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During the past decade, Environmental and Field Emission Scanning Electron Microscopes have been used at the
NASA/Marshall Space Flight Center to investigate freshly fractured interior surfaces of a large number of different
types of meteorites. Large, complex, microfossils with clearly recognizable biological affinities have been found
embedded in several carbonaceous meteorites. Similar forms were notably absent in all stony and nickel-iron
meteorites investigated. The forms encountered are consistent in size and morphology with morphotypes of known
genera of Cyanobacteria and microorganisms that are typically encountered in associated benthic prokaryotic mats.
Even though many coccoidal and isodiametric filamentous cyanobacteria have a strong morphological convergence
with some other spherical and filamentous bacteria and algae, many genera of heteropolar cyanobacteria have
distinctive apical and basal regions and cellular differentiation that makes it possible to unambiguously recognize the
forms based entirely upon cellular dimensions, filament size and distinctive morphological characteristics. For almost
two centuries, these morphological characteristics have historically provided the basis for the systematics and
taxonomy of cyanobacteria. This paper presents ESEM and FESEM images of embedded filaments and thick mats
found in-situ in the Murchison CM2 and Orgueil CI1 carbonaceous meteorites. Comparative images are also provided
for known genera and species of cyanobacteria and other microbial extremophiles. Energy Dispersive X-ray
Spectroscopy (EDS) indicates that the meteorite filaments typically exhibit dramatic chemical differentiation with
distinctive difference between the possible microfossil and the meteorite matrix in the immediate proximity.
Chemical differentiation is also observed within these microstructures with many of the permineralized filaments
enveloped within electron transparent carbonaceous sheaths. Elemental distributions of these embedded filaments are
not consistent with recent cyanobacteria or other living or preserved microbial extremophiles that have been
investigated during this research. The meteorite filaments often have a nitrogen content below the sensitivity level of
the EDS detector. Carbon, Sulfur, Iron or Silicon is often highly enriched and hence anomalous C/N and C/S ratios
when compared with modern cyanobacteria. The meteorite forms that are unambiguously recognizable as biological
filaments are interpreted as indigenous microfossils analogous to several known genera of modern cyanobacteria and
associated trichomic filamentous prokaryotes.
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