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Cretaceous Seas

Fossils from the banks of the Great Pee River in Florence county reveal life from the late Cretaceous Period, the last period of the age of dinosaurs. These Pee Dee sedimentary rock layers are one of the few windows to Cretaceous life in South Carolina. (South Carolina State Museum)

Text provided courtesy of the South Carolina State Museum:

Cretaceous Ocean Levels

During the Cretaceous Period the Earth’s climate was warmer, and the ocean level much higher, than today. Broad, shallow seas covered many areas that are now land. Continental drift, undersea mountain formation and melting polar ice contributed to the higher sea levels. The town of Florence was once near the coast, and a good portion of the Pee Dee’s coastal plain was underwater.
In South Carolina the sea level may have reached 1,000 feet above today’s level. Scientists can determine past ocean levels by studying the Pee Dee rock layers, which were form from deposits of the ancient seas.

Reading Rocks

Rock layers, or “strata,” were laid down in different time periods. This is why the oldest layers are always at the bottom and the youngest at the top. When scientists study past environments by examining rock strata, they are performing stratigraphic interpretation.

How old are these Fossils?

Exogyra costata

An index fossil, such as the Exogyra costata, can be used to determine the age of rock layers. Exogyra lived from 70 to 100 million years ago. When scientists find this fossil, they know the surrounding rock layers date to the period.

The hard shell of the marine mollusk Exogyra protected it from predators and helped preserve it as a fossil. Marine animals are especially useful index fossils because of their wide distribution.

Short-necked Plesiosaur (Cimoliasaurus magnus)

Because these marine reptiles preferred deep water, few of their fossils are found in the Pee Dee deposits at this site. Some plesiosaurs grew to 40 feet long, but the species found in South Carolina was a smaller variety. It was adapted for powerful, long swims with a streamlined body and flippers designed like wings for underwater flight. Fossil evidence indicates the plesiosaur fed on belemnites.

Early Crocodiles (Thoracosaurus neocesariensis)

The Cretaceous crocodile looked like its modern counterpart, found only in tropical regions today, and probably had similar habits. Crocodiles hunt mostly in rivers and brackish water, preying on turtles, fish, wading birds and mammals. The presence of crocodile fossils indicates this site was once a coastal mud flat.

Softshell Turtle (Trionyx sp.)

The turtle’s soft, leathery shell offered little protection from tidal-mud-flat predators like the crocodile. To escape, the turtle buried its body in the sandy bottoms of shallow tidal streams, keeping its nostrils above the water.

Cretaceous Bony Fish (Enchodus ferox)

This extinct bony fish commonly swam in warm open oceans. Since its body was not designed to withstand water pressure below 600 feet, it hunted in the upper waters of the open ocean. Occasionally it would swim into inland seas like South Carolina’s Cretaceous sea. This relative of trout and salmon used its large pointed teeth for biting and slicing. It grew to a length of 1 to 3 feet.

Crow Sharks (Squalicorax sp.)

These extinct predators grew to an average size of 9 to 13 feet. They probably hunted for prey in shallow coastal waters, occasionally entering inlets and large rivers.

Belemnite (Belemnitella Americana)

The extinct Belemnite’s internal shell, or “guard,” helped the squid-like cephalopod to ascend and descend in the water. When the hollow chamber in the end of the guard was empty, the guard served as a weight, and the animal descended. When the chamber filled with air, the animal ascended. If attacked, the Belemnite put up a smoke screen from its ink sac. Oxygen isotopes in the calcite molecules of the fossilized guards tell paleontologists that the Belemnite lived in a tropical sea.

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