The Proterozoic macrofossil Tawuia as a coenocytic eukaryote and a possible macroalga

Abstract

Proterozoic carbonaceous macrofossils are key to understand the evolution of early eukaryotes and how they acquired macroscopic sizes. However, phylogenetic interpretation of these macrofossils is challenging because most of them are morphologically simple and lack phylogenetically diagnostic features. Among Proterozoic carbonaceous macrofossils, the tomaculate fossil Tawuia is relatively rich in characters and is common in Proterozoic successions. However, its phylogenetic interpretation is uncertain and Tawuia has been variously interpreted as a bacterium or an alga (photosynthetic eukaryote), in part because certain aspects of its morphologies have not been consistently documented, hampering their functional interpretations. Here we report exceptionally preserved Tawuia specimens from the Tonian Liulaobei and Shiwangzhuang formations in North China. These fossils show that Tawuia is a tomaculate organism, sometimes with an axial stripe, terminal discoidal structures, and transverse constrictions, features that were previously documented but variously interpreted. We infer that Tawuia was a coenocytic organism with recalcitrant cell walls, asexual reproduction through asymmetric constrictions, and possible abscission structures. These features indicate that Tawuia is probably a coenocytic eukaryote, and possibly a macroalga, although more evidence is needed to confirm these interpretations, particularly the photosynthetic nature of Tawuia. A literature review reveals that the key features described here occur not only in many Tonian Tawuia specimens, but also in some possible Mesoproterozoic Tawuia fossils as much as ca. 1.6 Ga old. The new data and interpretations of Tawuia presented here provide important constraints on the origin and early evolution of coenocytic eukaryotes, primary plastids, and macroalgae, particularly if the Mesoproterozoic Tawuia fossils are confirmed as macroalgae.