Thursday, March 14, 2013

Four Fossils

There is nothing that will disappoint a kid more than telling them that they can't go outside their door and find a dinosaur fossil in their backyard.  No one has ever found a dinosaur fossil anywhere in Michigan.  Our fossils are either very old or very young.

We have many fossils that are younger than the retreat of the last glacier that covered Michigan (about 11 thousand years ago).  Our fossils of mammoths and mastodons are from this time period. 

Then there is a long time period for which Michigan has almost no rocks.  Our next youngest layer is over 200 million years old and dates to the Pennsylvanian Period (approximately 320 - 300 Million Years Ago).  Much of the coal found in Michigan formed during this time period, when parts of Michigan were covered by rich, mucky swamps.

Our richest layer of fossils is even older, dating to the Devonian (approx. 415 - 355 MYA) and Silurian (approx. 445- 415 MYA).  During much of this time Michigan was covered by a warm, salty, tropical ocean and located near the equator!  So the fossils that are found most often in Michigan are those of sea creatures.

Four Michigan fossils.  Each sample measures about 3 inches long.

The fossil on the upper right is the one that people in Michigan are probably most familiar with.  It is a type of extinct colony forming animal known as a Hexagonaria coral, better known as "Petoskey stone".  This polished form is Michigan's official state stone. Many people look for these fossils on the beaches along Lake Michigan and Lake Huron.  When smoothed out by waves, the familiar hexagon surrounding a star pattern is visible.

A Hexagonaria coral "Petoskey Stone" on a Lake Michigan beach
My sample of Hexagonaria was never polished by waves.  It was picked up by a glacier somewhere north of here and left behind when the glacier melted.  It was found in a farm field in Mid-Michigan.  It is probably more than 350 million years old.

An unpolished Hexagonaria

The next fossil is another extinct form of coral.  Unlike the Hexagonaria, this one usually did not form large colonies.  It liked to live as an individual instead of being crowded by its neighbors.  This one is called a "horn" or Rugosa coral.  It formed a horn shape that could grow up to three foot long.  This example is a cross section.  This sample was found on a Lake Michigan beach.

A cross-section of a Rugosa or "horn" coral in limestone

The third fossil is of another extinct species called a Stromatoporoid.  Like corals, Stromatoporoids formed large reefs in the tropical ocean that covered Michigan.  For a long time they were thought to be related to corals, but by examining structures within the fossils it was decided that they were more closely related to sponges.  The small regularly spaced structures are called mamelons and may have been used to take in and let out water.  This Stromatoporoid sample was found when digging a hole in Mid-Michigan.  Just like the Hexagonaria fossil, this one was picked up by a glacier and deposited when the glacier retreated.

A Stromatoporoid fossil

The final fossil is a limestone cobble containing broken pieces of Bryozoa.  Also known as "moss animals" Bryozoans are another type of aquatic colony-forming animals.  Bryozoa are not extinct.  There are over five thousand species that are alive today (including some that live in fresh water).  The Bryozoans in this rock formed colonies that branched out and looked like many species of coral do today.  Because of this resemblance, they are often confused with corals or plants.  This sample was also found on a Lake Michigan beach.

Bryozoa fossils in limestone

So while it may be disappointing to know that if you live in Mid-Michigan you won't find a dinosaur fossil in your yard, consider yourself lucky.  You might find something much, much older and more surprising.




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