New Morphological Structures of Eoandromeda Octobrachiata: Implications for Early Ctenophore Evolution and Locomotion

The octoradiate spiral fossil Eoandromeda octobrachiata is one of the most characteristic components of the Ediacaran Wenghui biota from the upper Doushantuo Formation in Wenghui, northeastern Guizhou Province, South China. As one of the earliest discovered macroscopic Ediacaran animal fossils in China, E. octobrachiata has been interpreted as a diploblastic-grade metazoan representing an early stage in animal evolution and provisionally assigned to the stem lineage of ctenophores, primarily due to the absence of crown-group ctenophore synapomorphies. However, the limited preservation of specimens in previous studies has left many specific morphological features poorly understood, fueling ongoing controversy regarding its classification. In this study, we report newly collected specimens of E. octobrachiata that exhibit exceptionally preserved anatomical details, including comb rows, comb plates, a distinct comb row nerve tract, and an apical organ. These features significantly expand the known morphological repertoire of the species and provide critical data for reassessing its phylogenetic position. Moreover, they offer novel insights into the origin and evolution of ctenophore body plans, as well as the locomotor coordination and ecological strategies of early ctenophores during the Ediacaran Period. Additionally, we examined the specimens using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) to investigate the microstructures and elemental composition of these carbonaceous metazoan fossils. EDS analysis reveals elemental enrichment patterns related to taphonomic conditions and diagenetic pathways, thereby helping to elucidate the preservation mechanisms of soft-bodied fossils.