The Three arms, Tribrachidium (1959)
Phylum : Tribozoa Family : Tribrachididae Genus : Tribrachidium Species : T. heraldicum
- Precambrian (550 Ma)
- 5 cm (size)
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.
Tribrachidium was originally described by Martin Glaessner as a problematic organism, one that is excluded from all known major groups of animals by its three-radial symmetry. However, Tribrachidium's superficial resemblance to edrioasteroid echinoderms was noted 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.
With the discovery of the closely related Albumares and Anfesta, along with the discoveries of much better-preserved Russian specimens, Mikhail Fedonkin proposed for these animals the new taxon, Trilobozoa – extinct group of the tri-radially symmetrical coelenterate-grade animals. Originally, Trilobozoa was erected as a separate class in the phylum Coelenterata, but since Coelenterata was divided into separate phyla Cnidaria and Ctenophora, the Trilobozoa have been transferred to the rank of phylum.
M. Fedonkin has shown that the Tribrachidium fossil is an imprint of the upper side of the animal's body, with some elements of its external and internal anatomy. The radial furrows on the fossil are radial grooves on the surface of the alive animal, while the three hooked ridges in central part of the fossil are imprints of cavities within the body. Tribrachidium was a soft-bodied benthic organism that temporarily attached (but did not accrete) to the substrate of its habitat (microbial mats).
