A new structurally preserved fern rhizome of Osmundaceae (Filicales) Ashicaulis wangii sp. nov. from the Jurassic of western Liaoning and its significances for palaeobiogeography and evolution

Permineralized osmundaceous rhizome with anatomical and phylogenetic information plays a significant role in understanding the origin, evolution, and diversity variation of the fern family Osmundaceae in geological history. The northern Hebei and western Liaoning region is one of the most important fossil localities for the Jurassic osmundaceous rhizome fossils in the Northern Hemisphere; however, the diversity character of osmundaceous rhizome fossil remains poorly known. A new structurally preserved fern rhizome species, Ashicaulis wangii sp. nov., is described from the Middle Jurassic Tiaojishan Formation in Beipiao City, Liaoning Province, northeastern China. The rhizome is composed of heterogeneous pith, an ectophloic-dictyoxylic siphonostele, a two-layered cortex, and a mantle of adventitious roots and petiole bases. The xylem cylinder, with complete leaf gaps, consists of 15–17 xylem strands. The petiole base is characterized by a heterogeneous sclerotic ring and numerous sclerenchyma masses in the petiolar cortex. Among five known Ashicaulis species with heterogeneous sclerotic ring, four of them are documented from China. Therefore, osmundaceous rhizome fossils from China show endemic anatomical characteristics and significances for palaeobiogeography. Comparisons of anatomical features suggest that A. wangii sp. nov. bears close similarities to Osmunda pluma Miller from the Paleocene of Dakota, USA. Fossil species of A. wangii provides new evidence for further understanding the species diversity of osmundaceous rhizome fossil in China and in the Northern Hemisphere, and contributes to exploring the macroevolution process of the Mesozoic osmundaceous plants.