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Mikhail Fedonkin, Paleontological Institute (Russian Federation)
Patricia Vickers Rich, Monash University (Australia)
Billie Swalla, University of Washington (United States)
Peter Trusler, Monash University (Australia)
Mike Hall, Monash University (Australia)
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The Vendian siliciclastic sediments exposed along the White Sea coast of northern Russia has yielded the richest record of early metazoans. Some species from this sequence are preserved in abundance, while others are extremely rare. Here we report the results of a study of two specimens from these marine rocks which represent a new genus, perhaps related to Ausia, a form from the Nama Group of Namibia. We interpret these fossils as the internal sand casts of a vast bag-like cavity, possibly a pharynx (branchial basket?) more than 90 mm across. The walls of the cavity were perforated by the oval fenestrae (smallest size is 2x5 mm) regularly spaced over the surface and forming rows. These rows are separated by structures which were probably grooves in the internal wall surface of the living organism, characteristic of the ascidians. The animals represented by these two fossils lived in the shallow waters of an epicontinental sea, slightly more than 555 million years ago and are likely the oldest evidence of the chordate lineage of metazoans, supporting molecular clock models predicting deep Precambrian roots of this group.
The ecological implication of this discovery is active filtering-feeding characteristic of tunicates had an early origin - at least by the late Neoproterozoic. Such a feeding habit, once it appeared, became a powerful regulator of plankton density (including small metazoans, eggs and larvae), increased the transparency of ocean waters, and would have had an influence on the rate of sedimentation.
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