Shuhai Xiao And James D Schiffbauer

Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA

1. Introduction

One of the major tasks of astrobiology is to critically examine evidence of past or present ecosystems beyond our planet. As Earth is the only planet that is known to have hosted life, perhaps as early as 3.8-3.5 billion years ago (or Ga, Giga anna) as illustrated by biologically-meaningful carbon isotopic signatures and prokaryotic microfossils (Mojzsis et al., 1996; Schopf, 2006; but see van Zuilen et al., 2002; Brasier et al., 2006; Fedo et al., 2006), it provides the only model for us to learn how traces of life can be preserved and recognized. In this contribution, we focus on fossil preservation through phosphate mineralization and discuss its implications for the identification of possible life (particularly ancient life if it did exist) on other planets.

If cellular organisms had once existed on other planets, by either independent origin or shared descent with life on Earth, they were most likely to be microscopic, soft-bodied life forms akin to microorganisms (microbial life: prokaryotes, protists, and microalgae) that have populated various environments on Earth. This inference is based on two arguments. First, the striking ecological and metabolic diversity of microbial life on Earth (Nisbet and Sleep, 2001) indicates that it has a better chance populating extreme environments on other planets. Second, consideration of phylogeny and morphological complexity requires that the earliest life be represented by simple forms that are microscopic, unicellular (single-celled), and lack skeletons. Indeed, the evolutionary history on Earth shows that non-skeletal microbial life preceded macroscopic life by slightly more than 1.5 billion years, so that the Archean-Proterozoic biosphere (3.5-0.54 Ga) was entirely dominated by microorganisms. Macro-organisms visible to naked eye (e.g., Grypania) did not appear until about 1.9 billion years ago (Han and Runnegar, 1992; Fralick et al., 2002; Schneider et al., 2002) and they did not become ecologically dominant until the Proterozoic-Cambrian transition at about a half billion years ago. Similarly, the Archean-Proterozoic biosphere consisted almost exclusively of non-biomineralizing, soft-bodied organisms; biologically controlled mineralization evolved in only a few Neoproterozoic (1.0-0.54 billion years ago) taxa (Allison and Hilgert, 1986; Grant, 1990; Grotzinger et al., 2000; Porter and Knoll, 2000; Wood et al., 2002; Hua et al., 2005) and did not become widespread until after the Cambrian Explosion (Bengtson, 1994; Knoll, 2003). Thus, the Precambrian world serves as a plausible model in the search for ancient ecosystems on other planets.

In this contribution, we ask the question how morphological evidence (as opposed to geochemical evidence) of microbial life - if it did exist - would be best preserved in extraterrestrial environments. We approach this question by briefly reviewing the taphonomic pathways in the Proterozoic (2.5-0.54 billion years ago) fossil record. This is followed by a more detailed analysis of three-dimensional phosphatization of non-biomineralizing microorganisms in the Neoproterozoic Doushantuo Formation. We focus on the phosphatization window of the Doushantuo Formation because it represents one of the most powerful taphonomic pathways through which soft-bodied microorganisms can be preserved. We then close our chapter by discussing the astrobiological relevance of phosphatization.

2. Preservation of Proterozoic Fossils

In contrast to the Phanerozoic fossil record that is characterized by macroscopic skeletal fossils, the Precambrian fossil record is dominated by microscopic, soft-bodied organisms. Thus, from a Phanerozoic point of view, the fossilization of such organisms is regarded as exceptional preservation. Butterfield identified six different taphonomic styles of exceptional preservation of Proterozoic-Cambrian non-biomineralizing organisms (briefly reviewed below, Fig. 1), which are named after well known biotas that exemplify each of these taphonomic styles (Butterfield, 2003).

Bitter Spring-type preservation is characterized by silicification of microorganisms in peritidal cherts (Schopf, 1968; Knoll, 1985; Zhang et al., 1998). Classical silicified biotas, for instance the Bitter Spring assemblage, typically show evidence of cellular preservation of microbial communities dominated by prokaryotic forms such as cyanobacteria, although unicellular and multicellular eukaryotes are often preserved as well (Xiao, 2004). In marine environments, this taphonomic window was open in its fullest in the Precambrian, particularly in the Proterozoic, most likely because of the higher availability of dissolved silica in marine waters before the rise of silica biomineralizers such as hexactinellid sponges, demosponges, and diatoms (Maliva et al., 1989, 2005).

Doushantuo-type preservation is known for the exquisite phosphatization of mostly eukaryotic microorganisms that thrived in shallow subtidal environments (Xiao and Knoll, 1999; Dornbos et al., 2006; Hagadorn et al., 2006). Often, labile cellular and subcellular structures are preserved in Doushantuo-type phosphati-zation. Similar to Doushantuo-type preservation is Orsten-type preservation, which represents a taphonomic pathway in which more recalcitrant tissues, such as ecdysozoan cuticles, are preserved through phosphatization (Müller, 1985; Walossek, 2003). The Orsten biota, however, is distinct from the Doushantuo biota in its carbonate (as opposed to phosphorite) depositional setting and the

Ediacara

peritidal subtidal (above wave base)

Depositional Environment subtidal (below wave base)

Doushantuo/Orsten Burgess Shale Ediacara

Bitter Spring

Algae Animals

Archean

Paleo-proterozoic

Meso-proter.

Neo-proter.

Phanerozoic

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