David C. Kopaska-Merkel, Geological Survey of Alabama
Alabama has a stunning diversity of fossils, ranging from tiny microfossils to giant prehistoric whales. The state is host to a number of sites that reveal fossil-bearing rocks from the ancient Cambrian period to almost modern-day sediments. These rocks range from the time period when trilobites swarmed in the oceans, more than half a billion years ago, to the more recent past when the world we recognize took shape. Alabama's best-known fossil is its state fossil,Basilosaurus cetoides, an ancient toothed whale, but the state is also home to fossil Paleozoic invertebrates, Pennsylvanian plants, Cretaceous-Tertiary mollusks and vertebrates, and a variety of dinosaurs. These include Lophorhothon atopus, a hadrosaur, and Appalachiosaurus montgomeriensis, a tyrannosaur that, so far, is known only from Alabama.
Fossils are intrinsically interesting because of their beauty and strangeness, but they are also important for what they reveal about the Earth's history. For example, because different creatures lived at different times, fossils can be used to determine the relative ages of rock layers. Generally speaking, the more closely fossils resemble the shells or bones or footprints of creatures that are alive today, the younger are the rocks, but this is not always the case. Because oysters have existed for many millions of years, it is sometimes difficult for scientists to determine the age of the rock in which their fossils are found. But if, for example, a shell is from an identified species of oyster, such as Agerostrea falcata, scientists can date the rocks because they know that the species lived during a particular period of the upper Cretaceous, about 80 million years ago. Fossils of animals like Agerostrea falcata, which lived during a specific span of time and were geographically widespread, are called index fossils and can be used to date the rocks that contain them.
Some rocks are practically made out of fossils. For example, when conditions in the sea were stable and favorable, microscopic organisms died in large numbers, rained down onto the sea floor, and over time fossilized to become chalk. Two processes make microscopic shells into chalk. First, the weight of overlying sediment actually fuses shells together where they touch. Second, shell material that dissolves at pressure points later grows as tiny crystals that cement particles to one another. Such is the case with the famous chalk deposits that underlie much of Alabama's Black Belt. To the naked eye, chalk appears to be a smooth or slightly roughened, solid white surface. Under a powerful microscope, however, it is revealed to be a conglomeration of thousands of tiny shells. Because microfossils are difficult to see, people tend to forget about them. However, wherever visible fossils appear, there are surely microfossils present too.
Alabama can be divided into geologic provinces based on the differing ages or formative processes of the rocks found in the various parts of the state. The Piedmont province, in east-central Alabama, consists of metamorphic rocks of Precambrian and lower-Paleozoic age. These rocks are part of the Appalachian mountain chain and have been so squeezed, twisted, and heated that they contain hardly any recognizable fossils. North Alabama rocks consist of strata, or layers, most of which were laid down on the floors of shallow seas between the Cambrian and Pennsylvanian Periods of the Paleozoic Era. Fossil sea creatures in these rocks are older than the dinosaurs. Some of them, such as crinoids, look vaguely familiar and are related to modern crinoids, but others, such as trilobites, have no close living relatives. The most common fossils in these rocks are brachiopods, which look like seashells. Brachiopods (Cambrian to Recent) are not close relatives of modern clams and oysters, however. They are more closely related to bryozoans (Ordovician to recent), which, like the brachiopods, are abundant in Ordovician through Pennsylvanian rocks of North Alabama. Bryozoans are still common today, but most are small and unobtrusive. They often can be found coating the seaweed that has washed up on Alabama's beaches after a high tide. What looks like very small pieces of lace made of stone is actually a series of tiny sticklike arms or segments perforated with minute pits. Each pit contained a bryozoan. Like corals, the individuals formed colonies made up of numerous individuals connected by their hard skeletons. Other common fossils in Alabama rocks, particularly limestones and shales, include crinoids (Ordovician to recent), blastoids (Ordovician to Permian), trilobites (Cambrian to Permian), and gastropods (Cambrian to recent).
Alabama is well known for its coal-age (Pennsylvanian) plant fossils. These are most abundant in the Warrior coal basin of northwest Alabama. The best plant fossils are preserved in shales associated with coal deposits and are found in coal mines. Common Alabama plant fossils include treelike lycopods, such as Lepidodendron, which are preserved as trunks, branches, leaves, flowers, and roots; ferns of many kinds; and giant horsetails. Fossil plants in Alabama record the former presence of swamps like those forming today in the Okefenokee Swamp. Similar Pennsylvanian plant fossils are found from Alabama to West Virginia. Petrified wood has been found in many parts of Alabama.
Much of the lower two-thirds of Alabama lies within the Coastal Plain physiographic province. The rocks in this area, which date to the Cenozoic Era, are much younger than those in the other sections. Coastal plain sediment was laid down in and near the ancient Gulf of Mexico and partly along rivers. These deposits also underlie the beaches and bays that border the modern Gulf. Fossil animals found in Coastal Plain sediment are more closely related to living forms than those in the Paleozoic rocks of North Alabama, and so they look more familiar to us. They include fish and sharks, snails, clams, sea urchins, and an almost incredible variety of oysters. Some of the best-known and largest marine creatures from Alabama are the state fossil, Basilosaurus cetoides, a toothed Eocene whale, and about a dozen species of mosasaurs (giant marine reptiles), which are common in the Cretaceous marine deposits of the Black Belt.
North Alabama is also a rich source of trace fossils, which is what scientists call the preserved remains of marks made by the activities of ancient creatures. Trace fossils include footprints, trails, burrows, and other signs of movement. Trace fossils are more visible in sandstone and shale than in limestone, and although they do not show as much of the physical appearance of ancient organisms, they do indicate what ancient creatures did, which can be even more useful to scientists. Alabama is home to one of the best Pennsylvanian trace-fossil sites in the world, the Stephen C. Minkin Paleozoic Footprint Site, where researchers have found a record of animal behavior on an ancient tidal flat. Creatures ran, walked, swam, crawled, and dug burrows in the mud at a time when reptiles began to replace amphibians as the dominant four-footed animals. Among the thousands of trackways recovered are some that capture interactions among fish and amphibians that lived and died more than 300 million years ago.
Alabama's remarkable fossil heritage is clear in the sheer number and the diversity of the fossil discoveries made in the state. The only dinosaur egg found east of the Mississippi was collected by pediatric immunologist and amateur fossil collector Prescott Atkinson, at the time a high school student, in 1970 in Alabama. The egg appears to have been laid by Lophorhothon, a hadrosaur, and is now being studied at the McWane Center in Birmingham. Cretaceous strata near Phenix City recently have yielded amber and fossil feathers. Cenozoic mollusks have been considered Alabama's greatest claim to fame among invertebrate paleontologists. The shell beds beautifully preserve hundreds of species, many of which were first found in Alabama. For the Paleocene and Eocene epochs especially, these shell beds rank among the very best in the world.
Alabama is a good place to go fossil collecting, and there are large collections of fossils on view in several museums. The National Museum of Natural History at the Smithsonian Institution in Washington, D.C., has an excellent and comprehensive collection of Alabama fossils. Within the state, the Anniston Museum of Natural History, the McWane Center in Birmingham, and the Alabama Museum of Natural History in Tuscaloosa have substantial holdings. Most museums have more material available for study and in storage than they do on display, but Alabama fossils are displayed in all four museums listed.
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Burdick, D. W., and H. L. Strimple. Genevievian and Chesterian Crinoids of Alabama. Bulletin 121. Tuscaloosa: Geological Survey of Alabama, 1982
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Rindsberg, A. K., D. C. Kopaska-Merkel, Doug Wymer, and J. C. Hall. A Guidebook to Cretaceous Rocks and Fossils of West-Central Alabama. Tuscaloosa: Geological Survey of Alabama, 2006.
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