MORPHOLOGICAL ANALYSIS OF THE NEOPROTEROZOIC CARBONACEOUS FOSSILS CHUARIA AND TAWUIA

Chuaria and Tawuia are carbonaceous compressions of spherical, spheroidal, or sausage-shaped structures. They are widely distributed in Proterozoic successions. However, the taxonomic distinction between these two taxa has been arbitrary and their phylogenetic affinity has been uncertain. For example, more than twenty taxa have been named from an assemblage of Chuaria- and Tawuia-like fossils in the early Neoproterozoic Jiuliqiao Formation, Anhui Province, North China. These fossils have been variously interpreted as large acritarchs or colonial cyanobacteria. We analyzed nearly a thousand specimens of Chuaria- and Tawuia-like fossils from a single bedding plane in the Jiuliqiao limestone. These specimens show no evidence of sheering deformation other than compaction. The data show that (1) the frequency distribution of width, length, and ratio (length/width) are continuous and all strongly right-skewed; (2) there is a strong correlation between width and length; and (3) the correlation between width (or length) and ratio is rather poor. The data suggest that the distinction of more than twenty taxa of Chuaria- and Tawuia-like fossils in the Jiuliqiao Formation is not justified. The data are also consistent with the previously proposed hypothesis that Chuaria and Tawuia may represent different ontogenetic stages of an allometrically growing organism. An allometric growth indicates that Chuaria and Tawuia are more likely to be eukaryotes than colonial prokaryotes.