Reefs of the Silurian formed as barriers, hindering the flow of currents across the shallow sea especially around Michigan. The main organisms that built and inhabited the reefs 425 million years ago have mostly gone extinct. The main reef “constructors” were the tabulate corals (A,B, and C in picture), a tabulate coral commonly called “Petoskey Stone” is the State Fossil of Michigan. The next organism in a reef is called the “binder”, which forms a stabilizing sheet. The Silurian binder was the stromatoporoid (not pictured), which is a calcareous sponge that would slowly grow over and encrust other organisms on the reef. The “bafflers” of a reef build up-right structures that slow the flow of water across the reef. In Silurian one of these bafflers was the now extinct horn coral, shown as E in the picture. Although these three groups built the reef structure about 90% of the organisms in a reef are called “reef dwellers” and they live in the reef taking advantage of this ideal living environment. These dwellers include the crinoids (letter G), bryozoans (letter D), brachiopods (letters F & H), trilobites (letter I), and the nautiloids (letter J). The crinoids and bryozoans build stalks and branches in the slower moving currents where they trapped microscopic floating organisms in their tentacles and feathery arms. The brachiopods anchored themselves to the reef or burrowed into the softer mud, extending feeding arms above the sediment to filter food from the passing water.
Trilobites crawled on their many legs across the sediment and used their well-developed eyes to scavenge food off the surface. The last organisms frequenting the reef are the swimmers (much like the tropical fish of today’s reefs) such as the giant nautilod, which could grow to lengths of over 20 feet long.
What’s New on the Website
Site Map