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

Abstract

<p id="d5603732e189">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. </p><p class="first" id="d5603732e192">Early members of the clade Pygostylia (birds with a short tail ending in a compound bone termed “pygostyle”) are critical for understanding how the modern avian bauplan evolved from long-tailed basal birds like <i>Archaeopteryx</i>. However, the currently limited known diversity of early branching pygostylians obscures our understanding of this major transition in avian evolution. Here, we describe a basal pygostylian, <i>Jinguofortis perplexus</i> gen. et sp. nov., from the Early Cretaceous of China that adds important information about early members of the short-tailed bird group. Phylogenetic analysis recovers a clade (Jinguofortisidae fam. nov.) uniting <i>Jinguofortis</i> and the enigmatic basal avian taxon <i>Chongmingia</i> that represents the second earliest diverging group of the Pygostylia. Jinguofortisids preserve a mosaic combination of plesiomorphic nonavian theropod features such as a fused scapulocoracoid (a major component of the flight apparatus) and more derived flight-related morphologies including the earliest evidence of reduction in manual digits among birds. The presence of a fused scapulocoracoid in adult individuals independently evolved in Jinguofortisidae and Confuciusornithiformes may relate to an accelerated osteogenesis during chondrogenesis and likely formed through the heterochronic process of peramorphosis by which these basal taxa retain the scapulocoracoid of the nonavian theropod ancestors with the addition of flight-related modifications. With wings having a low aspect ratio and wing loading, <i>Jinguofortis</i> may have been adapted particularly to dense forest environments. The discovery of <i>Jinguofortis</i> increases the known ecomorphological diversity of basal pygostylians and highlights the importance of developmental plasticity for understanding mosaic evolution in early birds. </p>