#rugose

silurfilur

The leaning tower of rugosa.

fossilseeker

A long gone reef

Corals were different in the earlier versions of the Earth, and not always formed mostly by cnidarians and the like but being made by different organisms and taking different shapes to those of the modern era. Reefs have always been delicate environments, and are often walloped hard in mass extinctions and take longer to recover than many other ecosystems. After the Great Dying 252 million years ago for example they took a full 10 million years to reappear. This chunk is even older still back in the Devonian era some 380 million years ago. reefs then were formed by calcareous algae, coral-like stromatoporoids, and tabulate and rugose corals (aka horn corals, also extinct). The specimen is some 25cm across, and was swum over and inhabited by very different organisms to those found on reefs today. Loz

Image credit: LGF Foundation https://lgfmuseum.org/

fossilsiowa

Petosky Stone

Michigan's stage stone is a lovely fossil coral dating from the late Devonian to early Carboniferous that was spread around parts of the state much more recently by grinding ice sheets during the recent ice ages, often as eroded rounded pebbles. Hexagonaria percarinata (named for the hexagonal patterns) belongs to an extinct group of corals called rugose that characterise reef building organisms of the Paleozoic. In those days what is now Michigan was the floor of a shallow equatorial sea filled with coral reefs.

In some areas then entire head of fossilised colonies transformed into limestone can be discovered in the Traverse group strata, though most are recovered on the beaches of Lake Michigan, where annual frosts heave and turn stones each winter revealing new specimens. A polish on the lapidary's bench is necessary to reveal the patterns in their full glory. Each hexagon was the home of a polyp that lived within the colony, who then fed via the tentacles that emerged from the central mouth. A small number of specimens have a rosy hue due traces of iron that infiltrated during the process of the reef turning into rock (called lithification or diagenesis).

The common name comes from that of an Ottawa chief called Pet O Saga (meaning rising sun), reputedly half French, who established himself as a fur trader in the last quarter of the 19th century. The stone has been a popular souvenir of the area since the Victorian era.

Loz

Image credit: Cobalt 123