Osmundastrum pulchellum

Osmundastrum pulchellum Rhizome includes several biotic interactions, including lycopsid roots, possible Peronosporomycetes oogonia, Fungal spores and oribatid mites.

Osmundastrum pulchellum is well known thanks to exceptional preservation of detailed anatomical structures (e.g., pith, stele, petiole base, adventitious roots, and even nuclei). As well is the only known case of fossilized ongoing mitosis.^[7] This is shown by the fact that the chromosomes and cell nuclei show marked structural heterogeneities compared to the cell walls during different stages of the cell cycle.^[7] A rapid calcite permineralization "froze" the organic molecules in time, which suggests the fern rhizome was fossilized probably on a very short time, perhaps even minutes thanks to a fast lahar deposit.^[7] The tissues show cells with nuclei, nucleoli, and chromosomes during the interphase, prophase, prometaphase, and possible anaphase of the cell cycle.^[11] Some cells also show pyknotic nuclei typical of cells undergoing apoptosis (programmed cell death).^[11] The subcellular detail is nearly unique, as other ferns preserved in similar conditions lack them, for example Ashicaulis liaoningensis.^[12] Several biotic interactions were recovered on the rhizome. Exotic roots were recovered on the petiole bases, with a level of preservation that matches that of the whole plant, bearing a similar vasculature as seen in modern lycophytes. They are interpreted as belonging to a small herbaceous epiphytic lycopsid, with its megaspores also linked with the specimen.^[6] Other sporangial fragments from other ferns (Deltoidospora toralis, Cibotiumspora jurienensis, etc.) were also recovered, known from the nearby deposits.^[13] A similar community was recovered on a Todea rhizome from the early Eocene of Patagonia, but with the epiphytic plants being in Osmundastrum pulchellum exclusively lycopsids and ferns, which may indicate that bryophytes had not yet evolved the epiphytic habit during the Jurassic.^[14] Possible oogonia of Peronosporomycetes are found in a parasitic or saprotrophic relation with the plant. If the identification of the oogonia of Peronosporomycetes is correct, then this implies regularly moist conditions for the growth of Osmundastrum pulchellum.^[6] Thread-like structures were found, identified as derived from a pathogenic or saprotrophic fungus invading necrotic tissues of the host plant. The interaction of the fungus with the plant was probably mycorrhizal.^[6] Excavations up to 715 μm in diameter are evident, filled with pellets that resemble the coprolites of oribatid mites, found also in Paleozoic and Mesozoic woods.^[6]

The Djupadal Formation was deposited in the Central Skane region, linked to the late Early Jurassic Volcanism. Several coeval Volcanic necks are recovered on the region, such as Eneskogen (A large hill covered by quaternary sediments. Some few boulders and basalt pillars were exposed), Bonnarp (5–6 m height and covers roughly 5,000 square meters, covered by Jurassic sediments) and Säte (Comprise two basalt pipes, each roughly 6–10 m high and some 10,000 square meters in area).^[15] The Korsaröd member includes a volcanic-derived lagerstatten where this fern was found, probably derived from a fast lahar deposition.^[13] Thanks to the data provided by the fossilized wood rings, it was found that the location of Korsaröd hosted a middle-latitude Mediterranean-type biome in the late Early Jurassic, with low rainfall. Superimposed on this climate were the effects of a local active Strombolian Volcanism and hydrothermal activity.^[13] This location has been compared with modern Rotorua, New Zealand, considered an analogue for the type of environment represented in southern Sweden at this time.^[4] The locality was populated mostly by Cupressaceae trees (including specimens up to 5 m in circunference), known thanks to the great abundance of the wood genus Protophyllocladoxylon and the high presence of the genus Perinopollenites elatoides (also Cupressaceae) and Eucommiidites troedsonii (Erdtmanithecales).^[13] The underlying Höör Sandstone Formation hosts abundant Chasmatosporites spp. pollen produced by plants related to cycadophytes, while the Djupadal volcanogenic deposits are dominated by cypress family pollen with an understorey component rich in putative Erdtmanithecales, both representing vegetation of disturbed habitats. The abundance of Protophyllocladoxylon sp. is also related with a sporadic intraseasonal and multi-year episodes of growth disruption, probably due to the volcanic action.^[13] Pollen, spores, wood and charcoal locally indicate a complex forest community subject to episodic fires and other forms of disturbance in an active volcanic landscape under a moderately seasonal climate.^[6] Osmundastrum pulchellum was a prominent understorey element in this vegetation and was probably involved in various competitive interactions with neighboring plant species, such as lycophytes, whose roots have been recovered inside the rhizome.^[6] The ferns were part of a fern- and conifer-rich vegetation occupying a topographic depression in the landscape (moist gully) that was engulfed by one or more lahar deposits.^[6]