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The Río Pichileufú (RP) fossil locality contains one of Patagonia’s only well-dated middle Eocene floras, deposited ca. 47.7 Ma during the onset of global temperature decline and South America's tectonic isolation. In 1938, Edward W. Berry described 135 species from RP based on compressed angiosperm leaves and rare reproductive structures. The flora was considered highly diverse and to have predominantly Neotropical affinities; however, many of Berry’s identifications were botanically incorrect, confusing interpretations of composition, diversity, and biogeography. Only a fraction of the flora has been studied since, and substantial new collections have remained unevaluated. Here, we reassess the fossil leaves from RP, creating a stable platform for systematic and ecological analyses. We use a morphotype approach to bypass the numerous prior taxonomic errors, while preserving nomenclatural links to specimens. We jointly consider the type and cohort (n = 696) and recent (n = 1286) collections. We validate 82 leaf morphotypes in the type collections, much lower than Berry’s estimate of 131, and consider 43 species as indeterminate. We find that 44 historical species were improperly split, lumped, or misaligned to existing names. At least 12 plant families and 30 plant genera initially reported from the site are unreliable, including Poaceae, Cannabaceae, Ericaceae, Hydrangeaceae, and Rosaceae. However, considering all the collections, we recognize 158 total leaf morphotypes. Reliable taxa include ginkgophytes, Norfolk Island pines (Araucaria), legumes (Fabaceae), soapberries (Sapindaceae), and laurels (Lauraceae). Although Berry's initial assessment of diversity at RP was significantly overestimated, including new material re-establishes the flora as exceptionally diverse. 

Abstract PremiseAcmopyle(Podocarpaceae) comprises two extant species from Oceania that are physiologically restricted to ever‐wet rainforests, a confirmed fossil record based on leaf adpressions and cuticles in Australia since the Paleocene, and a few uncertain reports from New Zealand, Antarctica, and South America. We investigated fossil specimens withAcmopyleaffinities from the early Eocene Laguna del Hunco site in Patagonia, Argentina. MethodsWe studied 42 adpression leafy‐shoot fossils and included them in a total evidence phylogenetic analysis. ResultsAcmopyle grayaesp. nov. is based on heterophyllous leafy shoots with three distinct leaf types. Among these, bilaterally flattened leaves uniquely preserve subparallel, linear features that we interpret as accessory transfusion tissue (ATT, an extra‐venous water‐conducting tissue). Some apical morphologies ofA. grayaeshoots are compatible with the early stages of ovuliferous cone development. Our phylogenetic analysis recovers the new species in a polytomy with the two extantAcmopylespecies. We report several types of insect‐herbivory damage. We also transferAcmopyle engelhardtifrom the middle Eocene Río Pichileufú flora toDacrycarpus engelhardticomb. nov. ConclusionsWe confirm the biogeographically significant presence of the endangered West Pacific genusAcmopylein Eocene Patagonia.Acmopyleis one of the most drought‐intolerant genera in Podocarpaceae, possibly due to the high collapse risk of the ATT, and thus the new fossil species provides physiological evidence for the presence of an ever‐wet rainforest environment at Laguna del Hunco during the Early Eocene Climatic Optimum. 

PremiseEocene floras of Patagonia document biotic response to the final separation of Gondwana. The conifer genusAraucaria, distributed worldwide during the Mesozoic, has a disjunct extant distribution between South America and Australasia. Fossils assigned to AustralasianAraucariaSect.Eutactausually are represented by isolated organs, making diagnosis difficult.Araucaria pichileufensisE.W. Berry, from the middle Eocene Río Pichileufú (RP) site in Argentine Patagonia, was originally placed in Sect.Eutactaand later reported from the early Eocene Laguna del Hunco (LH) locality. However, the relationship ofA. pichileufensisto Sect.Eutactaand the conspecificity of theAraucariamaterial among these Patagonian floras have not been tested using modern methods. MethodsWe review the type material ofA. pichileufensisalongside large (n= 192) new fossil collections ofAraucariafromLHandRP, including multi‐organ preservation of leafy branches, ovuliferous complexes, and pollen cones. We use a total evidence phylogenetic analysis to analyze relationships of the fossils to Sect.Eutacta. ResultsWe describeAraucaria huncoensissp. nov. fromLHand improve the whole‐plant concept forAraucaria pichileufensisfromRP. The two species respectively resolve in the crown and stem of Sect.Eutacta. ConclusionsOur results confirm the presence and indicate the survival of Sect.Eutactain South America during early Antarctic separation. The exceptionally complete fossils significantly predate several molecular age estimates for crownEutacta. The differentiation of twoAraucariaspecies demonstrates conifer turnover during climate change and initial South American isolation from the early to middle Eocene.