves in cross section. Great Basin National Park.
NPS photo by Gordon Bell.}

^{Retroceramus porrectus, a large bivalve, is common in Lake Clark National Park & Preserve.}

Permian bivalves. Grand Canyon-Parashant National Monument.

NPS/2020 Field Inventory.

Bivalve shells are among the most common invertebrate fossils found in national parks, being documented in at least 117 park units. Bivalves have been common since the Paleozoic and they have inhabited marine, brackish, and freshwater environments.

Bivalves are part of Paleozoic marine assemblages in many parks, although they aren’t as abundant as the outwardly similar brachiopods. Grand Canyon National Park and Grand Canyon-Parashant National Monument in Arizona have unusually ornamented Paleozoic scallops. The unusually large Silurian bivalve Pycinodesma giganteum was named from Glacier Bay National Park and Preserve in Alaska. Other notable Paleozoic bivalve assemblages have been found in Permian rocks in Carlsbad Caverns National Park in New Mexico and Guadalupe Mountains National Park, Texas, and Ordovician rocks in Mississippi National River and Recreation Area in Minnesota.

For the Mesozoic, significant bivalve assemblages are known from Jurassic and Cretaceous rocks in Alaska parks, including Katmai National Park and Preserve and Lake Clark National Park and Preserve. In the contiguous United States, Mesozoic marine sedimentary rocks are not as prevalent as Paleozoic ones. But marine incursions in the Jurassic and Cretaceous left rocks with abundant marine bivalves in several parks, such as Devils Tower National Monument in Wyoming and Yellowstone National Park in Idaho, Montana, and Wyoming (Jurassic), and Chaco Culture National Historical Park in New Mexico and Mesa Verde National Park in Colorado (Cretaceous).

Freshwater river mussels of the family Unionidae are common in Mesozoic rocks of several parks, such as Petrified Forest National Park in Arizona (Upper Triassic) and Dinosaur National Monument in Colorado and Utah (Upper Jurassic).

Bivalves are also abundant in Cenozoic rocks and deposits in the parks. Chesapecten jeffersonius from Colonial National Historical Park in Virginia, the site of the first permanent English settlement in North America, is the first published fossil from a NPS unit, with a scientific illustration of this Pliocene scallop published in 1687. Castillo de San Marcos National Monument in Florida contains the oldest masonry fort in the continental United States, which is also a fort made predominantly of an unusual building material: fossils. Coquina, a sedimentary rock made of small fragments of fossil bivalves, was its main building stone. Channel Islands National Park in California has an excellent bivalve record, predominantly from the Miocene and Quaternary, from all five islands. Freshwater bivalves are abundant in the Paleocene rocks of Theodore Roosevelt National Park in North Dakota, part of an ecosystem that existed a few million years after the great extinction at the end of the Cretaceous. Other significant Cenozoic bivalve records are known from Santa Monica Mountains National Recreation Area in California and Vicksburg National Military Park in Louisiana and Mississippi.

Cephalopods – Nautiloids, Ammonites, & Belemnites

^{Nautiloid fossil. Hot Springs National Park.}

^{Late Cretaceous fossil ammonites. Cliff House Sandstone Chaco Culture National Historical Park.
NPS photo by Phil Varela.}

^{Straight (orthoconic) nautiloid in Buffalo National River. Silurian.
Photo by the Arkansas Geological Survey.}

^{Middle Ordovician orthoconic nautiloid from the Lehman Formation. Great Basin National Park.
NPS photo by Gordon Bell.}

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

Paleozoic nautiloid. Mississippi National River and Recreation Area.

NPS photo by Justin Tweet.

Cephalopods are a diverse group of marine mollusks that are mobile predators with tentacles and large eyes. Today most cephalopods are octopuses or squids, both of which have poor fossil records because of their soft bodies. However, in the past many cephalopods had shells or large internal structures that easily fossilized. These cephalopods included:

• Nautiloids

• Ammonites

• Belemnites

Cephalopod fossils have been documented in at least 65 parks.

Nautiloids were dominant marine predators of the early Paleozoic and two species still survive today. Nautiloids have large external chambered shells. Both living species have shells that are coiled in a flat spiral, but many Paleozoic species had shells that were straight or gently curved. Some species could reach lengths of several meters. Nautiloid Canyon in Grand Canyon National Park is a popular stop during raft trips on the Colorado River. Straight (orthoconic) nautiloids are known from many parks with Paleozoic marine rocks. Some of the best nautiloid records in the NPS can be found at Chesapeake & Ohio Canal National Historical Park in Maryland and Mississippi National River and Recreation Area in Minnesota. Nautiloid fossils are much rarer after the Paleozoic.

Ammonites (more properly ammonoids) are found in Paleozoic and Mesozoic rocks, but they were dominant during the Mesozoic. Like nautiloids, ammonites had chambered shells, but they were actually more closely related to squids. Ammonite fossils can often be differentiated from nautiloids by more complex suture lines between the chamber walls of their shells. The classic ammonite shape is a flat tight spiral, but some ammonites took on many unusual forms, from open spirals to straight shells to “squiggles.” Cliffs along the Rio Grande in Big Bend National Park in Texas are studded with Cretaceous ammonites. Fossil Point in Lake Clark National Park and Preserve in Alaska has a great diversity of ammonites, some of which reach large sizes. The Cretaceous Pierre Shale at Badlands National Park in South Dakota yields lovely ammonites with mother-of-pearl coatings.

Belemnites were squid-like animals with internal hard parts that lived during the Mesozoic. The elongated bullet-shaped guard that extended to the rear of the body cavity is the most likely part to have been fossilized. Belemnites are abundant in Devils Tower National Monument in Wyoming, and are found in Katmai National Park and Preserve and Lake Clark National Park and Preserve, both in Alaska.

Learn More

• Grand Canyon—Coiled Nautiloid

Gastropods

^{Gastropods. Channel Islands National Park.
NPS photo by Justin Tweet.}

^{Fossil gastropods from the Cliff House Sandstone. Chaco Culture National Historical Park.
NPS photo by Phil Varela.}

^{A selection of gastropods from Great Basin National Park.
All photos NPS by Gorden Bell.}

Silicified Permian gastropod in Grand Canyon-Parashant National Monument.

Gastropods include snails and slugs. Slugs have no shells or only small vestigial shells and have a poor fossil record. But snail shells are among the most common invertebrate fossils reported in national parks. Gastropod fossils have been documented in at least 118 park units.

Most parks with Paleozoic marine rocks have at least a few species of fossil snails that can be easily distinguished by shape. Some gastropods in the Kaibab Formation in Grand Canyon-Parashant National Monument in Arizona have been replaced with red silica, making striking fossils. The Cretaceous Cliff House Sandstone in Chaco Culture National Historical Park in New Mexico contains a diverse assemblage of fossil gastropods. At Colonial National Historical Park in Virginia, gastropods are the second most common type of fossil, behind bivalves. Terrestrial gastropods are present in Quaternary sediments including loess in Vicksburg National Military Park in Mississippi and Louisiana, to go along with significant Oligocene marine gastropods.

Other Mollusks

Dentalium, an Oligocene scaphopod. Vicksburg National Military Park.

NPS photo by Charles Beightol.

There are several other groups of mollusks known from the fossil record, living and extinct. Those most frequently found in NPS units are scaphopods and chitons, which can be minor constituents of marine invertebrate faunas.

Scaphopods (also known as tusk shells for their shape) have been recorded in at least 31 park areas, with good examples known from Oligocene rocks at Vicksburg National Military Park in Louisiana and Mississippi and Pliocene rocks at Colonial National Historical Park in Virginia. Scaphopods live within the sediment on the ocean floor and have shells that are shaped like a hollow tube that are tapered at one end.

Fossils of chitons, which are small mollusks covered by eight plates, have been found in seven parks. By far the best record of chiton fossils in the National Park System is in the Quaternary deposits of Channel Islands National Park, California.

Coloring Book Pages

Related Parks

• Big Bend National Park (BIBE), Texas—[BIBE Geodiversity Atlas] [BIBE Park Home] [BIBE npshistory.com]

• Carlsbad Caverns National Park (CAVE), New Mexico—[CAVE Geodiversity Atlas] [CAVE Park Home] [CAVE npshistory.com]

• Castillo de San Marcos National Monument (CASA), Florida—[CASA Geodiversity Atlas] [CASA Park Home] [CASA npshistory.com]

• Chaco Culture National Historical Park (CHCU), New Mexico—[CHCU Geodiversity Atlas] [CHCU Park Home] [CHCU npshistory.com]

• Channel Islands National Park (CHIS), California—[CHIS Geodiversity Atlas] [CHIS Park Home] [CHIS npshistory.com]

• Chesapeake and Ohio Canal National Historical Park (CHOH), Washington DC, Maryland, and West Virginia—[CHOH Geodiversity Atlas] [CHOH Park Home] [CHOH npshistory.com]

• Colonial National Historical Park (COLO), Virginia—[COLO Geodiversity Atlas] [COLO Park Home] [COLO npshistory.com]

• Devils Tower National Monument (DETO), Wyoming—[DETO Geodiversity Atlas] [DETO Park Home] [DETO npshistory.com]

• Dinosaur National Monument (DINO), Colorado and Utah—[DINO Geodiversity Atlas] [DINO Park Home] [DINO npshistory.com]

• Glacier Bay Park and Preserve (GLBA), Alaska—[GLBA Geodiversity Atlas] [GLBA Park Home] [GLBA npshistory.com]

• Grand Canyon National Park (GRCA), Arizona—[GRCA Geodiversity Atlas] [GRCA Park Home] [GRCA npshistory.com]

• Grand Canyon-Parashant National Monument (PARA), Arizona—[PARA Geodiversity Atlas] [PARA Park Home] [PARA npshistory.com]

• Great Basin National Park (GRBA), Nevada—[GRBA Geodiversity Atlas] [GRBA Park Home] [GRBA npshistory.com]

• Guadalupe Mountains National Park (GUMO), Texas—[GUMO Geodiversity Atlas] [GUMO Park Home] [GUMO npshistory.com]

• Katmai National Park and Preserve (KATM), Alaska—[KATM Geodiversity Atlas] [KATM Park Home] [KATM npshistory.com]

• Lake Clark National Park and Preserve (LACL), Alaska—[LACL Geodiversity Atlas] [LACL Park Home] [LACL npshistory.com]

• Lake Mead National Recreation Area (LAKE), Arizona and Nevada—[LAKE Geodiversity Atlas] [LAKE Park Home] [LAKE npshistory.com]

• Mammoth Cave National Park (MACA), Kentucky—[MACA Geodiversity Atlas] [MACA Park Home] [MACA npshistory.com]

• Mesa Verde National Park (MEVE), Colorado—[MEVE Geodiversity Atlas] [MEVE Park Home] [MEVE npshistory.com]

• Mississippi National River and Recreation Area (MISS), Minnesota—[MISS Geodiversity Atlas] [MISS Park Home] [MISS npshistory.com]

• Petrified Forest National Park (PEFO), Arizona—[PEFO Geodiversity Atlas] [PEFO Park Home] [PEFO npshistory.com]

• Santa Monica Mountains National Recreation Area (SAMO), California—[SAMO Geodiversity Atlas] [SAMO Park Home] [SAMO npshistory.com]

• Theodore Roosevelt National Park (THRO), North Dakota—[THRO Geodiversity Atlas] [THRO Park Home] [THRO npshistory.com]

• Vicksburg National Military Park (VICK), Louisiana and Mississippi—[VICK Geodiversity Atlas] [VICK Park Home] [VICK npshistory.com]

• Yellowstone National Park (YELL), Idaho, Montana, and Wyoming—[YELL Geodiversity Atlas] [YELL Park Home] [YELL npshistory.com]

Body Fossils