The first Patagonian icehouse conifers: new fossil species of Austrocedrus (Cupressaceae) and Araucaria (Araucariaceae) from the earliest Oligocene of Argentina

ROSSETTO-HARRIS, G; ESCAPA, I.H.; IGLESIAS, A.; PASSALIA, M.G.; WILF, P.

Congreso; Conference Botanical Society of America (Botany 2023); 2023

During the Eocene greenhouse, northern Patagonian vegetation included abundant Gondwananrainforest-conifer lineages that are mostly endemic today to the West Pacific region, includingAraucaria Sect. Eutacta, Agathis, Papuacedrus, and Dacrycarpus. The Eocene-Oligocene transitionand the final breakup of Gondwana led to the earliest Oligocene (Oi-1) glaciation and an icehouseclimate state ever since. However, little is known about how and when the extant coniferassemblages of northern Patagonia evolved. For example, Araucaria araucana (Sect. Araucaria)and Austrocedrus chilensis are dominant conifers in the mesic forests to xeric woodlands of thesouthern Andes, but the sparse and poorly dated late Paleogene record from southern Patagoniaprovides no information on their origins. We analyzed new fossil conifers from the Prem fossil site,the first known earliest Oligocene (33.5 Ma, U-Pb) macroflora of South America, contemporaneouswith the Oi-1 glaciation. Prem is a small volcaniclastic lake deposit just 1.6 km east of the famous,highly diverse earliest middle Eocene (47.7 Ma) Río Pichileufú site in Río Negro, northernPatagonian Argentina, but remarkably different in composition. The Prem flora has low diversityand is dominated by Nothofagus, apparently showing a shift to a cooler climate. Conifers at thePrem site represent at least four species of Araucariaceae, Cupressaseae, and Podocarpaceae. Wehere present two of the species with leafy branches and associated reproductive structures, whichprovide new fossil links to the extant, endemic Andean flora. One species represents the first reliable fossil Austrocedrus from South America. We differentiate the Austrocedrus foliage frommorphologically similar Cupressaceae by its small leaves with blunt apices and lateral leaves thatare unfused, with concave/decurrent margins. The fossil ovulate cones, some of which are attached to leafy branches, are also consistent with Austrocedrus in having four bract-scales with valvate arrangement, two pairs being enlarged and striated longitudinally, and non-spinose bract tips extending to subtend the scale apex. Previously, fossil Austrocedrus was only reliably known from the early Oligocene of Tasmania, and thus the new species shows by this time a vast range for the genus, now restricted to South America. The second conifer species is a new species of Araucaria with broad, multi-veined leaves that resemble Sect. Araucaria and isolated ovuliferous complexes with reduced lateral extensions (wings) that resemble those of Sect. Eutacta more than the wingless seeds of living Sect. Araucaria. We used an affinity analysis to evaluate the influence of the isolated organs on the Araucaria species' phylogenetic position. The results indicate mosaic character evolution, suggesting that the wingless complexes of the living South American species evolved more recently.