A silicified Todea trunk (Osmundaceae) from the Eocene of Patagonia

Abstract

The Osmundaceae family is the sister group to all other extant leptosporangiate ferns. Its rich fossil record has enabled detailed reconstructions of the evolutionary history of the group. Fossil evidence for today’s Southern Hemisphere subtribe Todeinae, however, has been difcult to accumulate, with only two unambiguous fossil occurrences of modern Todea. Here, we describe a silicifed Todea stem from the Eocene Laguna del Hunco lagerstätte of Patagonia. The stem contains a heterogeneous outer cortex with distinct fbrous rings surrounding the leaf traces, indicative of subtribe Todeinae; nests of thick-walled fbres associated with departing leaf traces together with the sclerenchymatous pith and with abundant sclerenchyma fbres in the inner stipe cortex enable assignment to Todea. The stem difers from T. tidwellii—the only other known fossil Todea stem described thus far—mainly in lacking sclerenchyma lining the adaxial concavity of the stipe bundle. Co-occurrence with sterile foliage fragments indicates that the stem may belong to Todea amissa, a species that was initially described based on frond compressions from the same deposits. Our fnding corroborates the widely disjunct Gondwanan distribution of Todea across the Southern Hemisphere in the past, and provides an impetus to re-assess genetic divergence between the relictual African and Australasian populations of Todea barbara today in order to resolve the evolutionary history of the group more clearly. Finally, this frst report of structurally preserved plant remains from Laguna del Hunco highlights the thus far untapped potential for anatomical preservation in this important fossil lagerstätte.