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Geology
27, 519-522 (1999) Abstract.
Unlike the
celebrated Ediacara fossils, those from the roughly coeval localities of the
Kuibis Quarzite of Namibia are preserved not as imprints on the sandstone
bedding plane, but three-dimensionally, within the rock matrix. The pattern of
deformation and the presence of sand in lower parts of the bodies of Ernietta,
the most common and typical of those organisms, indicate that their
three-dimensional preservation is a result of a density-controlled sinking of
sand-filled organic skeletons within hydrated mud layers. Specimens of Ernietta
have preserved various stages of migration across the mud beds. Their wall
material, as documented by the mode of deformation, was not only flexible, but
also elastic, which makes it unlike chitin, The walls thus seem to be
proteinaceous, built probably of a collagenous fabric, The Ernietta
skeleton was built of series of parallel chambers, which excludes the
possibility that these were external body covers. The chambers apparently
represent walls of hydraulic skeleton units, resembling the basement membrane
of chaetognaths or the notochord sheath of primitive chordates, Such chambers
are widespread among the earliest fossil animals represented by fossils
preserved in sandstone. The rise and fall of the Ediacaran faunas thus seem to
be partially preservational artifacts. The range of its occurrence is a result
of two successive evolutionary events: the origin of an internal hydraulic
skeleton enclosed by a strong basement membrane, and the appearance of
decomposers with abilities to disintegrate such collagenous sheaths. |