A new clade of basal Early Cretaceous pygostylian birds and developmental plasticity of the avian shoulder girdle

We report the second most basal clade of the short-tailed birds (Pygostylia) from the Early Cretaceous. The new family Jinguofortisidae exhibits a mosaic assembly of plesiomorphic nonavian theropod characteristics, particularly of the fused scapulocoracoid and more derived flight-related features, further increasing the known ecomorphological diversity of basal avian lineages. We discuss the evolution of the scapula and coracoid in major tetrapod groups and early birds and hypothesize that the fused scapulocoracoid in some basal avian lineages, although rare, results from an accelerated rate of ossification and that the avian shoulder girdle likely was transformed by developmental plasticity along an evolutionary lineage leading to the crown group of birds.