First fossil record of Trichosteleum (Bryophyta: Sematophyllaceae) from mid-Miocene Zhangpu amber in Asia

The evolutionary history of terrestrial moss in Asia during the Cenozoic is poorly interpreted in largely because of very limited fossil evidence. The middle Miocene Zhangpu amber is the sole resin source of moss fossils in China, which shows a diverse moss population during the mid-Miocene Climatic Optimum. In this study, we reported the discovery of a gametophyte fragment of moss fossil in a well-preserved amber inclusion from the middle Miocene of Zhangpu, Southeast China. The observation methods involved light microscopy and synchrotron radiation–based X-ray microcomputed tomography analysis on the amber inclusions. The fossil has lanceolate leaves with involute margins, long linear laminal cells with unipapillae, well-developed and enlarged alar cells, and an absence of costae. The fossil is assigned to the extant Trichosteleum and represents the only known occurrence of a fossilized Trichosteleum member in Asia. This finding enhances our comprehension on the moss diversity within Zhangpu amber and establishes a first record of Trichosteleum in Asia.