Upside-down in volcanic ash: crown reconstruction of the early Permian seed fern Medullosa stellata with attached foliated fronds.

Abstract

Our understanding of fossil floras through geological time is mainly based on various differently preserved plant parts, often found isolated under restricted taphonomic circumstances. Preservation of whole plants is exceptionally rare in the geological record but provides the most reliable proof of ancient plants, especially those lacking a nearest living relative like the late Paleozoic pteridosperms. Among them, the medullosan seed ferns represent the largest and most essential systematic group. Medullosaleans are well known from the large Euramerican tropical swamp forests of Carboniferous age, but also from seasonally dry, clastic-soil habitats of Late Pennsylvanian–early Permian intramontane basins in central Europe. An exceptional taphonomic window that offered three-dimensional preservation of early Permian plants is located in Chemnitz, eastern Germany. Here, a forest ecosystem had been buried by pyroclastic deposits in a geological instant, 291 ± 2 Ma ago. Medullosaleans are both abundant and diverse in this striking autochthonous assemblage. The upper part of a Medullosa stellata var. typica individual broke at its top resulting from the overload of volcanic ash and was buried upside-down in the basal pyroclastics. The tree crown consists of the anatomically preserved apical stem, ten attached Alethopteris schneideri foliated fronds with Myeloxylon-type petioles and rachises. Though already discovered during the scientific excavation in 2010, the remarkable find required several years of preparation work, documentation, and reconstruction. The fronds were up to 3.5 m long, bifurcating, and bore numerous bipinnately compound pinnae preserved pulvinated in life position. The apical stem vascular system consists of a cylindrical peripheral vascular segment and up to 87 central accessory strands, each surrounded by manoxylic wood and secondary phloem. The reconstructed tree is supposed to have been self-supporting and of slender stature. Its architectural model is comparable to modern tree ferns or cycads and adapted to have grown in light-deficient lower-storey (sub-) tropical forests, usually not exceeding 15 m in height. Apical meristematic growth dominated, whereas only minor secondary growth occurred during ontogenesis. The densely attached frond bases followed a 3/8 phyllotaxis and were most likely abscised shortly after becoming photosynthetically inactive. A high water-conducting potential is assumed due to the tree’s cauline, petiolar and leaf vascular anatomies. Concerning the extensive leaf surface of the densely foliated fronds, considerable transpiration is hypothesised. Alethopteris schneideri foliage is stratigraphically significant for lower Permian (Asselian–Sakmarian) continental strata of central Europe, preferring habitats of wet clastic soils in sub-humid, seasonal palaeoclimate. The new insights provide a substantial step towards the first whole-plant concept of intramontane medullosaleans.