Trilobozoa
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Extinct (fossil)
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Trilobozoa ("three-lobed animals") is an extinct taxon of what are presumed to be animals which displayed tri-radial symmetry. Many, if not most, of the better-known trilobozoans lived prior to the Cambrian Explosion, during Ediacaran times.
According to Ivantsov and Fedonkin, 2002, because the Precambrian conulate Vendoconularia had a six-fold symmetry, and that the Conulata are regarded as a sessile sister group to the Cnidarian class Scyphozoa (jellyfish), the Conulata were nested within Trilobozoa, making Trilobozoa the sister clade of Scyphozoa. Thus, according to this, these three-fold beasts, such as the infamously enigmatic Tribrachidium, were cnidarians.
The most primitive trilobozoans were disk-shaped, typified by Tribrachidium. Through comparisons with the other discoidal trilobozoans, the different "arm" patterns on each genus/species occurred due to growth arresting or progressing at different stages of developmental growth.
The more advanced, and much longer-lasting, trilobozoans were cone-shaped, as typified by the long-lived genus Conularia, and had shell-like structures that resemble angular ice-cream cones which tended to have four corners. The conularids were once thought to be anthozoan cnidarians when they were first discovered. However, the lack of septa or other features diagnostic of anthozoans, along with their triradial symmetry lead some researchers to place the conularids within Trilobozoa. Conularids are not generally thought to be a part of the Vendian fauna, if only because of their fossil record, which begun a little before the Cambrian period, and ended at the close of the Permian. It is now also thought that the conulate trilobozoans derived their four-fold symmetry from a six-fold symmetry, as seen in Vendoconularia, which, in turn, was originally derived from the ancestral disk-like trilobozoans' three-fold symmetry.
[edit] References
- Ivantsov, A.Y. & M.A. Fedonkin 2002. Conulariid-like fossil from the Vendian of Russia: A metazoan clade across the Proterozoic/Palaeozoic boundary. Palaeontology 45(6): 1219-1229.
- McMenamin, M.A.S. The Garden of Ediacara. New York: Columbia University Press, 1998. ISBN 0-231-10559-2
- Anatomical Information Content in the Ediacaran Fossils and Their Possible Zoological Affinities, Jerry Dzik, Instytut Paleobiologii PAN, Twarda 51/55, 00-818 Warszawa, Poland
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