Fossils of the Coastal Plain

Paleozoic fossils

There are no rocks at the surface in Florida older than Eocene in age (approximately 55 million years old), but older rocks and fossils have been found in cores drilled deep below the surface. The oldest of these rocks dates to the early Ordovician period, about 480 million years ago, as determined by index fossils such as graptolites and brachiopods. The only trilobite ever found in Florida is a single specimen from one such core, drilled in Madison County, at a depth of around 1570 meters (4630 feet). It was described by Harry Whittington in 1953 and is known today as Plaesiacomia exsul.

Mesozoic fossils

Mesozoic marine fossils

During the Cretaceous, the Western Interior Seaway stretched across the center of North America from the Gulf of Mexico in the south to the Arctic Ocean in the north, and from the Rocky Mountain states in the west to Iowa in the east. Chalk was deposited in some areas of this seaway. Chalk is mostly made up of the calcium carbonate plates (coccoliths) formed by microscopic marine algae called coccolithophores.

Today, chalky sediments accumulate mainly in the deep sea. During the Cretaceous, when sea levels were much higher than they are today, thick chalk deposits accumulated in the shallow continental seas. The Cretaceous Period, in fact, is named for the abundance of chalk that was deposited during this time. (The Latin word for chalk is creta.) Cretaceous chalk is common across much of central Alabama. Chalk deposits also occur in northeastern Mississippi.

Cretaceous marine sediments of the Coastal Plain are often rich in both invertebrate and vertebrate fossils. Mollusks are especially common, including gastropods (snails), bivalves (clams and relatives), and cephalopods (the group including octopuses and squid), such as ammonites and belemnites. Crabs are also sometimes locally abundant. Vertebrates include sharks and marine reptiles, such as mosasaurs and, more rarely, plesiosaurs.

Mesozoic terrestrial fossils

Dinosaur fossils are not common in Coastal Plain sediments, thanks to the absence of Cretaceous-aged terrestrial deposits. Nevertheless, dinosaurs have been found in almost every state in the region. They include both herbivores (mostly hadrosaurs or "duckbills") and carnivores (theropods). Coastal Plain dinosaur fossils come from marine sediments and were preserved when dinosaur carcasses floated out to sea. Most of these fossils are isolated bones or partial skeletons, making them difficult to identify. Perhaps one of the most interesting dinosaur finds is an Ornithomimus egg from Harrell Station fossil site in Alabama. 

Other reptile groups are also known from Cretaceous sediments of the Coastal Plain. Pterosaurs (flying reptiles) have been found in North Carolina and Georgia. The giant crocodilian Deinosuchus, which reached lengths of up to 12 meters (39 feet), lived on both eastern and western sides of the Western Interior Seaway during the Cretaceous Period. In the eastern United States, it is found from New Jersey to Alabama.

Despite the lack of terrestrial sediments in the Coastal Plain, fossils of Cretaceous land plants are known from a number of localities. These deposits formed in nearshore environments where land plants washed into coastal marshes. They include fossil leaves, fruits, and flowers. Some plant fossils, like those from the Ingersoll Shale of eastern Alabama, are extremely well preserved. Tiny fossils of flowers and fruits have also been found in Cretaceous sediments in Virginia and Georgia. Late Cretaceous amber (fossil tree resin) is known from several places, including Russell County, Alabama, and Tishomingo County, Mississippi.

Cenozoic fossils

Cenozoic marine Fossils

Marine invertebrates

Paleogene sediments of the Coastal Plain have been studied since the early 1800s, longer than almost any other fossil-bearing sediments in the Americas. Coastal Plain deposits are often very fossiliferous and some are among the most famous Paleogene fossil beds in the world. 

Fossil mollusks—mainly clams and snails—of Paleocene, Eocene, and Oligocene age occur across the Gulf and Atlantic Coastal Plains, from Texas to Maryland. They are frequently found in beautifully preserved shell beds that may contain hundreds of different species. For example, the Middle Eocene Gosport Sand in southwestern Alabama contains more than 500 species of fossil mollusks. In total, more than 3000 species of mollusks have been described from the Paleogene of the Coastal Plain.

The Gulf and Atlantic coasts of North America are passive margins, formed when the supercontinent Pangaea broke apart between 200 and 100 million years ago. The continents did not break smoothly along a straight line, but rather along an irregular line with curves pointing outward (promontories) and inward (embayments). Embayments are semi-contained environments. In the Neogene, each embayment supported its own particular types of organisms.

As the Cenozoic progressed, siliciclastic sediments eroded from the land surface and were deposited in the embayments along the Atlantic coast. Geologists and paleontologists studying the Neogene geology of the Atlantic Coastal Plain frequently study the area’s sediments and fossils one embayment at a time, then attempt to connect them together to form a larger story. Shallow marine deposits from Neogene embayments contain many abundant and diverse fossil beds. They yield mollusks (clams and snails), corals, barnacles, and many other organisms.

Florida is distinct from the rest of the coastal plain because the sediment includes more carbonate. Florida's fossil-bearing deposits vary from shelly sand in the northern and central parts of the state to reef limestone in the far southern part of the state and throughout the Florida Keys. Fossils range from Paleogene to Pleistocene in age.

Sharks and rays

Sharks and rays have skeletons made of cartilage instead of bone, which means that their skeletons are rarely found as fossils. The teeth of sharks and rays, however, are much more resistant. They are common fossils in Cretaceous and Cenozoic sediments throughout the Coastal Plain. Shark and ray teeth are often found on beaches, where they are washed up from offshore or washed down rivers from uplands.

Sharks shed their teeth constantly, and a single individual can produce as many as 35,000 teeth during its lifetime. You can tell the difference between fossil and modern shark teeth by their color. Fossil teeth are often brown, gray, or black, whereas modern teeth are tan to white. Shark teeth come in many forms and sizes, and not all teeth in the mouth of a single individual look alike.

The largest shark that ever lived was a species related to the modern great white shark, but much bigger. Otodus megalodon—frequently called just "megalodon"—lived in the western Atlantic Ocean from approximately 16 to 2.6 million years ago (from the middle Miocene to the late Pliocene). Megalodon is regarded as one of the largest and most powerful ocean predators that ever lived.

The fossil record of megalodon consists almost entirely of teeth, which are common fossils in the Coastal Plain. Because of the lack of body fossils, size estimates for this shark vary widely. Nevertheless, a reasonable estimate is that megalodon reached lengths of up to 18 meters (59 feet). It likely fed on large whales.

Rays and skates are close relatives of sharks and also have a fossil record that consists largely of teeth. Many ray species feed on hard-shelled prey, such as clams that they dig up out of sand bottoms. Their teeth therefore frequently have flat, ridged surfaces suitable for grinding. Ray teeth are common fossils found on beaches throughout the Coastal Plain. Sometimes, larger pieces of intact dental plates can be found.

Marine mammals

Whales are most closely related to artiodactyls, or the even-toed hoofed mammals (for example, bison, deer, and hippopotamuses). Whales evolved from four-legged mammals that lived on land during the Eocene epoch, beginning around 55 million years ago. The early ancestors of whales were animals that resembled wolves. Fossils—especially from Egypt and Pakistan—have revealed many extinct forms that show the evolutionary steps in the remarkable transition from land-dwelling to truly aquatic animals.

The earliest whales are from the early to middle Eocene. Relatively small forms, called protocetids (a name meaning "early whales"), have been found in Eocene sediments of the Gulf and Atlantic coastal plains. The most completely known is Georgiacetus vogtlensis. It was about 3.4 m (11 ft) long and had four legs. Its skeleton shows that its rear legs probably could not have been used for walking on land. Examples of other protocetids from the Coastal Plain include Carolinacetus, Natchitochia, and Tupelocetus.

Basilosaurids (meaning "king lizards," because they were originally mistaken for reptiles) are mostly from the middle to late Eocene. These are the earliest animals that look whale-like. They could be very large—up to about 16 m (52.5 ft) long—and had elongated bodies with tiny rear legs. A small number of species lived in warm, low-latitude seas around the world. Basilosaurids became extinct in the early Oligocene. 

Basilosaurids were first discovered in Eocene sediments of the Coastal Plain during the early 1800s. Examples of species found in the southeastern United States include Basilosaurus cetoides, Dorudon serratus, and Zygorhiza kochii. Their large vertebrae and multi-cusped teeth with distinctive, yolk-shaped roots can be found from Maryland to Mississippi.

The two major groups of modern whales evolved from basilosaurids. The toothed whales or odontocetes (meaning "toothed whales") include dolphins, porpoises, and the sperm whale. The toothless baleen whales or mysticetes include the large, plankton-feeding whales such as humpback whales, blue whales, and right whales. Fossils of whales and dolphins can be found in Miocene and younger sediments along much of the Atlantic Coastal Plain and Gulf Coast of Florida.

Manatees and dugongs belong to a group of aquatic mammals called sirenians. Sirenians are not closely related to whales. Among living animals, they are most closely related to elephants. One species of manatee lives in Florida's rivers today. Many kinds of sirenians, however, are represented by fossils found in the Coastal Plain. They are especially found in Florida, where they are the most common fossil mammal.

Cenozoic Terrestrial Fossils

Birds

Many species of Pliocene birds are found in Florida. Most of these are marine birds, like cormorants. Hundreds of skeletons of one type of cormorant (Phalacrocorax filyawi) were found together in the Pinecrest Beds of Florida. The cormorants were probably killed by a single event, and then their bodies were deposited by a storm. One possible cause for the death of so many cormorants at the same time is a red tide. A red tide is a bloom of marine algae that produce toxins capable of killing vertebrates. Cormorants are often one of the types of birds affected by modern red tides.

Some Pliocene birds are among the largest flightless birds that have ever lived. Waller's terror bird (Titanis walleri) is one of the largest known terror birds, an extinct group of flightless, carnivorous birds. Most terror birds lived in South America during the Cenozoic Era. Waller's terror bird was the only terror bird that lived in North America and was probably the last member of its lineage. Bones of this giant bird are found at several sites in Florida.

Mammals

Paleogene mammals are not common in the Coastal Plain, but several species have been identified from central Mississippi. A primitive primate and an early horse are known from the late Paleocene to early Eocene and an early rhinoceros from the Oligocene. Fossils of Neogene mammals are abundant and diverse in the Coastal Plain. Florida has the richest fossil record of terrestrial mammals in the eastern United States.

Fossils of Pleistocene mammals are especially common throughout the Coastal Plain, from horse teeth on the coasts of the Carolinas to the bones of giant ground sloths, tapirs, horses, bison, and mastodons on the banks of the Mississippi River near Vicksburg, Mississippi. The Leisey Shell Pit in Hillsborough County, Florida (near Tampa Bay), has produced not only fossil shells, but was also one of the most important early Pleistocene fossil bone beds in the state (the quarry is now closed and flooded). Fossils of many species of mammals and birds have been found there, including saber-toothed cats, tapirs, horses, and llamas. 

Among the most common Pleistocene vertebrate fossils in the Southeast are those of mastodons and mammoths. Both are common in the Southeast and elsewhere in North America during the Pleistocene. Because they had different ecological preferences, and their fossils are usually found separately. Mastodons were browsers that fed on trees and shrubs, whereas mammoths were grazers that ate grass. (Learn more on the Earth@Home Mammoth vs. Mastodon page.)

Plants

Cenozoic plant fossils are surprisingly common across the Coastal Plain. Fossil leaves are especially abundant in deposits of early Eocene age in the Mississippi Embayment, which includes parts of Alabama, Mississippi, Arkansas, Louisiana, Texas, Kentucky, and Tennessee. Some of the most well-known Eocene plants come from clay pits in Kentucky and eastern Tennessee. These fossils are from the Claiborne Group. They include not only leaves, but also flowers and fruits. 

Plant fossils occasionally form localized deposits of brown coal, called lignite. Petrified wood also occurs in a number of places in the Coastal Plain. Mississippi even has its own petrified forest in the town of Flora. The forest is privately owned but is open to the public as a tourist attraction. It is about 36 million years old, or near the Eocene-Oligocene boundary in age.

Neogene plant fossils are scattered across the Coastal Plain, although only a few major plant fossil deposits have been described. The Hattiesburg flora from Mississippi and the Alum Bluff flora from the Florida panhandle are Miocene in age. The Pliocene Citronelle flora is known from several different locations in the Citronelle Formation of Alabama and Florida. Many of the plants in these Neogene floras still inhabit the United States, although a few types of plants are found only in Eurasia today. Examples include the wingnuts (Pterocarya), hong teng (Sargentodoxa), and crown of thorns (Paliurus).