COOPED UP IN AN EDIACARAN SHELL: WHY DID CLOUDINA BIOMINERALIZE?

The tubular calcified fossil Cloudina is one of the first biomineralizing organisms.  It is also one of the most abundant and widely distributed macroscopic Precambrian fossils, occurring on every continent except Australia.  Yet we know little about the environments in which it lived, what triggered biomineralization in this clade, or why it went extinct at the Ediacaran-Cambrian boundary.

To characterize environmental changes associated with the appearance and disappearance of cloudiniids, we analyzed the sedimentology and isotopic- and trace element-chemistry of six Cloudina-rich sections from Sonora, Nevada, and California.  In these sequences, cloudiniids are the dominant allochem in cm-m thick packstones that may extend laterally for several km.  Cloudiniids are preserved as calcite in limestone, dolomite or silica in dolostone, or as silica steinkerns.  More rarely they occur as isolated steinkerns in very fine grained sandstones.  Some shells exhibit postdepositional flexible deformation and others exhibit brittle fracture prior to lithification.  At least six Cloudina-like taxa have been described from this region; based on petrographic evidence for flaring and budding of shell walls, volume renderings from serially ground and CT-scanned samples, and shell size distributions, all of these can be synonymized to one taxon. 

Cloudiniids in these successions were transported into or within five distinct open carbonate platform or ramp facies.  Neither cloudiniid appearance nor cloudiniid disappearance appears to be linked to changes in hydraulic conditions, water depth, siliciclastic influence, or carbon isotopic or Mg/Ca composition of seawater.  Disappearance of cloudiniids in all three Sonoran successions is coincident with or immediately precedes a regional -3‰ d¹³C excursion.  Collectively, the lack of winnowing in Cloudina shellbeds, the lack of in situ specimens, the lack of evidence for attachment of cloudiniids to each other or to other allochems, and the wide variety of depositional environments in which cloudiniids occur suggests that Cloudina may have had a pelagic or planktonic mode of life.  Isotopic and trace element data from these successions suggests that Cloudina may have lived in marine waters that favored the precipitation of calcite.