Photograph of a group of crinoids from the Ordovician of Ohio. The photo shows randomly arranged crinoid calyces with attached arms. Some individuals also have stalks. The arms are well preserved and have pinnae.

Fossils of the Central Lowland

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Page snapshot: Introduction to the fossils of the Central Lowland region of the midwestern United States.


Topics covered on this page: Introduction; Paleozoic fossils; Cambrian fossils; Krukowski Quarry; Ordovician fossils; Silurian fossils; Devonian fossils; Carboniferous fossils; Mazon Creek biota; Mazon creek flora; Mazon creek fauna; Cretaceous fossils; Pleistocene fossils; Resources.

Credits: Most of the text on this page comes from "Fossils of the Midwestern US" by Alex F. Wall and Warren D. Allmon, chapter 3 in the The Teacher-Friendly Guide to the Earth Science of the Midwestern USedited by Mark D. Lucas, Robert M. Ross, and Andrielle N. Swaby (published in 2014 by the Paleontological Research Institution; currently out of print). The book was adapted for the web by Elizabeth J. Hermsen and Jonathan R. Hendricks in 2022. Changes include formatting and revisions to the text and images. Credits for individual images are given in figure captions.

Updates: Page last updated September 22, 2022.

Image above: Crinoids (Pycnocrinus dyeri) from the Orodovician Arnheim Formation, Dent, Ohio. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).

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.


Photograph of the pygidium of the trilobite Dikelocephalus from the Cambrian of Minnesota. The photo shows a the fossil preserved as an impression on dark brown rock. 

Trilobite pygidium (Dikelocephalus minnesotensis), Cambrian Trempealeau Formation, Washington County, Minnesota. Photo of USNM PAL 17863 by Department of Paleobiology (National Museum of Natural History, Smithsonian Institution, CC0/public domain).


Photograph of Skolithos burrows preserved in Cambrian sandstone in Minnesota. The photo shows beige sandstone with cross-bedding cut vertically by burrows filled with darker brown sediment. 

Burrows (Skolithos), Cambrian Mazomanie Formation, eastern Minnesota. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).


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.


Photograph of the trace fossil Climactichnites from the Cambrian of Wisconsin. The photo shows multiple trackways criss-crossing one another on the surface of a beige rock. Each trackway consists of two parallel lines with light ridges running perpendicularly between them.

Trace fossils (Climactichnites), Cambrian, Krukowski Quarry, Wisconsin. Photo by Kennethcgass (Wikimedia Commons, public domain).


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.


Photograph of fossil algae from the Ordovician of Wisconsin. The photo shows a chunk of light brown rock with impressions of algae preserved on its surface. The algae are frond-like, with a central axis and lateral, up-curved branches. 
Photograph of bryozoans from the Ordovician of Indiana. The photo shows branching bryozoans preserved in a gray rock matrix. Some bits of branchiopod shell can be seen around the bryozoans.

Bryozoans and brachiopods, Ordovician Whitewater Formation, Indiana. Photo by Mark A. Wilson (Wikimedia Commons, CC0 1.0 Universal/public domain dedication).


Photograph of a group of solitary rugose corals from the Ordovician of Wisconsin. The photo shows a group of randomly arranged, horn-shaped corals preserved in a dense aggregation. 

Rugose corals (Streptelasma corniculum) from the Ordovician Platteville Limestone, Wisconsin. Photo of YPM IP 547681 by Jessica Utrup (Yale Peabody Museum of Natural History on GBIF.org, CC0 1.0 Universal/public domain dedication).


Photograph of a brachiopod from the Ordovician of Iowa. The photo shows a brachiopod shell from above and below. From above, the shell has a series of concentric ridges overlain by fine radiating striations. From below, it is relatively smooth, with a pair of depressions near the hinge. The overall shape of the shell is nearly a half-circle.

Upper (left) and lower (right) surface of a brachiopod (Leptaena) shell, Ordovician, Iowa. Photo by Mark A. Wilson (Wikimedia Commons, CC0 1.0 Universal/public domain dedication).


Photograph of brachiopods from the Ordovician of Oho. The photograph shows a dense aggregation of brachiopods preserved on the surface of a gray rock. Some have radiating ridges and some are relatively smooth.

Brachiopods, Ordovician Waynesville Formation, Warren County, Ohio. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).


Photograph of a straight-shelled cephalopod from the Ordovician of Ohio. The photo shows part of a straight cephalopod shell with a series of regularly spaced straight sutures.

A straight-shelled cephalopod, Ordovician Waynesville Formation, Warren County, Ohio. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image reconfigured, cropped, resized).


"Fossil specimen of the cephalopod Actinoceras beloitense from the Ordovician Platteville Limestone of Ogle County, Illinois (PRI 70326). Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Length of specimen is approximately 9 cm. Model by Emily Hauf." Source: Digital Atlas of Ancient Life on Sketchfab.


A series of photographs showing four views of a specimen of an enrolled trilobite from the Ordovician of Indiana. The photos show, from left to right, a view from above, a view from the front (with the trilobite facing the viewer), and a view from each side (trilobite facing left and right). 

An enrolled trilobite (Flexicalymene meeki) in four views: from above (left), from the front (second from left) and from each side (two right photos), Ordovician Waynesville Formation, Franklin County, Indiana. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image reconfigured, cropped, resized).


Photograph of a starfish from the Ordovician of Wisconsin. The photo shows a small starfish with five arms preserved on the surface of a yellowish-beige rock along with fragments of other fossils. The starfish measures about 1.5 centimeters in diameter based on a scale at the bottom of the image.
Photograph of crinoids from the Ordovician of Ohio. The photo shows a group of crinoids exposed on the surface of a beige rock. One crinoid has much of its stalk, calyx, and arms preserved. The other crinoids consist mostly of parts of the calyx and/or arms. The arms on this species of crinoid bifurcate repeatedly.

Crinoids (Iocrinus subcrassus) from the Ordovician McMillan Formation, Hamilton County, Ohio. Photo of USNM PAL 33475 by Bruce Martin (National Museum of Natural History, Smithsonian Institution, CC0/public domain).


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.


Photograph of a branching bryozoan from the Silurian of Illinois. The photo shows a heavily branched, gray structure that his completely weathered out of the rock matrix. The surface of the bryozoan is bumpy.

A bryozoan (Fistulipora neglecta), Silurian, Illinois. Photo by James St. John (flickr, Creative Commons 2.0 Generic license, image resized).


Photograph of the tabulate coral Halysites from the Silurian of Ohio. The photo shows a beige rock with a gray, chain-link-like network covering part of its upper surface.

The tabulate coral Halysites, Silurian Brassfield Formation, Ohio. Photo by James St. John (flickr, Creative Commons 2.0 Generic license, image resized).


Photograph of internal molds of pentamerid brachiopods from the silurian of Illinois. The photo shows a yellowish rock with internal molds on its surface. The molds appear beak-like.

Internal molds of pentamerid brachiopods, Silurian Racine Formation, Grafton, Illinois. Photo by James St. John (flickr, Creative Commons 2.0 Generic license, image resized).


Photograph of a rock with three trilobites on its surface from the Silurian of Illinois. The photo shows an off-white rock with three relatively complete trilobites on top, each trilobite with its head pointed toward the top of the image. The trilobites do not have spines or other ornamentation.
3-panel figure showing photos of animals from the Silurian Waukesha Biota of Wisconsin. Panel 1: Photo of a trilobite preserved in off-white rock. The trilobite is oriented vertically with its head toward the top of the image. The cephalon has two long, backwardly pointed spines. Panel 2: Photo of the arthropod Parioscorpio. The arthropod is oriented vertically with its head toward the top of the image. It has two appendage emerging from the head, and multiple body segments. Panel 3: Photo of a worm identified as a possible leech. The photo shows an impression of a worm on brown stone. The worm's body is folded over on itself, and it has a series of horizontally oriented dark brown bands on it.

Fossils from the Silurian Waukesha Biota, Brandon Bridge Formation, southeastern Wisconsin. Left: A dalmatid trilobiteCenter: An arthropod (Parioscorpio venator). Right: A worm (leech?). Photos by Kennethcgass (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image rotated and resized).


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.


Photograph of a stromatoperoid from the Devonian of Iowa. The photo shows an off-white, spherical fossil with a light pattern of swirling lines on its surface. The fossil is about 13.5 centimeters in diameter.
Photograph of the colonial rugose coral Hexagonaria from the Devonian of Michigan. The photo shows part of a curved gray rock covered with a series of shallow circular pits. 

The colonial rugose coral Hexagonaria, Devonian, Michigan. Photo by James St. John (flickr, Creative Commons 2.0 Generic license, image resized).


Photograph of the colonial rugose coral Hexagonaria from the Devonian of Michigan. The photo shows a detail of the polished surface of the coral. The coral is yellowish beige in color and made up of six-sided polyps. Radiating septa can be seen within each polyp. 

Detail of a Petoskey stone (Hexagonaria percarinata), Devonian, Michigan. Photo by Mark A. Wilson (Wikimedia Commons, CC0 1.0 Universal/public domain dedication).


Photograph of a fossiliferous rock from the Devonian Milwaukee Formation of Wisconsin. The photo shows a chunk of light gray rock with dark gray branching bryozoans on its surface. The bryozoans are perforated by round holes. Branchiopods and crinoid columnals can also be seen on the rock's surface.

Bryozoans (Sulcoretepora) with brachiopods and crinoid columnals (stalk segments), Devonian Milwaukee Formation, Wisconsin. Photo by Kennethcgass (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image resized).


Photograph of a brachiopod from the Devonian of Ohio. The photo shows the upper surface of a brachiopod shell with radiating ridges. The shell is roughly half-circular in shape.

A brachiopod (Mucrospirifer mucronatus), Devonian Silica Shale Formation, Paulding County, Ohio. Photo by Mark A. Wilson (Wikimedia Commons, CC0 1.0 Universal/public domain dedication).


Photograph of a brachiopod from the Devonian of Ohio. The photo shows the lower and upper  surfaces of a brachiopod shell with with fine radiating ridges. The shell is roughly half-circular in shape.

Lower (left) and upper (right) surfaces of a brachiopod (Strophodonta demissa), Devonian Silica Shale Formation, Michigan. Photo by Mark A. Wilson (Wikimedia Commons, CC0 1.0 Universal/public domain dedication).


Photographs showing four views of a brachiopod from the Devonian of Wisconsin on a black background. Clockwise from upper left: View from above, view from below, view of the hinge, view from the side (hinge to the right). The shell is unornamented expect for a series of fine radiating ridges and furrows.
2-panel figure made up of photographs of nautiloid cephalopods from the Devonian of Wisconsin. Panel 1: Straight-shelled cephalopod. The photo shows a shells that is wide at the base, gets wider about 1/3 of the way up from the base, and tapers near the apex. A series of regularly spaced sutures occurs on the shell. Panel 2: Loosely coiled cephalopod shell. The photo shows a shell coiling in a single plane, with the coils not touching each other. Regularly spaced sutures occur on the shell. 

Nautiloid cephalopods from the Devonian Milwaukee Formation, Wisconsin. Left: A straight-shelled cephalopod (Acleistoceras whitfieldi). Right: A coiled cephalopod (Gyronaedyceras eryx). Left photo and right photo by Kennethcgass (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image resized).


Photograph of a slab of trilobites from the Devonian of Ohio. The photo shows a group of dark brown trilobites exposed on the surface of a light gray rock. More than 20 trilobites are preserved. The trilobites have no spines or other ornamentation.
Photograph of a dermal bone from a placoderm fish from the Devonian of Ohio. The photo shows a roughly rectangular bone that is broken at the left end. The bone is dark brown in color and its surface is pebbly.

Dermal plate from a placoderm fish (Aspidichthys clavatus), Devonian Olentangy Shale, Delaware County, Ohio. Photo of USNM V 20730 by Michael Brett-Surman (National Museum of Natural History, Smithsonian Institution, CC0/public domain).


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.


Photograph of a crinoid from the Mississippian of Indiana. The photo shows a crinoid exposed on the surface of a light gray rock. The crinoid is folded back on itself, with in arms pointed to the right and the stalk arching over the top of the calyx and arms. The arms have delicate pinnules preserved.

A crinoid (Platycrinites), Mississippian Keokuk Limestone, Montgomery County, Indiana. Photo of USNM S 6075 by Bruce Martin (National Museum of Natural History, Smithsonian Institution, CC0/public domain).


Photograph of a crinoid from the Mississippian of Indiana. The photo shows a crinoid exposed on the surface of a beige rock. The crinoid is arranged in roughly an oval shape, with the base of the stalk on the left turning 90 degrees and then proceeding horizontally before making a u-turn, with the stalk ending in the calyx and arms oriented horizontally near the top of the image, with the arms pointed left. The calyx is small relative to the long, graceful arms of the crinoid.

A crinoid (Springericrinus sacculus), Mississippian Ramp Creek Formation, Montgomery County, Indiana. Photo of USNM S 5999 by Crinoid Type Image Project (National Museum of Natural History, Smithsonian Institution, CC0/public domain).


Photograph of a slab of crinoid fossils from the Mississippian of Iowa. The photo shows a light beige slab of rock with multiple crinoids exposed on its surface. Some of the specimens include a stalk, calyx, and arms. At least 10 specimens appear to be on the slab.

Slab of crinoids and a blastoid, Mississippian Le Grand Formation, Marshall County, Iowa. Species are Cactocrinus nodobrachiatusDichocrinus inornatusPlatycrinitesRhodocrinus, and an unnamed blastoid. Photo of USNM S 6084_1 by Bruce Martin (National Museum of Natural History, Smithsonian Institution, CC0/public domain).


Photograph of a crinoid exposed on the surface fo a rock. The photo shows a beige to yellow rock with a single large crinoid laying horizontally on its surface, with arms to the left and stalk to the right. Arms and calices of four other smaller crinoids also occur on the slab.
Photograph of a tooth plate from a shark, Mississippian of Indiana. The photo shows a roughly oval, light brown tooth plate with what appear to be small pores regularly distributed over its surface. 

Tooth plate from a shark (Psephodus) that ate shelly invertebrates, Mississippian Warsaw Shale, Lawrence County, Indiana. Photo of USNM V 13906 by Michael Brett-Surman (National Museum of Natural History, Smithsonian Institution, CC0/public domain).


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.


Photograph of a Mazon Creek nodule split in half to reveal the impression of a large lycophyte cone on two surfaces (both part and counterpart shown). The cone is an elongated structure with a rounded tip. The base of the cone is missing. Cone scales can be seen on the sides of the cone; the scales look like short spines. 

Cone of an extinct lycophyte tree (Lepidostrobus), Pennsylvanian, Grundy County, Illinois. Photo of YPM PB 000679 by Fiona O'Brien (Yale Peabody Museum of Natural History on GBIF.org, CC0 1.0 Universal/public domain dedication).


Photograph of a Mazon Creek nodule split in half to reveal the impression of whorls of leaves on its surface. The photo shows Annularia leaf whorls arranged on a thin branch. Each whorl consists fo more than 10 leaves. The leaves elongated-obovate in shape. The whorls are widely spaced along the branch. The whole specimen is more than 10 centimeters in length.

Leaves of an extinct sphenophyte tree (Annularia stellata), Pennsylvanian, Grundy County, Illinois. Photo of YPM PB 000719 by PB Staff (Yale Peabody Museum of Natural History on GBIF.org, CC0 1.0 Universal/public domain dedication).


Photograph of a Mazon Creek nodule split to expose a Pecopteris fern frond. The photo shows only one side of the nodule; the counterpart is not shown. The frond has a central axis bearing pinnae with are further divided in pinnules. The frond is overall oblong in shape, taper toward its apex.

A frond of a marattioid tree fern (Pecopteris), Pennsylvanian Francis Creek Shale Member, Carbondale Formation, Illinois. Photo of USNM PAL 444624 by Chip Clark (National Museum of Natural History, Smithsonian Institution, CC0/public domain).


Photograph of a Mazon Creek specimen of seed fern foliage. The photo shows a close-up of a dark brown frond preserved on a medium brown rock. The frond has a central axis and lateral pinnae. The pinnae are subdivided into pinnules.

A frond of a seed fern (Neuropteris flexuosa), Pennsylvanian, Illinois. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).


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.


Photograph of a Mazon Creek nodule split to show part and counterpart of a horseshoe-crab-like animal.

A horseshoe crab-like animal (Euproops danae), Pennsylvanian, Grundy County, Illinois. Photo of USNM PAL 386656 by Michael Brett-Surman (National Museum of Natural History, Smithsonian Institution, CC0/public domain).


Photograph of a Mazon Creek nodule split in half to reveal the impression of a fossil crustacean on one surface. The animal has an elongated body made up of multiple segments. It has multiple long legs and antennae emerging from its head. The overall length of the animal is more than 4 centimeters.
Photograph of a Mazon Creek nodule split in half to reveal the impression of a myriopod on one surface. The photo shows a brown rock with a fossil impression that varies from light brown to white in color. The fossil is an animal with an elongated body made up of multiple segments, each with legs. Neither the head or the tail end of the body appear to be preserved.
Photograph of a Mazon Creek nodule split in half to reveal the impression of a cockroach on one surface. The photo shows an insect impression in a medium brown stone. The insect is facing right and the wings are folded back on the body. The animal is about 8 centimeters long.
Photograph of a Mazon Creek nodule split in half to reveal the impression of a shark on one surface. The rock is grayish-white in color. The shark as a elongated snout and short pectoral fins. Two large eyes can be seen behind the snout.

A shark (Bandringa rayi), Pennsylvanian, Illinois. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).


"Tully: Monster vs Method." Video by the Field Museum (via Vimeo).

Cretaceous fossils

While an expansive record of Paleozoic life is present in the Midwest, the Mesozoic Era is relatively poorly represented. Cretaceous-aged rocks in Minnesota, Iowa, and southern Illinois provide some glimpses into life during this time period. A few fragmentary dinosaur fossils have been found in riverine deposits in the lower parts of the Dakota Formation of Minnesota and Iowa. Abundant fossil leaves of flowering broadleaf plants have been found in the Dakota Formation of south-central Minnesota.

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. In southern and western Minnesota, similar layers have produced shark teeth, sea turtles, and scattered marine invertebrates, including ammonites, bivalves, and microfossils (foraminifera). Fragments of a Creatceous crocodile (Terminonaris robusta) and teeth of a crow shark (Squalicorax) have been found In the Mesabi Iron Range of northeastern Minnesota.


Photograph of Late Cretaceous gastropods from the Mesabi Iron Range, Minnesota. The photo shows two gastropod (snail) shells, one viewed from the apex so that looks like a spiral, and one from the side. The shells are orange in color and do not appear well preserved. The rock matrix around them is brown and appears coarse-grained.

Gastropods from the Late Cretaceous Coleraine Formation, Calumet, Minnesota. Photo by Rylan Bachman (Wikimedia Commons, Creative Commons Attribution 4.0 International license, image resized).


Photograph of Late Cretaceous shark tooth from the Mesabi Iron Range, Minnesota. The photo shows a roughly triangular shark tooth missing its root. The tooth is orange in color with serrated edges.

Crow shark tooth (Squalicorax), Late Cretaceous Coleraine Formation, Calumet, Minnesota. Photo by Rylan Bachman (Wikimedia Commons, Creative Commons Attribution 4.0 International license, image resized).

Pleistocene fossils

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 in the region are due to glaciers eroding bedrock away.

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. Many fossils from the Quaternary are found in pond and stream sediment dating from the receding of glaciers. 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 the region today. Many 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 form 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, mastodons, giant beavers, peccaries, tapirs, foxes, bears, seals, deer, caribou, bison, and horses. Numerous mastodon skeletons have been found throughout the Midwest.


Photograph of Pleistocene spruce cones and snail shells from Michigan. The photo shows small black seed cones of a spruce intermixed with very small white snail shells.

Pleistocene spruce (Picea) seed cones mixed with gastropod shells, Oakland County, Michigan. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).


Photograph of the skull of a beaver from the Pleistocene of Indiana. The photo shows the upper part of a beaver skull with its nose pointed to the left. The skull preserved the characteristic large front teeth of the animal.
Photograph of a bear skull from the Pleistocene of Ohio. The photo shows the upper part of a bear skull with its nose pointing toward the left. The bone is medium brown in color, and the teeth are all missing or broken off.
Photograph of an American mastodon from the Pleistocene of Indiana on display in a museum. The photo shows the skull, tusks, neck, and shoulders of a mounted mastodon skeleton. The bones are dark brown and the tusks are light brown. The characteristic teeth with pointed cusps can be seen in the open mouth.

An American mastodon (Mammut americanum), Pleistocene, DeKalb County, Indiana. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).

Resources

Resources from the Paleontological Research Institution & partners

Cretaceous Atlas of Ancient Life, Western Interior Seaway (Colorado, Iowa, Kansas, Montana, Nebraska, New Mexico, North Dakota, Oklahoma, South Dakota, Texas, Utah, Wyoming): https://www.cretaceousatlas.org/geology/

Digital Atlas of Ancient Life Virtual Collection: https://www.digitalatlasofancientlife.org/vc/ (Virtual fossil collection featuring 3D models of fossil specimens sorted by group)

Digital Encyclopedia of Ancient Life: https://www.digitalatlasofancientlife.org/learn/

[email protected]: Earth Science of the Northwest-central U.S.: Fossils of the Central Lowland (covers the Central Lowland in Nebraska, North Dakota, and South Dakota): https://earthathome.org/hoe/nwc/fossils-cl/

[email protected]: Earth Science of the South-central U.S.: Fossils of the Central Lowland and Interior Highland (covers the Central Lowland in Kansas, Missouri, Oklahoma, and Texas): https://earthathome.org/hoe/sc/fossils-cl-ih/ 

[email protected]: Quick guide to common fossils: https://earthathome.org/quick-faqs/quick-guide-common-fossils/

Pennsylvanian Atlas of Ancient Life, Midcontinent United States (Iowa, Illinois, Indiana, Kansas, Missouri, Nebraska, Oklahoma, Texas): https://pennsylvanianatlas.org/


Go to the full list of resources about fossils in the midwestern U.S.

Go to the full list of resources about fossils