Ashicaulis macromedullosus sp. nov. (Osmundaceae) is described on the basis of one silicified rhizome, collected from the Middle Jurassic Tiaojishan Formation in Hebei, China. This specimen consists of a small, upright stem surrounded by frond bases, and adventitious roots. The stem consists of pith, xylem cylinder, a two-layered cortex and petiole bases. In cross section, the stem has a dictyoxylic xylem cylinder composed of 24-32 xylem strands separated by many leaf gaps with some incomplete leaf gaps. This species is characterized by continuous cylindrical phloem, absence of medullary traces in the pith, no sclerenchyma in leaf trace concavity, homogeneous cortical sclerenchyma, and one mass of sclerenchyma in each stipular wing. This is the third structurally preserved rhizome of Ashicaulis from the Mesozoic in northern China and adds to the knowledge of fern diversity in China. Northern Hemisphere species from the Jurassic have a tendency to larger stele diameter than comparable Mesozoic species in the Southern Hemisphere.
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