SIZING UP THE LEAVES OF AN EOCENE PATAGONIAN PALEORAINFOREST AND ITS AUSTRALIAN ANALOGS

Leaf size is a valuable indicator of environmental conditions and can help to infer paleoenvironments when measured from fossils. Leaf size is also one of the essential traits defining rainforest type. Although no ancient forest has a single, identical modern analog, the Laguna del Hunco (LH) paleoflora from early Eocene Patagonia, Argentina, possesses remarkable similarities to extant subtropical montane rainforests in eastern Australia (e.g., Simple Notophyll Vine Forest) in terms of leaf size, floristic composition, diversity, and inferred forest structure and climate. Here, I report a new method of ecosystem reconstruction using leaf size that will be applied to LH to compare specific modern Australian analogs and test previous paleoecological inferences determined from well-characterized members of the paleoflora.

Fossil leaf sizes were measured directly or estimated from fragmented specimens. Intact leaves were used to test three methods of leaf area estimation: the Cain & Castro formula, the Raunkiaer-Webb size categories, and a new technique applicable to more fragmented leaves that uses global negative scaling relationships between 2° vein density and leaf area (Sack et al. 2012). Areas determined with the Cain & Castro formula fit best with actual values, and those from the Raunkiaer-Webb categories and 2° vein scaling gave comparable results of lower accuracy. All methods were applied to LH specimens, producing datasets of four confidence levels for a robust characterization of assemblage-wide and taxon-specific leaf size. Modern analog forests will be selected as those with similar leaf sizes and forest structure to LH, as inferred from the life forms of well-described fossil taxa and photosynthetic productivity suggested by leaf vein density. The most similar potential analog forests will be statistically compared to discover common ecological features, such as canopy closure and soil drainage, that may be representative of the LH paleorainforest. Reconstructing LH will refine our understanding of a mid-latitude, extinct rainforest biome that was present during the highest sustained global temperatures of the Cenozoic. This new method of using leaf size for ecosystem reconstruction can be used on many other paleofloras with significant numbers of close living relatives.