Mississippi is rich and diverse in fossils—the preserved remains of ancient life. Paleontology (the study of fossils) has a long history in the state, dating back nearly two hundred years. Among the first fossils to be described from Mississippi are petrified wood, giant oysters and other mollusks, the elephantine mastodon, an ancient serpentine whale, giant marine lizards, and giant ground sloths. Perhaps most unusual to early Mississippi fossil collectors was the discovery of ancient marine life throughout the state as far north as the northeastern corner of the state, well inland from the modern coastline. This was an indication that Mississippi had spent a large part of its geologic history under water. In fact, only sixty-six million years ago, a small, shallow inland sea called the Mississippi Embayment finally began its slow retreat from the Lower Mississippi River Valley to expose the area on its eastern shores we now call Mississippi.
Six particularly fossil-rich time periods are recorded in Mississippi’s geologic history. From oldest to youngest, these are (in millions of years) the Devonian period (409–363), Mississippian epoch (350–320), Cretaceous period (95–66), Eocene epoch (55–34), Oligocene epoch (34–24), and Pleistocene epoch (0.10–0.01). Although fossiliferous deposits representing other periods occur in the geologic record of the state, these are the most fossiliferous in Mississippi. Most fossils are recovered from rocks and sediments exposed at the surface, although fossils are also recovered from drilling projects that go deeper. In Mississippi, fossils are found only in sedimentary deposits, which get younger moving from the far northeastern corner of the state (Tishomingo County) to extreme southwestern and southern Mississippi. These sedimentary layers, laid down across the Southeast over the past four hundred million years, include deposits of limestone, chalk, gravel, quartz sand, clay, silt, mud, organic debris, and various combinations thereof. Most ancient rocks and sediments lying beneath Mississippi’s blanket of soils were deposited by the Mississippi Embayment, an extension of the Gulf of Mexico. However, thinner layers of nonmarine deposits also exist, largely formed by streams. All fossil-bearing Pleistocene deposits found north of the modern coastline are of stream origin.
The oldest rock formations exposed at the surface in Mississippi are found in in Tishomingo County. Millions of years of erosion have weathered and eroded fragments of these rocks of Devonian and Mississippian age, carrying them all over the state and depositing them as thick gravel lenses. These gravel deposits are mined today for use in paving rural roads and driveways. Many people with gravel driveways know about the bounty of fossils contained within the pebbles, which are primarily composed of chert (petrified siliceous ooze). Marine invertebrates such as corals, crinoids (sea lilies), gastropods (snails), bryozoans (moss animals), brachiopods (lampshell), and trilobites populate this nearly ubiquitous chert gravel.
Moving to the west (Boonville, Ripley, and New Albany) and south (Tupelo, Aberdeen, and Columbus), deposits of Late Cretaceous age are encountered. The Cretaceous is the last period in the Age of Dinosaurs, or Mesozoic era. During this period, the sea floor supported a variety of bottom-dwelling invertebrates, including a variety of clams, gastropods, urchins, and bryozoans. The most abundant and easily recognized bottom-dwellers common to the Cretaceous chalks of northeastern Mississippi are the large, thick-shelled oysters Exogyra and Pycnodonte. Fragments of thick-shelled aberrant clams called rudists are not uncommon, identified by their unusual macrocellular (honeycombed) texture. Fish teeth and bones are common in these deposits, especially teeth belonging to several species of sharks, like the crow shark Squalicorax and the goblin shark Scapanorhynchus. Shell fragments of sea turtles and individual backbones of giant marine lizard-snakes called mosasaurs are common in Mississippi’s Cretaceous deposits. As dinosaurs were strictly terrestrial animals, very few bones of this extinct group are found in marine Cretaceous deposits. However, dinosaurs were occasionally washed to sea by hurricanes and rivers.
Moving further west (Holly Springs, Oxford, and Grenada) and south (Jackson, Meridian, and Quitman), overlying deposits of Cretaceous age are deposits dating to the Eocene epoch. In Mississippi, Eocene deposits frequently contain archaeocetes, or premodern whales. Several different types have been discovered to date, the oldest (forty-one million years) with well-developed hind limbs. Zygorhiza is the smallest of the Late Eocene archaeocetes, which also include Basilosaurus, the largest known archaeocete, and Cynthiacetus, a medium-sized form named for Cynthia, Mississippi, a small community on the edge of Jackson. Because of the abundance of fossil whales in central Mississippi, the Mississippi state legislature designated archaeocetes the State Fossil in 1981. Exposed along several watercourses in the central part of the state from Yazoo County in the west to Clarke County in the east are layers of sand rich in the Eocene sand dollars Protoscutella and Periarchus.
Fossil-rich lime pits scattered throughout central Mississippi expose limestone dating to the Oligocene epoch. Like the underlying and thus preceding sedimentary layers, this relatively soft limestone contains a variety of bottom-dwelling creatures, including many of the same basic groups that lived as far back as the Cretaceous period. The species, however, are quite different—much closer to modern forms. Ubiquitous in the Oligocene age limestone are the wafer-like, coin-sized tests of the giant foraminiferan Lepidocyclina. Foraminiferans are a very diverse group of single-celled animals that inhabited the oceans in great abundance for hundreds of millions of years. The same soft limestone contains the sand dollar Clypeaster and heart urchin Schizaster.
The last important time period in the state’s fossil record is the Pleistocene (100,000 to 10,000 years ago), the most recent Ice Age, which some scientists believe is still ongoing today. The major compositional difference between the animal life of the present and that of the Pleistocene is the extinction of many large terrestrial vertebrates from the Northern Hemisphere 10,000 years ago. Otherwise, the animals (and plants) are basically the same. Most of the extinct megafauna were mammals. In Mississippi, this group includes the mammoth, mastodon, horse, tapir, stag moose, llama, sabertooth cat, and American lion as well as giant forms of bison, ground sloth, armadillo, beaver, short-faced bear, and wolf. The disappearance of most of North America’s large mammals at the end of the Pleistocene is thought to have been caused by warming climate coupled with the arrival of the first migrants from Asia.
- Alvin R. Bicker Jr., Geologic Map of Mississippi (1969)
- Eleanor Daly, A List, Bibliography, and Index of the Fossil Vertebrates of Mississippi (1992)
- David T. Dockery III, Windows into Mississippi’s Geologic Past (1997)
- David T. Dockery III, James E. Starnes, David E. Thompson, and Laura Beiser, Rocks and Fossils Found in Mississippi’s Gravel Deposits (2008)
- Earl M. Manning and Michael B. E. Bograd, Mississippi Geology (December 1999)