Osmundastrum pulchellum is an extinct species of Osmundastrum, leptosporangiate ferns in the family Osmundaceae from the lower Jurassic (Pliensbachian-Toarcian?) Djupadal Formation of Southern Sweden. It remained unstudied for 40 years. It is one of the most exceptional fossil ferns ever found, preserving intact calcified (thus dead) tissue with DNA and cells. Its exceptional preservation has allowed the study of the DNA relationships with extant Osmundaceae ferns, proving a 180-million-year genomic stasis. It has also preserved its biotic interactions and even ongoing mitosis.
History and discovery
The only known specimen was recovered at the mafic pyroclastic and epiclastic deposits of the Djupadal Formation, dated Pliensbachian-Toarcian(?), that are present near Korsaröd Lake, at the north of Höör, central Skåne, southern Sweden. The location was studied first by Gustav Andersson, a local farmer, who was a passionate follower of scientific discoveries. Through his interest in geology, he identified several coeval volcanic plugs, and motivated by the presence of volcanic soils, he excavated a location at the south of the Korsaröd lake. Initially nothing was found, but a second deeper dig revealed a series of aggregated wood remains on volcanic lahar-derived stones. Samples taken from the location were sent to the geologist Hans Tralau, who carried out palynological research on them, estimating an age of deposition of Late Toarcian-Aalenian(?). A petrified rhizome was sent to Tralau, who understood the significance of the fossil and intended to publish it formally, but his untimely death in March 1977 made it impossible. The rhizome, along with the fossil wood, was archived at the Swedish Museum of Natural History, where the geologist Britta Lundblad tried also to publish it formally, what was also impossible due to her retirement in 1986. The fossil was lying forgotten in the archives of the museum until 2013, when it was discovered again and studied, finding that it preserved spectacular cellular detail, rarely seen on fossils. In 2015, it was finally published as Osmunda pulchella by B. Bomfleur, G. W. Grimm and S. McLoughlin. The specific epithet pulchella (Latin diminutive of pulchra, 'beautiful', 'fair;) was chosen in reference to the exquisite preservation and aesthetic appeal of the holotype specimen. The name Osmunda pulchella was mostly used in the main publications referring to it until in 2017 a revision of the cladistic status of the fossil Osmundales showed that the fossil was in fact a member of the genus Osmundastrum, so it became Osmundatrum pulchellum.
Description
The Osmundastrum pulchellum holotype is a calcified rhizome fragment about 6 cm long and up to 4 cm in diameter that probably come from a small (approx. 50 cm tall) fern. It is composed of a small central stem surrounded by a compact mantle of helically arranged petiole bases and interspersed rootlets that extend outwards perpendicular to the axis, indicating a low rhizomatous rather than arborescent growth. This, together with the asymmetrical distribution of the roots, points to a creeping habit. The stem is around about 7.5 mm in diameter and the pith about 1.5 mm in diameter and entirely parenchymatous. In the pith, cell walls lack the presence of an internal endodermis or internal phloem, considered to be an original feature, rather than a loss due to inadequate preservation. Traces of leaves and associated rootlets are present traversing the outer cortex. This specimen is well known for the quality of its preservation, quality revealing cellular and subcellular detail: from tracheids with preserved wall thickenings, to parenchyma cells containing preserved cellular contents. Some of the parenchyma cells contain oblate particles about 1–5 μm in diameter, interpreted as putative amyloplasts.
Classification The exceptional preservation of Osmundastrum pulchellum has allowed the establishment of an evolutionary overview of royal ferns since the lower Jurassic. At its description as Osmunda pulchella, it was compared with Todea, Leptopteris, Plenasium and Claytosmunda, and found as a bridge in the morphological gap between extant Osmundastrum and the subgenus Osmunda inside Osmunda – the closest species to Osmundastrum. It was shown that this species and the extant Osmundaceae share the same chromosome count and DNA content. In 2017, a re-examination of the phylogeny of the fossil Osmundales showed it to be a member of the genus Osmundastrum and a probable precursor of the modern Osmundastrum cinnamomeum. Latter, a new species, Osmundastrum gvozdevae from the Middle Jurassic of the Russian Kursk Region was recovered as a possible sister taxon.
Biology
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