The anatomically preserved tripinnate frond rothwellopteris pecopteroides gen. Et sp. nov. from the latest permian of south China: timing the stem to crown group transition in Marattiales

Premise of research. Fern fronds are common in Late Paleozoic and Mesozoic strata. Large tri-or multi-pinnate fronds are mainly preserved as impression/compressions, while anatomically preserved specimens are typically smaller and comprise dispersed petioles, rachides, ultimate pinna, and pinnules. Here, we describe a large, anatomically preserved tripinnate frond from the latest Permian of southwest China that provides the first detailed histological information for a Cathaysian marattialean with pinnule morphology of the cosmopolitan Carboniferous-Permian pecopteriod type, but with different frond anatomy. Methodology. Specimens were prepared by the cellulose acetate peel technique and studied by light microscopy. Pivotal results. The tripinnate frond has a main rachis and a primary pinnae rachis with many (>70) small vascular bundles arranged in cycles and abundant tanniferous cells. Pinnules are small, and their bases are entirely attached to the ultimate rachis. Abundant vascular bundles in the rachises are distinct from previously recognized marattialean genera, justifying the establishment of Rothwellopteris pecopteroides gen. et sp. nov. Comparison of pinnule morphology with impression/compression fossils shows the specimen to be an anatomically preserved equivalent of Pecopteris marginata Li et al. Conclusions. Rothwellopteris pecopteroides displays a novel combination of marattialean characters from the extinct Paleozoic family Psaroniaceae and the extant family Marattiaceae. Its frond morphology resembles Psaroniaceae including Psaronius but differs from extant Marattiaceae that are monopinnate, palmate, or, as in Angiopteris, bipinnate and that have large pinnules with contracted bases. By contrast, its anatomy, with abundant vascular bundles, is similar to Marattiaceae, especially Angiopteris, but it is distinct from members of the Psaroniaceae in which the rachis possesses one or two tangentially elongate vascular bundles. Pecopteris marginata shows that, by the latest Permian, Marattiales had already evolved frond anatomy typical of extant genera, demonstrating that the stem group to crown group transition commenced prior to the Triassic.