Regions 1–2: The Central Lowland & Interior Highlands

The geological core of the Interior Highland 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, surrounded by vast inland seas and ringed by coral reefs. Sandstone and limestone was deposited near the shore, while shale formed in deeper water.

See Chapter 1: Geologic History for more about the formation of continents in the Proterozoic and Paleozoic.

The sediments accumulating in these shallow seas preserved an abundant record of the marine animals that lived there during the early Paleozoic. In southeastern Missouri, the Upper Cambrian Lamotte Sandstone formed in a near-shore environment, and contains brachiopods (Figure 3.2 and see box on p. 86) and mysterious trace fossils called Climactichnites (Figure 3.3), thought to have been made by large mollusks, perhaps similar to gastropods (snails). Other Cambrian rocks in Missouri contain trilobites (Figure 3.4 and see box on p. 87) and hyoliths (Figure 3.5).

Fossils from the Ordovician rocks of the Ozarks in southern Missouri and northern Arkansas include many mollusks, such as gastropods, bivalves, cephalopods (Figure 3.6), and monoplacophorans—animals with one cap- or cone-shaped shell, which, in some forms, is somewhat coiled back on itself (Figure 3.7). Brachiopods (Figure 3.8), trilobites (Figure 3.9), corals (Figure 3.10), bryozoans, and echinoderms—including crinoids, cystoids, and sea stars—are also found here. In southeastern Missouri, Ordovician rocks have produced the remains of conodonts (see box)—primitive vertebrates with small, eel-like bodies and complex arrangements of teeth. Conodonts are very important index fossils for rocks of this age.

Figure 3.1. The history of life in relation to global and regional geological events and the fossil record of the South Central US. (Time scale is not to scale).

Figure 3.1. The history of life in relation to global and regional geological events and the fossil record of the South Central US. (Time scale is not to scale).

Figure 3.2: Cambrian brachiopod, <em class='sp'>Dicelomus</em>. About 1.6 centimeters (0.6 inches) wide.

Figure 3.2: Cambrian brachiopod, Dicelomus. About 1.6 centimeters (0.6 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).

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 3.3: <em class='sp'>Climactichnites</em>. The maker of this trackway is unknown, but may have been a mollusk. Each trackway is about 10 centimeters (4 inches) wide, and can extend for many meters.

Figure 3.3: Climactichnites. The maker of this trackway is unknown, but may have been a mollusk. Each trackway is about 10 centimeters (4 inches) wide, and can extend for many meters.

Figure 3.4: Enrolled Cambrian trilobites from Missouri. A) <em class='sp'>Cliffia wilsoni</em>. B) <em class='sp'>Sulcocephalus candidus</em>. Specimens are about 0.5 centimeters (0.2 inches) wide.

Figure 3.4: Enrolled Cambrian trilobites from Missouri. A) Cliffia wilsoni. B) Sulcocephalus candidus. Specimens are about 0.5 centimeters (0.2 inches) wide.

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.

Figure 3.5: Hyolithid. About 1 - 2 centimeters (less than 1 inch) long.

Figure 3.5: Hyolithid. About 1 - 2 centimeters (less than 1 inch) long.

Figure 3.6: Cephalopods.

Figure 3.6: Cephalopods. A) Straight (orthocone) nautiloid shell and restoration. These animals reached lengths of more than 4 meters (12 feet), making them among the largest invertebrate animals that ever lived. Specimens up to 25 centimeters (1 foot) long are frequently found. B) Restoration of an ellesmeroid nautiloid from the Ordovician of Missouri. About 10 - 15 centimeters (4 - 6 inches) long.

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 dinosaurs, also eliminated belemnites and ammonoids, which had been extremely diverse during the Mesozoic. Ammonoids are useful index fossils, especially in Mesozoic rocks.

Figure 3.7: Monoplacophorans in limestone (highlighted), Lower Ordovician, Barry County, Missouri. Pocketknife at bottom of photo is about 8.5 centimeters (3.5 inches) long. Inset shows top and sides views of an individual shell in detail.

Figure 3.7: Monoplacophorans in limestone (highlighted), Lower Ordovician, Barry County, Missouri. Pocketknife at bottom of photo is about 8.5 centimeters (3.5 inches) long. Inset shows top and sides views of an individual shell in detail.

Figure 3.8: Ordovician brachiopods from Missouri. A) <span class='sp'>Strophomena</span>. 1 - 2 centimeters (0.5 - 1 inch) wide. B) <span class='sp'>Resserella</span>. About 1 centimeter (0.5 inches) wide. C) <span class='sp'>Pionodema</span>. About 1 centimeter (0.5 inches) wide.

Figure 3.8: Ordovician brachiopods from Missouri. A) Strophomena. 1 - 2 centimeters (0.5 - 1 inch) wide. B) Resserella. About 1 centimeter (0.5 inches) wide. C) Pionodema. About 1 centimeter (0.5 inches) wide.

Figure 3.9: Ordovician trilobites. A) Isotelus iowensis, Missouri. B) Homotelus bromidensis, Oklahoma.

Figure 3.9: Ordovician trilobites. A) Isotelus iowensis, Missouri. B) Homotelus bromidensis, Oklahoma.

Figure 3.10: Ordovician corals from Missouri. A) Tabulate, <em class='sp'>Favistella</em> (colonial). About 8 centimeters (3.2 inches) wide. B) Solitary rugose (“horn coral”), <em class='sp'>Streptelasma subregulare</em>. About 3 centimeters (1 inch) tall.

Ordovician corals from Missouri. A) Tabulate, Favistella (colonial). About 8 centimeters (3.2 inches) wide. B) Solitary rugose (“horn coral”), Streptelasma subregulare. About 3 centimeters (1 inch) tall.

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 domed 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.

The oldest rocks in the Central Lowland are Devonian deposits in southwestern Missouri and northeastern Oklahoma, where marine invertebrates, such as corals, trilobites, crinoids, and cephalopods, shared the waters with fishes including shark-like acanthodians (Figure 3.11).

After the mass extinction in the late Devonian (see Figure 3.1), trilobites never regained their previous abundance and diversity, and post-Devonian trilobites are relatively rare. In the Mississippian of Missouri, however, and in the Pennsylvanian of Texas, small trilobites are locally common (Figure 3.12).

Conodonts

Conodonts are tiny tooth-shaped fossils (0.2 - 5 millimeters long) found in marine rocks of Cambrian through Triassic age. They have long been among the most important index fossils in these rocks, allowing the rocks to be dated through biostratigraphy. For many years, paleontologists did not know what kind of animal they belonged to, but in 1983 a very well-preserved fossil was found in Scotland that showed conodonts to belong to small fish-like animals just a few centimeters long that were distant relatives of bony fish.

Isolated conodont elements (Silurian).

Isolated conodont elements (Silurian).

Restoration of what the conodont animal may have looked like alive. Length 2 - 4 centimeters (1 - 2 inches).

Restoration of what the conodont animal may have looked like alive. Length 2 - 4 centimeters (1 - 2 inches).

Figure 3.11: Silurian acanthodian fish (life restoration and spine). These fish reached lengths of up to 30 centimeters (1 foot).

Figure 3.11: Silurian acanthodian fish (life restoration and spine). These fish reached lengths of up to 30 centimeters (1 foot).

Figure 3.12: Carboniferous trilobites. A) Tail (pygidium) of <span class='sp'>Breviphillipsia swallowi</span>, Mississippian, Pettis County, Missouri; about 5 millimeters (0.2 inches) wide. B) <span class='sp'>Ditomopyge</span> sp., Pennsylvanian, San Saba County, Texas; about 1 centimeter (0.4 inches) long.

Figure 3.12: Carboniferous trilobites. A) Tail (pygidium) of Breviphillipsia swallowi, Mississippian, Pettis County, Missouri; about 5 millimeters (0.2 inches) wide. B) Ditomopyge sp., Pennsylvanian, San Saba County, Texas; about 1 centimeter (0.4 inches) long.

Mississippian and Pennsylvanian rocks in Missouri, western Arkansas, and eastern Oklahoma contain ammonoids (Figure 3.13) and graptolites, both pelagic groups that were once common and diverse but are now extinct. Mississippian limestone beds in Missouri and Arkansas preserve the inhabitants of an ancient reef system, including corals, bryozoans (Figure 3.14), brachiopods, and fishes. Abundant echinoderms are also present, including blastoids (Figure 3.15), crinoids (such as Missouri’s state fossil, Delocrinus missouriensis [Figures 3.16 and 3.17]), and large echinoids (sea urchins) (Figure 3.18). The Mississippian is sometimes called “the age of crinoids” because they were so abundant in the shallow seas of this period.

Figure 3.13 Pennsylvanian ammonoids, Missouri. A) <span class='sp'>Solenochilus</span>. About 10 centimeters (3 inches) at the largest diameter. B) <span class='sp'>Gonioloboceras</span>. About 2 centimeters (1 inch) in diameter.

Figure 3.13 Pennsylvanian ammonoids, Missouri. A) Solenochilus. About 10 centimeters (3 inches) at the largest diameter. B) Gonioloboceras. About 2 centimeters (1 inch) in diameter.

Graptolites

Graptolites (meaning “rock writing”) are an extinct group of colonial, free-floating organisms. They lived from the Cambrian to the Carboniferous, and were relatives of modern hemichordates such as acorn worms. Graptolites are frequently preserved as thin black sawblade-like streaks across black shale; tiny cups along these structures held individual animals. Graptolites are often useful as index fossils.

A) Specimen with many fragments of colonies of <span class='sp'>Climacograptus</span>. Slab is 7.5 centimeters (3 inches) on each side. B) Restoration of what graptolite colonies may have looked like when they were alive, floating in the water.

A) Specimen with many fragments of colonies of Climacograptus. Slab is 7.5 centimeters (3 inches) on each side. B) Restoration of what graptolite colonies may have looked like when they were alive, floating in the water.

Figure 3.14: The bryozoan <span class='sp'>Archimedes</span> sp., Carboniferous. A) <span class='sp'>Archimedes</span> colonies consisted of a screw-shaped axis, with a spiral fan connected to the “threads” of the screw. The tiny bryozoan animals lived in chambers on the fan. In some localities, thousands of these “fossil screws” cover the ground. They are less than 2.5 centimeters (1 inch) long. B) <span class='sp'>Archimedes</span> life restoration.

Figure 3.14: The bryozoan Archimedes sp., Carboniferous. A) Archimedes colonies consisted of a screw-shaped axis, with a spiral fan connected to the “threads” of the screw. The tiny bryozoan animals lived in chambers on the fan. In some localities, thousands of these “fossil screws” cover the ground. They are less than 2.5 centimeters (1 inch) long. B) Archimedes life restoration.

Figure 3.15: Blastoid. <span class='sp'>Pentremites</span> sp., Carboniferous. About 1 - 2 centimeters (0.75 inches) long.

Figure 3.15: Blastoid. Pentremites sp., Carboniferous. About 1 - 2 centimeters (0.75 inches) long.

Figure 3.16: Crinoids, Upper Mississippian, Arkansas. A) <span class='sp'>Phanocrinus</span>. Crown and part of stem. Specimen about 5 centimeters (2 inches) tall. B) <span class='sp'>Linocrinus</span>. Crown and part of stem. Specimen about 6 centimeters (2.4 inches) tall.

Figure 3.16: Crinoids, Upper Mississippian, Arkansas. A) Phanocrinus. Crown and part of stem. Specimen about 5 centimeters (2 inches) tall. B) Linocrinus. Crown and part of stem. Specimen about 6 centimeters (2.4 inches) tall.

Figure 3.17: Crinoid. <em class='sp'>Delocrinus missouriensis</em>. A) Restoration of the entire animal, attached to the sea floor. About 30 centimeters (1 foot) tall. B) Specimen of the crown (calyx) with arms. About 5 centimeters (2 inches) tall. C) Bottom view of the cup portion of the calyx. About 2 centimeters (0.8 inches) in diameter.

Figure 3.17: Crinoid. Delocrinus missouriensis. A) Restoration of the entire animal, attached to the sea floor. About 30 centimeters (1 foot) tall. B) Specimen of the crown (calyx) with arms. About 5 centimeters (2 inches) tall. C) Bottom view of the cup portion of the calyx. About 2 centimeters (0.8 inches) in diameter.

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 were attached to the sediment by a stalk that ended in a root-like structure called the holdfast—however, some forms were 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.

Figure 3.18: Slab with many individuals of the echinoid <em class='sp'>Melonechinus multiporus</em>, Mississippian, near St. Louis, Missouri. Slab is about 45 centimeters (18 inches) across.

Figure 3.18: Slab with many individuals of the echinoid Melonechinus multiporus, Mississippian, near St. Louis, Missouri. Slab is about 45 centimeters (18 inches) across.

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, including giant horsetails, tree ferns, scale trees (lycopods or “club mosses”), and the conifer-like Cordaites (Figure 3.19). Conversely, the marine deposits here contain abundant bivalves, bryozoans, echinoids, crinoids, gastropods, corals, trilobites, and brachiopods (Figure 3.20).

Figure 3.19 (AT LEFT): Restorations of coal swamp plants. A) <em class='sp'>Lepidodendron</em>, a lycopod (club moss); reached 30 meters (100 feet) tall. B) <em class='sp'>Medullosa</em>, a tree fern; reached 10 meters (35 feet) tall. C) <em class='sp'>Calamites</em>, a sphenopsid (horsetail); reached 20 meters (65 feet) tall. D) <em class='sp'>Cordaites</em>, a conifer-like seed plant; reached 10 meters (35 feet) tall.

Figure 3.19 (AT LEFT): Restorations of coal swamp plants. A) Lepidodendron, a lycopod (club moss); reached 30 meters (100 feet) tall. B) Medullosa, a tree fern; reached 10 meters (35 feet) tall. C) Calamites, a sphenopsid (horsetail); reached 20 meters (65 feet) tall. D) Cordaites, a conifer-like seed plant; reached 10 meters (35 feet) tall.

Figure 3.20: Pennsylvanian brachiopods from Missouri. A) Dictyoclostus sp., about 5 centimeters (2 inches) wide. B) Derbyia sp., about 3 centimeters (1.5 inches) wide. C) Echinoconchus sp., about 5 centimeters (2 inches) wide.

Figure 3.20: Pennsylvanian brachiopods from Missouri. A) Dictyoclostus sp., about 5 centimeters (2 inches) wide. B) Derbyia sp., about 3 centimeters (1.5 inches) wide. C) Echinoconchus sp., about 5 centimeters (2 inches) wide.

Several fossil lagerstätten deposits 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 (Figure 3.21), one of the earliest known diapsid reptiles, can be found at this site.

Figure 3.21: Restoration of Petrolacosaurus, about 40 centimeters (16 inches) long.

Figure 3.21: Restoration of Petrolacosaurus, about 40 centimeters (16 inches) long.

The Elmo Limestone in Dickinson County is famous for its fossil insects and xiphosurids (the group of arthropods that includes horseshoe crabs) (Figures 3.22 and 3.23). Other examples of deposits containing similar faunas include the Robinson locality in Brown County and the Hamilton locality in Greenwood County.

Anapsid, Diapsid, Synapsid...

The skulls of reptiles are distinct from our own in several important respects. One of the most striking differences is that the typical reptile skull looks like a “box within a box.” Unlike the arrangement in our own skulls, the braincase of most reptiles is not built of the bones that make up the outermost layer of the skull. Instead, there is a second set of interior bones (these are largely absent in mammals like us), that surround the braincase; the space between these two layers is filled mostly with muscles. Different groups of reptiles have modified this continuous outer wall of bone by evolving openings, or “apses,” in the skull. The resulting structures are the basis for terms that describe the types of reptilian skulls and the reptile groups that possess them. Anapsid skulls (such as those of turtles) have no openings. Synapsid skulls (such as those of mammal ancestors) have one opening. Diapsids (such as lizards, crocodilians, and dinosaurs) have two openings.

Figure 3.22. <em class='sp'>Paleolimulus</em>. Late Pennsylvanian-Early Permian of Kansas. About 1 centimeter (0.4 inches) long.

Figure 3.22. Paleolimulus. Late Pennsylvanian-Early Permian of Kansas. About 1 centimeter (0.4 inches) long.

Figure 3.23: <em class='sp'>Meganeuropsis</em> is an extinct genus of griffinfly, Order Meganisoptera, and includes the largest known insect that ever lived, with an estimated wingspan of up to 70 centimeters (28 inches), and a body length from head-to-tail of almost 43 centimeters (17 inches). This wing, from Noble County, Oklahoma, in the collection of Harvard’s Museum of Comparative Zoology, is about 33 centimeters (13 inches) long.

Figure 3.23: Meganeuropsis is an extinct genus of griffinfly, Order Meganisoptera, and includes the largest known insect that ever lived, with an estimated wingspan of up to 70 centimeters (28 inches), and a body length from head-to-tail of almost 43 centimeters (17 inches). This wing, from Noble County, Oklahoma, in the collection of Harvard’s Museum of Comparative Zoology, is about 33 centimeters (13 inches) long.

See Chapter 2: Rocks for more information about Permian red beds in Texas.

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. 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 (Figure 3.24). 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, and the enigmatic Diadectes dominate the fauna (Figures 3.25 - 3.27).

Figure 3.24: <em class='sp'>Captorhinus</em>. Reconstructed skeleton and life restoration, about 30 centimeters (12 inches) long.

Figure 3.24: Captorhinus. Reconstructed skeleton and life restoration, about 30 centimeters (12 inches) long.

Figure 3.25: <em class='sp'>Eryops</em>. Reconstructed skeleton and life restoration, 1.5 - 2.0 meters (5 - 6.5 feet) long.

Figure 3.25: Eryops. Reconstructed skeleton and life restoration, 1.5 - 2.0 meters (5 - 6.5 feet) long.

Figure 3.26: <em class='sp'>Dimetrodon grandis</em>, one of the larger species of <em class='sp'>Dimetrodon</em>. Skull and life restoration, about 3 meters (9 feet) long.

Figure 3.26: Dimetrodon grandis, one of the larger species of Dimetrodon. Skull and life restoration, about 3 meters (9 feet) long.

Figure 3.27: <em class='sp'>Diadectes</em>. Skull and life restoration, about 2 meters (6.5 feet) long.

Figure 3.27: Diadectes. Skull and life restoration, about 2 meters (6.5 feet) long.

Above the Permian layers are fossil-bearing Triassic strata, forming a ribbon from the western part of the Texas panhandle southeast to near Abilene. The temporal boundary between the Permian and Triassic marks the greatest mass extinction known (see Figure 3.1). While its effect on marine ecosystems was particularly devastating, life on land was dramatically affected as well. Many kinds of tetrapods disappeared, which may have cleared the path for a new group of tetrapods―dinosaurs (which first appeared in the middle Triassic) ―to dominate the land.

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 crocodile-like (at least in appearance) phytosaurs, and synapsids (Figure 3.28).

Figure 3.28: Phytosaur restoration, 2 - 3 meters (6 - 9 feet) long.

Figure 3.28: Phytosaur restoration, 2 - 3 meters (6 - 9 feet) long.

See Chapter 6: Glaciers to learn more about the history of glaciers in the South Central.

Cretaceous rocks exposed in southern Oklahoma and north central Texas contain a variety of marine mollusks, including bivalves, gastropods, and ammonites. Cenozoic rocks are largely absent from this area, but Pleistocene gravels in southwestern Oklahoma (Tillman County) have produced the bones of glyptodonts (see Figure 3.42). Glacial-age deposits in Dallas County, Texas also contain the bones and teeth of mammoths and mastodons. Late Pleistocene mammals are abundant at several localities in Kansas, including along the banks of the Kansas River from Topeka to Kansas City, and several finds have been made in greater Kansas City itself for more than the past century. Another famous site is the Kimmswick Bone Bed in Jefferson County in eastern Missouri (at Mastodon State Historic Site), which not only contains abundant fossil mastodon and other bones, but also stone tools made by some of the earliest human residents of this part of North America.

Mastodons and Mammoths

These two kinds of ancient elephants (or, more technically, proboscideans) are frequently confused. Both were common during the Pleistocene, but they had different ecological preferences and are usually found separately. Mammoths are close cousins of modern African and Asian elephants; mastodons are more distant relatives, from a separate line of proboscideans that branched off from the modern elephant line in the Miocene. Mastodons have a shorter, stockier build and longer body; mammoths are taller and thinner, with a rather high “domed” skull. In skeletal details, the quickest way to tell the difference is with the teeth: mastodons have teeth with conical ridges, a bit like the bottom of an egg carton; mammoths, in contrast, have teeth with numerous parallel rows of ridges. The teeth are indicative of the two species’ ecological differences. Mastodons preferred to bite off twigs of brush and trees, while mammoths preferred tough siliceous grasses. Thus, mastodon teeth are more suitable for cutting, while mammoth teeth are more suitable for grinding.

A mammoth tooth, suitable for grinding grass and softer vegetation. About 25 centimeters (1 foot) long.

A mammoth tooth, suitable for grinding grass and softer vegetation. About 25 centimeters (1 foot) long.

A mastodon tooth, suitable for chewing twigs and tree leaves. About 20 cm (8 - 9 inches) long.

A mastodon tooth, suitable for chewing twigs and tree leaves. About 20 cm (8 - 9 inches) long.