Tribrachidium
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Tribrachidium heraldicum Temporal range: Ediacaran, 558–555 Ma |
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| Fossil of Tribrachidium heraldicum | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Trilobozoa Fedonkin, 1985 |
| Family: | Tribrachididae Runnegar, 1992 |
| Genus: |
Tribrachidium Glaessner, 1959 |
| Species: | T. heraldicum |
| Binomial name | |
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Tribrachidium heraldicum Glaessner, 1959 |
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Tribrachidium heraldicum is a tri-radially symmetrical fossil animal that lived in the late Ediacaran (Vendian) seas. In life, it was hemispherical in form. T. heraldicum is the best known member of the extinct group Trilobozoa.
The generic name Tribrachidium is derived from combination of the Greek τρία (tria, "three") + Latin brachium ("arm") + diminutive suffix -idium. The specific name T. heraldicum references the similarity of the pattern of this fossil with the well-known heraldic triskelion design, such as the coat of arms of the Isle of Man.
Tribrachidium fossils were first discovered in the Ediacara Member of the Rawnslay Quartzite, Flinders Ranges in South Australia. This fossil is also known from the Mogilev Formation in the Dniester River Basin, Podolia, Ukraine and from the Verkhovka, Zimnegory an Yorga Formations in the White Sea area of the Arkhangelsk Region, Russia.
T. heraldicum is preserved as negative impressions on the base of sandstone beds. These fossils have a circular, three-lobe form, with straight or trefoil-like edges; they are usually covered by numerous radial branched furrows. The central part of the fossil has three hooked ridges ("arms"). The lobes are twisted into weak spirals.
The diameter of specimens ranges from 3 to 50 millimetres (0.3 to 5.0 cm).
Tribrachidium was originally described by Martin Glaessner as a problematic organism, one that is excluded from all known major groups of animals by its tri-radial symmetry. However, Tribrachidium's superficial resemblance to edrioasteroid echinoderms was well known to researchers and discussed. Later, Glaessner rejected any putative affinities of this animal with any known phyla, leaving the status of its taxonomy uncertain. Originally, the various structures on the poorly preserved Australian specimens were interpreted as tentacles, peculiar arms and mouth, but later this interpretation was rejected. Its mode of locomotion in life also remains unknown.
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