Microfossils as biosignatures

Abstract

As technology advances, we have more possibilities to search for past life, looking for biosignatures in the rock record. Even when these biosignatures can be chemical compounds, the irrevocable evidence of past life would be life itself preserved as fossils. Besides the pure shape, extra information such as the paleoenvironment, biotic associations, and the age of the rocks, can be retrieved from fossils to understand their significance and the context in which they developed. However, much of that information is rarely well preserved, giving fossils a wide range for speculation. Furthermore, abiotic structures can sometimes be easily mistaken as fossils, leading to wrong interpretations. Depending on the mode of fossilization and the type of organism, more or fewer characteristics would be available for interpretation, and depending on the techniques used for observation, those characteristics would be more or less appreciated. In permineralized organisms, internal and external structures can be observable with simple techniques (e.g. thin sections), revealing basic anatomic features that can yield trusty identifications. An example of remarkable organisms with these characteristics is found in the North-East region of Sonora, Mexico. Cretaceous sequences (70-72 Ma old) from the Tarahumara Formation contain several chert horizons where palm roots, freshwater aquatic plants, pollen grains, flowers, seeds, small crustaceans, algae, and cyanobacteria (among others) are commonly found, reflecting the extense diversity existing at that time. One of the Formation's localities, the Huepac Chert, exhibits several stromatolitic horizons in close relation with black chert that harbors a number of permineralized microfossils. About 65 different morphotypes have been described and some have been identified as cyanobacteria, chlorophyceans, and diatoms. These comparisons allowed us to conclude that some had benthonic habits, being perhaps stromatolite constructors. Others appear to be strict freshwater planktonic dwellers, which constrain even more the paleoenvironmental conditions and support geological studies leaning toward freshwater environments. Taking the Huepac microfossils as an example, it is strongly highlighted that morphology, size, and spatial arrangement are key for the identification of microfossils, and should be considered as essential resources of information when looking for life in the geological record on Earth and other planets.