THE ELABORATE SKELETON OF CLOUDINA, ONE OF THE EARLIEST BIOMINERALIZED METAZOANS

Cloudina, a milimetric tubular fossil consisting of stacked funnel-shaped elements, is the best known of the biomineralized metazoans that appear in the terminal Ediacaran. However, many aspects of its diversity and detailed morphology remain poorly understood. In central Spain, Cloudina occurs in various preservational modes: silicified, phosphatized, pyritized, as moulds in siliciclastic sediments and preserving a carbonatic composition. The silicified, pyritized and some of the phosphatic specimens seem to represent replicas of the original skeleton, while some of the phosphatized specimens are internal moulds. Wrinkles, ridges, folds and crests occur abundantly in these variously preserved specimens, providing new information on the diversity of Cloudina. The silicified specimens, as well as the moulds in siliciclastic rocks and some of the pyritized fossils, are characterized by bell-shaped funnels with prominent longitudinal crests and a thickened apertural rim. The consistent occurrence of these features in many specimens led to the recent description of the new species C. carinata. Most pyritized specimens show another morphotype characterized by closely spaced, fine transverse annulations (less than 10 µm wide) and essentially cylindrical funnels. In some specimens, there are also longitudinal ridges of a similar size; the presence of both types of ridges result in a reticulated ornamentation. Several phosphatized specimens present small annulations similar to those occurring in the pyritized ones, while others show large transverse undulations (c. 30 µm wide) and conical funnels, apparently representing a third morphotype. The presence of three distinct morphotypes of Cloudina in the terminal Ediacaran of central Spain, each with its characteristic ornamentation pattern, emphasizes the morphological complexity and diversity within this taxon.