Restudy of string fossils from the Ediacaran-Cambrian Liuchapo Formation in Guizhou Province, South China

Abstract

String fossils, Horodyskia and Palaeopascichnus are globally distributed in Proterozoic strata. They are composed of uniform segments with specific intervals, and their origins have been debated for decades. Horodyskia consists of spherical or subspheroidal segments, and has been interpreted as eukaryote, seaweeds, foraminifera, and pseudofossils; Palaeopascichnus is composed of crescent or sausage-shaped segments and has been interpreted as trace fossils or protozoans. A large number of string fossils are found from the Liuchapo Formation in South China, which are similar to Horodyskia minor and Palaeopascichnus jiumenensis. However, the classification of H. minor and P. jiumenensis remains debatable because of their small size, inter-segment connection, and the external quartz halo. In order to further explore the biological attributes and affinities of these two string fossils, we studied the chert samples of the Liuchapo Formation from Jiumen, Jinlongcun and Bahuang sections in Guizhou Province, and reconstructed the morphological characteristics of the string fossils using the methods of thin section observation and serial grinding. Based on the reconstruction, we renamed the string fossils and erected two new genera, Parahorodyskia and Ordinilunulatus. Parahorodyskia consists of even-sized spherical and ellipsoidal segments with consistent spacing, surrounded by a peapod-shaped halo. According to the absence or presence of the ribbon-like connection, two species are identified: P. disjuncta, and P. minor. Ordinilunulatus consists of uniform, evenly spaced disk-shaped segments with a terminal spherical structure, surrounded by a sausage-shaped halo, and segments are generally jointed by connection. On the basis of three-dimensional reconstruction, string fossils Parahorodyskia and Ordinilunulatus of the Liuchapo Formation are more likely to be of biological rather than diagenetic origin. In addition, a strip-like fossil is also discovered in abundance in the Liuchapo Formation. This strip-like fossil is described as a new erected genus Jiumenia, and it shares similar outer halo structure with the Liuchapo string fossils. Both Parahorodyskia and Ordinilunulatus might be affiliated to protist. Finally, the wide occurrences of Parahorodyskia and Ordinilunulatus in South China may imply their potential biostratigraphic significance for the bedded cherts in the Ediacaran-Cambrian transition.