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Stable isotopes in early Eocene mammals as indicators of forest canopy structure and resource partitioning

¹ Ross Secord.* Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Post Office Box 37012, NHB MRC 121, Washington, D.C. 20013-7012
² Scott L. Wing. Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Post Office Box 37012, NHB MRC 121, Washington, D.C. 20013-7012
³ Amy Chew. Department of Anatomy, School of Medicine, Stony Brook University, T8-040 HSC, Stony Brook, New York 11794-8081
⁴ *Present address: Florida Museum of Natural History, 206 Dickinson Hall, Museum Road and Newell Drive, Gainesville, Florida 32611. rsecord{at}umich.edu

The three dimensional structure of vegetation is an important component of ecosystems, yet it is difficult to reconstruct from the fossil record. Forests or woodlands prevailed at mid-latitudes in North America during the early Eocene but tree spacing and canopy structure are uncertain. Here we use stable carbon isotope values (¹³C ) in early Eocene mammalian faunas to infer canopy structure. We compare ¹³C values in two diverse fossil assemblages from the central Bighorn Basin to values predicted for mammals in a variety of open and closed habitats, based on modern floras and faunas. We conclude that these early Eocene faunas occupied an open canopy forest. We also use carbon and oxygen isotopes to infer diet and microhabitat. Three higher level taxa have significantly different mean ¹³C values, and values are negatively correlated with body mass. The pattern suggests diets high in leaves for larger mammals, and fruit or other non-foliar plant organs for small ones. A preference in the larger mammals for wetter habitats with high water availability to plants may also have contributed to the pattern.