NEOPROTEROZOIC LAGERSTÄTTEN (AND THE SEARCH FOR PAST LIFE on MARS)

Megascopic fossils are well known from Phanerozoic Lagerstätten -- to note a few, primitive land plants permineralized in the Devonian Rhynie Chert, cellularly preserved floras permineralized in Carboniferous calcitic coal balls, and fish and insects preserved in tuffaceous shales of the Eocene Green River Formation. Lagerstätten are known also from the Precambrian, common especially in the Proterozoic, where their component fossils are microscopic, rather than megascopic, and the exquisitely preserved microorganisms are permineralized in fine-grained cherts.

Studies of such chert-permineralized microfossils have long been hampered by two deficiencies, an inability to document accurately their three-dimensional organismal morphology and cellular anatomy at high spatial resolution; and a lack of means to analyze directly the chemistry of the coaly kerogen of which they are comprised. These needs have now been met by three techniques recently introduced to paleontology: confocal laser scanning microscopy (CLSM), Raman imagery, and fluorescence-spectral imagery. Use of these techniques, together, provides information in three dimensions at sub-micron spatial resolution about fossil morphology, cellular anatomy, taphonomy, molecular-structural composition, and mode of preservation that is not available by any other means.

We here demonstrate the usefulness of these techniques by applying them to microscopic fossils chert-permineralized in two Neoproterozoic Lagerstätten: "scale fossils" and cyanobacteria from the ~750-Ma-old Lower Tindir Group, Yukon Territory, Canada; and acritarchs and cyanobacteria from the ~775-Ma-old Chichkan Formation, southern Kazakhstan. We have also applied these techniques to studies of primary gypsum, a rock-type widely assumed to be barren of fossils. The unit studied is the Miocene (Messinian, ~6-Ma-old) Vena del Gesso Formation of northeastern Italy, uplifted after deposition into the Italian Alps and a unit replete with gypsum-permineralized microorganisms (bacteria, cyanobacteria, phytoplankton, diatoms). A Miocene Lagerstätte preserved in a mineral matrix largely neglected by paleontologists, this finding has implications for the search for evidence of past life on Mars where bedded deposits of gypsum and other sulfates are widespread.