A PALEOPROTEROZOIC DIVERSIFICATION OF EARLY EUKARYOTES: FOSSIL EVIDENCE FROM THE CHANGZHOUGOU FORMATION, NORTH CHINA

The ~1800 Ma Changzhougou Formation (Changcheng Group) contains numerous acritarchs that range in size from a few dozen to a few hundred micrometers in diameter. Along with sphaeromorphs, a diversity of morphologies that exhibit varying degrees of complexity exists. In 1991, Yan Yuzhong reported on acritarchs from the lowermost Changzhougou Formation in the Pangjiapu region (Hebei Province), which is especially rich in these microfossils. In 2005, new samples were collected as part of a collaborative research project and have been subsequently processed by palynological techniques refined for the recovery of delicate Paleoproterozoic specimens.

Biomarker research suggests that eukaryotes originated as early as 2700 Ma and molecular clock studies predict that organisms with more than 2-3 cell types (the maximum number in prokaryotes) originated ~2000 Ma. There is some morphological evidence for eukaryotes by 1900-1800 Ma; this interpretation rests primarily on the large size of the sphaeromorphs. The morphological complexity of the Pangjiapu acritarchs demonstrates a much more reliable indicator of eukaryotic affinity.

The exquisite preservation of the Pangjiapu specimens allows extremely informative optical microscopy. Single, double, and multiple-layered walls are resolved at high magnification (optical microscopy is still the best friend of paleomicrobiology). In addition, differential peeling of wall layers are distinguished from folding and compression features. This peeling and the flexibility of the walls result in a variety of features, some of which were previously misinterpreted to be primary features (i.e., not taphonomically induced). It is likely that the splitting of vesicles, and subsequent peeling and enrollment of two basic shapes (spherical and ovoidal), resulted in the wide range of morphologies. At least 4 morphotypes are recognized, some of which exhibit excystment structures and other features that further support a eukaryotic affinity.

The Changzhougou Formation microfossils are the oldest best preserved eukaryotic acritarchs. They exhibit a variety of complex morphologies, many of which are primary. This complexity indicates that an early phase of eukaryotic diversification, likely at a stem group taxonomic level, was well underway in the Paleoproterozoic.