Plagiochila ikiensis (Plagiochilaceae), a new species of leafy liverwort from the Miocene formation of Iki Island, Nagasaki Prefecture, Japan

Abstract

  A new leafy liverwort, Plagiochila ikiensis T.Katag., sp. nov., is described based on a re-examination of Late Miocene fossils from the Monobe Formation on Iki Island, Nagasaki Prefecture, Japan. The new species is characterized by a simple ramification pattern in the presence of lateral-intercalary and terminal branches, distant leaf arrangement, broadly ovate to suborbicular leaves with entire margins, and hexagonal laminal cells of the leaf. Among the fossil species of Plagiochila, P. ikiensis is most similar to P. saportana Schimp., another Miocene species from France, sharing entire leaf margins, rounded leaf apices, and lacking recognizable underleaves. However, P. saportana can be distinguished from P. ikiensis based on its imbricate and obovate leaves and subpinnate ramification pattern. The lack of reproductive characters prevents a more detailed comparison of P. ikiensis with extant species. This is the first fossil species of the genus Plagiochila and family Plagiochilaceae in Southeast Asia to provide clues regarding the evolution of the genus in Southeast Asia.