Stromatolites Lake Michigan Discovery

Lake Michigan Stromatolite Fossil (Same as sample below, wet to bring out layers)

Stromatolites Lake Michigan Beach Fossil






You’re strolling along the shoreline of Lake Michigan combing the beach for interesting stones and driftwood or perhaps beach glass. You find a gray beach stone commonly found. You admire it for the smooth way it feels in your hand, ground down by the wind, wave and sand action. It even smells of the fresh outdoors. But upon a closer look, you can see layers of striations interesting and beautiful. When wet, they suddenly pop out and there’s no mistaken this is not an ordinary mineral rock. It’s a stromatolite.

Lake Michigan Beach Stromatolite Fossil

Lake Michigan Beach Stromatolite (Same as fossil left wet)

What Are Stromatolites?

For us laymen, simply put, they’re fossils of bacteria. You need a firm understanding of biology, geology and chemistry to fully understand them. Nevertheless, I will attempt to delve into their fascinating formation.

Forming in water, scientists today generally agree stromatolites are layered structures formed by cyanobacteria, single-cell microorganisms capable of photosynthesis producing their own food. Cyanobacteria are prokaryotic cells (the simplest form of modern carbon-based life) in that they lack a DNA nucleus. Bacteria, including the photosynthetic cyanobacteria, were the only form of life on Earth for the first two billion years that life existed on Earth.

Forming The Layers

The stromatolite bacteria live in between thin sheets of filament bound together by a sticky substance. Photosynthesis in the bacteria depletes carbon dioxide in the surrounding water making it less acidic and initiating the release of calcium carbonate. Calcium carbonate and other minerals and grains of sediment settle, then get trapped on the outside sticky layer. The cyanobacteria thus rises to the top of the stromatolite structure over the sediment and the layers recycle repeatedly building the solid structures that can take several forms such as mounds, sheets or columns which appear like giant mushrooms.

While the microbes that construct the layered mats generally are not preserved, the wrinkled calcium carbonate, mineral rich layers remain in the fossilized forms.

Stromatolites are the oldest discovered fossils dating as far back as 3.5 billion years. First appearing during the Archean Eon, their hay day was during the Upper Proterozoic Eon long before multi cellular Cambrian creatures evolved.

Geologic time scale showing stromatolites being most abundant during the Late Proterozoic (Condie and Sloan, 1997)


The stromatolites forming today in the shallow waters of Shark Bay, Australia are built by colonies of microbes. Credit: University of Wisconsin-Madison

Modern stromatolites were first discovered growing in the salty waters of Shark Bay, Australia in 1956. Before then, scientists believed they were extinct. Other locations discovered around the globe include the shallow waters of Yellow Stone, Mexico, Canada, Brazil, Oregon and most uniquely, Bahamas. Stromatolites lost out when animals such as snails evolved that ate them. Modern stromatolites thus live in water too salty or hot for those predators, except in the Bahamas. Watch the video to find out why!

Studies of modern stromatolites have shone they are not uniform in shape and form, but also host a variety of bacteria and archaea (bacteria like microorganisms). Archaea usually live in extreme, often very hot or salty environments such as hydrothermal vents or mineral hot springs, ie Yellow Stone. In any event, various biological environmental conditions may attribute to the differences in their make-up and shapes. Some form a round ball or lumpy mass. The example below shows one of these such forms found on a Lake Michigan beach.

Lake Michigan Stromatolite Fossil

Some of the most ancient stromatolite fossils found are in the 3.35 billion year old Strelley Pool chert of Western Australia, part of a fossilized ocean reef. Seven different types were identified, so there was already a variety of stromatolite shapes even back then. The stromatolite fossils found in Michigan are typically younger, dating from 2.2 billion years ago. During the great ice age 10,000 years ago, glaciers cut the Great Lakes digging up time-buried layers of sediment containing many varieties of fossils we find on the beaches today. This could explain how I picked up the stromatolite fossil pictured above on the beach in Southwestern Michigan.


Lake Michigan Stromatolite Fossil

Lake Michigan Stromatolite Fossil (Same as sample on left, wet to bring out layers)








Why Are Stromatolites Important To All Life?

Cyanobacteria that make up stromatolites were ultimately responsible for one of the most important global changes that the Earth has undergone. Being photosynthetic, cyanobacteria produce oxygen as a by-product. Photosynthesis is the only major source of free oxygen gas in the atmosphere. As stromatolites became more common 2.5 billion years ago, they gradually changed the Earth’s atmosphere from a carbon dioxide-rich mixture to the present day oxygen-rich atmosphere. This major change paved the way for the next evolutionary step, the appearance of life based on the eukaryotic cell (cell with a nucleus).

Stromatolites in the Soeginina Beds near Kübassaare, Saaremaa, Estonia

Stromatolites at Highborne Cay, in the Exumas, The Bahamas


Recent Find (Halysite Coral)

Golden Beach grass dune

Oval Beach Saugatuck, Michigan  Winter 2012


I was super excited to discover some fossils on Oval Beach in Saugatuck, Michigan (USA) that are highly unusual to find in winter. The sample below is an extinct tabulate coral, a reef building colony type coral called halysite. But we often find these saltwater varieties from the Paleozoic Era on our fresh water Lake Michigan shores. To explain this phenomenon I’ve written an article  entitled “I Found a Fossil on the Beach and Wondered” at where I’m also the Fossillady. A link is provided in the box to the right.

This halysite coral is easy to distinguish due to the chain link appearance coining it with the common name “chain coral”. It was lying under layers of sand that aren’t usually exposed during the winter months. But the mild weather with little snow has allowed winter winds to push sand off the under layers. It’s also due to low water levels from a long dry spell during summer 2012, there’s more beach to explore.

The extinct halysite corals displayed small tubes from which resided the jelly-like coral animals called polyps.  The coral polyps contained stinging cells for protection and also grasped plankton food that passed by in the ocean currents. As the halysite corals grew, they built up walls of tube-like chambers called theca which steadily multiplied adding more links to the chain. In their hay-day as they continued to multiply, they built large limestone reef structures on the seabed. They thrived mostly during the Silurian period up to 425 million years ago!


Kingdom: Animal

Phylum: Cnidaria (means stinging animal)

Class: Anthozoa (means flower animal)

Order:  Tabulata (possess inner horizontal dividing walls from growth patterns)

Family: Halisitidae (means chain coral)

Genus: Halysite  Species:unknown

Halysite Coral

Halysite Coral Fossil


Halysite Colony “Chain Coral” Rendering