Diversity and disparity in mammalian fossil assemblages of the John Day Formation, Oregon, USA

Abstract

The John Day Formation offers a remarkably complete terrestrial record of Eocene-, Oligocene-, and Miocene-aged mammalian faunas in central Oregon. Sites in the John Day Formation have yielded numerous mammalian species spanning across many taxonomic orders, offering a unique opportunity to closely examine changes in mammalian diversity and disparity across a geographically constrained region. However, a comprehensive review of the full diversity and disparity of John Day Formation faunas has not yet been undertaken. In this study, we examine species richness (i.e., taxonomic diversity) alongside morphological disparity in sites in the John Day Formation to assess how these measures vary with each other and with stratigraphy and sampling effort. We did this by gathering a broad dataset of mammalian occurrences across nine sites in the John Day Formation and collected first-molar area for each occurrence, followed by coverage-based rarefaction analyses to assess sampling completeness and calculations of Chao and shareholder quorum subsampling (SQS) richness. We found that sampling appears to have a stronger relationship with taxonomic diversity than with morphological disparity, and that diversity and disparity patterns do not appear to be strongly correlated. We assess these diversity and disparity patterns and discuss implications for the effects of taphonomic and collection biases on calculations of diversity and disparity in fossil assemblages.