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Project Progress

Research in the Knoll lab has focused on three major issues relevant to early animal evolution. First, Knoll and colleagues have developed a hypothesis that potentially explains the observed diversification of eukaryotes in mid-Neoproterozoic oceans, invoking the evolution of eukaryovorous protists in a way that is analogous to the role of carnivores in driving Cambrian animal diversification. This hypothesis is supported by functional and phylogenetic evaluations of the Neoproterozoic fossil record (Knoll, 2014) and gains additional strength through the mapping of feeding mode onto eukaryotic phylogenies (Knoll and Lahr, in press; Lahr and Knoll, in review). The comparative biological research shows that the last common ancestor of extant eukaryotes fed on bacteria-sized particles, and that eukaryovory (feeding on larger eukaryotic cells) is a derived feature evolved independently in four major Neoproterozoic clades. Multicellularity can arise as a defense against predation, making the evolution of feeding mode directly relevant to the early evolution of animals. Moreover, from this functional perspective, every instance of complex multicellularity can be seen as an evolutionary escape from phagotrophy.

In research led by postdoctoral fellow Erik Sperling, the Knoll lab has worked to integrate ecological data from oxygen-minimum zones with paleontological and geochemical data on early animal evolution. Consistent with reports from others, we find that small, thin (and essentially unfossilizable) animals exist at exceedingly low oxygen concentrations, but that one key functional class of animals – carnivores – occurs only when oxygen levels are higher (Knoll and Sperling 2014). Statistical analysis of nearly 5000 analyses of the iron-speciation chemistry of ancient shales shows that (1) Proterozoic oxygen minimum zones were generally anoxic and ferruginous, but with a tendency toward euxinia not observed in the younger Proterozoic; (2) early animals likely experience little if any sulfide stress in later Neoproterozoic oceans, enabling them to thrive at lower oxygen concentrations than would otherwise have been possible; and (3) Cambrian oceans remained incompletely oxygenated placing limits on the extent of any end-Proterozoic increase in pO2 (Sperling et al., in revision). Oxygenated oxygen minimum zones existed locally as early as 1400 Ma (Sperling et al., 2014), and a range of subsurface conditions in younger basins, as well, stressing the importance of treating geochemical data statistically. A forthcoming review attempts to integrate geochemical and physiological data relevant to early animal evolution (Sperling et al., in preparation).

Members of the Knoll and Johnston labs have also worked to understand the taphonomic processes that gave rise to the Cambrian record of animal evolution, specifically the phosphatization of minute invertebrate skeleton known as small shelly fossils. The phosphatization window opens and closes within the Cambrian period, a consequence of changing biology and ocean chemistry (Creveling et al., 2014a, 2014b). In other work, the Knoll and Macdonald labs have collaborated on the analysis of a new record of diverse vase-shaped protists in Neoproterozoic rocks from northwestern Canada (Strauss et al., 2014), and Knoll has collaborated with others to generate a record of phylogeny and character evolution in complex multicellular red algae (Yang et al., in press). And finally, Knoll postdoctoral fellow Kristin Bergmann initiated a new field research program on the Neoproterozoic carbonate record of Spitsbergen, with the goal of placing our growing paleobiological understanding of this succession into a modern environmental framework informed by sequence stratigraphy and clumped isotope measurements of depositional temperature.