Abstract
This paper describes two species of the leptosporangiate fern genus Osmunda Linnaeus (Osmundaceae) based on permineralized rhizomes from late Middle Miocene sediments in the Mosanru Formation, Shimokawa Group, in Shimokawa, north-central Hokkaido, Japan. Osmunda shimokawaensis sp. nov. has an ectophloic dictyoxylic siphonostele radiating numerous leaf traces in spiral order, and stipulate leaf bases characteristic of the Osmundaceae. Mode of leaf-trace departure and sclerenchyma distribution at the leaf base strengthen its affinity with Osmunda (Osmunda). This species is important in being the first record of the subgenus from Asia based on permineralized rhizomes. One rhizome is attributable to the extant species Osmunda cinnamomea L. of subgenus Osmundastrum Presl. Osmunda shimokawaensis and O. cinnamomea provide new evidence for better understanding the evolution and phytogeography of the genus Osmunda.
