Nuclei and nucleoli in embryo-like fossils from the Ediacaran Weng’an Biota

Zongjun Yin,John A Cunningham,Kelly Vargas,Stefan Bengtson,Maoyan Zhu,Philip C.J Donoghue

doi:10.1016/j.precamres.2017.08.009

Abstract

The embryo-like microfossils from the Ediacaran Weng’an Biota (ca. 609 million years old) are among the oldest plausible claims of animals in the fossil record. Fossilization frequently extends beyond the cellular, to preserve subcellular structures including contentious Large Intracellular Structures (LISs) that have been alternately interpreted as eukaryote nuclei or organelles, degraded remains, or abiological structures. Here we present new data on the structure, morphology, and development of the LISs in these embryo-like fossils, based on Synchrotron Radiation X-ray Tomographic Microscopy (SRXTM) and quantitative computed tomographic analysis. All the lines of evidence, including consistency in the number, shape, position, and relative size (LIS-to-cytoplasm ratio) of the LISs, as well as their occurrence within preserved cytoplasm, support their interpretation as cell nuclei. Our results allow us to reject the view that nuclei cannot be preserved in early eukaryote fossils, offering new potential for interpreting the fossil record of early eukaryote evolution.

Highlights

As one of the oldest plausible claims of animals in the fossil record, the embryo-like microfossils from the Ediacaran Weng’an Biota (ca. 609 million year old (Zhou et al, 2017)) have long been expected to afford new insights into the developmental evolution of animal body plans (Chen et al, 2006, 2009; Xiao et al, 1998; Yin et al, 2016)
The large cell containing two Large Intracellular Structures (LISs) has a volume of 0.124 mm3, which is approximately double the volume of the remaining small cells (0.057–0.082 mm3, mean = 0.068 mm3)
The consistent size, shape and position of the large intracellular structures within embryo-like fossils Tianzhushania and Spiralicellula and their occurrence within mineralized cytoplasm preclude their interpretation as artefacts resulting from taphonomic or diagenetic processes

Summary

Introduction

As one of the oldest plausible claims of animals in the fossil record, the embryo-like microfossils from the Ediacaran Weng’an Biota (ca. 609 million year old (Zhou et al, 2017)) have long been expected to afford new insights into the developmental evolution of animal body plans (Chen et al, 2006, 2009; Xiao et al, 1998; Yin et al, 2016). The LISs have a consistent size and location, sometimes occur paired in cells, positioned parallel to an anticipated plane of cell division (Hagadorn et al, 2006), and can be elongated or dumbbell-shaped, suggesting ongoing division (Huldtgren et al, 2011) They were originally considered as nuclei, spindle bundles, or other organelles (Hagadorn et al, 2006) and most subsequent biological interpretations have focussed on a nucleus interpretation (Chen et al, 2009; Huldtgren et al, 2011, 2012; Schiffbauer et al, 2012; Xiao et al, 2012). We sought to test the established taphonomic models for the LISs and to reassess their origin to constrain affinities of these embryo-like fossils and to better understand the early eukaryotic fossil record

Methods

Results

Discussion

Conclusion