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 difficult to accumulate, with only two unambiguous fossil occurrences of modern Todea. Here, we describe a silicified Todea stem from the Eocene Laguna del Hunco lagerstätte of Patagonia. The stem contains a heterogeneous outer cortex with distinct fibrous rings surrounding the leaf traces, indicative of subtribe Todeinae; nests of thick-walled fibres associated with departing leaf traces together with the sclerenchymatous pith and with abundant sclerenchyma fibres in the inner stipe cortex enable assignment to Todea. The stem differs 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 finding 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 first report of structurally preserved plant remains from Laguna del Hunco highlights the thus far untapped potential for anatomical preservation in this important fossil lagerstätte.

中文翻译：

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巴塔哥尼亚始新世的硅化Todea树干（Osmundaceae）

夹竹桃科是所有其他现存的细孢子脉蕨类植物的姐妹群。它丰富的化石记录使该小组进化史的详细重建成为可能。然而，如今的南半球Todeinae分支的化石证据很难积累，只有现代Todea发生了两次明确的化石。在这里，我们描述了来自巴塔哥尼亚的始新世Laguna del Huncolagerstätte的硅化Todea茎。茎包含一个异质的外皮层，在叶迹周围有明显的纤维环，表明亚氏To科。与离开叶片痕迹相关联的厚壁纤维巢，以及硬柄皮髓和内部柄皮层中的丰富的硬皮组织纤维，使得能够分配给Todea。茎与T.tidwellii（迄今为止唯一已知的其他化石Todea茎）不同，主要在于在柄束的正向凹面内没有巩膜内膜。与不育的叶子碎片同时出现表明该茎可能属于Todea amissa，该物种最初是基于来自相同沉积物的叶状压缩而描述的。我们的发现证实了过去在南半球广泛分布的Todea冈瓦南分布，并为重新评估Todea barbara的非洲和澳大拉西亚遗迹人口之间的遗传差异提供了动力。今天，以便更清晰地解决小组的进化历史。最后，这份来自Laguna del Hunco的关于结构保存的植物遗骸的第一份报告强调了这一重要的化石lagerstätte迄今为止尚未进行的解剖保存的潜力。