GLOBALLY WIDESPREAD APATITIC SCALE MICROFOSSILS OF THE MID-TONIAN

We report here apatitic scale microfossils from shallow marine Tonian units of the Black River Dolomite of Tasmania, the Pahrump Group of Death Valley, the Chuar Group of the Grand Canyon, and the Gotia Group of Svalbard. At least seven morphotypes of these microfossils have been recorded (e.g., pennate, oval, 5 to 7-sided shields), with lengths of 5–34 microns and widths of 4.5­–21 microns. Confocal Laser Scanning Microscopy, Raman spectroscopy and Raman imaging indicate that these fossils are composed of two distinct materials: an inner kerogenous region and an outer apatitic region.

The apatite is considered likely to be of primary biological origin rather than diagenetic as clusters of scales do not share a single apatitic layer, and co-occurring fossils of acritarchs, prokaryotic filaments and vase-shaped microfossils do not show evidence of coating. Further, although broken and abraded two-part scales are found, there are no instances of the apatite envelope surrounding broken scales.

These newly discovered microfossils are morphologically distinct from the apatitic scale microfossils described from the Tonian Fifteenmile Formation of Canada in both their apparent lack of ornate sculpture and in their two-part composition. Like the Fifteenmile scales, however, they are interpreted to have encompassed a protistan cell. Scales are found today in many phylogenetically distant groups of single-celled eukaryotes (protists), varying in size from <1 micron to many tens of microns across and in composition (organic, calcium carbonate, or silica).

These apatitic scale microfossils are globally widespread in mid-Tonian units, reported here from Tasmania, western North America, and Svalbard and potentially similar to forms recently described from Sweden. These differ morphologically from the older scales of the Fifteenmile Formation, indicating a higher biomineralizer diversity than previously recognized in Tonian seas.