Fossils


Fossils can tell us what life was like on Earth in ancient geologic time, helping geologists describe ancient depositional environments and understand past climates. Fossils also show us that life on Earth has changed, or evolved, through time—from primitive algae and bacteria to fish, reptiles, mammals, and more.

In Minnesota, fossils can be found in deposits as old as about 2 billion years (such as the stromatolites that occur in the Precambrian iron-rich rocks of the Mesabi Iron Range) to as young as about 10,000 years (like the Pleistocene mammals that have been found in glacial deposits). Below is a brief summary description of some of the more common fossils that can be found in Minnesota.

Common fossils in Minnesota

Cyanobacteria (stromatolites)

Cyanobacteria are a diverse group of photosynthetic bacteria. Modern forms can have symbiotic relationships with plants, helping convert nitrogen gas in the soil into other forms that the plants can absorb. Some ancient forms of cyanobacteria (and other microbes) built mounded structures in shallow marine environments that are now left as fossil stromatolites. Stromatolites are among the world's oldest known fossils and are still present today in near-similar environemnts, such as along the coastline of Shark's Bay in Australia. Fossil stromatolites occur in the Precambrian iron-rich rocks in northern Minnesota and in the carbonate rocks of the Ordovician Prairie du Chien Group in southeastern Minnesota.

Fossil stromatolites in Precambrian bedrock of northern Minnesota.
Fossil stromatolites in Precambrian bedrock of northern Minnesota.

Photo courtesy of Mark A. Jirsa

Sponges and sponge-like animals

The fossil sponges in Minnesota largely occur in the Ordovician shale and carbonate rocks, such as the Decorah Shale, Cummingsville Formation, Prosser Formation, and Stewartville Formation, in the southeast. Fossil sponges had hard skeletons made of calcium carbonate or silica. Like modern sponges, they would attach to the seafloor in shallow marine environments and filter their food out of the water column. Receptaculites, a sponge-like fossil organism, is fairly common in the Ordovician Galena Group. However, paleontologists are uncertain whether this organism can truly be classified as a sponge. Current understanding is that they may actually be calcareous algae. The fossil organism's radiating disc-shape looks similar to a sunflower, thus the common name "sunflower coral".

A fossil receptaculitid from Ordovician bedrock in Estonia.
A fossil receptaculitid from Ordovician bedrock in Estonia. While placed under the sponge and sponge-like category here, current understanding is that they may actually be calcareous algae.

Photo courtesy of Mark A. Wilson (Department of Earth Science, The College of Wooster) / Public domain

Anthozoans (corals)

Corals commonly found in the Decorah Shale are called horn corals because their skeleton, which is preserved, looks like a tiny horn. Corals are simple animals that feed by capturing small floating sea life with their tentacles. Most modern corals are colonial, which means that many animals live together in one skeleton. Ordovician horn corals were solitary (having only one animal per coral skeleton). Many types of corals are abundant in today's oceans, but horn corals are extinct.

Solitary rugose coral fossil (Grewingkia canadensis) in three views from Ordovician bedrock in Indiana.
Solitary rugose coral fossil (Grewingkia canadensis) in three views from Ordovician bedrock in Indiana.

Photo courtesy of Mark A. Wilson (Department of Earth Sciences, The College of Wooster) / Public domain

Bryozoans

These fossils commonly resemble a twig, a ribbon, or a small fan with tiny pores. Others are biscuit or gumdrop shaped or encrust other fossils. Like corals, bryozoans live by filtering food from surrounding water with special tentacles. Unlike the horn corals, bryozoans are colonial organisms; each pore in the skeleton is home to one tiny animal. They were abundant during the Ordovician Period, but they are scarce in modern seas.

Prasopora, a trepostome bryozoan from Ordovician bedrock in Iowa.
Prasopora, a trepostome bryozoan from Ordovician bedrock in Iowa.

Photo courtesy of Mark A. Wilson (Department of Earth Sciences, The College of Wooster) / Public domain

Brachiopods (lampshells)

Brachiopods are shelled organisms; their shell is composed of two unequal halves called valves. Commonly, thin growth lines and ribs are preserved on the valves. Brachiopods usually attach themselves to the seafloor by a fleshy stalk that extends through one of the valves. They are filter feeders; however, their tentacles remain inside their shell. Like bryozoans, they were abundant during the Ordovician Period, but are now rare.

Fossil molds of several brachiopods in Ordovician bedrock in southeastern Minnesota.
Fossil molds of several brachiopods in Ordovician bedrock in southeastern Minnesota.

Photo courtesy of Andrew J. Retzler

Gastropods (snails)

Gastropods most commonly possess one tightly coiled shell. Some shells are coiled in a plane (like a garden hose), and some are coiled in a spiral (like a cone). Snails without shells are called slugs. Although some modern snails have lungs and live on land, many others live under water. They are scavengers, finding food along the seafloor.

Two fossil gastropod shells from Ordovician bedrock in southeastern Minnesota.
Two fossil gastropod shells from Ordovician bedrock in southeastern Minnesota.

Photo courtesy of Andrew J. Retzler

Cephalopods

Cephalopods of today include octopuses, squid, and the only living representative with an outer shell—the pearly nautilus. The bodies of Ordovician cephalopods were like those of modern squid or octopuses, but the ancient cephalopods had an external shell that grew as a series of chambers. The animal occupied only the last, largest chamber. Ancient cephalopods were able to jet themselves rapidly through the water like the modern octopuses and squid, and like them, captured their prey with their tentacles. Another group of ancient cephalopods, known as ammonites, occur in the Cretaceous deposits of western Minnesota. Ammonites are distinguished from other cephalopods by their dividing walls that separate the various chambers in their shell. These dividing walls, or septa, leave patterns (known as suture patterns) on the outer portion of the shell and can be rather ornate.

Straight-shelled fossil cephalopod from Ordovician bedrock in southeastern Minnesota.
Straight-shelled fossil cephalopod from Ordovician bedrock in southeastern Minnesota.

Photo courtesy of Andrew J. Retzler

Bivalves (clams)

Bivalves, like brachiopods, are shelled creatures. Generally, their two valves are the same size and are mirror images of each other. Bivalves are filter feeders that can move around by means of a fleshy foot. Commonly, the form of the animal is preserved as a cast of the internal cavity between the two valves. The shells are usually not preserved, and fine growth lines and details are lost.

Fossil bivalve shell from Ordovician bedrock in Ohio.
Fossil bivalve shell from Ordovician bedrock in Ohio.

Photo courtesy of Kyle Hartshorn / CC BY (https://creativecommons.org/licenses/by/2.0) / cropped from original

Trilobites

Trilobites are long-extinct relatives of modern arthropods such as crabs and lobsters. Like these modern relatives, they shed or molted their hard external skeleton as they grew. Many of the fragments of trilobites found in rocks may be molted segments. Most commonly, the fossilized fragments are from the head or tail of the animal. Trilobites fed along the sea bottom or burrowed for food. Trilobite fossils are typically the most sought after fossils, and the most difficult to find intact.

Fossil molds of several trilobite fragments from drill core of Cambrian bedrock in Minnesota.
Fossil molds of several trilobite fragments from drill core of Cambrian bedrock in Minnesota.

Photo courtesy of Anthony C. Runkel

Ostracods

Ostracods are very tiny crustaceans—like crabs, shrimp, and crayfish—that are still abundant today in ocean and freshwater environments. The animal is enclosed in a pair of shells, often ornamented in a number of ways, and fossil ostracods can be important indicators of geologic time intervals and depositional environments.

Examples of several fossil ostracod valves from Ordovician bedrock in southeastern Minnesota.
Examples of several fossil ostracod valves from Ordovician bedrock in southeastern Minnesota (Magnification: 33x; modified from Burr and Swain, 1965, plate 4).

Crinoids (sea lilies)

Crinoids, relatives of sea stars and sea urchins, are spiny skinned animals with five-fold body symmetry. The animal has a small, cup-shaped body made of calcified plates. Five arms, each with many branchlets, extend from this cup. The crinoid attaches itself to the sea floor by a stem, and feeds itself by filtering food from surrounding water with its branchlets. The columnals which made up the stem are common fossils. The body plates are less common, but also may be found.

Several fossil crinoid columnals from Ordovician bedrock in southeastern Minnesota.
Several fossil crinoid columnals from Ordovician bedrock in southeastern Minnesota.

Photo courtesy of Andrew J. Retzler

Conodonts

Conodonts are an extinct eel-like, jawless fish that is mostly known in the geologic record by their tooth-like microfossils, known as conodont elements. Conodonts were prevalent in ocean environments from the Cambrian to Late Triassic, making them excellent stratigraphic markers and indicators of geologic time during this portion of Earth's history. Conodont elements are rather abundant in the formations from the Glenwood Shale to the Dubuque Formation, but, due to their microscopic size, require a fair amount of sample processing to extract and identify them.

Several fossil conodont elements from Ordovician bedrock in southeastern Minnesota.
Several fossil conodont elements from Ordovician bedrock in southeastern Minnesota. Note: Formal fossil names shown may now be obsolete (modified from Hogberg and others, 1965).

Shark and other fish teeth or material

Fossilized shark teeth and other fish material have been recovered in Cretaceous deposits of western Minnesota and around the Mesabi Iron Range. These organisms once thrived within the Western Interior Seaway, the large inland sea that once bisected North America. Sharks continually shed their teeth throughout their lifetime. Because of the hardiness of these teeth, they are more readily fossilized and preserved than other material. Other fossilized fish teeth and even fish scales are also known to occur in these Cretaceous deposits.

Fossil shark tooth, genus Squalicorax, from Cretaceous bedrock drill core in Aitkin County.
Fossil shark tooth, genus Squalicorax, from Cretaceous bedrock drill core in Aitkin County.

Photo courtesy of Andrew J. Retzler

Mammoths, mastodons, and other Pleistocene mammals

The Pleistocene sands, gravels, and clays, as well as old peat and lake beds, can host the teeth and bones of mammoths, mastodons, bison, musk oxen, elk, and giant beaver that once roamed the Quaternary landscape.

Drawings of a fossil mammoth tooth and mastodon tooth.
Drawings of a fossil mammoth tooth and mastodon tooth (modified from Hogberg and others, 1965).

Plant material

Some plant fossils have been found in the Cretaceous deposits of Minnesota, including: petrified wood, leaves, and pollen. Oftentimes these deposits represent swampy or marginal-marine environments that were associated with the eastern margin of the Western Interior Seaway at this time. Wood, leaves, and peat are also found within the Quaternary glacial sediments that indicate a thriving plant life during the intervals between advancements of the glacial ice sheets. MGS geologists often sample drill core and outcrops containing unoxidized clay or possible lake sediment to analyze for pollen, as this can help determine the age of the deposit as Quaternary vs. Cretaceous.

Drawings of fossil leaves from Cretaceous deposits.
Drawings of fossil leaves from Cretaceous deposits. Note: Formal fossil names shown may now be obsolete (modified from Hogberg and others, 1965).

Trace fossils

Trace fossils are not the body remains of an animal. Instead, they are traces, or impressions, made when an animal burrowed, rested, or crawled upon the seafloor.

Trace fossils in Cambrian bedrock of southeastern Minnesota.
Trace fossils in Cambrian bedrock of southeastern Minnesota.

Photo courtesy of Anthony C. Runkel


Common pseudofossils in Minnesota

Pseudofossil is a term used for any inorganic (nonliving-sourced) structure that is commonly mistaken for a fossil. Typically these structures form through chemical precipitation in rock and sediment from mineral-rich waters. Below is a brief description of the more common pseudofossils found in Minnesota.

Manganese dendrites

Manganese dendrites are manganese oxide concentrations that exhibit a dendritic pattern, which can occur internally (within the rock matrix) or along fractures and surfaces of a rock. Manganese dendrites form when manganese oxides precipitate out of hydrous solution.

Manganese dendrites on limestone from Germany (Scale in mm).
Manganese dendrites on limestone from Germany (Scale in mm).

Photo courtesy of Mark A. Wilson (Department of Earth Sciences, The College of Wooster) / Public domain

Concretions

Concretions are ovoid, smooth, hard rocks that are often mistaken for fossil eggs or bones. They form within sedimentary rock as mineral-rich waters move through the sediments and precipitate out, often in successive layers around a nucleus. Some of the carbonate bedrock deposits in southeastern Minnesota are known to commonly host concretions of various sizes. Because they have been mineralized and hardened more than the surrounding sedimentary bedrock containing them, concretions are often left behind long after extensive weathering of the bedrock deposit.

An example of a concretion (light gray rock) still contained within the original bedrock it formed in (dark gray rock).
An example of a concretion (light gray rock) still contained within the original bedrock it formed in (dark gray rock).

Photo courtesy of John Vonderlin / CC BY (https://creativecommons.org/licenses/by-nc/2.0/)

Chert nodules

Like concretions, chert nodules form when mineral-rich waters move through bedrock sediments, crevices, cracks, fractures, or voids and precipitate out. In this case, the mineral-rich water is carrying silica (or SiO2), which is the primary constituent of chert (also known as chalcedony or flint). When chert weathers out of the original bedrock deposit, their shapes can appear similar to that of fossils. Chert has a microcrystalline texture, meaning it appears very smooth and does not have any discernible crystal grains to the naked eye.

A chert nodule from Indiana.
A chert nodule from Indiana.

Photo courtesy of The Children's Museum of Indianapolis / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Soft sediment deformation structures

Soft sediment deformation structures form in liquidized, hydroplastic, and sometimes more competent sediments when they are stressed during or shortly after deposition. These structures are then preserved within the sediments as they become more consolidated sedimentary bedrock. Some forms of stress that may lead to these structures include ancient earthquakes, water currents and storms, and density differences between two layers of sediments deposited sequentially.

A soft sediment deformation feature in Dead Sea sediments, Israel.
A soft sediment deformation feature in Dead Sea sediments, Israel.

Photo courtesy of Mark A. Wilson (Department of Earth Sciences, The College of Wooster) / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)


Fossil collecting in Minnesota

Geologic maps of the various rock formations in the state, including those that produce fossils, can be viewed and downloaded either through the Map & Data Library or the Open Data Portal. These maps can help you determine where the fossil-rich formations may be exposed at the land surface. Natural exposures are commonly found along riverbanks and eroded hillslopes. Artificial excavations like road cuts and rock quarries can also be very productive.

Most land in Minnesota is private property. Always obtain permission before entering. Collecting the kinds of fossils described on most of this page is allowed on some state property, but places such as State Parks and Scientific and Natural Areas do not allow people to collect rocks or fossils. Local, city, and county parks, as well as private camping areas, may have access to fossil-collecting areas. Contact the operators of these areas to receive permission.

After you have obtained permission to collect from a site that yields fossils, a few simple tools will prove useful:

  1. Brick-layer's hammer or geologic hammer
  2. Chisels
  3. Magnifying glass or hand lens
  4. Collecting bags
  5. Tissue paper for wrapping delicate specimens
  6. Labels to note locality, formation, date, and collector
  7. Pencil, pen, or camera to note the location of the fossil
  8. Work gloves
  9. Safety glasses or goggles to wear when chipping at a rock surface

Always exercise caution when collecting fossils.