Abstract

The ―Ratites‖ are secondarily flightless Paleognathae that have lost their keel. The main body of the sternum develops paired sternal ossifications, resembling the primitive pattern of other Ornithodira who never evolved a keel. In contrast, Neognathae form a single medial ossification, from which the keel develops. Basal enantiornithes (Pengornithidae) show paired ossifications, with no evidence (so far) for the presence of a keel, but other enantiornithes show a unique pattern of two medial ossifications, with a keel developing from the most caudal. Here, we present the post- hatching ossification patterns of a keeled paleognath, the Chilean Tinamou Nothoprocta perdicaria. It shows a primitive pattern of paired sternal plates, with ossification and fusion of plates proceeding from caudal to cranial, as in basal Pygostylia. Nothoprocta may reflect the primitive pattern for Neornithes, consistent with evidence that a median keel-forming ossification evolved convergently in Enantiornithes and Neognathae. Nothoprocta shows that a median ossification can no longer be assumed for all keeled fossil forms, and that evolution of the keel may have preceded that of a median ossification. We predict that keeled ancestors of Neornithes (basal Ornithuromorpha) may have presented a pattern similar to Nothoprocta.