Region 2: Fossils of the Central Lowland

The Paleozoic is well represented in the Central Lowland region of the Midwest, from the Cambrian through the Pennsylvanian periods. Together, fossils from across the region record some of the most important chapters in life’s story. The Central Lowland’s fossils take us on a journey from when complex organisms first became abundant, through the development of reefs and other recognizable components of marine ecosystems, to the invasion of land. To the west, a few Cretaceous fossils of giant marine reptiles and sharks may be found. Terrestrial fossils from the most recent ice age, beginning just two million years ago, have also been discovered in every state in the Midwest, preserved in sediment left by the glaciers.

Cambrian

The Cambrian period is represented in the Central Lowland by an irregular strip that cuts east and west through Wisconsin, crossing into neighboring parts of Minnesota and Michigan. The beginning of this period is marked by the relatively sudden appearance of an unprecedented diversity of creatures. Within a span of roughly 30 million years, all of the major animal groups that we know today, such as mollusks, arthropods, echinoderms, and vertebrates, appeared within the Earth’s oceans. This time of rapid diversification and evolution is known as the “Cambrian Explosion.” These new kinds of animals interacted with each other and their environments radically in new ways. It was during this time that animals quickly evolved a suite of innovations, including mobility, vision, and hard mineralized parts.

While the Cambrian period was the beginning of complex life, its cast of characters is very unlike the animals of today. Cambrian-aged rocks in Wisconsin and Minnesota preserve the hard parts of trilobites (Figure 3.4), brachiopods, trace fossils of worms, algae, and jellyfish, and the mysterious hyoliths (Figure 3.5). Hyoliths are animals with cone-shaped shells that existed throughout the Paleozoic Era. Their affinities to other animals are uncertain, with some scientists classifying them as mollusks and others placing them in their own phylum. The Krukowski Quarry in central Wisconsin reveals an extraordinary fossil assemblage of Cambrian marine invertebrate trackways and jellyfish in cream-colored sandstone that looks like it formed on a beach.

Figure 3.4. Trilobite. Dikelocephalus. Figure 3.5. Hyolithid. About 1–2 cm Length about 2 cm (1 inch).

Figure 3.5. Hyolithid. About 1–2 cm Length (less than an inch) long.

The Krukowski Quarry

The Krukowski Quarry, near Mosinee, Wisconsin, is a locality famous for its unique preservation of trace fossils from a 510-million-year-old beach. Fossilized trackways found there give paleontologists clues about the organisms and the way they moved around under the shallow water. Amazingly, the quarry also contains fossilized traces of scyphozoan jellyfish. Preservation of such soft creatures is rare enough, but these are both the oldest and, at a diameter of up to 50 centimeters, several times larger than any other fossil jellyfish! At a time when few animals had hard parts, these jellyfish were likely top predators. Their extraordinary preservation seems to indicate that these unfortunate individuals had become stranded on the beach, long before any life (except perhaps bacteria) had colonized land.

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

Jellyfish impression from the Cambrian of Wisconsin. Impression is about 30 cm (1 foot) in diameter.

Ordovician to Pennsylvanian

Trilobites and brachiopods, which had dominated the Cambrian period, expanded somewhat in diversity and abundance in the post-Cambrian. As the Paleozoic Era progressed, they were joined by many other new or expanding groups of animals. Many of these were filter feeders. Trilobites (which probably fed on seafloor mud) developed defenses against threats from new kinds of predators and competitors: spines, acute vision, and the ability to swim or burrow.

Ice ages occurred at the ends of both the Ordovician and Devonian periods, dramatically affecting life in the then-tropical Central Lowland by sequestering water in polar ice sheets. This caused dramatic changes in sea level, which resulted in devastating mass extinctions, allowing new successions of organisms to come to the fore.

Ordovician fossils are found in all seven Midwestern states. Life in the tropical sea that covered much of the central United States at the time experienced a burst of diversity, which increased fourfold compared with that of the Cambrian. Beginning in the Ordovician and expanding through the rest of the Paleozoic, great reefs were formed by organisms like bryozoans, corals, algae, and sponges, which are found in rocks in the Central Lowland region. These reefs played a major role in forming the ecosystems that were also home to straight-shelled cephalopods (Figure 3.6), trilobites (Figure 3.7), bivalves, gastropods, crinoids, graptolites (Figure 3.8), brachiopods (Figures 3.9–3.11), and fish (Figure 3.12). By the Devonian, fish had become increasingly prominent and diverse. Meanwhile, plants and arthropods had begun to populate the hitherto barren land.

Figure 3.6: Straight (orthocone) nautiloid, shell and animal reconstruction. These animals reached lengths of more than 4 meters (12 feet), making them among the largest invertebrate animals that ever lived. Specimens 30–90 cm (1–3 feet) long are frequently found.

Figure 3.7: Ordovician trilobites. A) Isotelus maximus, state fossil of Ohio. This species reached more than 30 cm (1 foot) long. B) Calymene celebra, state fossil of Wisconsin. About 2 cm (1 inch) long.

Rock with many fragments of Climacograptus colonies.

Restoration of what graptolite colonies may have looked like when they were alive, floating in the water.

Figure 3.9: Ordovician brachiopods from Ohio and Minnesota. A) Strophomena incurvata. B) Platystrophia biforata. C) Rhynchotrema capax. D) Rafinesquina alternata. Each about 4 cm (1.5 inches) wide.

Figure 3.10: The Silurian brachiopod Pentamerus is often preserved as an internal mold. A) The “slots” show the location of supports for internal organs that extended into the interior of the shell. These strange-looking fossils are sometimes called “pig’s feet.” B) The exterior of the shell. Specimens are about 2 cm (1 inch) long.

Figure 3.11. Devonian brachiopods from Iowa. A) Platyrachella sp. [about 8 cm (3 inches) wide]. B) Atrypa devoniana [about 3.5 cm (1.5 inches) wide].

Figure 3.12: Silurian acanthodian fish reconstruction and spine. These fish reached lengths of up to 30 cm (1 foot). Spines are 5–10 cm (2–4 inches).

Mazon Creek

The Mazon Creek lagerstätte is a fossil deposit of Pennsylvanian age exposed in coal mines in northeastern Illinois. It’s hematite concretions (or nodules) preserve hundreds of fossil plant and animal species in beautiful detail,(both terrestrial and marine), with many found nowhere else in the world. Among the amazing fossils of Mazon Creek is Illinois’ state fossil, the Tully Monster or Tullimonstrum, an unusual invertebrate that is thought to have swum using its three-finned tail and hunted using its eight-toothed proboscis to bring prey to the mouth. Its place on the tree of life is uncertain, though some scientists suspect it is a mollusk. Without the exceptionally preserved specimens at Mazon Creek, this soft-bodied predator would be unknown to science. Tully Monsters reached lengths of up to 30 cm (1 foot), but most specimens are less than half that length.

Tullimonstrum gregarium, the state fossil of Illinois.

A reconstruction of the Tully Monster in life.

During the Mississippian and Pennsylvanian, the expansion and contraction of glaciers far to the south caused sea levels to fluctuate. In the Central Lowland, these periods produced cyclothems, which are cycles of alternating marine and terrestrial rocks, often including coal. By that time, the first vertebrates had crawled onto the land, joining numerous arthropods that already lived in expansive, swampy forests. In the ocean, brachiopods, trilobites, and reef-builders were decimated by the mass extinction at the end of the Devonian.

Reef Builders

Through the Silurian and Devonian periods, reefs expanded across the shallow sea that covered the Midwest. While these reefs performed ecological functions similar to those of modern coral reefs, many of the animals that constructed them were very different. Now-extinct tabulate corals, like Halysites (Figure 3.13) found in Ohio, formed elaborate honeycomb-like colonies. Bryozoans (Figure 3.14) are an entire phylum of colonial marine animals, and during the Paleozoic, their erect, branching skeletons formed vast thickets that are fossilized in the Ordovician rock around Cincinnati. Michigan’s state rock, the Petoskey Stone, is actually a colonial rugose coral of the genus Hexagonaria (Figure 3.15), a reef-builder that is also the namesake of Coralville, Iowa. Other rugose corals were solitary—composed of only one coral polyp (Figure 3.16). All tabulate corals were colonial.

The Cedar Valley Group in Iowa yields abundant remains of Devonian reefs composed of layered sponges called stromatoporoids (Figure 3.17). The mass extinction event at the end of the Devonian left bryozoans as the major reef-building group until the Mesozoic.

Halysites sp. About 10 cm (4 inches) across.' href='/images/tfgg/halysites_lg.png'>

Figure 3.13: Tabulate coral, Halysites sp. About 10 cm (4 inches) across.

Figure 3.14: Bryozoans. A) Rhombopora sp., Ordovician. About 5–10 cm (2–4 inches) long. B) Archimedes sp., Carboniferous. 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 usually less than an inch long. C) Archimedes life reconstruction.

Figure 3.15: Surface of the colonial rugose coral Hexagonaria, Devonian. When polished, this fossil is called the Petoskey Stone, the state rock of Michigan. Hexagonaria colonies were sometimes bowling ball-sized; most Petoskey stones are the size of ping pong balls. Individual corallites are about 12 mm (0.4 inches) in diameter.

Figure 3.16: A solitary rugose, or “horn” coral. Some horn corals reached lengths of 20 cm (8 inches), but most were less than 2 cm (about 1 inch) long.

Figure 3.17: Stromatoporoid sponge. About 30 cm (1 foot) across.

Fishes and Filter-Feeders

Sharks trace their lineage to the Ordovician, more than 420 million years ago, and they became increasingly diverse throughout the remainder of the Paleozoic Era. Ohio’s Cleveland Shale preserves specimens of the well-studied primitive shark Cladoselache (Figure 3.18). This shark was relatively common during the late Devonian and possessed an interesting blend of primitive and derived characteristics. Superficially, its body looked like that of a modern shark—but the 1.2-meter-long (4-foot-long) Cladoselache had a terminal mouth (rather than a mouth somewhat under the “nose” as in modern sharks), almost no scales, and no claspers—leaving scientists to wonder how they reproduced.

Figure 3.18: Cladoselache, a shark about 1.2-meter-long (4-foot-long).

The Devonian saw fishes explode in diversity, from tiny scavengers to huge hunters. The 9-meter-long (30-foot-long) Dunkleosteus (Figure 3.19) did not have a bony internal skeleton, but its head was covered in bony armor. While remains of its soft parts, including its unarmored rear two-thirds, have yet to be discovered, the plates of numerous specimens have been found in the Cleveland Shale of Ohio, and the Cleveland Museum of Natural History houses the largest known specimens. The plates that formed the jaw functioned as huge blades, rubbing against each other to stay sharp as the fish chomped its prey: sharks, other fish, and large invertebrates. Along with T. rex, it is estimated to have had one of the most powerful bites in the history of life, and it was the apex predator of the late Devonian seas. Despite its apparent advantages, it and all of its its placoderm kin became extinct at the end of the Devonian.

The Mississippian is sometimes known as the “Age of Crinoids” because of the increase in the abundance and diversity of crinoids (Figure 3.20), as well as starfish, edrioasteroids (Figure 3.21), urchins, and other echinoderms (Figure 3.22) during this time. Sites near Crawfordsville, Indiana are world-famous for containing abundant specimens of more than 60 well-preserved crinoid species. While it is common to find stem pieces of crinoids throughout the Midwest, discoveries of the head, or calyx, are much rarer. Crawfordsville provides a wealth of unbroken lengths of stem and calyxes from these filter-feeding animals.

Figure 3.19: Dunkleosteus. The dotted lines show inferred shape of the unpreserved part of the body. Total length was probably about 9 meters (30 feet).

Figure 3.20: Crinoids from the Mississippian of Indiana. A) Crown and stem; about 15 cm (6 inches) long. B) Stem fragments. C) Holdfast; about 8 cm (3 inches) across.

Figure 3.21: Edrioasteroid, Isorophus cinicinnatiensis, attached to a bivalve shell. Ordovician. About 2 cm (about 1 inch) across.

Figure 3.22: A) Blastoid. Pentremites sp. 1–2 cm (about .75 inches) long. B) Cystoid. Caryocrinites sp. 1–2 cm (about .75 inches) long.

Land Ho!

After the appearance of land plants in the Ordovician and the presence of established forests by the beginning of the Mississippian, much of the Midwest’s land through the remainder of the Paleozoic consisted of swampy deltas. Wetland forests were home to a tangle of lycopod and sphenopsid trees, early conifers, the now-extinct seed ferns (Figure 3.23), and other plants. By the Pennsylvanian, huge lycopods like Lepidodendron (Figure 3.24) and other plants grew thickly on the Midwestern landscape. (Lycopods survive today but only as very small plants on the forest floor, sometimes called “ground pines.”) These ancient ecosystems are sometimes called “coal swamps” because the stagnant, wet environments in which they thrived protected huge volumes of vegetation from decomposing. As the decaying plant matter accumulated for tens of millions of years, it formed thick, extensive deposits of coal underlying much of the Central Lowland as well as the parts of the Inland Basin.

Cretaceous

While an expansive record of Paleozoic life is present in the Midwest, the Mesozoic Era is relatively poorly represented. Still, Cretaceous-aged rocks in Minnesota, Iowa, and southern Illinois provide some glimpses into life during this time period.

Figure 3.23: Neuropteris, a seed fern in a Mazon Creek nodule. Length about 9 cm (4 inches).

Figure 3.24: Lepidodendron. Left: close-ups of leaf scars on the trunk. Right: reconstruction of the entire tree, which reached 30 meters (100 feet) in height.

A few dinosaur fossils have been found in riverine deposits in the lower parts of the Dakota Formation of Minnesota and Iowa (and also in nearby Kansas and Nebraska). Later layers were deposited under marine conditions, as the huge Western Interior Seaway invaded farther into the upper Midwest. In Iowa, these marine strata (including the Graneros, Greenhorn, Carlile, and Niobrara formations) have yielded a few bones of large sea reptiles called plesiosaurs (Figure 3.25). In southern and western Minnesota, similar layers have produced shark teeth (Figure 3.26), sea turtles, and scattered marine invertebrates, including ammonites (Figure 3.27), bivalves, and microfossils (foraminifera). Abundant fossil leaves of flowering broadleaf plants have been found in south-central Minnesota (Figure 3.28).

These Midwestern fossils provide a snapshot of the dramatic changes in marine fauna once the Paleozoic ended. The once-dominant trilobites were now extinct, brachiopods and bryozoans had become very scarce, and tabulate and rugose corals, once abundant, were gone. Reefs remained a crucial ecosystem for marine life, but they were now formed almost exclusively by scleractinian corals and bizarre rudist clams. Other mollusks like ammonoid cephalopods, snails, and clams were also much more diverse and abundant than they had been during the Paleozoic. Life on land also changed dramatically: by the mid-Cretaceous, flowering plants and insects had suddenly become ubiquitous. In addition to dinosaurs, which dominated the land, various other kinds of reptiles, including mosasaurs, ichthyosaurs, and plesiosaurs, dominated the sea, and pterosaurs as well as the first birds filled the air. The oceans teemed with newly abundant and diverse bony fish, sharks, and rays.

Figure 3.25: A) Reconstruction of a plesiosaur in life. Some plesiosaurs reached 15 meters (50 feet) long. B) Plesiosaur vertebrae. About 20 cm (8 inches) across.

Figure 3.26: Cretaceous shark teeth from the west end of the Minnesota River valley in west-central Minnesota. Tooth on far left about 2.5 cm (1 inch) long.

Figure 3.27: Jeletzkytes, a scaphitid ammonite cephalopod. Fossils of these animals have been found in Cretaceous rocks in southern Minnesota. Specimen is about 10 cm (4 inches) across.

Figure 3.28: Cretaceous fossil leaves from Minnesota. A) Metasequoia sp. Leaves about 0.25 cm (0.1 inch) long. B) Magnolia sp. Leaf about 12 cm (5 inches) long.

Ice Age Fossils in Pleistocene Deposits

About 2.6 million years ago, permanent ice sheets formed in the Northern hemisphere, marking the beginning of the Quaternary period (which extends to the present). During this time, glaciers repeatedly scraped their way southward across the Midwest and melted back northward. Some gaps in the fossil record are due to glaciers eroding bedrock away. Fossils from the Quaternary are found either in pond and stream sediment dating from the receding of glaciers, or they exist as isolated tooth or bone fragments found in glacial till. Some important animal fossils are preserved in Pleistocene caves. The glaciers began to retreat from the Midwest about 15,000 years ago, leaving behind the landscape we see today as well as the sediment in which we find fossils and sometimes human artifacts.

As the glaciers melted away from the Midwest, some of the geographic features they created filled with water and formed many of the lakes and ponds present in those states today. Even the Great Lakes were formed by ice sheets, though their geologic underpinnings go back much farther. Many smaller bodies of water left by the glaciers have since been filled with sediment and are virtually invisible at the surface. When flooding or construction exposes these pond sediments, the organisms preserved in them are suddenly revealed. Nearly all glacial-age ponds contain a rich fossil record of small freshwater mollusks, pieces of wood, pollen, and seeds, many of which increased over time as plant communities recolonized land freed from the ice. Since the shape of pollen indicates the kind of plant it came from, the pollen record can give a detailed account of how vegetation moved into an area as the climate changed. As plants returned, so did large animals: large vertebrate remains include those of mammoths (Figure 3.29), mastodons (Figure 3.30), giant beavers, peccaries, tapirs, foxes, bears, seals, deer, caribou, bison, and horses. Numerous mastodon skeletons have been found throughout the Midwest, especially in Michigan, Illinois, Ohio, and Indiana.

Figure 3.29: A Pleistocene woolly mammoth, Mammuthus primigenius.

Figure 3.30: A Pleistocene mastodon, Mammut americanum.

Mastodons & Mammoths

Among the most common Pleistocene vertebrate fossils in the Midwest are those of mastodons and mammoths. People frequently confuse these two kinds of ancient elephants (or, more technically, proboscideans). Both were common during the Pleistocene, but they had different ecological preferences and are usually found separately. Mammoths are from the same line of proboscideans that gave rise to African and Asian elephants; mastodons are 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 cm (almost a foot) long.

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