Min Wang, Zhiheng Li, and Zhonghe Zhou (2017)
Insight into the growth pattern and bone fusion of basal birds from an Early Cretaceous enantiornithine bird.
Proceedings of the National Academy of Sciences
We report an Early Cretaceous bird from 120 My ago that has a completely fused carpometacarpus and pelvis, pushing back the date for these avian traits by over 40 My. We suggest that this taxon grew more rapidly than other basal birds, but the degree of bone fusion is not causally linked with growth pattern in primitive birds. We hypothesize that the surprisingly high degree of bone fusion in basal birds may have been environmentally induced pertinent to flight or alternatively resultant from genetic modifications. Future developmental studies focusing on the development of bone fusion are needed to test these hypotheses and illustrate how the skeletal system of living birds achieves its modern shape.
Bird skeletons exhibit remarkable modifications that allow for flight. The most distinguishable features are the fusion of the bones in the hand, feet, and pelvis into composite rigid and bony structures. However, the historical origins of these avian bone fusions remain elusive because of the rarity of transitional fossils and developmental studies on modern birds. Here, we describe an Early Cretaceous bird (120 Mya) that has fully fused alular-major metacarpals and pelvis. We discuss the manus and pelvis fusions across Paravian phylogeny and demonstrate that these features evolved independently across nonavian theropods, Enantiornithes, and Ornithuromorpha. The fusions of these bones are rare in known nonavian theropods and Early Cretaceous birds but are well established among Late Cretaceous and modern birds, revealing a complicated evolution pattern unrecognized previously. We posit that the developments of bone fusion were polymorphic close to the origin of birds, resulting in the varying degrees of fusion in Paraves. However, that development polymorphism appears to be fundamentally restricted along the line to modern birds by the Late Cretaceous, where all birds have a completely fused manus and pelvis. Such changes likely correspond to a refinement of flight capability. Alternatively, the degree of bone fusion in this primitive bird may have been related to modifications in genes or developmental paths. Future studies and fossil discoveries are required to clarify these hypotheses and pinpoint the developmental pathways involving the bone fusions in early avian evolution through to their modern pattern.