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

From Tree to Stone

Araucarioxylon, arizonicum (Petrified Forest National Park) Drawing Rendition

The drawing above is a rendition I drew with colored pencil of Araucarioxylon, arizonicum, an extinct conifer tree identified from the petrified wood discovered at the famous Petrified Forest National Park located in the US State of Arizona. Originally, it was thought to be a distant relative of the Araucaria tree or the Norfolk Pine, which you often see in stores at Christmas time as potted plants. Without detailed microscopic examination of the wood, its link to present day trees is only speculation at this point. Also, modern scientific discoveries indicates there were more species than originally thought lying on the dry Arizona plateau.

Araucarioxylon, arizonicum petrified wood from Arizona Petrified Wood National Park


Triassic Landscape

Triassic Beginnings

During the Triassic Period, around 225 million years ago, the present day continents were melded together as one supercontinent called Pangea. By the end of the era, around 200 million years ago, widespread volcanic activity began to break the continents apart.  The first dinosaurs and mammals had evolved and there had been a boom in cycads (palm like trees), giant tree ferns and conifers . . .  enter the Araucarioxylon of Arizona, a type of conifer or pine. It differs from modern day conifers with its sporadic branch growth pattern around the trunk rather than growing in whorls. It grew up to 200 feet (60 meters) high with a 9 foot diameter. Compare that to the tallest current living conifers that grow up to 188 feet (57 meters). To help put that into perspective, maple trees reach up to about 50 feet (15 m) and oak trees top out at about 80 feet (24m) tall.

From Tree To Stone

The high and dry tableland in northern Arizona where the National Petrified Forest Park rests, was a vast flood plain during the Triassic Period overflowing with streams and widespread ponds. Prehistoric, extinct plants and trees concentrated along the spilling bodies of water. Dying or blown over trees were washed down the streams into the flood plains where they gathered and decayed. If they got buried under mud, silt or volcanic ash deep enough to cut off oxygen, it slowed the decaying process. Silica laden groundwater gradually seeped through the logs replacing the original wood tissue with silica deposits. As the process continued over the ages, the wood was replaced, atom by atom, with silica crystallized into mineral rich quartz, turning it to stone.



Petrified Wood Araucarioxylon, arizonicum from Petrified Forest National Park


Petrified Wood Colors

Various colors, often striking, are produced depending on the mineral contents in the stone:

PINK or RED – Hematite present – a form of oxidized iron

This interesting process is well explained from Iron dissolves in ground water when no oxygen is present. The ground water becomes re-oxygenated as it moves though the tree trunks causing oxygen to bond with the iron. The iron then precipitates to produce a solid form of iron called hematite. This hematite is incorporated into the log’s cell walls. The same process occurs when iron stains porcelain sinks. The soluble iron in ground water becomes oxidized into a solid form when it comes in contact with air, causing a reddish stain.

YELLOW BROWN or ORANGE – Goethite present- a weathered hydrated iron oxide that becomes crystallized

GREEN – Pure native iron present

WHITE  – Pure Silica present – Silicon, Si, and oxygen, O, are the two most abundant elements in the earth’s crust which together form silica dioxide quartz

BLACK – Carbon or Pyrite or Iron Sulfide (the most wide spread sulfide) present – The wood was affected as hydrogen sulfide from decaying organic matter interacted with iron forming pyrite.

PURPLE or BLUE  Manganese present – This is a secondary material formed when water leaches manganese from igneous rock and re-deposits it as a concentration of manganese dioxide.

TAN   Silica Dioxide present – naturally found in water, plants, animals, and the earth


How did the petrified wood of Arizona become uncovered? First, millions of  years ago, the area sank to the point which completely flooded everything with freshwater sediments. It continued to sink deeper becoming completely buried. Millions of years later, the area was lifted far above sea level from westerly continental plate pressure. The uplift created stresses that cracked the giant logs. Over time, wind and water have worn away the layers of hardened sediments, exposing the fossilized wood.

Lake Michigan Petrified Driftwood

Petrified Driftwood Lake Michigan Beach

While the Petrified Forest of Arizona has the highest concentration of petrified wood in the world, it can be found in every US state and other countries around the world. I found this sample of petrified driftwood on a Southwest Michigan beach off Lake Michigan called Oval Beach. It’s predominantly gray with a slight bluish green cast and has a few streaks of rust. In certain light, it casts a very bright sheen or luster. It’s difficult to capture the sheen in a photograph, but you can get a better idea from its flip side in the photo below.

Petrified Driftwood Lake Michigan Beach

After researching the possible mineral contents of my sample, according to its dense property, color and luster, my best guess tells me that it’s hematite specularite. At any rate, it’s an exciting find. I collect driftwood on the beach all the time, but never before a piece that was petrified.



Timetable of Clams


Timetable of Bivalve Drawing (Six Clams and One Scallop)

What is a clam and how do they live? Clam can be a term that covers all bivalves. Bivalves such as oysters and mussels attach themselves to hard objects, and scallops can free swim by flapping their valves together. Some clams bury themselves in sand and breathe by extending a tube to the water’s surface. Those varieties usually possess a stronger foot for digging that looks like a tongue. Clams feed by filtering plankton with their adapted gills, although the digging varieties use their siphoning tubes and more primitive species used special tentacles. Bivalves lack a head or brain and usually have no eyes, although scallops are a notable exception. All bivalves possess a heart, kidneys, a mouth and anus, as well as a circulatory system.

Middle Left Orange Colored Rendering: Ctendonta has smooth surface with fine concentric growth lines and teeth along the hinge plate. Ordovician – Silurian (505 million years ago (mya) to 408 mya)

Top Right Pinkish Colored Rendering: Modiolopsis has asymmetrical thin valves crossed by an oblique depression. Ordovician-Silurian (505 mya to 408 mya)  


MODIOLOPSIS Fossil Clam Shell

Middle Bluegreen Colored Rendering: Byssonychia has sharp steeply inclined beak near end of hinge; usually has strong radial ribs. Mostly Upper Ordovician (400 mya)


BYSSONYCHIA Source: Wendell Rickets (All Rights Reserved)

Top Middle Yellow/Brown Colored Rendering: Goniophora has a lopsided shell with a prominent beak and ridge extending to the rear margin. Silurian – Devonian (438 mya – 360 mya)  

Bottom Left Goldish/Brown Colored Rendering: Pteria has thin inequilateral shaped valves with a long straight hinge merging into large unequal wings. Jurassic to recent (245 mya – today)

Bottom Right Tanish/Peach Colored Rendering: Glycimeris has symmetrical circular outline with pointed beak. Cretaceous to Recent (144 mya – today)

Top Left Deep Blue Colored Rendering: Pecten is a genus of many well known bivalves otherwise know as large scallops. Valves have strong radial ribs and are symmetrical except for slightly unequal wings. Mississippian to Recent (360 mya – today)

Bivalves feed the world. Bivalve oysters, scallops and clams are near the bottom of the food chain which many marine and freshwater species depend on for a food source. But don’t forget about land creatures such as otters, for one, and we humans who especially enjoy a treat of clam chowder. Their shells, secreted from calcium carbonate, can be beautiful and are used to adorn our homes in many creative ways.  Scan_Pic0009


Coral or Sponge; a process of identification

Sponge or Coral (Mystery Florida Fossil)

Sponge or Coral (Mystery Florida Fossil)

I’ve been very perplexed by this Florida fossil whether it’s a coral or sponge, but my sense of curiosity doesn’t give up easily.  Follow along as I come to a conclusion as to its identity.  To begin with, the mystery fossil  possesses meandering ridges resembling the various types of brain corals, yet it is dotted with numerous protruding openings very much like the encrusting  sponge species below, also found in Florida.

Brown Encrusting Octopus Sponge Source:

Brown Encrusting Octopus Sponge
Source: Wikipedia Commons



Florida Brain Coral Fossil


The problem with the encrusting type of sponge is that they cover hard surfaces much the same way in which moss covers the ground.  I don’t know if this type of sponge, with its unique growth pattern, would break loose and end up on a beach where my mother-in-law, Winkie, picked it up in Florida during the 70’s and 80’s.

Rose Brain Coral Fossil

Rose Brain Coral Fossil

Pink Lumpy Sponge Source:

Pink Lumpy Sponge
Source: Wikipedia Commons









If you’ll notice, the “rose brain coral” above has a few pitted holes like the mystery fossil but not nearly as many; and if you’ll also notice, the “pink lumpy sponge” to the right possesses a pattern of ridges, but only sort of like the mystery fossil.  Hmmmm . . . a point I’d like to make for a second here, is how this demonstrates what fossil hunters do. They learn to observe things with a keener eye in order to identify their findings. Anyway, we’ve concluded our mystery fossil has similar characteristics as the above corals and sponges, but not exactly the same as any of them.

We’re on the bloodhound trail, let’s continue observing.  On the one hand, the mystery fossil is quite lighter and airier compared to the other Florida corals in my collection and it has more openings visible on all sides which leads me to think it could be a sponge. But, the patterns of ridges, plus the color, shape, and texture is quite similar to the other corals from the collection.

Christmas Tree Tube Worms Source:

Christmas Tree Tube Worms on Coral Source: Wikipedia Commons


Tube Worms On Clam

Tube Worms On Clam









So, if the mystery fossil, on most levels, appears to be a coral with the one exception of the deeply pitted holes, what could have caused those holes and why so many? Observe the Christmas tree tube worm image above. The tube worms have completely covered the top of a coral.  Could they have made the holes in question? I’ve decided not because the openings on the mystery coral look quite different from the tube worm burrowing on the clam fossil above. I’m no expert, but I decided the mystery fossil is a type of coral, likely related to the brain corals. Feel free to disagree in the comments if you know of think otherwise.

Check out the tube worms link to the right listed under CLASSIFICATIONS to learn more about them and how they can actually be quite pretty.

Coral Sponge

Florida Sponge Fossil

Florida Sponge Fossil


My title “Coral Sponge”, in this case, refers to the beautiful coral color of this sample and because I’m 95% certain it’s a sponge and not a coral fossil. The main reason is because of the lack of vertical septa walls inside the cups, or in the case of sponges, pores. You can click on the image above to enlarge and observe its finer details. (If you believe it’s not a sponge, feel free to comment)


Coral cups showing septa walls lacking in sponges


There are some 5,000 to 10,00 known species of sponges and identification usually depends on the patterns and shapes of their spicules (tiny rods used for defense) often only visible enough through the microscope in order to distinguish.

In lieu of this, I can only wager a guess as to its exact identity. It’s a rather attractive piece from my mother- in-law’s collection she gathered in the 70’s and 80’s off Florida beaches. So, I’m guessing it’s some kind of calcareous type which forms a hard calcium carbonate skeleton; and I would also say that it’s a tube type of some kind as well.


Below, I found an image of a tube type of sponge from a Florida reef. It looks fairly close in comparison. See what you think . . .



Tube Coral Sponge (Reverse Side)

Tube Coral Sponge
(Reverse Side)






  • Sponges are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them.
  • Sponges do not have nervous system, nor digestive or circulatory systems; instead their water flow system perform all the necessary functions.
  • For defense, sponges shed rod-like spicules forming a dense carpet several meters deep that keep away echinoderms (i.e. starfish) which prey on them. They also may produce toxins that prevent other prey from growing on or near them.
  • Their bodies have two outer layers, separated by a non-living gel layer which contains the tiny rod-like spicules.
  • Sponges are sessile (attached to a substrate or hard surface).
  • Most sponges live in quiet, clear waters because sediment stirred up by waves or currents would block their pores making it difficult for them to feed and breathe.
  • Sponges  improve water quality as effective biological filterers, extracting microscopic food and bacteria from the current.
  • Sponges evolved over 500 million years ago.
  • Sponges form different shapes, including tubes, fans, cups, cones, blobs, barrels, and crusts.

Sponge Classification

Kingdom: Animalia (animals)

Phylum: Porifera (having pores)


Demosponges  Largest class; Inner structure reinforced with collagen fibers and spine-like spicules made of silica minerals; Usually barrel shaped; Can live in a wide variety of habitats; Some are bath sponges

Hexactinellida – Glass Sponges; Spiny spicules made of silica minerals forming inner scaffolding structure with gelatin substance weaved in between framework; likes Polar Regions

Calcareous – Outer exoskeleton and inner spicules made of calcium carbonate. Restricted to shallow marine waters where production of calcium carbonate is easiest to obtain.

Scleropongiae (Coralline or Tropical Reef Sponges) soft body that covers a hard, often massive skeleton made of calcium carbonate, either aragonite or calcite.  The layered skeletons look similar to reef corals, therefore are also called coralline sponges.

Branching and/or Finger Coral

Branching Type Coral Fossil

Branching Type Coral Fossil
(View Large to see minute details)

Branching finger-like corals are a dominant species in the Caribbean, Florida and Bahama ocean reefs and form some of the largest colonies extending as far as eight meters in height. They are very slow growing and therefore some may be a thousand years old.

Club Tip Finger Coral

Club Tip Finger Coral

Because the fossil/skeleton sample in my possession has broken off branches (very typical) I was unable to identify the exact species, but am certain it belongs to the genus called Porites. Three Western Atlantic Porites species have features that overlap so can be difficult to identify.

  • Club Tip Finger Coral (Porites, porites) possess thick, stubby branches growing upright or spread wide apart. Often gray, occasionally bright blue
  • Branching Finger Coral (Porites, furcata) possess elongated, tightly compact branches with rounded tips. Usually grey
  • Thin Finger Coral (Porites, divaricata) possess most slender branches, widely spaced apart, often divided at their tips. Colors vary from purple, yellowish brown, grey and brown.



Branching Finger Coral (Close-Up)


  • Kingdom: Animalia
  • Phylum: Cnardia (stinging cells)
  • Class: Anthozoa (flower animl)
  • Order: Scleractinia (stony coral)
  • Family: Poritidae (massive reef builders)
  • Genus: Porites (finger-like)
Branching Finger Coral Fossil (Opposite Side)

Branching Type Coral Fossil (Opposite Side)
(View Large to see minute details)

Rough Star Coral

Rough Star Coral, Isophyllastrea rigida on the left and Symmetrical Brain Coral, Diploria strigosa on the right. Source:

Rough Star Coral, Isophyllastrea rigida on the left and Symmetrical Brain Coral, Diploria strigosa on the right.

Rough Star Coral,  Isophyllastrea rigida is commonly found in the Atlantic Ocean along the shores of the Caribbean Islands, Bahamas and Florida. The fossil sample below is part of a collection gathered in the 70’s and 80’s during Florida vacations my husband’s parents took from their busy lives. Winkie, my mother-in-law (may she rest in peace) faithfully collected sea shells and coral adrift on the beaches, which today is not allowed. Now in my care, it has been my pleasure to research their origins and share them with you on the information highway which she never knew about, but would have been very proud to share.

Winkie with my late husband Joseph at age three

Winkie with my late husband Joseph at age three

Rough Star Coral is easily recognized by its small dome-shaped colonies and closely spaced corallites having only a thin margin between them and the polyps are rounded to polygonal in outline. Living colonies of the rough star coral range in color from varying degrees of light green to yellow, usually with the ridges being lighter in color than the darker groove floors.

rough star coral fossil_edited-1

Rough Star Coral (Isophyllastrea rigida)


It belongs to a family of corals called Mussidae originating during the Cenozic Era beginning 65 million years ago. Mussidae is a relatively small family of coral consisting of roughly 13 genera with a wide geographical distribution. Eight genera from Mussidae are found in the Indo-Malayan Western Pacific seas and the remaining four genera, including the Rough Star Coral, Isophyllastrea rigida are Atlantic species.

Below are living Mussidae corals from the genera of Acanthastrea, a showy and colorful breed with meaty polyps living in the Indo-Pacific regions. The similarities and differences from my Atlantic species, Rough Star Coral, Isophyllastrea, rigada, and Acanthastrea requires a close study to discern.

Acanthastrea lordhowensis

Mussidae (Acanthastrea lordhowensis)


Mussidae (Acanthastrea)









Mussidae (Acanthastrea) Source:


  • Kingdom: Animalia
  • Phylum: Cnidaria (stinging cells)
  • Class: Anthozoa (flower animal)
  • Subclass: Hexacorallia (polygon shape)
  • Order: Scleratina (stony coral)
  • Family: Mussidae (large fleshy polyps)
  • Genus: Isophyllastrea
  • Species: rigida

Coral: A Simple Animal Simply Explained

Corals exist at the tissue level lacking organs, such as a heart. On the evolutionary ladder, corals are one step above the sponges. They are the simplest animals to have a nervous system, and a connected muscular system, and a dedicated reproductive system.

A video beautifully photographed with vivid colors and depictions of the coral reefs geared for kids, but great for adults as well.