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Hexagonaria, showing the separate corallites of a colonial rugose coral
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Hexagonaria sp.
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As the name suggests, Hexoganaria are hexagonal - i.e. have six, more-or-less equilateral sides. This is not unique amongst corals, for in a tightly packed colonial community, it's an efficient way of organising yourself. It does, however, help to make it one of the easier genuses to recognise.
Hexagonaria is a colonial rugose coral, so - unlike in tabulate corals - the individual corallites operated as separate entities with their own wall. This is evident in close examination of the image above, where the division between each corallite is clear. Also, typically of rugose corals, the septae are distinct and long. For the same reason, however, growth of the corals doesn't always proceed in a totally organised way. Instead, corallites compete with each other for space, and the more tardy ones find that their opportunities for growth are limited. As a consequence, some corallites fail to achieve a hexagonal shape. Together with their size, this can be used, to deduce the order of growth within a corallum. The larger and more complete corallites formed first, the smaller and more distorted ones later. Allocation of the species shown here to species level is more uncertain, but it seems probable that it belongs to Hexagonaria allani, which has previously been reported from the Reefton Formation. Access to good, comparative material would be needed to confirm this suggestion. Distinguishing features may include the number of septae (ca. 48) and the diameter of the corrals (ca. 25 mm), both of which are large for Hexagonaria species. There is only a limited literature on Hexagonaria in New Zealand, but some interesting examples of how to study colonial rugose corals like this are given by Rozkowska (1960) - see Further Reading, below . Further Reading: Rozkowska, M. 1960 Blastogeny and individual variations in tetracoral colonies from the Devonian of Poland. Acta Paleontologicala Polonica V(1), 1-64. |
Trivia: In the USA, Hexagonaria is sometimes called the Petoskey Stone, and has been adopted as the state stone of Michigan. The name comes from an old American Indian word 'Petosegay', meaning ' sunbeam', or 'the rays of the rising sun'. According to legend a French fur trader named Antoine Carre came to Michigan where he met and married an Odawa princess. In time he was adopted by the local Odawa tribe and eventually was made their chief. In the spring of 1787, while traveling with him, his wife gave birth to a son. According to the story, in the morning, as the sun rose over their camp, the rays fell upon the infant’s face, provoking his father to pronounce his name to be Petosegay and two predict that he would be an important person. His prediction came true. Petosegay became wealthy and bought up much of the land in the area. Ul;timely, the area was called after him Petoskey. It's an area of Devonian rocks, where water-rounded pebbles are often found, formed from the fossil Hexagonaria. Because of their attractiveness, these became known as Petoskey stones and in 1965 were formally adopted by Michigan as the state stone. |