Ontogenetic niche shifts in the Mesozoic bird Confuciusornis sanctus

Synthetic analysis of trophic diversity and evolution in Enantiornithes with new insights from Bohaiornithidae

Enantiornithines were the dominant birds of the Mesozoic, but understanding of their diet is still tenuous. We introduce new data on the enantiornithine family Bohaiornithidae, famous for their large size and powerfully built teeth and claws. In tandem with previously published data, we comment on the breadth of enantiornithine ecology and potential patterns in which it evolved. Body mass, jaw mechanical advantage, finite element analysis of the jaw, and traditional morphometrics of the claws and skull are compared between bohaiornithids and living birds. We find bohaiornithids to be more ecologically diverse than any other enantiornithine family: Bohaiornis and Parabohaiornis are similar to living plant-eating birds; Longusunguis resembles raptorial carnivores; Zhouornis is similar to both fruit-eating birds and generalist feeders; and Shenqiornis and Sulcavis plausibly ate fish, plants, or a mix of both. We predict the ancestral enantiornithine bird to have been a generalist which ate a wide variety of foods. However, more quantitative data from across the enantiornithine tree is needed to refine this prediction. By the Early Cretaceous, enantiornithine birds had diversified into a variety of ecological niches like crown birds after the K-Pg extinction, adding to the evidence that traits unique to crown birds cannot completely explain their ecological success.

Fossil evidence sheds light on sexual selection during the early evolution of birds

The impact of sexual selection on the evolution of birds has been widely acknowledged. Although sexual selection has been hypothesized as a driving force in the occurrences of numerous morphological features across theropod evolution, this hypothesis has yet to be comprehensively tested due to challenges in identifying the sex of fossils and by the limited sample size. Confuciusornis sanctus is arguably the best-known early avialan and is represented by thousands of well-preserved specimens from the Early Cretaceous Jehol lagerstätte, which provides us with a chance to decipher the strength of sexual selection on extinct vertebrates. Herein, we present a morphometric study of C. sanctus based on the largest sample size of this taxon collected up to now. Our results indicate that the characteristic elongated paired rectrices is a sexually dimorphic trait and statistically robust inferences of the sexual dimorphism in size, shape, and allometry that have been established, providing the earliest known sexual dimorphism in avian evolution. Our findings suggest that sexual selection, in conjunction with natural selection, does act upon body size and limb length ratio in early birds, thereby promoting a deeper understanding of the role of sexual selection in large-scale phylogenetic evolution.

A new confuciusornithid bird with a secondary epiphyseal ossification reveals phylogenetic changes in confuciusornithid flight mode

The confuciusornithids are the earliest known beaked birds, and constitute the only species-rich clade of Early Cretaceous pygostylian birds that existed prior to the cladogenesis of Ornithothoraces. Here, we report a new confuciusornithid species from the Lower Cretaceous of western Liaoning, northeastern China. Compared to other confuciusornithids, this new species and the recently reported Yangavis confucii both show evidence of stronger flight capability, although the wings of the two taxa differ from one another in many respects. Our aerodynamic analyses under phylogeny indicate that varying modes of flight adaptation emerged across the diversity of confuciusornithids, and to a lesser degree over the course of their ontogeny, and specifically suggest that both a trend towards improved flight capability and a change in flight strategy occurred in confuciusornithid evolution. The new confuciusornithid differs most saliently from other Mesozoic birds in having an extra cushion-like bone in the first digit of the wing, a highly unusual feature that may have helped to meet the functional demands of flight at a stage when skeletal growth was still incomplete. The new find strikingly exemplifies the morphological, developmental and functional diversity of the first beaked birds.