Fossils of the Central Lowland

Introduction

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.

Paleozoic fossils

Cambrian fossils

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, brachiopods, trace fossils of worms, algae, and jellyfish, and the mysterious hyoliths. 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

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. 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, Krukowski 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.

Ordovician fossils

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. Bryozoans 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. These reefs played a major role in forming the ecosystems that were also home to straight-shelled cephalopods, trilobites, bivalves, gastropods, crinoids, graptolites, brachiopods, and fish.

Trilobites and brachiopods, which had dominated the Cambrian period, expanded somewhat in diversity and abundance in the post-Cambrian. 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.

Silurian fossils

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 (found in Ohio), formed elaborate honeycomb-like colonies. 

In Wisconsin, the Waukesha Biota, known from two quarries in Milwaukee County, is a fossil Lagerstätte (exceptionally preserved fossil deposit). This biota includes brachiopods, trilobites, soft-bodied arthropods, enigmatic conularids, a variety of worm-like animals, and graptolites.

Devonian fossils

Devonian rocks occur in all states in the region, but are limited in extent in Minnesota and Wisconsin. The Cedar Valley Group in Iowa yields abundant remains of Devonian reefs composed of layered sponges called stromatoporoids. Michigan’s state rock, the Petoskey Stone, is actually a colonial rugose coral of the genus Hexagonaria that is found in Devonian-aged rocks (this reef-builder that is also the namesake of Coralville, Iowa). Other rugose corals were solitary, composed of only one coral polyp. All tabulate corals were colonial. The mass extinction event at the end of the Devonian left bryozoans as the major reef-building group until the Mesozoic.

The Silica Shale in western Ohio yields well preserved invertebrates like brachiopods and trilobites. A small sliver of fossil-bearing Devonian rocks of the Milwaukee Formation occurs in Wisconsin between Milwaukee and Sheboygan.

Carboniferous fossils

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.

The Mississippian is sometimes known as the “Age of Crinoids” because of the increase in the abundance and diversity of crinoids, as well as starfish, edrioasteroids, urchins, and other echinoderms 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. Spectacular crinoids, and shark teeth are known from both Indiana and Iowa.

A Mississippian-aged site near Delta, Iowa, has yielded some of the best-preserved examples of early land-dwelling vertebrates. For example, Whatcheeria deltae, described in 1985, is considered a “reptile-like amphibian,” having some anatomical features more like those of amphibians and some more like those of amniotes. This animal measured about 1 meter (3 feet) long, and dates to around 340 million years ago.

Mazon Creek biota

The Mazon Creek lagerstätte is a fossil deposit of Pennsylvanian age exposed in coal mines in northeastern Illinois. Its hematite concretions (or nodules) preserve hundreds of terrestrial and marine plant and animal species in beautiful detail, with many found nowhere else in the world.

Mazon Creek flora

Plants in the Mazon Creek assemblage include those typical of Carboniferous forests, also known as "coal swamps." These wetland forests were home to a tangle of lycopod and sphenopsid trees, early conifers, the now-extinct seed ferns, and other plants. By the Pennsylvanian, huge lycophytes like Lepidodendron and other plants grew thickly on the Midwestern landscape. (Lycophytes survive today but only as very small plants on the forest floor, sometimes called “ground pines.”) Peat built up in these stagnant, wet environments. As 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.

Mazon Creek fauna

Mazon creek also includes many types of animals, such as jellyfish, horseshoe crabs, crustaceans, eurypterids (sea scorpions), millipedes, insects, and bivalves. 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.