SIZE DISTRIBUTION CONSTRAINS AFFINITY OF CHUARIA FROM EARLY NEOPROTEROZOIC SUCCESSIONS IN NORTH CHINA

The body size is one of the main attributes of organisms. The knowledge of body size distributions of the fossils and the related factors can contribute to the understanding of many biological and geological processes. In this study, we accept the common held view that body sizes of the same organism show normal distribution at a certain developmental stage and combine the normal distribution of body size at each stage with the other three factors, i.e. growth, survivorship and taphonomy, to establish a growth-survivorship-taphonomy (GST) model that can simulate body size distributions of a certain type of fossils when organisms have various growth and survivorship patterns given different taphonomic conditions. Forty-eight GST distribution curves have been generated altogether to offer valuable information for disentangling biological and geological processes in particular fossils.

Chuaria, which is diagnosed as millimeter-sized, organic-walled spherical vesicles, is one of the most common carbonaceous compression fossils found in the Proterozoic Eon (ca. 1000–720 Ma). Many mutually contradictory affinities have been intensively suggested for this globally distributed macrofossil over the last 100 years, but no consensus regarding the phylogeny of Chuairia has not yet reached because of its simple morphology, poorly preserved cellular structure and lack of fossil evidence showing recognizable complete life cycle. In this study, we conducted statistical analysis of size distribution of Chuaria and compare the pattern with GST distribution models, with the hope to shed some light on the affinity of Chuaria. Fossils were collected from the Jiuliqiao Formation at the Shouxian section in Anhui Province, North China. Measurements (N=2906) were gathered from several bedding surfaces of argillaceous limestones. Our results show that the body size of Chuaria is subject to standardized normal distribution. This pattern dovetails nicely with the GST model under the condition when organisms stay the same size from birth. Thus the result indicates that Chuaria probably belong to a certain developmental stage in their life cycle, leading to the resting cysts interpretation for the affinity of Chuaria.