Fossil Friday #9 – Stromatoporoids vs. Stromatolites

One of the hardest sets of terminology and fossils/structures for students to remember is stromatoporoids and stromatolites.  Not only are the names painfully similar, but they also look very similar, until you get your nose next to them.  Both can be massive (tens to hundreds of metres), both appear finely laminated, and both can be round or bulbous in shape.  The short version is that stromatoporoids (left image below) are body fossils, and stromatolites are more sedimentary structures (right image below).  Hard to tell apart, right?

The longer version:

Stromatoporoids

Stromatoporoids are considered to be an extinct group of sponges that were particularly common during the Silurian and Devonian.  In Alberta, stromatoporoids formed giant reefs during the Devonian.  A large reef formation (biostrome) can be observed in the Moberly Member of the Waterways Formation, which outcrops around Fort McMurray, right underneath the oil sands (McMurray Formation).

Sponges, including stromatoporoids, are the basal group for our own kingdom (Animalia).  Stromatoporoids are usually classified within the sclerosponges, a group which secretes a calcareous skeleton (most other sponges do not have a solid skeleton, but instead make their skeletons from distinct hard pieces called spicules).  The skeletons of stromatoporoids are put down in layers called laminae, and separated into distinct chambers (galleries) by upright pillars.  Water would have flowed through the galleries, where ciliated cells would have drawn nutrients from the water.

Image result for cross section of stromatolite vs stromatoporoid
Image credit: https://www.geol.umd.edu/~tholtz/G331/lectures/331porifera.html

 

p1040577-2
A stromatoporoid showing laminae (horizontal lines), pillars (vertical lines), and galleries (chambers formed from intersections of laminae and pillars).

Often times, a microscope or hand lens is required to see the laminae, pillars, and galleries of stromatoporoids, but occasionally you get really nice preservation, as in the image above (a specimen from the Waterways Fm.), or this specimen below (from the Potter Farm Fm. of Michigan – Devonian).  Another, sometimes more easily observed feature of stromatoporoids, at least when present, are the small bumps that would have been on the outer wall or surface of the animal.  These structures are called mamelons, and would have probably assisted with drawing water into the galleries.

Stromatolites

Stromatolites are sedimentary structures which are created when a layer of sticky cyanobacteria traps sediment layer by layer, sometimes creating huge mounds or sheets.  Stromatolites are one of the earliest organically formed structures, dating back to the Precambrian. A new paper by Nutman et al. (2016) found stromatolites in Greenland which they determined to be 3.7 billion years old!

stromatolite
Stromatolite

Because stromatolites are formed by algae/cyanobacteria, they are still technically considered fossils, but what is preserved is layers and layers of sediment.  When you look closely at a stromatolite, you will see laminae, but they are not well defined or regular as in stromatoporoids, and they do not have any kind of vertical structure such as pillars or galleries.

Image result for cross section of stromatolite
Image credit: http://weeklysciencequiz.blogspot.ca/2012/06/stromatolites.html

One other cool thing about stromatolites is that they still exist today.  There are some in Shark’s Bay, Australia that are about 2,000 – 3,000 years old!  Check out their website for some more pictures and videos of stromatolites.

Image result for modern stromatolites
Modern stromatolites of Shark’s Bay.  Image credit: https://en.wikipedia.org/wiki/Stromatolite

References:

https://www.geol.umd.edu/~tholtz/G331/lectures/331porifera.html

http://www.sharkbay.org.au/nature-of-shark-bay/fact-sheets/stromatolites-of-shark-bay.aspx

Nutman, A., P., Bennett, V., C., Friend, C. L. R., Van Kranendonk, M., J., and Chivas, A., R. 2016. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature. 537:535–538.

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