Combining phenotypic functional traits and metabolic theory to reconstruct Miocene equatorial palaeoclimates from the reptile fossil record.

Understanding the relationship between palaeoenvironmental change and vertebrate evolution and ecology through deep time has historically been examined in the context of faunal responses to climate parameters estimated from lithologic, palaeobotanical and stable isotopic proxies. Conversely, recent advances in the application of ecometrics, trait-environmental relationship models rooted in functional factors such as mechanical performance and metabolic tolerance, provide taxon-free estimates of palaeoclimate that can be applied across multiple temporal and spatial scales. The Miocene sedimentary sequences of the Nyanza Rift in Western Kenya includes a temporally-constrained and dense vertebrate fossil record as well as climate proxies that can be combined to examine equatorial palaeoenvironments during globally warm intervals. We reconstructed body size distributions and quantified axial skeletal morphologies for fossil reptile communities at multiple localities to constrain minimum ambient temperatures necessary for efficient metabolism based on metabolic theory as well as infer palaeoprecipitation values based on locomotory modes reconstructed from skeletal morphometrics. Estimates based on the reptile record are consistent with values derived from local palaeobotanical data and demonstrate warmer, wetter climates in the Early Miocene, with shifts toward slightly drier climates by the early Middle Miocene. These results demonstrate the utility of the vertebrate fossil record for reconstructing palaeoclimates and provide new proxies for non-analog environments of the past.