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Premise of research. The fossil assemblage of the Macquarie Harbour Formation (MHF) in Tasmania, Australia, represents a subpolar (65°S–70°S) lowland forest during the early Eocene climatic optimum (∼53–50 Ma) and bears fossils of Araucarioides linearis Bigwood et Hill, 1985 emend (Araucariaceae). New fossils, including isolated leaves, female ovuliferous complexes (=bract/scale complexes), a possible seed, and dispersed pollen, were recovered from the Lowana Road site near the type locality and offered the opportunity to clarify the previously poorly constrained relationship of Araucarioides within the Araucariaceae. Methodology. Macrofossils and cuticles were described and compared with other Araucariaceae. Palynological analyses were conducted to identify associated pollen. Tree Analysis Using New Technology (TNT) was used for phylogenetic analyses. To confirm conspecificity of leaves and reproductive remains, the whole-plant hypothesis was tested using multiple methods, including optical and fluorescence photo(micro)graphy, scanning electron microscopy, and X-ray computed tomography. Paleobiogeographic comparisons with other Araucarioides assemblages were undertaken. Pivotal results. New Araucarioides linearis fossils included two distinct leaf populations and the first putative female reproductive remains of this species. The generic and specific diagnoses of A. linearis were emended based on new data. Co-occurring Araucarioides sinuosa was designated a junior synonym of A. linearis. The fossils are associated with abundant Dilwynites tuberculatus pollen. The phylogenetic analyses revealed affinity with the agathioid clade (Wollemia + Agathis) of Araucariaceae. The whole-plant hypothesis was supported. Conclusions. Phylogenetic analyses indicated that A. linearis was a non- Agathis agathioid that represents the sister to the clade formed by Agathis and Wollemia. The polar Campanian Araucarioides falcata in New Zealand supports the Cretaceous divergence of Araucaria and agathioid lineages. The restriction of Araucarioides to high paleolatitudes and their adaptations to seasonal environments such as broad leaves and possibly winter deciduousness may have facilitated survival through the impact winter and photosynthetic crisis during the end-Cretaceous extinction. Continuous northward movement of Australia and New Zealand and resulting climatic and light regime changes likely contributed to its extinction. [ABSTRACT FROM AUTHOR]