Exploring the stem to crown group transition in Marattiales: a new species of frond from the late Permian of China with features of the Psaroniaceae and Marattiaceae

A new species of the Marattialean fern frond Rothwellopteris is proposed for specimens preserving both morphology and anatomy from the Wuchiapingian-Changshingian (late Permian) aged Xuanwei Formation of South China. Fronds are at least bipinnate and bear falcate, pecopteroid pinnules with a thin lamina. The penultimate pinna rachis has a polycyclic vascular system with endarch primary xylem and longer vascular bundles located abaxially in a single radial row, and shorter vascular bundles adaxially arranged in two radial rows, with a vascular bundle sheath of sclerenchyma cells. Ground tissue comprises parenchyma cells and numerous secretory ducts. Features of the new species suggest that it was adapted for mesomorphy, while the previously recognized species Rothwellopteris pecopteroides was adapted for xeromorphy or was a sun-leaf. Like R. pecopteroides, the new species displays a mosaic of morphological and anatomical features characteristic of both the Paleozoic family Psaroniaceae and the extant family Marattiaceae; we interpret it to be a member of the evolutionary stem-group from which stratigraphically younger Marattiaceae evolved. Our findings show that the stem- to crown-group transition within Marattiales commenced before the end of the Permian and that a greater diversity of transitional forms can now be recognized from the late Permian and include the genera Compsopteris, Rothwellopteris, Tietea and Tuvichapteris.