Fossil evidence for Patagonian Araucaria Sec. Eutacta and its implications for conifer turnover during the Eocene initial isolation of South America.

ROSSETTO, GABRIELA; WILF, PETER; ESCAPA, IGNACIO; ANDRUCHOW-COLOMBO, ANA

Tucson

Congreso; Botany 2019; 2019

Botanical Society of America

The iconic genus Araucaria, distributed worldwide during the Mesozoic, now has a relict, disjunct distribution between South America (2 species) and Australasia (18 species). Australasian Araucaria Section Eutacta is the most diverse clade with 16 species, all but two of them endemic to New Caledonia. Fossils assigned to Sect. Eutacta usually are based on single dispersed organs, making it difficult to diagnose the section and test molecular estimates of its crown age, which are generally near 20-25 Ma. Araucaria fossils thought to belong to Sect. Eutacta are abundant in early and middle Eocene Patagonian caldera-lake deposits from Laguna del Hunco (LH; ~52.2 Ma; Early Eocene Climatic Optimum) and Rio Pichileufu (RP; ~47.7 Ma; earliest middle Eocene, when initial opening of the Drake Passage had begun, and climatic cooling was underway). Araucaria pichileufensis Berry 1938 was described from the type locality RP and has been reported from LH. Although there is increasing evidence of angiosperm species turnover between these floras via loss of some rainforest taxa, the diverse conifers found at LH and RP are thought to represent the same set of species. However, the relationship of A. pichileufensis to Sect. Eutacta and the conspecificity of the Araucaria material in these floras have not been rigorously tested. Large new fossil collections from LH and RP include the multi-organ preservation of Araucaria leafy branches, cuticle, bract-scales, and pollen cones, which allows for the development of a more complete plant concept for Araucaria pichileufensis at RP and for the recognition of a new species from LH. Analysis of characters, including those of an attached terminal pollen cone discovered from RP, establishes a relationship of both species to Eutacta, suggesting presence and survival in Patagonia of this group during initial separation from Antarctica. A total evidence phylogenetic analysis places both Eocene species within the crown group of Sec. Eutacta, confirming the taxonomic treatment and adding to the Gondwanan connection of Patagonian fossil floras to Australasia. Furthermore, these Araucaria fossils comprise one of the most complete representations of fossil Eutacta in the world, and they predate the molecular age estimates for the crown of Eutacta by ~30 million years. The differentiation of two Araucaria species is the first documentation of a change in the conifer species composition between LH and RP, adding to the evidence for turnover between the two floras during the climate change and movement of landmasses of the earliest middle Eocene.