PALEOPROTEROZOIC EUKARYOTES FROM THE CHANGCHENG GROUP, NORTH CHINA

The Paleoproterozoic Changzhougou Formation (Changcheng Group, ~1800-1600 Ma), Yanshan Range, North China, contains circular, elliptical, and elongate carbonaceous compression-like structures. Most are a few cm's in length and hundreds of um to a few mm's in thickness, have variable morphology, and are found on sandstone bedding planes. Zhu and others (2000) interpreted the structures as carbonaceous compression, body fossils and placed them into three morphological types: Chuaria-, Shouhsienia-, and Tawuia-like. They also found what they called “cellular tissue.” The compression-like structures were interpreted as megascopic algal eukaryotes with cellular preservation.

Further investigation, including field work and petrographic, SEM, EDS, CHN, and XRD analyses, has shed new light on the “compressions.” The Changzhougou examples lack many of the hallmarks of younger, bona fide carbonaceous compressions. They (1) lack regularity in shape, (2) are commonly much larger than most younger compressions, (3) lack wrinkles or other distinguishing features, (4) have relatively small amounts of carbon (0.6 to 1.6% by weight), and (5) are clay- and occasionally phosphorous-rich. No indigenous cellular structure has been verified. These results, coupled with sedimentological and paleoenvironmental considerations, indicate the Changzouguo “compressions” are rounded chips composed of clay to fine sand with varying amounts of organic matter and phosphate that were deposited as clasts in or near a flat tidal environment. Hence they are pseudofossils.

Although the Changzhougou does not contain the algal megafossils once thought, it does contain large (up to 300 um) sphaeromorph acritarchs that are likely eukaryotes. In addition, other formations in the Changcheng Group contain fossils considered to be eukaryotes (acritarchs, large and morphologically complex microbial fossils in chert, and carbonaceous compressions and films). For example, more convincing eukaryotic megafossils (0.5 to 3.5 mm x 5 to 10 mm) may occur in the ~1700 Ma Tuanshanzi Formation.

The Mesoproterozoic is considered the time of major eukaryote diversification. If a robust and diverse eukaryote record can be confidently established for the Changcheng Group, then models on early eukaryote evolution will need to be modified.