Riccardiothallus devonicus gen. et sp. nov., the earliest simple thalloid liverwort from the Lower Devonian of Yunnan, China

This paper describes a fossil liverwort in the Posongchong Formation, Lower Devonian (Pragian), Yunnan, China preserved as a partially permineralized compression in gray arenaceous mudstone. The plant comprises a regularly-bifurcating flat ecostate multilayered thallus with entire margins. Following comparisons of this fossil with gametophytes of extant and fossil liverworts, a new genus Riccardiothallus was established in the family Aneuraceae (Jungermanniopsida). The fossil, which appears to be closely similar to several members of the extant genus Riccardia, is the earliest unequivocal megafossil evidence of a liverwort. Considering the sedimentary environment of Riccardiothallus devonicus and the habitats of many extant Riccardia species, we conclude that Riccardiothallus devonicus probably lived in a warm and humid riverine environment. The age (Pragian, 407–411Ma) of Riccardiothallus devonicus suggests that the differentiation of Jungermanniopsida and Metzgeriidae was in the Early Devonian, significantly earlier than the assumptions derived from analyses of chloroplast DNA sequences of modern liverworts and current total evidence phylogenies (Late Devonian, 359–385Ma; Late Carboniferous, 299–307Ma).