Structure and possible ventilatory function of unusual, expanded sternal ribs in the Early Cretaceous bird Jeholornis

ABSTRACT Many specimens of the basal bird Jeholornis from the Early Cretaceous Jehol Biota of northeast China include one or two distinctive paddle-shaped skeletal elements preserved in the thoracic region. These ossifications have generally been identified as lateral trabeculae, paired processes of the sternum that are common within the derived avian clade Ornithothoraces. In extant birds, lateral trabeculae define membrane-filled embayments or fenestrae in the caudal portion of the sternum that contribute to the area available for attachment of the pectoralis musculature, which drives the downstroke in flight. The presence of lateral trabeculae in Jeholornis would thus suggest a proportionally larger M. pectoralis, and a more powerful downstroke, than in other non-ornithothoracine avians. However, previously undescribed specimens of Jeholornis reveal that the paddle-shaped elements are actually anomalously expanded sternal ribs, the caudalmost of four pairs in the ribcage. Accordingly, lateral trabeculae are absent in Jeholornis, dovetailing with other evidence that basal birds lack many components of the sophisticated flight apparatus typical of ornithothoracines. The expanded sternal ribs represent a striking, and somewhat functionally enigmatic, autapomorphy of Jeholornis. In many pterosaurs the sternal ribs bear multiple small prominences, the sternocostapophyses, that probably improved the mechanical advantage of ribcage musculature involved in ventilation and increased the area for muscle attachment. The sternal rib expansions seen in Jeholornis presumably served a similar purpose, and are among a suite of derived features of this taxon that appear to represent adaptations for the demands of powered flight but only partially parallel those independently acquired by ornithothoracines.