Photographs of fossils from the Ordovician of Kentucky, including a brachiopod, a sponge, a trilobite, and a cephalopod.

Fossils of the Inland Basin

Page snapshot: Introduction to the fossils of the Inland Basin region of the Southeastern US from the Paleozoic Era to the Quaternary Period.

Topics covered on this page: Paleozoic fossils, Paleozoic marine invertebrates, Paleozoic sharks and bony fishes, Paleozoic terrestrial fossils, Cenozoic fossils, Cenozoic terrestrial fossils, Gray Fossil Site (Tennessee), Big Bone Lick State Park (Kentucky)

Credits: Most of the text of this page is derived from "Fossils of the Southeastern US" by Warren D. Allmon, chapter 3 in The Teacher-Friendly Guide to the Geology of the Southeastern U.S., 2nd. ed., edited by Andrielle N. Swaby, Mark D. Lucas, and Robert M. Ross (published in 2016 by the Paleontological Research Institution; currently out of print). The book was adapted for the web by Elizabeth J. Hermsen and Jonathan R. Hendricks in 2021–2022. Changes include formatting and revisions to the text and images. Credits for individual images are given in figure captions.

Updates: Page last updated January 6, 2022.

Image above: Ordovician fossils from Kentucky. Top left:A brachiopod (Zygospira cincinnatiensis), Eden Formation, in four views. Top right:A sponge (Heterospongiasubramosa), Richmond Formation. Bottom left:A trilobite (Flexicalymene granulosa), Kope Formation. Bottom right:A cephalopod (Endoceras proteiforme), Kenton Formation. All images from the Atlas of Ordovician Life.

Paleozoic fossils

Paleozoic marine fossils

Marine invertebrates

The Inland Basin region primarily contains the story of Paleozoic mountain-building events, associated sediment deposited in the inland sea, and changes in relative sea level, superimposed on the evolution of Paleozoic marine and coastal plant life.

Exposures of Cambrian rocks with fossils are uncommon in this region, but a few can still be found. The early Cambrian Antietam Sandstone of West Virginia and Virginia contains Skolithos trace fossils (see below), as well as occasional trilobites and brachiopods. Early Cambrian fossils, especially trilobites, are also found in western Virginia, eastern Tennessee, and northwestern Georgia. In eastern Tennessee (near Knoxville), trilobite trace fossils called Rusophycus and Cruziana are common in Cambrian rocks that appear to have been deposited in intertidal environments.

Drawings of three trilobites. Left to right: Elrathia antiquata, Aphelaspis brachyphasis, and Modocia sp.

Cambrian trilobites from northwestern Georgia and northeastern Alabama. A. Elrathia antiquata, approximately 2 centimeters (0.8 inches) long. B. Aphelaspis brachyphasis, approximately 2.5 centimeters (1 inch) long. C. Modocia sp., approximately 2 centimeters (0.8 inches) long. Drawings by Alana McGillis.

Drawings of trace fossils associated with trilobites from Palaeontology of New York volume 2, published 1852.

Trace fossils commonly associated with trilobites. Rusophycus are resting traces, recording the outline of the organism while it remains still. Cruziana are elongate, bilaterally symmetrical paths with repeated striations, recording the animal’s movement through the mud. Drawing by F.J. Swinton/lithograph by R.H. Pease, from plate 8 in J. Hall (1852) Palaeontology of New York, vol. 2 (public domain).

Trace fossil specimen of Rusophycus bilobatus from the Silurian Rose Hill Shale of Scott County, Virginia (PRI 76852). This resting trace was likely made by a trilobite. Specimen is from the research collections of the Paleontological Research Institution, Ithaca, New York. Longest dimension of rock is approximately 9.5 cm (Sketchfab; Creative Commons 0 license).

Shales and concretions in the middle Cambrian Conasauga Formation of northeastern Alabama and northwestern Georgia contain body and trace fossils that preserve the record of diverse soft-bodied organisms. There are also many mineralized skeletons. Fossils include algae, sponges, arthropods, brachiopods, echinoderms, mollusks, and trace fossils. Some of the most curious Conasauga fossils are Brooksella, also called "star cobbles." These enigmatic fossils have been thought of as medusae (jellyfish), algae, trace fossils, or inorganic structures. Recent research suggests, however, that they are most likely sponges with siliceous (SiO2) skeletons.

Drawings of star cobbles, genus Brooksella, from an 1898 monograph by Walcott.

"Star cobbles" (Brooksella) from the Cambrian Conasauga Formation. Fossils are 2.5–5 centimeters (1–2 inches) across. Images from plate I in Walcott (1898) Monographs of the U.S.G.S, 30 (public domain).

Ordovician, Silurian, and Devonian marine fossils are widespread in the Inland Basin and are typically abundant, diverse, and beautifully preserved. These fossil assemblages are nearly always dominated by brachiopods, but also contain trilobites, graptolites, corals, clams, crinoids, Skolithos trace fossils, and many others.

Photographs of 4 species of Ordovician brachiopods  from Kentucky and Indiana, each shown in several views.
Examples of Ordovician brachiopods from the Cincinnati region, which includes northern Kentucky. Featured species include Vinlandstrophia laticosta (McMillan Formation of Crestview Hills, KY), Rafinesquina alternata(Arnheim formation of Franklin County, IN), Hiscobeccus capax (Waynesville formation of Franklin County, IN), Strophomena nutans (Whitewater Formation of Maysville, KY). Images are from the Ordovician Atlas.

Photograph of a stone preserving multiple Ordovician trilobites (genus Homotelus) from Virginia.

Slab of trilobites (Homotelus sp.) from the Ordovician of Virginia, approximately 38 centimeters (15 inches) across (PRI 40023; photograph by Wade Greenberg-Brand).

Middle Ordovician limestones in Tennessee are well known for containing numerous crinoids and fossils of the large gastropod Maclurites.

Image showing photographs of two Ordovician crinoids from Tennessee. The calyx, arms, and a portion of the stem of each specimen is illustrated.

Crinoid calyces with arms and stem from the Ordovician Lebanon Limestone of Central Tennessee. Top: Reteocrinus alveolatus, approximately 6 centimeters (2.4 inches) long. Bottom: Cremacrinus sp., approximately 16 centimeters (6.3 inches) long. Images from Guensburg (1984) in Bulletins of American Paleontology; copyright Paleontological Research Institution.

Fossil snail Maclurites sp. from the Ordovician Chazy Limestone of Clinton County, New York (PRI 76290). Specimen is on public display at the Museum of the Earth, Ithaca, New York. Diameter of shell is approximately 11 cm (Sketchfab; Creative Commons 0 license).

In Kentucky and Tennessee, trilobites, brachiopods, bryozoans, corals, and the distinctive sponge Brachiospongia are also found in Ordovician rocks.

Drawings of Ordovician organisms, including a sponge, a stromatoporoid sponge, a bryozoan, and a tabulate coral.

Common Ordovician reef-building organisms of the Interior Basin. A. Sponge (Brachiospongia digitata) approximately 25 cm (10 in) wide. B. Stromatoporoid sponge, approximately 30 cm (12 in) across. C. Bryozoan (Dekayia aspera) approximately 1 inch (2.54 centimeters) across. D. Tabulate coral (Favistella sp.) approximately 8 cm (3.2 in) wide. Image credits: A, Nettleroth (1889; Kentucky Fossil Shells: a Monograph of the Fossil Shells of the Silurian and Devonian Rocks of Kentucky, Kentucky Geological Survey, 245 pp., 36 pls.), public domain; B and D, copyright Christi Sobel; C, Alana McGillis.

Fossil sponge (Brachiospongia digitata) from the Ordovician of Franklin County, Kentucky (PRI 49856). Specimen is on public display at the Museum of the Earth, Ithaca, New York. Longest dimension of specimen is approximately 23 cm (Sketchfab, Creative Commons 0 license).

Late Ordovician fossils are abundant in central Tennessee (around Nashville), central Kentucky (southeast of Louisville), and northern Kentucky. Ordovician limestones in the area around Strasburg, Virginia, contain abundant trilobites. The fossils are preserved as silica (SiO₂), and can therefore be extracted from the surrounding carbonate rock by treating it with dilute acid. This allows for the examination of delicate fossils that would otherwise be impossible to study.

In Alabama, the Silurian Red Mountain Formation, which is rich in the iron mineral hematite, contains reefs made largely of stromatoporoid sponges. Skolithos traces can be found throughout Silurian deposits in Tennessee, while Silurian deposits in Virginia contain abundant corals and brachiopods.

Color photograph of Skolithos burrows from the Silurian of Tennessee. Image shows a beige rock with slanting, vertical, dark streaks.

Skolithos burrows from the lower Silurian Clinch Formation at Clinch Mountain, Tennessee. Rocks with abundant Skolithos are sometimes called "pipe rock." The organism that made these burrows is unknown, but their shape suggests a worm-like creature that lived in the vertical burrows. Rock containing burrows is approximately 15 x 30 cm (6 x 12 in). Photograph by James St. John (flickr; Creative Commons Attribution 2.0 Generic license).

Early to middle Devonian marine fossils are spectacularly exposed in the 390-million-year-old rocks at Falls of the Ohio in northern Kentucky. These beds of limestone are among the largest naturally exposed Devonian fossil deposits in the world. They are packed with the skeletal remains of countless corals, stromatoporoid sponges, echinoderms, brachiopods, mollusks, arthropods, and microscopic organisms. Well-preserved Devonian marine invertebrate fossils also occur in Tennessee, Virginia, and West Virginia.

The Mississippian is sometimes referred to as the "age of crinoids" because crinoids were so abundant that entire rocks are made up of bits of their calcite skeletons. Mississippian limestones from West Virginia to Alabama contain numerous fossils, including corals, gastropods, bivalves, bryozoans, blastoids, and fossilized shark teeth. West Virginia’s official state "gemstone" is Lithostrotionella, a Mississippian fossil coral from the Hillsdale Limestone found in portions of Greenbrier and Pocahontas counties. Carboniferous trilobites have also been found in northwestern Georgia and northern West Virginia.

Drawings of a blastoid. The left drawing shows the whole organism in life position, the right drawing shows a detail of a calyx.

Blastoid (Pentremites sp.). Blastoids were a group of stalked echinoderms similar to crinoids. The calyx of a blastoid did not have long arms, but instead a series of holes called brachioles that held much shorter arms. Left: Restoration, approximately 15 centimeters (6 inches) tall. Right: Calyx. Image credits: Left, image by Alana McGillis, from image by Illinois State Geological Survey; Right, copyright Christi Sobel.

Fossil specimen of a blastoid (Pentremites godani) from the Mississippian Paint Creek Formation of St. Clair County, Illinois (PRI 70771). Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Specimen is approximately 2 cm in length (Sketchfab; Creative Commons 0 license).

Photograph of a crinoid from the Mississippi Period of Kentucky.

Gilbertsocrinus typus, a crinoid from the Mississippian Borden Formation of north-central Kentucky, 8.5 cm (3.4 in). Photograph by Tim Evanson (flickr; Creative Commons Attribution-ShareAlike 2.0 Generic license).

Sharks and body fishes

Sharks and bony fishes evolved rapidly during the Devonian and Carboniferous periods, and their fossils can sometimes be found in the rocks of the Inland Basin. Fossils of heavily armored bony fishes called placoderms are found in Kentucky and Tennessee, including the enormous Dunkleosteus.

Cast of a Dunkleosteus skull on display at the Museum of the Earth, Ithaca, New York.

Sharks first appeared in the late Silurian period, and their teeth are also preserved in these rocks. Among these are the bizarre and puzzling coiled tooth whorls of edestid sharks. These are sometimes found in shales overlying coal seams, indicating that the sharks inhabited shallow waters near shore.

Paleozoic terrestrial fossils

The oldest known fossils of land plants are isolated spores dating to the Ordovician Period. By the late Silurian, fossils of simple, leafless stems just a few centimeters tall are found in sediments that accumulated in low, wet, coastal environments. The first trees date to the early part of the Devonian period, and the first seeds to the middle Devonian. Devonian plant fossils are found in northeastern Alabama (Cherokee County). By the end of the Devonian, extensive forests containing many different kinds of trees and other plants had developed.

During the Mississippian and especially the Pennsylvanian, what is now the Interior Basin region of the Southeast was a broad, low coastal plain near the Equator. It was home to extensive swamp forests. The swamp forests that grew on Carboniferous coastal plains west of the Appalachian Mountains were dominated by four major groups of plants, each of which had independently evolved tree-like forms. These trees were tall plants held erect by a single stem made of stiff tissue; most produced wood.

The trees found in these forests did not belong to the familiar groups that dominate modern forests, such as conifers and flowering broadleaf plants. Instead, these early forests were made up of groups of plants that are either extinct or much less conspicuous today. These groups were the lycopods (club mosses), the sphenopsids (horsetails), the marratioid ferns, and the cordaites (a group related to conifers). Lycopods and horsetails survive today, but often small, non-woody plants. Marratioid ferns are restricted to the tropics. Cordaites are extinct.

Four drawings showing reconstructions of Paleozoic trees. From left to right: Lepidodendron, a tree fern, the giant horsetail calamites, and cordaites, a plant related to conifers.

Restorations of Pennsylvanian coal swamp trees. A. Scale tree (Lepidodendron), a lycopod that reached 30 m (100 ft) tall. B. Psaronius, a marratioid tree fern, reached 3 m (10 ft) tall. C. Calamites, a sphenopsid (horsetail), reached 20 m (65 ft) tall. D. Cordaites, a conifer-like seed plant; reached 10 m (35 ft) tall. Illustration by Christi Sobel.

Fossil specimen of the lycopophyte fossil Stigmaria from the Carboniferous of Morgan County, Tennessee (PRI 50355). Stigmaria is a rhizomorph, or root-like anchoring structure for ancient lycophyte trees. Note the helically arranged, circular rootlet scars. Specimen is on display at the Museum of the Earth, Ithaca, New York. Length of specimen is approximately 26.5 cm. Model by Emily Hauf (Sketchfab, CC0 1.0 Universal, public domain dedication).

4 photographs of coal swamp fossil plants. From left to right: A. A seed fern frond. B. Portion of a calamites stem. C. The outer bark of a lepidodendron trunk. D. Cordaites leaves.

Coal swamp plant fossils. A. Seed fern frond (Alethopteris) slab approximately 10 x 10 cm (4 x 4 in). B. Calamites trunk, approximately 25 cm (10 in) tall. C. Lycopod bark impression (Lepidodendron sp.). Pennsylvanian lycopods reached huge sizes, up to 50 m (150 ft) tall and 1 m (3 ft) in diameter. D. Cordaite leaves (Cordaites sp.), slab approximately 1.2 m (4 ft) long. Image credits: A. Photograph by Wade Greenberg-Brand, specimen PRI 49906 in the collection of the Paleontological Research Institution, Ithaca, NY; B. "Verisimilus" via Wikimedia Commons, modified from original (CC BY 3.0 license); C. James L. Stuby via Wikimedia Commons (public domain); D. "Jebulon" via Wikimedia Commons (public domain).

The Carboniferous coal swamps were also home to a great diversity of animals, including insects, amphibians, and early reptiles. Although fossil bones have been found in Mississippian and Pennsylvanian rocks in West Virginia, Virginia, and Alabama, trace fossils are more common. One particular trackway site in Walker County, Alabama is one of the largest known Carboniferous track sites in the world (learn more).


Cenozoic Terrestrial Fossils

The Interior Basin contains no fossil-bearing outcrops of Mesozoic age. It does, however, have a rich fossil record from the Neogene, including numerous sites at which Pleistocene and older terrestrial vertebrates have been found.

Spotlight: Gray Fossil Site, Tennessee

Gray Fossil Site is located near the small town of Gray in Washington County, northeastern Tennessee. It was discovered in 2000, when a road project unearthed fossil bones in a thick bed of clay that had accumulated in a flooded sinkhole. Subsequent excavation revealed the bones of numerous species of mammals, reptiles, and amphibians. The particular species present constrain the age of the fossil-bearing sinkhole deposit to the early Pliocene.

The fossils recovered from the site are noteworthy for several reasons. The site is an isolated Neogene deposit in the Appalachian region, and thus provides some of the only direct evidence for the types of animals and plants that lived there during the Neogene. It is also the world’s largest fossil tapir find, and it has yielded the most complete skeleton of Teleoceras (an ancient rhinoceros) found in eastern North America. A species of red panda from Gray Fossil Site is only the second (and most complete) record of this group in North America. Other fossil mammals present at Gray Fossil Site include salamanders, turtles, beaded lizards, alligators, shrews, moles, rabbits, rodents, weasels, short-faced bears, saber-toothed cats, mastodonts, camels, peccaries, ground sloths, and three-toed horses.

2-Panel figure of Gray Fossil Site tapirs. Panel 1. Photograph of tapir skeletons on display at Gray Fossil Site. Panel 2. Drawing showing a reconstruction of a living tapir underneath a tapir skull.

Ancient tapirs (Tapirus polkensis). Left: Specimen found at Gray Fossil Site, Tennessee, on display in the museum at the site (photograph by Elizabeth J. Hermsen). Right: Reconstructions of the skull and complete animal (skull drawing by Christi Sobel; animal drawing by Pearson Scott Foresman).

2-Panel figure. Panel 1. Photograph the the skeleton of the North American rhino Teleoceras on display. Panel 2. Drawing showing a reconstruction of Teleoceras underneath a drawing of a Teleoceras skull.

The ancient rhinoceros Teleoceras. Left: Specimen found at Gray Fossil Site, Tennessee, on display in the museum at the site (photograph by Elizabeth J. Hermsen). Right: Reconstructions of the skull and complete animal (skull drawing by Christi Sobel; animal drawing by Pearson Scott Foresman).

2-Panel figure. Panel 1. Photograph of a skeleton of Pristinailurus bristoli, an extinct red panda. Panel 2. Drawing of a reconstruction of Pristinailurus showing its scale next to a human (about knee-high) under a drawing of a Pristinailurus skull.

The red panda Pristinailurus bristoli. Left: Specimen found at Gray Fossil Site, Tennessee, on display in the museum at the site (photograph by Elizabeth J. Hermsen). Right: Reconstructions of the skull and complete animal; drawings by Christi Sobel.

"Gray Fossil Site" by ETSU Natural History Museum & Gray Fossil Site (YouTube).

Gray Fossil Site is best known for its fossil vertebrates, but numerous plant fossils have also been found at the location. These plant fossils—which mostly consist of pollen, fruits, and seeds, but also include fragments of other plant parts—help paleontologists reconstruct the ancient environment at this locality. They also inform our understanding of the evolutionary histories of the plants themselves. A presentation about the plant fossil record of Gray Fossil Site by PRI's Dr. Elizabeth Hermsen is available to view here and is part of a series of "Paleo Talks" that covers all aspects of the biota of Gray Fossil Site.

2-Panel figure. Panel 1: Photograph of plant fossils in a petri dish on the stage of a dissecting microscope. Panel 2: Image of a plant fossil taken through the eyepiece of a dissecting microscope.

Left: Fossil plant remains, including seeds and fruits, from Gray Fossil Site; many are small and require a microscope to study. Right: An unidentified fossil viewed through a microscope eyepiece. Photographs by Elizabeth J. Hermsen.

Spotlight: Big Bone Lick State Park, Kentucky

Bones of Pleistocene mammals are widespread and locally abundant throughout Kentucky, Tennessee, western Virginia, West Virginia, and northern Alabama. They occur mainly in caves, sinkholes, and boggy deposits, and include mastodons, mammoths, giant sloths, and peccaries. Fossils of bats from the late Pleistocene (38,000 years ago) have been found in guano deposits in Mammoth Cave, Kentucky.

Perhaps the most famous Pleistocene mammal site in the region is Big Bone Lick, located less than 2 kilometers (1.2 miles) south of the Ohio River in Boone County, Kentucky. Big Bone Lick is sometimes called the "birthplace of American vertebrate paleontology." The first published illustration of a mastodon fossil (in 1756) was based on a tooth collected there in 1739, probably by Native American members of a French expedition. US President Thomas Jefferson acquired fossils from the site in 1808 that are now held at the Academy of Natural Sciences of Philadelphia, the Natural History Museum in Paris, and as part of Jefferson's personal collection at Monticello (an upper jaw bone from a mastodon remains on display at Monticello; see here).

Photograph of the sign at the entrance to Big Bone Lick State Park

Entryway sign at Big Bone Lick, Kentucky. Photograph by "albedo20" via flickr (CC BY-NC-ND 2.0 license).

2-Panel figure showing two plates illustrating a mastodon molar from Big Bone Lick. The plates were published in 1756.

Illustrations of a mastodon tooth from Big Bone Lick, Kentucky, collected in 1739. These images were published in 1756. Drawings by J. Ingram, plates III and IV from Guettard (1756) Histoire de l'Académie royale des sciences, avec les mémoires de mathématique et de physique 1972: 182–220. (via Biodiversity Heritage Library, Creative Commons Attribution-NonCommercial 3.0 Unported license, images cropped and resized).

Big Bone Lick gets its name from the large number of natural salt springs, or salt licks, in the area. During the late Pleistocene, these saline springs attracted numerous large mammals, many of which died and were buried there. The fauna includes an extinct horse (Equus complicatus), two giant ground sloths (Megalonyx jeffersonii and Paramylodon harlani), mammoths (Mammuthus sp.), mastodons (Mammut americanum), and caribou (Rangifer tarandus). Radiocarbon dating shows that the age of the bones at Big Bone Lick are between 11,000 and 12,300 years old.

2-Panel photographic figure. Panel 1: Photo of a historical marker about Big Bone Lick: "Big Bone Lick: Scientists consider William Clark's dig at Big Bone Lick in 1807 as establishing American Vertebrate paleontology. Bones found here by Clark included mastodon and mammoth. Prehistoric native American artifacts found were given to Dr. Wm. Goforth in Cincinnati." Panel 2: Photo of a salt spring showing cloudy water caused by the presence of salt.

Big Bone Lick, Kentucky. Left: Historical marker explaining why Big Bone Lick is the birthplace of American vertebrate paleontology. Right: Salt spring. Photographs by Jonathan R. Hendricks & Elizabeth J. Hermsen.