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The Middle Miocene Mosanru Formation of the Shimokawa Group in north-central Hokkaido, Japan, contains well-preserved silicified plant remains showing fine and histological structure. This paper elucidates the morphological anatomy of silicified plants found in the Shimokawa chert of the Mosanru Formation and provides the data for phylogenetic reconstruction of Osmundaceae. Furthermore, geology, flora and vegetation of the Mosanru Formation has been investigated for paleoenvironmental context.
Based on lithologic and sedimentary features, this Formation is interpreted to represent some environments of deposition : terrestrial sediments, fluvial channel, flood plain (remains of fossil forests), and lacustrine sediments. The upper part of this Formation has seven silicified beds (S1-S7) and nearly in situ well-preserved plant parts are mostly obtained from the S3 bed.
The assemblage is composed mainly of coniferous and broad-leaved elements that include with two now-extinct taxa. Most o
f the taxa probably grew in a swampy temperate environment : the fern genus ; Osmunda, the conifer genera ; Abies, Picea, Tsuga and Glyptostrobus, and the angiosperm genera ; Decodon and Alnus.
The quality of preservation is exquisite, and several remains allow precise study of taxonomy. Available anatomical characters of both fossil and extant species were conducted to most significant data for phylogenetic relationship. The detailed data on anatomical structure of both vegetative and reproductive organs provides the data for phylogeny. Even when the reproductive structure of above mentioned taxa is unknown, the phylogenetic results can be gained from their vegetative structures.
Since Osmunda were widely distributed and possess several characteristic features in the rhizomes, their phylogeny are better understood using both fossil and modern spccies. As the result, Osmunda cinnamomea is the most basally positioned species in Osmundaceae because fossil species with almost the same morphology as this species were recorded from the Triassic. Greater anatomical variation was found between Osmunda cinnamomea and O.claytoniana. They possibly belong to biologically differentiated groups. Less