WERE MOUNTAIN PASSES HIGHER DURING THE MIDDLE MIOCENE?

In a classic paper, “Why mountain passes are higher in the tropics,” Janzen hypothesized that low temperature seasonality in the tropics leads to narrow thermal tolerances of organisms and strong thermal zonation, thereby limiting species’ elevational ranges to a greater degree than in temperate montane regions. Mountain passes in the tropics are thus highly effective barriers to dispersal, influencing community assembly and turnover over ecological timescales and resulting in high allopatric speciation over evolutionary timescales. We expand on Janzen’s hypothesis to test drivers of species composition and richness patterns of mammals in western North America in relation to Neogene tectonic activity and climate change. Specifically, we 1) quantify faunal similarity among six fossil-rich regions from 17 to 6 Ma, 2) assess whether elevated spatial turnover in species composition contributed to peak regional diversity during the middle Miocene global warming interval (17-14 Ma), and 3) test which landscape variables best explain the dynamics of mammalian assemblages. To do so, we estimated regional paleotopography through quantitative integration of crustal kinematic and deformation models in the Basin and Range Province through the Neogene. We then produced elevational profiles between each fossil region for 1-myr intervals and calculated four landscape variables: paleo distance, maximum elevation, and two relief indices. We found a strong signal of decay-by-distance in faunal similarity between regions today and during the warm middle Miocene, but not from 13 to 6 Ma. Furthermore, the best-fit model for the middle Miocene (explaining 82% of the variance in similarity) included paleo distance, maximum elevation, and relief. These results suggest that physical barriers and corresponding temperature and precipitation gradients were key to structuring species turnover in the tectonically active Basin and Range Province. We conclude that mountain passes were more effective barriers from 17 to 14 Ma during the warm Miocene Climatic Optimum, shaping the regional species pool and influencing processes of community assembly. Future paired topography and climate models will help distinguish the relative influence of relief versus thermal gradients on faunal diversity and turnover patterns.