Rare leaf fossils of Monimiaceae and Atherospermataceae (Laurales) from Eocene Patagonian rainforests and their biogeographic significance

Two Eocene fossil sites in Patagonia, Argentina, Laguna del Hunco (ca. 52.2 Ma) and Río Pichileufú (ca. 47.7 Ma), produce some of the most diverse fossil floras known, representing angiosperm-dominated, Gondwanan rainforests. We focus on rare, toothed fossil leaves representing the families Atherospermataceae and Monimiaceae (Laurales), which currently exhibit broad, often disjunct southern distributions and hold much interest for Gondwanan biogeography. For Laurelia guinazui Berry 1935 (Atherospermataceae), we report 24 new specimens and propose reassignment to Atherospermophyllum gen. nov. and A. guinazui (Berry) comb. nov. The species was thought to be a South American element of the Eocene floras, but we find that it shows greater similarity to the extant, closely related Australian genera Daphnandra and Doryphora than to Laureliopsis (South America) and Laurelia (South America and New Zealand). Monimiaceae are represented by a single fossil specimen from Laguna del Hunco, here assigned to Monimiophyllum callidentatum sp. nov. This fossil shows greatest similarity to Wilkiea, a derived genus extant in Australia, in apparent contrast with molecular analyses placing the divergence of the Wilkiea clade in Australasia at 16-38 Ma. Thus, the Wilkiea lineage may be older and have a broader biogeographic history across Gondwana. Our findings significantly improve the scarce fossil records for Atherospermataceae and Monimiaceae. The fossils were found at great modern distance from their apparent closest living relatives and, remarkably, with similar associated genera, increasing the links of Eocene Patagonian floras to Australasia and further weakening their surviving signal in extant South American forests.