PRECAMBRIAN PROTOZOA

The Precambrian fossil record of protists is dominated by algae. Body fossils representing red, green, xanthophyte, and, possibly, brown algae have been found in Precambrian rocks, and acritarchs – organic-walled microfossils common in Late Precambrian rocks – are usually assumed to represent algal cysts. Protozoa must have been present in Precambrian oceans, but direct fossil evidence for their existence has been limited. New populations of exceptionally well-preserved vase-shaped microfossils (VSMs) preserved in carbonate nodules from the 742 ± 6 Ma Chuar Group, Grand Canyon, provide the best fossil evidence for early protozoa. At least seventeen species of VSMs have been described, including several that can be reliably assigned to two modern protozoan taxa, the euglyphid testate amoebae and the arcellid testate amoebae. VSMs are among the most widespread, abundant, and diverse fossils in middle Neoproterozoic rocks, suggesting that by this time, protozoa were an important component of protistan communities. Interestingly, the first appearance of VSMs coincides with increases in both average ð¹³C values and acritarch diversity. This has been interpreted to suggest that increasing primary productivity and algal diversity allowed those organisms that consumed algae (i.e., the protozoa) to flourish. Alternatively, however, it is possible that the appearance of diverse and abundant protozoa fueled -- rather than was fueled by -- algal diversification. In other words, increases in food web complexity associated with the diversification of protozoa may have helped drive early protistan diversification.