Publication Date:
2011-06-01
Description:
Despite their abundance, diversity, and importance today, organisms with mineralized skeletons are a relatively recent introduction. For the first three billion years of its history, life was soft-bodied, inducing mineralized structures passively, if at all. Beginning ca. 550 Ma, however, more than two dozen clades—primarily animal, but also protistan—independently evolved mineralized skeletons within a geologically short interval of time (Fig. 1; Bengtson, 1992). Now a new report by Cohen et al. (2011; p. 539 in this issue of Geology) describing beautifully intricate scale-like microfossils from the Fifteenmile Group, Yukon Territory, provides definitive evidence for mineralized skeletons some 150–250 m.y. earlier. These scale-like microfossils were first reported over two decades ago (Allison and Hilgert, 1986), but neither their age nor their mineralogy were well constrained. Work by Cohen and her colleagues has now shown that these scales (which perhaps enveloped a single-celled green alga) are between ca. 717 and ca. 812 Ma in age and composed of primary phosphate (Macdonald et al., 2010; Cohen et al., 2011). This adds to earlier suggestive evidence for mineralization at this time: the ca. 770–742 Ma vase-shaped microfossil (VSM) Melicerion poikilon, interpreted on the basis of taphonomic models to be a euglyphid amoeba whose organic-walled test was embedded with mineralized scales, possibly siliceous (Figs. 1B and 1C; Porter and Knoll, 2000; Porter et al., 2003); the mid-Neoproterozoic Tenuocharta cloudii, a multicellular, sheet-like fossil whose calcareous cell walls may reflect primary (Horodyski and Mankiewicz, 1990) or early diagenetic (Knoll, 2003) mineralization; and ca. 650 Ma millimeter- to centimeter-scale asymmetric bodies permeated with a network of canals and interpreted to be sponge-like organisms perhaps lightly mineralized with carbonate (Maloof et al., 2010b)...
Print ISSN:
0091-7613
Electronic ISSN:
1943-2682
Topics:
Geosciences
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