Morphologically simple enigmatic fossils from the wheeler formation: A comparison with definitive algal fossils

摘要

Study of noncalcareous algal fossils is problematic due to their broadly defined taxonomy and lack of preserved features by which modern algae are classified. Four distinct morphologically simple, enigmatic fossils from the Wheeler Formation were analyzed using Raman spectroscopy, and elemental mapping by electron microbeam techniques. These fossils have been interpreted as dissociated algal fragments, and accordingly, were compared to known algal fossils: Yuknessia simplex (green alga; Spence Shale), Marpolia spissa (cyanobacteria; Burgess Shale), and Margaretia dorus (green alga; Burgess and Wheeler formations). All fossils examined were composed of carbonaceous films, at least in part, but varied with respect to secondary mineral coatings: iron oxides were associated with the surfaces of all three algal species to some degree, and in addition, Margaretia dorus exhibited silicification. Fossils characterized by a thin, wispy, filamentous form (Linear Morphotype 2) display mineralogical and morphological similarities with Marpolia spissa, but lack the characteristic longitudinal striping of this species. Filamentous fossils with a coiled form were the only fossils found to contain chlorite. These fossils are interpreted to be fecal strings. Stubby, linear fossils (Linear Morphotype 1) that commonly have been interpreted as fragmented Yuknessia simplex exhibit neither the mineralogical nor the microtextural features of this algal fossil, making this interpretation unlikely. As with Linear Morphotype 1, fossils of Morania fragmenta lack secondary iron oxides and phyllosilicates. Accordingly, they are interpreted to have been composed of labile material that preserves only under inhospitable conditions such as sustained anoxia.