Pellet microfossils: Possible evidence for metazoan life in Early Proterozoic time

Abstract

(14)(15)(16). The origin of metazoans, multicellular animals that relied on microbes as a food resource, is less well documented. The oldest proven metazoan body fossils are in the Ediacaran System (0.55-0.67 Gyr) of southeast Australia (17). The complexity and diversity of the fauna suggest that these animals had a history that extended further back than Ediacaran time. Furthermore, amino acid sequences in 02-binding proteins suggest the radiation of animals took place at or before 0.9-1 Gyr (18). Although older body fossils have not been identified, trace fossils such as tubes of benthic animals range to 2 Gyr (19), and fecal pellets, burrows, and trails of benthic animals are widespread in rocks younger than 0.68 Gyr (20).Detection of the time of metazoan origin has several constraints. The first is ecological: the abundance of a consumer is less than its resource, suggesting that metazoans initially were rare. The probable small size and soft-bodied nature of early consumers is another constraint that likely resulted in poor preservation in the fossil record. Metazoans, just like algae, may have begun as plankton (10,19,21). Plankton are less likely than benthos to leave body fossils or traces because of their small size, light structure, and aquatic habitat. Today, remains of planktonic animals consist primarily of fecal pellets. Microcrustacean fecal pellets predominate in modem sediment, but pellets of microscopic worms and salps as well as bodies of ciliates that resemble fecal pellets also have been identified (refs. 22-26; M. Silver, personal communication). Pellets that are predominantly from copepods accumulate today in low-energy anoxic waters of either silled marine basins or rift-valley lakes (Fig. la) (27, 28). Body parts are rare both in modem sediments and in the fossil record, although there are exceptions such as those of the Ediacara and Burgess faunas. The usual explanation for their absence is that chitin is rapidly degraded (29)(30)(31). We confirmed this by aging modem copepod chitin under nonsterile and sterile conditions at 20'C and observed complete dissolution by 16 and 32 weeks, respectively. Therefore, the history of such animals is probably going to be written by their fossilized fecal pellets.In this study, we applied a palynological technique that isolates soft microfossils in our search for remains of the oldest metazoans. Instead of studying oxide, silicate, or carbonate facies rocks (32) sampled by earlier authors, we sampled the pyrite-bearing rocks of anoxic basins. We studied kerogen in the sulfide facies because anoxic basins are modern depositional analogues that enhance organic preservation in sediments (28, 33). MATERIALS AND METHODSSamples. We solicited samples of dark gray and black laminated shale and slate from Cambrian, Proterozoic, and Archean rocks. These were from rocks that are thought to have been formed in anoxic, nonbioturbated environments. Table 1 shows ages, lithologies, and depositional environments of rocks used in this study.Acid Residues. Te...

Zooplankton fecal pellets, marine snow and sinking phytoplankton blooms

“…Similar rocks of Archaean age (2.68 × 10 9 to 3.8 × 10 9 yr old) did not contain pellet-like microfossils. Robbins et al (1985) argued that if the Proterozoic microfossils were fossilized fecal pellets, then they provided evidence of metazoan life and a complex food web 1.9 × 10 9 yr ago, which predates macroscopic metazoan body fossils 0.67 × 10 9 yr ago, animal trace fossils 0.9 × 10 9 to 1.3 × 10 9 yr ago and fossils of unicellular eukaryotic algae 1.4 × 10 9 yr ago. Fecal pellets may have facilitated oxygenation of the ocean, leading to the Cambrian 'explosion' of multicellular life (Walter 1995, Oliwenstein 1996.…”

“…Porter & Robbins (1981) suggested that zooplankton fecal pellets contributed to the deposition of organic matter in anoxic basins, contributing to formation of the black shale deposits. Robbins et al (1985) described microfossils resembling fecal pellets of modern zooplankters in Middle Cambrian and Early Proterozoic shale. These microfossils were in rocks that also preserved sulfides analogous to modern anoxic environments that preserve fecal pellets.…”

“…If this were the case, then the drop in carbon ratios would be a consequence, rather than a cause, of the Cambrian explosion. Kitchell (1983) and Haberyan (1985) have suggested that if zooplankton fecal pellet sedimentation is a dominant means by which phytoplankton remains reach the seafloor and become part of the micropaleontological fossil record, then selective feeding by the zooplankton producing the fecal pellets might heavily bias the fossil record toward phytoplankton species preferred by grazers.…”

The development of early terrestrial ecosystems

“…Evidence for the existence of Metazoa on land during phase 1 is controversial (Robbins, Porter & Haberyan, 1985;Chaloner, Scott & Stephenson. 1991) and conjecture is rife.…”

Pelletal black shale fabrics: their origin and significance

“…Porter & Robbins 1981;Patton et al 1984;Hakes 1985;Robbins et al 1985;Sageman 1985;Cuomo & Rhoads 1987). They are most often found in dark gray to black laminated mudstones and have been attributed to planktonic organisms.…”