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Silicified Horodyskia and Palaeopascichnus from upper Ediacaran cherts in South China: tentative phylogenetic interpretation and implications for evolutionary stasis

Horodyskia is one of the earliest known macroscopic life forms, with a fossil record dating from c. 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 moulds in siliciclastic rocks. Silicified specimens from the upper Ediacaran Liuchapo Formation in eastern Guizhou, South China, are described as Horodyskia minor sp. nov. and Palaeopascichnus jiumenensis sp. nov. Their taxonomic assignments are based on their uniserial arrangement of spheroidal or discoidal units, which are connected by a filament and surrounded by a quartz halo. They are unlikely to be brown algae, animal traces, faecal pellets, colonial metazoans, or giant sulphide-oxidizing bacteria. Instead, we propose that Horodyskia and Palaeopascichnus may be phylogenetically related, and their collective morphologies allow tentative comparison with agglutinated foraminifers: the segments can be compared with cytoplasm-filled chambers, connecting filament with small passage between chambers, and quartz haloes with agglutinated tests. However, their ontogeny appears to be distinct from that of modern foraminifers. The occurrences of Horodyskia fossils in Mesoproterozoic and Ediacaran rocks indicate an extremely long range (c. 900 Ma) and echoes the proposition of extended evolutionary stasis in the Proterozoic.