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The Rugosa or rugose corals are an extinct order of solitary and colonial corals that were abundant in Middle Ordovician to Late Permian seas. [ 3 ] Solitary rugosans (e.g., Caninia , Lophophyllidium , Neozaphrentis , Streptelasma ) are often referred to as horn corals because of a unique horn-shaped chamber with a wrinkled, or rugose , wall.
A rugose coral seems an unlikely common ancestor because these corals had calcite rather than aragonite skeletons, and the septa were arranged serially rather than cyclically. However, it may be that similarities of scleractinians to rugosans are due to a common non-skeletalized ancestor in the early Paleozoic.
Hexagonaria is a genus of colonial rugose coral. Fossils are found in rock formations dating to the Devonian period, about 350 million years ago. Specimens of Hexagonaria can be found in most of the rock formations of the Traverse Group in Michigan. Fossils of this genus form Petoskey stones, the state stone of Michigan. [1]
Heliophyllum is an extinct genus of corals that existed predominantly in the Devonian. Heliophyllum is of the order Rugosa and can be referred to as horn corals. [1] [2] They were mostly solitary animals, although some were colonial. [1] The genus had a wide distribution.
The rugose corals existed in solitary and colonial forms, and were also composed of calcite. [89] Both rugose and tabulate corals became extinct in the Permian–Triassic extinction event [88] [90 (along with 85% of marine species), and there is a gap of tens of millions of years until new forms of coral evolved in the Triassic.
Like rugose corals, they lived entirely during the Paleozoic, being found from the Ordovician to the Permian. With Stromatoporoidea and rugose corals, the tabulate corals are characteristic of the shallow waters of the Silurian and Devonian. Sea levels rose in the Devonian, and tabulate corals became much less common.
A Petoskey stone is a rock and a fossil, often pebble-shaped, that is composed of a fossilized rugose coral, Hexagonaria percarinata. [1] Such stones were formed as a result of glaciation, in which sheets of ice plucked stones from the bedrock, grinding off their rough edges and depositing them in the northwestern (and some in the northeastern) portion of Michigan's lower peninsula.
The class includes important coral reef builders such as the stony corals, sea anemones, and zoanthids. The recognized orders are shown below: [4] Actiniaria – sea anemones; Antipatharia – black corals; Corallimorpharia – corallimorpharians aka "false corals" †Rugosa – rugose corals; Scleractinia – stony corals †Tabulata ...