Fossils Found at Fort Pulaski National Monument

Kelly Cronin, Christy Visaggi and Katy Smith at Fort Pulaski National Monument, May 2023. National Park Service photo.

A fort on old sediments

Fort Pulaski stands at the mouth of the Savannah River in Chatham County, Georgia. It was built after the War of 1812 to guard the port of Savannah, and its thick masonry walls, raised in the first half of the 19th century, were thought impossible to breach. Confederate troops held it during the Civil War until 1862, when U.S. forces retook it with rifled cannon, a new weapon that could break its walls. President Calvin Coolidge established Fort Pulaski National Monument in 1924. Besides the fort, it protects salt marshes on Cockspur and McQueens islands, and the layers beneath the islands record millions of years of coastal Georgia's marine, estuarine and land environments.

The monument is not known for fossils, but it sits in promising ground. Parts of the fort were built from local shell, and fossils had turned up in cores and construction ditches on Cockspur Island. Fossil hunters know the Savannah River area for shark teeth, including those of megalodon (Otodus megalodon), and nearby islands have yielded marine invertebrates and Pleistocene mammals — among them some of the first giant ground sloth fossils described from North America, in 1824. Until recently, though, nearly all the region's known fossils came from outside the park.

At the park's request, and with support through a partnership between the National Park Service and the American Geoscience Institute, three paleontologists from Georgia State University and Georgia Southern University surveyed the monument for fossils in 2023 — its first field-based paleontological inventory.

What lies beneath Cockspur Island

A 1,017-foot core that the U.S. Geological Survey drilled on the island in 2010 passed through sediments from 43 million years old to the present. From the bottom up:

The upper layers under Cockspur Island and the depths of known fossil beds, adapted from Swezey and others (2018). Image from the National Park Service's page.

Some fossil beds were already known underground. A Georgia Geological Survey core at the monument yielded foraminifera — single-celled organisms with shells — and other microfossils from the early Pliocene Coosawhatchie Formation. Digging for the fort exposed a layer 15 to 20 feet down with estuarine diatoms, phytoliths (bits of silica formed in plant cells), and fragments of sponges and other marine invertebrates. Mollusk shells, especially oysters, are abundant in several places; the fort's mortar was made of lime, sand and oyster shell, probably from Native American oyster middens near the river.

The 2023 finds

Led by Christy Visaggi, with Kelly Cronin and Katy Smith, park archeologist Laura Seifert and park staff, the team looked especially for fossils common nearby but never recorded in the park, such as shark teeth and Pleistocene bones. The richest spot was a beach of dredge spoils placed to protect the fort's cultural resources. It is closed to the public because of strong tides, currents and big wakes from cargo ships; the closure also protects shorebird and migratory bird roosting, feeding and nesting sites, and may help preserve the fossils.

Georgia paleontologists and National Park Service staff surveying for fossils at the monument. National Park Service photo.

Marine fossils included teeth of requiem sharks (Carcharhinus) and lemon sharks (Negaprion), whose fossil records reach back to the Late Eocene; white sharks (Carcharodon), back to at least the Middle Miocene; and tiger sharks (Galeocerdo), also to the Middle Miocene. The team also found ray tooth plates and barbs, a drum fish tooth plate and the mouth plate of a diodontid fish.

Marine vertebrate fossils from the survey. National Park Service photo.

Land mammals included several Pleistocene species: cheek teeth of an extinct horse (Equus) and a piece of proboscidean tusk, probably from an American mastodon (Mammut americanum). Another bone fragment may belong to a further Pleistocene giant, but its identity is not yet confirmed.

A lower cheek tooth of an extinct horse, Equus sp., seen from its chewing surface. National Park Service photo.

A cross-section of fossil tusk, probably from an American mastodon. National Park Service photo.

All these fossils were found loose, not in place, so none can be tied to a particular layer. Many of the species span the ages of the formations beneath the park, and the fossils could also come from other young units exposed nearby. They were probably washed in by storms or brought up by dredging of the Savannah River shipping channel. Even out of context, they are new records for the monument and add to what is known of Georgia's fossils.

More to come

The 2023 finds are likely just a first sample. The Army Corps of Engineers dredges the river twice a year, and every dredging, storm or deepening of the channel could bring more fossils ashore, as could digging or construction on the island. Further surveys and monitoring could reveal more, and the fossils open the way to interpretive programs on the park's prehistory as well as further research.

Sources

• National Park Service, Park Paleontology News 16(1), Spring 2024: "Uncovering the Fossil Resources of Fort Pulaski National Monument," by Kelly E. Cronin, Christy C. Visaggi, Kathlyn M. Smith and Laura Seifert, citing Swezey and others (2018), Weems and Edwards (2001), Tweet and others (2009) and Mitchell (1824).
• The source spells the Coosawhatchie Formation two ways; this page uses its first spelling.