Fossils of the Coastal Plain

Introduction

The sediments that accumulated to form the Coastal Plain of Texas and Louisiana were deposited by numerous rivers that drained the land to the north and northwest. This process began in the Late Cretaceous, and the Mississippi River continues to drain the central US today, contributing to the coastline of the Gulf of Mexico. Late Creatceous sediments define the edge of the Coastal Plain, forming an irregular northeast-southwest-oriented band farthest from the modern coastline. Roughly parallel bands of sediments of decreasing age outcrop toward the coast, with the youngest sediments accumulating right at the shore today. In a few places on the edge of Coastal Plain, Early Cretaceous rocks that yield fossils are exposed.

The northward extension of the Coastal Plain into regions far from the Gulf Coast today (like Missouri and Arkansas) is called the Mississippi Embayment. This region was a low spot (basin) that was flooded during the Cretaceous, and sediments of Cretaceous age and younger have been filling in the basin since Cretaceous time.

Mesozoic fossils

Cretaceous marine fossils

In Texas and Oklahoma, fossil-bearing Cretaceous sediments are found from the base of the Edwards Plateau near the Mexican border to Dallas and into the sliver of Coastal Plain on the eastern half of the Oklahoma-Texas border. These Cretaceous sediments are frequently rich in fossils, including shark teeth and ammonites, both the usual coiled forms and unusual irregular or uncoiled forms known as heteromorphs. Oysters and other bivalves (including Inoceramus) and echinoids are also present, as well as rare mosasaur and plesiosaur remains. 

The line of Late Cretaceous layers continues along the landward edge of the Coastal Plain into Arkansas, bordering the Interior Highlands. This boundary forms a line from the southwest to northeast corners of the state. Cretaceous layers, however, only crop out in the corners of Arkansas, as they are buried by younger sediment in the state’s center. Cretaceous strata contain large clams and bivalves, ammonites, gastropods, echinoids, shark teeth, and the occasional marine reptile.

A few pockets of Cretaceous-aged rock are also found in Louisiana, where salt domes have pushed up through the overlying Cenozoic sediment, occasionally carrying Cretaceous shark teeth to the surface.

Cretaceous dinosaurs

Dinosaurs of Oklahoma and Texas

The Texas-Oklahoma border area yields rare dinosaur skeletons, most spectacularly the gigantic sauropod (long-necked, herbivorous dinosaur) Sauroposeidon proteles, known from the Early Cretaceous of Oklahoma and Texas. Skeletal remains of the predatory theorpod dinosaur Acrocanthosaurus atokensis have also been found in the Early Cretaceous of southeastern Oklahoma and Texas. Dinosaur footprints that are common in Cretaceous rocks near the border between the Coastal Plain and the Great Plains regions in eastern Texas may have been made by Sauroposeidon, Acrocanthosaurus, or similar dinosaurs (see examples on the South-central U.S.: Fossils of the Great Plains page). A variety of other dinosaur remains have been found in this region, including those of tyrannosaurids.

Dinosaurs of Arkansas and Missouri

Rare dinosaurs are also known from Arkansas and Missouri. The Early Cretaceous dinosaur Arkansaurus fridayi, a small theropod (predatory dinosaur with three-toed feet), was named the state dinosaur of Arkansas in 2017. Parrosaurus missourenisis, a duck-billed dinosaur, comes from the Late Cretaceous Chronsiter Vertebrate Site in southeastern Missouri. Tail vertebrae belonging to the dinosaur were first discovered at the site in the 1940s, when Lula Chronister owned the land. Interest in and excavations at the Chronsiter Vertebrate Site were rekindled in the 1970s, with parts of dinosaur skeletons recovered from the site as recently as 2021. In addition to Parrosaurus, Albertosaurus (a carnivorous dinosaur) teeth and turtle remains have been found at the site. 

Cenozoic fossils

Paleogene

The great majority of sediment exposed in the Coastal Plain is Cenozoic in age. Paleogene-aged strata are, for the most part, sandwiched between the western rim of Cretaceous deposits, and the youngest layers of Neogene and Quaternary sediment, which are found nearest the Gulf and along the Mississippi River Valley. Paleogene marine deposits in Texas, southern portions of Arkansas, and northern parts of Louisiana are abundantly fossiliferous, yielding shark teeth, corals, clams, snails, and the bones of early whales.

Abundant tiny fossils called foraminifera are found throughout the Coastal Plain’s Cretaceous and Cenozoic sediments. Foraminifera, or “forams,” as they are frequently called, are single-celled organisms (protists) with shells made of calcium carbonate. They live in the ocean in huge numbers, both at the bottom and floating in the water column, and are extremely important as index fossils and paleoenvironmental indicators in Cretaceous and Cenozoic sediments.

Paleogene whales

Whales are most closely related to artiodactyls, or the even-toed hoofed mammals (for example, bison, deer, and hippopotamuses). Whales evolved from four-legged animals that resembled wolves and lived on land during the Eocene epoch, beginning around 55 million years ago.

The earliest whales are from the early to middle Eocene. Relatively small forms, called protocetids (a name meaning "early whales"), have been found in the middle Eocene sediments of the Coastal Plain. In the south-central region of the U.S., the protocetid Natchitochia is known from Louisiana, and protocetid bones have also been found in the Eocene Yegua Formation of Texas.

Another type of ancient whale known from the Gulf Coast is Basilosaurus. Richard Harlan first described Basilosaurus in 1843. He believed that the few bones he found belonged to a marine reptile (hence, the name that means "king lizard"). Basilosaurus was a member of an early group of whales known as basilosaurids, which included the ancestors of the two major groups of modern whales. These are the toothed whales or odontocetes (for example, porpoises and sperm whales), and the baleen whales or mysticetes (for example, humpback whales and blue whales).

Protocetids and basilosaurids inhabited the equatorial oceans during the Paleocene and Eocene. The vertebra pictured below was found in Arkansas in 1945, and is one of the northernmost early whale fossils ever found.

Paleogene terrestrial fossils

Plant fossils occasionally occur in the Paleogene of the coastal plain in Louisiana and Texas. Fossil fruits and seeds are known from the Whitsett Formation in southeastern Texas. They include some types of plants that grow in the Coastal Plain today, like tupelo (Nyssa). Others are found only in Eurasia, like Yua, a member of the grape family found today in eastern to southeastern Asia. Leaf fossils are also known from Louisiana and Texas.

A small deposit of Eocene amber occurs in the Claiborne Group near Hot Springs, Arkansas. Insects are preserved in this amber, including members of the groups that include beetles, flies, and ants.

Neogene

Terrestrial deposits in eastern Texas, dating from the early Miocene (e.g., Fleming Formation, exposed near Toledo Bend Reservoir; Lampara Creek Fauna, southeastern Texas), include abundant bones and teeth of amphibians, reptiles, and rodents. Larger mammals are also present, including carnivores, tapirs, pigs, rhinos, horses, and bizarre extinct forms such as the large herbivore Moropus and the gomphothere Blancotherium. Miocene vertebrate remains also come near Fort Polk in western Louisiana.

The Coastal Plain’s Miocene sediments also contain petrified wood, including palm (Palmoxylon), the Texas state rock and Louisiana state fossil.

Quaternary

By the Quaternary period, the glaciation of North America was underway. Many Neogene mammals had gone extinct by this time, yet mastodons, horses, and camels remained common. Fossils from this time are found on the Coastal Plain of Texas and Louisiana, and in the Mississippi River Valley in Louisiana, Arkansas, and the southeastern-most corner of Missouri. Pleistocene manatees are also known from Texas. 

The youngest deposits of the Coastal Plain―for example, in the counties around Corpus Christi, Texas―contain the fossils of camels, saber-toothed cats, mammoths, glyptodonts, and dire wolves. Because they were the dominant life on land, and because their relatively large, heavy bones preserve more easily, it is easy to focus on Quaternary mammals; however, major groups of flora and fauna shared the landscape with these beasts, and their remains are occasionally found as well. For example, shells of freshwater bivalves are common, as well as those of land and freshwater gastropods. Petrified wood and pollen are also found.

Spotlight: Waco Mammoth National Monument

Waco Mammoth National Monument in McLennan County, central Texas, contains the fossil bones of 24 Columbian mammoths (Mammuthus columbi) and other Pleistocene mammals. The site, discovered in 1978, is the largest known concentration of a single herd of mammoths dying from the same event, which is believed to have been a flash flood. The site was designated a U.S. National Monument in 2015. (You can take a virtual tour of the Waco Mammoth National Monument shelter here).