Region 1: The Central Lowland
The Central Lowland region composes the middle part of the North American continent, centered on the Mississippi River Valley. This region as a whole includes abundant and extensive fossils from the early Paleozoic (for example, in Ohio, Iowa, and Wisconsin), demonstrating that the area was covered by a warm, shallow sea during much of this time.
The portion of the Central Lowland region represented in the Northwest Central States—eastern Nebraska, eastern South Dakota, and eastern North Dakota— has very few Paleozoic rocks at the surface. The majority of fossils found in this region are from the Cretaceous period, which is the youngest bedrock in the area, although younger Quaternary sediments also yield fossils (see, for example, Figure 3.50). Cores drilled from the subsurface in eastern parts of the Dakotas, however, have yielded marine fossils of early and middle Paleozoic age similar to those found elsewhere in the region (Figure 3.2), revealing that these areas were also covered by the same warm, shallow sea. Fossils and other subsurface information indicate that coral reefs were well developed in what is now North Dakota during parts of the Silurian and Devonian.
Figure 3.2: Cambrian trilobite and brachiopod found in cores from the Dakotas and Montana. A) Trilobite pygidium (tail), Lloydia valmyensis. B) Brachiopod, Nanorthis perilla. Both fossils are about 5 millimeters (0.25 inches) wide.
Pennsylvanian rocks outcrop in easternmost Nebraska (in Cass, Otoe, and Sarpy counties), and contain a diversity of marine fossils, including foraminifera, brachiopods, bryozoans, cephalopods, crinoids, gastropods, bivalves, trilobites, corals, and the teeth of early sharks (Figures 3.3 - 3.12). The Beil Limestone, for example, is a rock layer containing abundant corals; it occurs in Cass County, Nebraska, and also extends into Mills and Montgomery counties in Iowa, Holt County in Missouri, and Doniphan, Atchison, Greenwood, and Douglas counties in Kansas (Figure 3.13).
Figure 3.3: Pennsylvanian brachiopods from eastern Nebraska. A) Hemipronites crassus. B) Parajuresania nebrascensis. C) Syntrilasma hemiplicata. D) “Productus” costatus. E) Punctospirifer kentuckensis. All specimens are 3 - 4 centimeters (1.25 - 1.5 inches) wide.
Figure 3.4: Pennsylvanian bivalves from eastern Nebraska. A) Allorisma subcuneata, about 3 centimeters (1.25 inches) wide. B) Edmondia aspinwallensis, about 9 centimeters (3.5 inches) wide.
Brachiopods
Brachiopods are filter-feeding animals that have two shells and are superficially similar to bivalves (such as clams). Instead of being mirror images between shells (symmetrical like your hands), brachiopod shells are mirror images across each shell (symmetrical like your face). Internally, brachiopods are substantially different from bivalves, with a lophophore (filter-feeding organ made of thousands of tiny tentacles), and a small and simple gut and other organs. Bivalves, in contrast, have a fleshier body and collect their food with large gills.
The difference between the shells of a typical brachiopod (left) and a typical bivalve mollusk (right). Most brachiopods have a plane of symmetry across the valves (shells), whereas most bivalves have a plane of symmetry between the valves.
Figure 3.5: Single-celled fusulinid foraminifera from the Permian. A) A cluster of the shells, about the size and shape of large rice grains. B) Photograph of a cross-section through a single fusulinid, as seen through a microscope. Fusulinids can be a major component of carbonate rocks, composing up to 70% of some limestones in eastern Nebraska.
Figure 3.6: Pennsylvanian bryozoan from eastern Nebraska. Fenestella sp., about 4 centimeters (1.5 inches) wide.
Figure 3.7: Pennsylvanian crinoid crowns from eastern Nebraska. A) Stellarocrinus. B) Apographiocrinus. C) Stenopeerinus. D) Exaetocrinus. All specimens are about 4 - 5 centimeters (1.75 - 2 inches) tall.
Figure 3.8: Pennsylvanian gastropods from eastern Nebraska. A) Hypselentoma perhumerosa, about 2 centimeters (0.8 inches) tall. B) Platyceras nebrascensis, about 3 centimeters (1.25 inches) tall.
Crinoids
Crinoids are echinoderms, related to sea urchins and sea stars. These invertebrate animals feed by using their arms to filter food out of the water. Most are attached to the sediment by a stalk that ends in a root-like structure called the holdfast—however, some forms are free floating. Crinoid fossils are most commonly found as “columnals,” pieces of the stalk that hold the head (calyx) above the surface. The calyx and the holdfast are only occasionally preserved as fossils.
(A) Crown and stem, about 15 centimeters (6 inches) long. (B) Stem fragments.
The northeastern corner of North Dakota (Pembina County) contains the Central Lowland’s only Jurassic bedrock. By that time, a shallow sea had flooded the region again, and fossil marine gastropods, bivalves, and crinoids are found in the state’s Jurassic deposits. Cretaceous rocks occurring in the Central Lowland, as well as the few Cenozoic deposits that extend into northeastern Nebraska, are identical to those found in the Great Plains region and will be discussed in detail in the next section.
Cephalopods
Cephalopods, such as squid, octopods, nautiloids, ammonoids, and belemnites, are mollusks with tentacles and beak-shaped mouths for catching prey. Some cephalopods such as belemnites and living cuttlefish have internal shells, while others have straight or coiled shells, such as those of ammonoids or nautiloids. Still other cephalopods, such as the octopus, have no shell. The mass extinction at the end of the Cretaceous (famous for eliminating the non-avian dinosaurs), also eliminated belemnites and ammonoids, which had been extremely diverse during the Mesozoic.
Figure 3.9: Pennsylvanian nautiloid cephalopod from eastern Nebraska, Titanoceras ponderosus, about 15 centimeters (6 inches) in diameter.
Figure 3.10: Early sharks from the Pennsylvanian of Nebraska. A) Helodus simplex, teeth and restoration. Teeth about 2 centimeters (0.8 inches) wide; body about 30 centimeters (1 foot) long. B) Orodus sp., tooth and restoration. Tooth about 1 centimeter (0.4 inches) wide; body about 1 meter (3 feet) long. C) Cladodus occidentalis, tooth, about 1.5 centimeters (0.5 inches) tall.
Figure 3.11: Skeleton and restoration of Xenacanthus. Xenacanths were freshwater sharks that lived during the Pennsylvanian and Permian periods. Body about 30 centimeters (1 foot) long.
Figure 3.12: Pennsylvanian trilobites from eastern Nebraska. A) Ditomopyge, about 2 centimeters (0.8 inches) long. B) Ameura cephalon (head) and pygidium (tail), each about 1 centimeter (0.4 inches) in maximum width. C) Anisopyge cephalon (head) and pygidium (tail), each about 1 centimeter (0.4 inches) in maximum width.
Figure 3.13: Common corals of the Beil Limestone. A) Solitary rugose (“horn”) coral, Caninia torquata, Upper Pennsylvanian. Specimen about 14 centimeters (4.5 inches) long. B) Colonial tabulate coral, Syringopora sp. Specimen is about 6 centimeters (2.3 inches) tall.
Trilobites
Trilobites are iconic Paleozoic fossils, but were more common in the Cambrian and Ordovician than in later periods. They were arthropods, and had well-defined head, tail, and thoracic (leg-bearing) segments. Most had large compound eyes, often with lenses that are visible to the naked eye. In life, they had antennae like many other arthropods, but since these were not mineralized, they only fossilize under exceptional circumstances. Many could roll up for protection, and several species also had large spines.
Corals
Corals are sessile relatives of jellyfish and sea anemones. They possess stinging tentacles, which they use to feed on small planktonic prey. Each group of coral possesses distinctly shaped “cups” that hold individual animals, or polyps. Colonial corals live in colonies of hundreds or even thousands of individuals that are attached to one another. Solitary coral lives independently, as a single isolated polyp.
Rugose corals were both colonial and solitary (solitary forms are often called “horn corals”). Tabulate corals were exclusively colonial and produced a variety of shapes, including sheetlike and chainlike forms. These corals receive their name from the table-like horizontal partitions within their chambers. Both rugose and tabulate corals went extinct at the end of the Permian. Modern corals—scleractinians—appeared in the Triassic, and include both solitary and colonial species. Many scleractinian corals have photosynthetic symbiotic algae in their tissues, called zooxanthellae. This algae provides nutrition to the coral polyps, helping them to grow more rapidly.
