THREE-DIMENSIONALLY SILICIFIED HORODYSKIA AND PALAEOPASCICHNUS FROM UPPER EDIACARAN CHERTS IN CENTRAL GUIZHOU OF SOUTH CHINA AND THEIR IMPLICATIONS FOR EVOLUTIONARY STASIS

Horodyskia is one of the earliest known macroscopic life forms with a fossil record dating from ~1.4 Ga. Palaeopascichnus represents a key Ediacaran element with world-wide distribution. However, their body constructions and affinities are poorly understood, partly because previously described species are mostly preserved as casts and molds in siliciclastic rocks. Three-dimensionally silicified specimens of Horodyskia and Palaeopascichnus from the upper Ediacaran Liuchapo Formation in central Guizhou, South China show that both genera share a bodyplan characterized by uniserially arranged, spheroidal or discoidal segments, which are connected by a filament and surrounded by a quartz halo. They are unlikely to be giant sulfide oxidizing bacteria, animal fecal pellets, or polyp-like colonies. Instead, their morphologies draw tentative comparison with agglutinated foraminifera: the segments can be compared with cytoplasm-filled chambers, connection filament with small passage between chambers, and quartz halo with agglutinated test. However, their ontogeny appears to be distinct from modern foraminifers. The occurrence of Horodyskia fossils in Mesoproterozoic and Ediacaran rocks indicates an extremely long range (~900 Myr) and echoes the proposition of extended evolutionary stasis in the Proterozoic.