Tribrachidium

Tribrachidium is a genus of early multi-cellular animal from the Ediacaran period of history. Now extinct, it is known only from fossils. One known species, T. heraldicum, makes up the entire genus.

Tribrachidium fossils are found in numerous locations throughout the world, including Newfoundland, the Northwest Territories, Russia, and Australia. They show a disc-shaped creature about 5cm in diameter on average, with three curved "arms" extending from the centre to nearly the edge. These arms were probably hollow and could be inflated or deflated. This three-fold symmetry is almost unique in animals, which are usually either bilaterally or radially symmetric. Fleshy ridges, or possibly loose filaments, extended from the edges of the arms and formed the rest of the body.

Like many Ediacaran-era fossils, the relationship of Tribrachidium to other animals is poorly known. To some extent it has become a poster child for the problem in general, often being shown as an example of that era's peculiar lifeforms. Among others, Tribrachidium has been described variously as a cnidarian, lophophore, echinoderm, ecdysozoan or even as an odd, outlying member of the dipleurozoa -- a proposed ancestor of the chordates. Some have even speculated that it is not a complete animal at all, but rather the holdfast of a larger creature.

Tribrachidium heraldicum was named and described by Martin Glaessner in 1959 in The Geology and Late Precambrian Fauna of the Ediacara Fossil Reserve. Records South Australian Museum 13: 369-401.

Australia Post issued a 50 cent stamp featuring Tribrachidium on 21 April 2005 in a series entitled Creatures of the slime.

References

• Anatomical Information Content in the Ediacaran Fossils and Their Possible Zoological Affinities, Jerry Dzik, Instytut Paleobiologii PAN, Twarda 51/55, 00-818 Warszawa, Poland
• The Ediacaran Biotas in Space and Time, Ben Waggoner, Department of Biology, University of Central Arkansas, Conway, Arkansas 72035-5003