Insight into the early evolution of the aviansternum from juvenile enantiornithines

摘要

The sternum is one of the most important and characteristic skeletal elements in living birds,highly adapted for flight and showing a diverse range of morphologies. new exceptional materialof young juvenile specimens from the Early Cretaceous Jehol Group in northeastern Chinareveals the unique sequence of development in the sternum of Enantiornithes, the dominantclade of Cretaceous birds. We recognize six ossifications that together form the sternum, threeof which were previously unknown. Here we show that although basal living birds apparentlyhave retained the dinosaurian condition in which the sternum develops from a bilateral pair ofossifications (present in paravian dinosaurs and basal birds), the enantiornithine sternal bodyprimarily develops from two unilateral proximo-distally arranged ossifications. This indicatesthat although superficially similar, the sternum formed very differently in enantiornithines andornithuromorphs, suggesting that several ornithothoracine sternal features may representparallelism. This highlights the importance of ontogenetic studies for understanding homologyand the evolution of skeletal features in palaeontology.