Data from: Maidenhair ferns, Adiantum, are indeed monophyletic and sister to shoestring ferns, vittarioids (Pteridaceae)

Across the tree of life, molecular phylogenetic studies often reveal
surprising relationships between taxa with radically different
morphologies that have long obscured their close affiliations. A
spectacular botanical example is Rafflesia, a holoparasite that produces
the largest flowers in the world, but that evolved from tiny-flowered
ancestors within the Euphorbiaceae. Outside of parasitic lineages, such
abrupt transformations are rarely seen. One exception involves the
“maidenhair ferns” (Adiantum), which are quintessential ferns: beautifully
dissected, terrestrial, and shade loving. The closely related “shoestring
ferns” (vittarioids), in contrast, have an extremely simplified
morphology, are canopy-dwelling epiphytes, and exhibit greatly accelerated
rates of molecular evolution. While Adiantum and the vittarioids together
have been shown to form a robust monophyletic group (adiantoids), there
remain unanswered questions regarding the monophyly of Adiantum and the
evolutionary history of the vittarioids. Here we review recent
phylogenetic evidence suggesting support for the monophyly of Adiantum,
and analyze new plastid data to confirm this result. We find that Adiantum
is monophyletic and sister to the vittarioids. With this robust
phylogenetic framework established for the broadest relationships in the
adiantoid clade, we can now focus on understanding the evolutionary
processes associated with the extreme morphological, ecological, and
genetic transitions that took place within this lineage.