Minnesota’s common Paleozoic Era fossils and how to identify them

Minnesota is not the first place most people think of when “tropical sea” comes up for discussion, but hundreds of millions of years ago, during the Paleozoic Era, Minnesota and its neighboring states were south of the Equator and submerged under a warm continental sea. Today, if we travel in the Mississippi River gorge in the Twin Cities, we can see resistant limestone beds sticking out over less durable sandstone beds (think Minnehaha Falls or Dayton’s Bluff, for example). If we were to go back to when these rocks were deposited, about 455 million years ago during the Ordovician Period of the Paleozoic, we would find a shallow sea teeming with life. Some organisms would be familiar; others might look familiar but would actually be unrelated to their modern equivalents; and some things would be completely unfamiliar. Many of these ancient lifeforms left various kinds of fossils in the rocks, usually shells and other hard parts that have a head start to becoming fossils because they are mineralized or otherwise durable to begin with. (Original shell material can and does become fossilized without much change, but with rocks as old as those around here, usually there’s been at least some change in minerals.) Similar geological stories to Minnesota’s can be found in places throughout Michigan, Wisconsin, and the Ontario Peninsula, from between roughly 500 and 340 million years ago. Here are some of the most abundant groups and how to recognize them.

A quick note: While I’ve focused on Minnesota in this article, the fossils in general are pretty similar across the western Great Lakes region at the coarse level of “snail”, “brachiopod”, etc. The major exception is the corals; in Minnesota we’ve basically got horn corals and some generally humble tabulates (see below for what these are), but others may be encountered in Wisconsin and Michigan. Canada, unfortunately, is pretty sparse for Paleozoic rocks fossils near the western Lakes apart from Manitoulin Island and some sites on the west side of the Ontario Peninsula; its rocks are generally much older, representing much different conditions.

Corals
The Paleozoic coral fossils of Minnesota belong to extinct lineages. The most abundant are a type of solitary coral known as horn corals, because their pointed conical shape resembles an animal’s horn. The wide end is where the polyp (coral animal) lived, in a cup or depression. This end in a well-preserved specimen will show a starburst of thin walls, which can also be seen in planar cross-sections. Horn coral fossils in Minnesota are often about the size and shape of acorns minus the cap, and frequently have a corduroy-like surface appearance.

Another common type of coral are small colonial corals, belonging to the tabulate corals. In the local rocks, the colonies often look something like small wasp nests encrusting other fossils. They are easy to overlook at first, but are frequently minor members of “hash plates.” Tabulate colonies have corallites (the chambers where the polyps lived) that are usually a couple of millimeters across or less. Other kinds of colonial coral fossils have much larger corallites, a centimeter or more across (like the Petosky stones of Michigan).

Bryozoans
Bryozoans are also known as “moss animals” because many modern forms cover and encrust surfaces like mosses do. They are much less abundant and diverse than during their Paleozoic heyday. Bryozoans are tiny aquatic colonial animals that are sometimes confused with corals, although they are much different. Corals are predatory, with some also hosting photosynthetic algae, whereas bryozoans are filter-feeders. Bryozoans are also more complex despite their much smaller size, featuring a one-way digestive system.

Bryozoan fossils are usually pieces of mineralized colonies, which took on an amazing variety of shapes: twig-like, flat branching, disc-like to dome-like, flag-like, perforated-cracker-like, net-like or lacey, irregular or massive, encrusting forms that resembled whatever they had encrusted, screw-like, and more. Many of the kinds we see frequently in Minnesota are of the twiggy or flat-branching forms. Many species had variable colonies and cannot be distinguished except by thin sections. An exception is the “gumdrop” Prasopora. Fossils of bryozoan colonies can be distinguished from corals by the tiny “pinprick” pore-like chambers for the individual bryozoan animals.

Brachiopods
Brachiopods are also known as “lamp shells” because some of them are similar in appearance to ancient oil lamps. They are shelled animals that resemble some bivalve mollusks (particularly scallops), but are anatomically very different. Brachiopods are more closely related to bryozoans and, like them, are filter-feeders using fine tentacles. Also like bryozoans, they still exist today but were much more abundant and diverse during the Paleozoic. After the great mass extinction at the end of the Permian Period about 252 million years ago, brachiopods were replaced in many niches by bivalve mollusks. Brachiopod shells can often be distinguished from bivalve shells by symmetry. Brachiopod shells anatomically represent the top and bottom sides of the animal, and each shell has a line of symmetry. However, the two shells of a brachiopod are often different sizes and shapes, and therefore are not symmetric to each other. Bivalve shells anatomically represent the left and right sides of the animal, and the line of symmetry goes between the two shells, like butterfly wings.

Brachiopod fossils come in many shapes and sizes. Common varieties found in Minnesota rocks include shells that are something like fingernails in size, shape, and composition; cap-like shells encrusting other fossils; small nut-like fossils with strong ridges from the hinge to the shell edge; and fairly large D-shaped shells (up to a couple of inches across) that may be strongly arched and have muscle scars in the form of diamond-shaped ridges. Unlike most brachiopods, the owners of these D-shaped shells did not have a stalk-like organ to attach to surfaces, and may have been able to move around like scallops by clapping their shells.

Nautiloids
Today, most cephalopods are octopuses or squid, which have almost no hard parts. In the Paleozoic, though, there were many kinds of cephalopods with mineralized outer shells. Many were distantly related to the modern chambered nautilus, and are called nautiloids. Nautiloids, unlike most Paleozoic animals of Minnesota that we find as fossils, were free-swimming. They are generally interpreted as predators, catching other animals with their tentacles.

Unlike the modern nautilus, the nautiloid fossils we find in Minnesota are generally not coiled. Most are straight shells or are slightly curved like a banana. The straight shells are also called “orthocones,” and are gently tapered, something like a hennin (long pointed “princess hat”). When there were limestone quarries in the Twin Cities, quarry workers sometimes found large orthocones, which they called “stone snakes.” The largest orthocones can be more than 10 feet long, but most are much smaller, finger- to hand-sized. Nautiloid shells are divided into chambers, and the chambers are connected by an internal passage called a siphuncle. Because of the composition of nautiloid shells, the original shell material has often been lost, so fossils may be natural casts of the outside or internal casts (known as steinkerns) of the chambers and siphuncle.

Snails
Snails and snail-like animals are well-represented by fossils in Minnesota. Their fossil shells come in a bewildering variety of shapes and sizes. Some are smaller than a fingernail, others are several inches across. Some are simple cap-like shapes with no coiling, similar to today’s limpets. Many Paleozoic snails grew shells in a flat or nearly flat spiral. Some of these are flared at the aperture, and appear to be swallowing their coil. Others are flat-sided, sometimes with ornate frilly ridges. Still others are flat only on one side and ridged on the other, more like a cinnamon roll. There are also numerous species with stacked coils. Some of these have bulbous whorls, other have smooth lines giving them a cone-like appearance. The snails with the tightest stacks look something like snowmen. Something these fossil shells usually share is their preservation: like nautiloids, snail shells are susceptible to replacement and loss due to their composition. In Minnesota, the great majority of snail shell fossils are actually external molds and impressions, or casts of the filled internal space (with the shell gone). It is thought that these ancient snails mostly grazed on algae.

Trilobites
Trilobites are iconic fossils, well known for their pillbug-like shapes and amazing complete specimens with all of the appendages, sometimes replaced by pyrite. In Minnesota, trilobites that well-preserved have not been found, but we do have our share of trilobites, including the giant Isotelus that could reach lengths of more than a foot. Wisconsin is more fortunate: exceptionally preserved trilobites about 435 million years old are known from Waukesha County. Trilobites peaked in diversity and abundance in the early part of the Paleozoic and went extinct in the great end-Permian extinction (252 million years ago). They inhabited many different niches: some were scavengers, some were predators (there are trilobite tracks that meet worm burrows, with only the trilobite continuing), some were bottom-dwellers, some were planktonic.

The name “trilobite” means “three lobes,” corresponding to the axial lobe running down the midline of the body and the two lobes flanking it (pleural lobes). Trilobites, being arthropods like insects, spiders, crustaceans, and many other animals, were also divided into three main sections: a head section (cephalon), a body section (thorax, jointed for movement and to allow the trilobite to roll up on itself when threatened), and a tail section (pygidium). Like other arthropods, trilobites had exoskeletons that they shed to grow larger, which is one reason most trilobite fossils are just fragments from one part of the body. When you find these pieces, you’re finding castoff pieces from a growing trilobite. In addition to their durable exoskeleton, trilobites also had antennas and numerous jointed limbs with gills. These appendages are rarely preserved.

Crinoids
Crinoid fossils are abundant in some of Minnesota’s Paleozoic rocks, but complete specimens are extraordinarily rare. Crinoids are still around today and are also known as “sea lilies” because of their general plant-like shape. They are not plants, though—they are echinoderms, related to sea stars, sea urchins, sea cucumbers, and other marine groups that share five-sided symmetry, an internal water vascular system, and a skeleton of many small mineralized pieces. Like most echinoderms, crinoid skeletons usually fall apart soon after death, dispersing the individual bits and pieces.

Crinoids today may be attached to the sea floor by stalk or be stalkless and free-swimming, but the Paleozoic crinoids found in Minnesota were all stalked forms. The basic body plan of these crinoids included a holdfast attaching the body to a hard object; a long slender stalk made up of numerous cylindrical pieces called “columnals;” a cup-shaped body called a calyx, made up of polygonal plates; and a set of feathery arms. The most abundant fossils are columnals, which often look like rings or gears a few millimeters across. Columnals are very difficult to assign to crinoid genera or species. Calyx plates are more distinctive but much rarer. Ridged plates of Carabocrinus are fairly abundant as plates go, and represent an unusual crinoid with a short stalk and a big calyx.

Out in the field: fossil hunting and collecting
There are many other groups than these seven, of course, but most of them are rarer or difficult to spot. An experienced fossil-spotter would not be surprised to see fossils of any of these seven groups in most of the region’s Paleozoic rocks. Finding them is often as simple as going to any place where rocks of the right age are exposed (it’s not nearly as difficult at finding dinosaurs, for example!). Collecting them is a different matter: places where fossils can be collected have dwindled for various reasons, especially liability and landowners’ perception of overly zealous fossil collectors causing problems. I recommend checking with local geological societies to find out what they recommend. You may be able to join a group trip, which can be very helpful if you are new at this and want to learn the basics. Of course, do not collect where you do not have permission!

You should record information about your finds, such as location, geologic unit (the scientific name for the rock layer, such as “Platteville Formation” or “Decorah Shale,” which can be found on geologic maps like those of the Minnesota Geological Survey), and date. This is very helpful if you want to understand your collection, and also if you ever plan to donate specimens or otherwise want a better fate for them than curios that go into the trash when you go to join the original inhabitants of your fossils.

In the field, fossils may be loose whole specimens, “hash plates” with accumulations of many fossils, and/or isolated and firmly embedded in outcrops. It depends on the kind of rock (soft rocks such as mudstone yield individual fossils, whereas fossils in limestone usually stay there). It may take some time to develop a search image, but once you do, the fossils will begin to “pop.” Once collected, most fossils in this region fortunately do not require much preparation beyond water and a toothbrush (preferably one you no longer use) to be presentable. If you’re feeling up to it, dental picks can be used to remove less resistant sediment from specimens, and you may be able to get some for free if you ask the next time you’re in for a cleaning. If you are practicing “catch and release” fossil-hunting, an object for scale is an essential part of taking photos. It can be something as simple as a coin or pen.

Looking for and photographing fossils without collecting them is an easy pursuit to add to hikes and nature walks where sedimentary bedrock is exposed, like observing birds, butterflies, or flowers. Whether you’re collecting fossils or documenting them in place, good luck and have fun!

Justin Tweet grew up in Cottage Grove, Minnesota and got his undergraduate degree at the University of St. Thomas, where he became interested in the rocks and fossils of the Twin Cities from wandering the Mississippi River Gorge. After earning a master’s degree in geology at the University of Colorado-Boulder by fighting a recalcitrant brachylophosaur named Leonardo to a draw, he came back to the Twin Cities and became involved in the National Park Service’s Paleontology Program, for which he now helps to manage and protect the fossil resources of the NPS. He also writes about paleontology at his blog, Equatorial Minnesota . All fossil images © the author.