The Matoniaceae are among the most ancient lineages of extant leptosporangiate ferns, with an evolutionary history that extends into the early Mesozoic. They have long been considered to be a systematically isolated group that, along with Gleicheniaceae and Dipteridaceae, occupies a basal position in the phylogeny of leptosporangiate ferns. Although the extant taxa of Matoniaceae are today restricted to Malesia, a diverse assemblage of matoniaceous ferns occurred on every continent, including Antarctica, throughout the Mesozoic. As one of the few localities in which early Mesozoic ferns are anatomically preserved, the flora of the early Middle Triassic Fremouw Formation in the central Transantarctic Mountains of Antarctica provides an unparalleled opportunity to explore patterns of character evolution through time in a number of modern fern families. Here we described anatomically preserved, detached fern sori and sporangia from the Fremouw Formation with a combination of characters that affiliates them with the Matoniaceae. The sori are peltate with more than twenty-five crowded sporangia displaying simple maturation. The indusium is multiseriate and centrally attached to a massive receptacle, with margins that extend over or inroll around the sporangia. Scalariform tracheids occur in the receptacle and form strands that radiate from the center to the margins of the indusium. Sporangia are globose to ovoid with vertical, meandering, incomplete annuli, slightly flattened in the lateral plane, and helically attached to the receptacle in three gyres. This report places the early Middle Triassic of Antarctica as the earliest known occurrence of the Matoniaceae in the fossil record. Additionally, the presence of a peltate indusium in the earliest known representative of the family contradicts the hypothesized evolutionary sequence in development of this structure in the Matoniaceae.

Key words: Antarctica, ferns, Matoniaceae, Middle Triassic