Fossils of the Central Lowland and Interior Highlands

Paleozoic fossils

The geological core of the Interior Highlands and Central Lowland regions is formed by a Proterozoic mountain range that formed around 1.5 billion years ago. Today, these igneous and metamorphic rocks crop out in the St. Francois Mountains across an area of around 13,000 square kilometers (5000 square miles) in southeastern Missouri. During the Paleozoic, this ancient mountain range formed an archipelago of islands and seamounts ringed by coral reefs in vast inland seas. Sandstone and limestone were deposited near the shore, while shale formed in deeper water. The sediments accumulating in these shallow seas preserved an abundant record of the marine animals that lived there during the early Paleozoic.

Marine animal fossils

Cambrian fossils

In southeastern Missouri, the Upper Cambrian Lamotte Sandstone formed in a near-shore environment and contains brachiopods and mysterious trace fossils called Climactichnites. Climactichnites is thought to have been made by large mollusks, perhaps similar to gastropods (snails). Other Cambrian rocks in Missouri contain trilobites and hyoliths.

Ordovician fossils

Fossils from the Ordovician rocks of the Ozarks in southern Missouri and northern Arkansas include many mollusks, such as gastropods (snails), bivalves (mollusks with two-valved shells, like clams), cephalopods (relatives of squid and octopuses), and monoplacophorans. (Monoplacophorans are animals with one cap- or cone-shaped shell, which, in some forms, is somewhat coiled back on itself.) Brachiopods, trilobites, corals, bryozoans, and echinoderms—including crinoids, cystoids, and sea stars—are also found here. In southeastern Missouri, Ordovician rocks have produced the remains of conodonts—primitive vertebrates with small, eel-like bodies and complex arrangements of teeth. Conodonts are very important index fossils for rocks of this age.

Devonian fossils

The oldest rocks in the Central Lowland are Devonian deposits in southwestern Missouri and eastern Oklahoma. Marine invertebrates, such as corals, trilobites, crinoids, and cephalopods, shared the waters with fishes including shark-like acanthodians. After the mass extinction in the late Devonian, trilobites never regained their previous abundance and diversity, and post-Devonian trilobites are relatively rare. In the Mississippian of Missouri and the Pennsylvanian of Texas, however, small trilobites are locally common (there are photos of some Pennsylvanian trilobites further down on this page).

Mississippian and Pennsylvanian (Carboniferous) fossils

Mississippian limestone beds in Missouri and Arkansas preserve the inhabitants of an ancient reef system, including corals, bryozoans, brachiopods, and fishes. Abundant echinoderms are also present, including blastoids, crinoids (such as Missouri’s state fossil, Delocrinus missouriensis), and large echinoids (sea urchins). The Mississippian is sometimes called “the age of crinoids” because they were so abundant in the shallow seas of this period.

Mississippian and Pennsylvanian rocks in Missouri, western Arkansas, and eastern Oklahoma contain ammonoids and graptolites, both pelagic groups (animals that lived in the water column rather than on the seafloor) that were once common and diverse but are now extinct. The marine deposits in western Missouri, eastern Kansas, and Oklahoma contain abundant brachiopods, ammonoids, bivalves, gastropods, bryozoans, echinoids, crinoids, corals, and trilobites. The horseshoe crab Paleolimulus is known from Kansas. 

Permian fossils

The beginning of the Permian saw continued sea level fluctuations, and shallow marine deposits from this time in eastern central Kansas contain faunas similar to those of the Pennsylvanian. As the period continued, however, the climate became much drier, and shallow seas were restricted to central Kansas. To the west of these beds, Permian rocks contain the remains of lungfish, sharks, and other fishes.

Terrestrial fossils

Pennsylvanian fossil plants

In the Pennsylvanian, the sea began to drain away from the Interior Highlands and Central Lowland. Pennsylvanian rocks in northern and western Missouri and eastern Kansas and Oklahoma indicate cyclical fluctuations between shallow marine and terrestrial environments, preserved in repeating layers of shale, limestone, and sandstone interspersed with thick seams of coal. These coal beds, representing terrestrial near-shore swamps, were formed from great thicknesses of decomposing plants.

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 lycophytes (club mosses and relatives), the sphenophytes (horsetails and relatives), the marattioid ferns, seed ferns (seed-producing plants with fern-like leaves) and the cordaites (a group related to conifers). Lycophytes and horsetails survive today as small, non-woody plants. Marattioid ferns are restricted to the tropics. Cordaites are extinct.

Kansas lagerstätten

Several fossil lagerstätten deposits (deposits of exceptionally well-preserved fossils) are known from the Pennsylvanian and Permian of Kansas. These deposits, which formed along the shoreline of a shallow sea, contain abundant and beautifully preserved terrestrial tetrapods and insects as well as many marine invertebrates and fish. The Garnett locality in Andersen County preserves an incredible record of life during the Pennsylvanian in a series of stream channel infills. Petrolacosaurus, one of the earliest known diapsid reptiles, can be found at this site.

The Elmo Limestone in Dickinson County is famous for its fossil insects and xiphosurids (the group of arthropods that includes horseshoe crabs). Other examples of deposits containing similar faunas include the Robinson locality in Brown County and the Hamilton locality in Greenwood County.

Fossils of Oklahoma and Texas

In Oklahoma and Texas, the shallow sea retreated to the west, and thick layers of gypsum and salt were deposited, indicating that evaporation rates were high. Rare fossils of insects, amphibians, and reptiles, as well as vertebrate footprints, have been collected from the youngest Paleozoic rocks in Oklahoma. The Dolese Quarry near Lawton (Comanche County), for example, has produced a great diversity of tetrapods in soft white claystone, including the common small anapsid reptile Captorhinus aguti.

Permian “red bed” deposits in western and north-central Texas (such as the Quartermaster Formation) are famous for their abundance of early Permian reptiles including synapsids—a group that includes the ancestors of modern mammals. Here, well-known tetrapods like the sail-backed Dimetrodon and Edaphosaurus, the giant amphibian Eyrops, the bizarre amphibian Diplocaulus, and the enigmatic Diadectes dominate the fauna. Plants also occur, including sphenophytes, seeds ferns, and early conifers.

Mesozoic fossils

Triassic vertebrate fossils

A ribbon of fossil-bearing Triassic strata occur above the Permian layers in the western part of the Texas panhandle southeast to near Abilene. The temporal (time) boundary between the Permian and Triassic marks the greatest mass extinction known. While its effect on marine life was particularly devastating, life on land was dramatically affected as well. Many kinds of tetrapods (four-legged vertebrates) disappeared, which may have cleared the path for a new group of tetrapods, the dinosaurs, to dominate the land. (Dinosaurs first appeared in the middle Triassic.)

The Triassic rocks of Texas crop out along the border between the Great Plains and Central Lowland regions. A variety of early dinosaurs are found in here, along with phytosaurs (extinct animals that look like crocodiles, although the two groups are not related), and synapsids (mammal-like reptiles).

Cretaceous marine invertebrate fossils

Cretaceous rocks exposed in southern Oklahoma and north central Texas contain a variety of marine mollusks, including bivalves, gastropods, and ammonites.