Elucidating the evolution and biogeographic affinities of Cretaceous Antarctic rainforests will help us piece together how these extinct forests served as a center for diversification and/or corridor for dispersal in the assembly of Southern Hemisphere floras. To date, pollen, wood and leaf records have pointed to the presence of Araucariaceae, Podocarpaceae, and Cupressaceae as canopy components in these temperate rainforests, with angiosperms, ferns, and bryophytes forming the understory. New data from permineralized Late Cretaceous (Campanian) plants recovered from marine calcareous concretions from the Baculites Hill locality, James Ross Island (Beta Member, Santa Marta Formation), offer an opportunity to contribute insights in the biodiversity of these extinct forests based on anatomically preserved reproductive structures. One ovulate cone was identified and studied using serial cellulose acetate peels. The cone is complete, about 7 mm long and 6 mm wide, and ellipsoid in shape. There are about 50 helically-arranged cone scale complexes that are attached to the cone axis at right angles and distally expand, thicken, and bend upwards. Cone scales are asymmetric and are comprised of a narrow sclerotic proximal stalk and an expanded distal half that is slightly fleshy. Each cone scale contains a single vascular bundle and an abaxial resin canal. The vascular bundle divides vertically, providing a small ovular trace to the seed that later divides within their integuments. Each cone scale has a single seed oriented with the micropyle facing towards the cone axis and attached to the scale at the chalazal end by a short stalk on an elevated pad of tissue. Seeds have symmetric integumentary lateral wings and a chalazal wing. Spheroidal pollen grains with granular sculpturing were observed in or near the micropyle of several seeds. The presence of fungal hyphae within the cone axis, perithecia on some cone scales, and cyanobacterial filaments between the cone scales suggests an ecosystem rich with microorganisms. Cone scale morphology and the single stalked winged seed per scale indicate affinities within agathioid Araucariaceae. Bayesian and parsimony phylogenetic analyses were performed using a matrix of 57 characters and 49 taxa of living and extinct Araucariaceae. In both analyses, the new cone forms a monophyletic group with the extinct Wairarapaia and Emwadea and the extant Wollemia. This clade is sister to Agathis. The fossil described here adds to the diversity of Cretaceous agathioids, which are not as well-known as extinct Araucaria, and provides strong support that conifers closely allied with Wollemia were components of Cretaceous Antarctic rainforests.