The diet of enantiornithine birds : a quantitative approach

Abstract

Enantiornithines or "opposite birds" were the dominant birds of the Mesozoic. While there have been many hypotheses about their diet and broader ecology, very little has been done to test these hypotheses. The qualitative aspects of enantiornithines pointed to when constructing these hypotheses have had contradicting interpretations between researchers, which calls for a quantitative approach. Reviewing work examining the diet of living birds and other theropod dinosaurs, I identify seven lines of evidence which show promise in determining enantiornithine diet: dental microwear, cervical muscle reconstruction, stable isotope analysis, body mass estimation, traditional morphometrics, mechanical advantage analysis, and finite element analysis. Due to limitations on in-person sampling, only the latter four lines of evidence could be incorporated into this work. The three best-known enantiornithine families (Longipterygidae, Pengornithidae, and Bohaiornithidae), all of whom have established dietary hypotheses to test, were chosen as test subjects. The four lines of evidence were compared to over 200 extant bird species to reconstruct the diet of these groups. By combining four lines of evidence in a single study, much more robust and precise diet reconstructions were possible than using any single line alone. Longipterygids were found to all be best-adapted for invertebrate feeding, with possible diversity in specific invertebrate preference and hunting location. Pengornithids overall show adaptations for vertebrate feeding, though Pengornis was more likely a generalist feeder and shows the earliest known evidence of macrocarnivory in a bird. Bohaiornithid diet is recovered as diverse, with Bohaiornis and Parabohaiornis resembling extant avian herbivores, Longusunguis showing distinct carnivorous adaptations, and Zhouornis most likely being a generalist. This level of trophic diversity was achieved within roughly 25 million years of the divergence of Enantiornithes, a similar rate of ecological diversification to crown birds. This suggests that the traits which gave rise to the ecological success of crown birds may be present much deeper in time than previously suggested, particularly in the common ancestor of Ornithothoraces (Enantiornithes + the lineage that gave rise to crown birds).