Dicksoniaceous ferns from the Cretaceous of Patagonia, Argentina

CÚNEO, RUBÉN; ESCAPA, IGNACIO; ZAMALOA, MARIA; GANDOLFO, MARIA A.; HERMSEN, ELIZABETH

New Orleans

Congreso; Botany 2013; 2013

Dicksoniaceous ferns are rare in the fossil plant assemblages of South America, with only a few records of microspores and fragmentary macrofossil remains. In this contribution, we describe dicksoniaceous fronds preserved nearly in their entirety and bearing both sterile and fertile pinnae from two rock formations in Patagonia: the Lower Cretaceous Kachaike Formation (Albian) of Santa Cruz Province and the Late Cretaceous La Colonia Formation (Maastrichtian) of northern Chubut Province. The fronds from the Kachaike Formation are bipinnate with sterile pinnules of the sphenopteroid type; some pinnae are entirely fertile or bear fertile segments basally; fertile parts occur either basally or entirely on fertilesegments. Sori are indusiate, marginal, ca. 1 mm in diameter, and cup-shaped. The sporangia have clearly differentiated annuli. Associated spores are triangular with concave sides and verrucate sculpture; they are of the Concavissimisporites type. The La Colonia Formation specimens include tripinnate fronds with pecopteroid sterile pinnules. Fertile pinnae represent complete segments or well bear lateralfertile segments intercalated with sterile segments basally. Sori are indusiate, cup-shaped, ca. 1.2 mm in diameter, and bear numerous (6-10) leptosporangia 0.3 mm in diameter. Associated dispersed spores and massulaeare triangular, psilate, and of the Cyathidites type. On the basis of the preserved characters,the two Patagonian species can be preliminarily assigned to the widespread Mesozoic fossil genus Coniopteris (Brongniart) Harris. The origin, diversification, and phylogenetic relationships of Coniopteris are not entirely clear, and the character combinations of the numerous species need to be compared in greater detail. For instance, most morphological traits of the two new species from Argentina partially fit the modern fern Thyrsopteris, a monotypic genus from the Juan Fernández Islands (Chile). Dicksoniaceous ferns were not very diverse or widespread in southern South American fossil floras, and they probablyplayed a secondary role in plant communities. The sedimentary and palynological context in both areas reflect freshwater-dominated coastal environments, in which these ferns occupiedsubtropical wetlands as understory.