Every Michigander knows about Petoskey stones – rare treasures that can be found on countless beaches on Michigan’s lower peninsula. Some more savvy rockhounds may even know that they are fossil corals in the genus Hexagonaria, dating back to the Devonian Period some 350 million years ago.
While most common on up north beaches, they can also be found farther south, with known collecting localities from the Sleeping Bear Dunes to Fort Gratiot County Park.
They can also be found inland and have been uncovered in many sand and gravel mines across the state.
While Petoskey stones may be among the most prized fossil finds on Michigan beaches, there are a wealth of other fascinating fossils to be found while combing for ancient treasures.
If you look at a geologic bedrock map of Michigan, you will immediately notice a distinct “bullseye” pattern. This is what geologists refer to as the Michigan Basin, and it’s one of the most unique geologic structures on the planet. It covers the entire lower peninsula, and roughly half of the upper peninsula.
The edge of the basin is roughly defined by the ~541to 486 million year old (Cambrian age) Munising Formation, and at its center is the ~300 million year old (upper Pennsylvanian age) “Red Beds” of the Pewamo Formation.
Rocks get younger toward the center of the basin, making the youngest bedrock in Michigan over 300 million years old! The Michigan Basin was formed by a combination of tectonic forces, and possible reactivation of geologic faults of the 1.1 billion-year-old failed “Mid-Continent Rift,” which lies at the center of the basin, buried under more than 4 kilometers of rock and sediment.
Given these brutal past couple winters in Michigan, it may be difficult to imagine that for many millions of years, the state was covered by vast, shallow tropical seas that inundated the low-laying Michigan Basin. It is from these ancient seas that (almost) all of the fossils you can find on Michigan’s beaches originate.
Arthropods represent the most diverse group of organisms on the planet, accounting for an estimated over 80% of all species on Earth. They include groups like insects, spiders, scorpions, crabs, lobsters, isopods, and many, many more.
While crabs and lobsters may reign supreme in modern oceans, in the ancient seas of the Michigan Basin, trilobites were king. You’re are far more likely to find trilobites than any other type of fossil arthropod in Michigan. On Michigan beaches, you are more likely to find broken fragments of trilobites than a whole one.
Look for segmented fossils that might remind you of other arthropods, or the distinct three-lobed appearance from which they get their name. You may even find remains of their large compound eyes!
Brachiopods may look similar to modern clams or bivalve mollusks, but are almost entirely unrelated.
They consist of two shells or valves, and come in many shapes and sizes. An easy way to distinguish a brachiopod from a clam is the symmetry. Looking at a brachiopod top-down, the left and right side should be a mirror image of each other, which is never the case with clams.
You are far more likely to find a fossil brachiopod in Michigan than a fossil clam, so if you find a fossil that looks like a clam, it’s most likely a brachiopod!
Bryozoans, also called moss animals, are a group of filter-feeding colonial organisms that are sometimes confused with corals, but are actually more closely related to brachiopods.
To identify a bryozoan, you’ll have to look very closely for a fossil covered in tiny, near pin-prick sized holes. They can form independent mounds or other shapes, and can also encrust other fossils.
Cnidarians include corals, sea anemones, jellyfish, and hydra.
One of the most unique features of cnidarians is that they tend to be colonial organisms, meaning that they form colonies of genetically identical individuals which work together as a whole. There are also solitary varieties, often called horn corals for their distinct horn shape.
Corals are among the most common types of fossil you will find on Michigan beaches because they formed vast reefs, and had a hard calcium carbonate skeleton likely to be preserved. Although superficially similar to modern scleractinian corals, Michigan corals represent two entirely extinct groups called tabulate corals and rugose corals.
When identifying a fossil as a coral, look for repeating, sometimes geometric structures like seen in rugose coral Hexagonaria percatinata (aka Petoskey stones).
Other common Michigan fossil corals include:
...and various types of solitary rugose horn corals.
Echinoderms are a diverse group with a great many fossil and modern representatives.
In Michigan, you’re most likely to find fossils from a group called crinoids (also known as sea-lilies). Other types of echinoderms include echinoids (think sea-biscuits and sand-dollars), starfish, sea urchins, and sea cucumbers.
In life, crinoids appear to be plant-like (but they are in fact, animals), consisting of a holdfast, which fastens them to the seafloor, connected to a long stem or stalk, connected to a cup or calyx, with many arms and pinnules used for filter feeding.
All these parts are made of many connected segments, which quickly come apart when the crinoid dies. Because of this, you are much more likely to find disarticulated segments than a whole crinoid.
The most common type of segments are ones that make up the stem, called columnals, which can be round, donut-shaped, star-shaped, or pentameral (having five parts, like a starfish).
Mollusks are a diverse group that include clams, snails, slugs, cephalopods, and others. One fascinating group of fossil mollusk you may find in Michigan is called nautiloids.
Relatives of the modern coiled nautilus, fossil nautiloids are cephalopods with chambered shells they used to control buoyancy. While fossil coiled nautiloids can be found, the straight-shelled variety is more common.
Look for cylindrical fossils, with clear chambers. You can also find relatives of nautiloids called ammonoids, which come in many shapes and sizes. There are straight-shelled ammonoids as well, but you’re more likely to find the coiled variety.
Porifera are sponges. You can find a few types of sponge fossils in Michigan, but are more likely to find fossils from an enigmatic group called stromatoporoids.
To identify a stromatoporoid, look for a fossil with thin concentric layers and round bumps called mamelons.
This page covers only a very small number of the countless types of fossils that can be found on Michigan’s beaches and beyond. Here are a few useful tips for finding fossils on Michigan beaches - our geologist's most closely guarded secret for finding fossils? Get out there and look for them!
Go fossil collecting after a storm or high wave event; the bigger the better.
Storms cause shoreline erosion and can reveal new fossils that were not previously exposed. If you go for a walk along the beach soon after a storm, you will get first dibs on the newly exposed fossils.
High lake levels also cause shoreline erosion and can expose new fossils in the same way as storms.
Our geologist found more Petoskey stones during the record high lake level years than all previous years of his life combined!
Bring a snorkel and go offshore. Fossils underwater are far less likely to be picked up than ones easily seen walking on shore.
Using a snorkel and goggles can give you access to fossils most people miss.
Look for areas with larger cobbles near the foot of steeper bluffs. Often, fossils will be eroded out of dunes or bluffs along the shoreline.
Steeper areas generally have higher rates of erosion, so a steady supply of new fossils can be found near them.
Look for areas where small creeks or streams enter the lake.
Streams also cause erosion, and new material can be washed down them, especially after storms.
Do your research.
There are a wealth of resources and highly knowledgeable local people who can help you find the best collecting localities wherever you may go.
This content was compiled by Evin Maguire, senior geologist in the Materials Management Division at the Michigan Department of Environment, Great Lakes, and Energy (EGLE).
Photos from Cassandra Tiensivu used with permission.
