The Role of Eocene Climatic and Environmental Changes in Avian Diversification and Evolution

  • Carolina Karoullas

Student thesis: Phd


The Eocene saw the largest diversification event in avian history with half of all extant avian orders appearing during this time. The event coincides with extreme climatic and environmental instability during the early Eocene, which has been hypothesised to be the cause the observed diversification. However, this has not been tested despite avian diversification being previously linked to niche creation in passerines through time and, potentially, the group overall after the K-Pg mass extinction. Testing this potential link involves examining temporal taxonomic and ecological diversity changes and assessing these against climatic and environmental proxies, i.e., isotope proxies. Consequently, extinct avian ecology must be predicted. However, such predictive models vary in reliability and accuracy due to, e.g., differing methods, implementation of those methods and sample size. Consequently, this thesis firstly uses error analysis, predictive modelling, and phylogenetic comparative methods to assess the utility of various predictors of avian flight and feeding ecology including aerodynamic theory (aerodynamic power curves (APCs)), brachial index (BI), proximal humeral shape, bill shape and phylogeny itself. With this information it was then possible to use multi-layered network analyses to investigate the ultimate aim of this thesis, which was to evaluate the impact of Eocene climatic and environmental instability on avian taxonomic and ecological diversity. APCs were shown to unreliably predicted extinct avian flight capability due to large amounts of error associated with calculating them for extinct taxa caused by the need to estimate of multiple morphological variables. However, it was shown that while APCs cannot be currently used, future improvements to the underlying equations used to predict the morphological metrics may reduce this error enough to make them viable. BI poorly predicted individual flight capability metrics but was strongly predicted by the metrics together, implying the relationship requires further study. Also, changes in BI, the flight metrics, and related morphology through evolutionary time showed a shift towards increased flight performance through evolutionary time. However, diversification in these metrics was shown to have potentially began in the Eocene rather than the immediately after the K-Pg. Consequently, there is mixed evidence for the hypothesis that neornithines rapidly improved flight capability to fill forest niches vacated by non-neornithine Mesozoic avialans after the K-Pg extinction. Proximal humeral and bill shape were good predictors of flight and feeding ecology, respectively, in non-phylogenetically corrected models. However, order and family strongly predicted morphology and ecology and both were subject to strong phylogenetic constraint and inertia. This implies that the relationship between morphology and ecology in avians is dependent on their concurring phylogenetic relationships. Taxonomic information was thus used to predict Eocene avian flight and feeding ecology. Taxonomic and ecological diversity increased from the late Paleocene to early Eocene with no large-scale changes thereafter, which mirrored changes in biome diversity but not temperature. At a smaller scale, more orders and species used terrestrial herbivory in the Late Eocene compared to before, coinciding with the advent of open grassland biomes in North America. This study highlights the importance of environmental instability and subsequent niche creation in avian diversification and evolution. The opening of forest niches after the K-Pg extinction to primitive neornithines enabled avians to improve flight capability and begin diversifying. This laid the foundations for them to be able to exploit global environmental instability and niche creation during the early Eocene and led to the largest diversification event in avian history.
Date of Award1 Aug 2022
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorJonathan Codd (Supervisor) & Robert Nudds (Supervisor)


  • phylogenetic constraint
  • morphology
  • phylogenetic inertia
  • humerus
  • bill
  • PETM
  • shape
  • networks
  • diversity
  • Paleocene Eocene Thermal Maximum
  • geometric morphometrics
  • beak
  • morphometrics
  • Eocene
  • flight
  • bird
  • error propagation
  • predictive limits
  • adaptive radiation
  • ancestral states
  • ecology
  • evolution
  • phylogeny
  • avian
  • early burst
  • evolutionary models
  • evolutionary rates

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