A Fish-Eating Enantiornithine Bird from the Early Cretaceous of China Provides Evidence of Modern Avian Digestive Features.

Modern birds differ from their theropod ancestors in lacking teeth and heavily constructed bony jaws, having evolved a lightly built beak and a specialized digestive system capable of processing unmasticated food [1, 2]. Enantiornithes, the most successful clade of Mesozoic birds, represents the sister group of the Ornithuromorpha, which gave rise to living birds [3]. Nevertheless, the feeding habits of enantiornithines have remained unknown because of a lack of fossil evidence. In contrast, exceptionally preserved fossils reveal that derived avian features were present in the digestive systems of some non-enantiornithine birds with ages exceeding 125 million years [4, 5]. Here, we report a new piscivorous enantiornithine from the Early Cretaceous Jehol Biota of China. This specimen preserves a gastric pellet that includes fish bones. The new enantiornithine, like many modern piscivores and raptors, seems to have swallowed its prey whole and regurgitated indigestible materials such as bones, invertebrate exoskeletons, scales, and feathers. This fossil represents the oldest unambiguous record of an avian gastric pellet and the only such record from the Mesozoic. The pellet points to a fish diet and suggests that the alimentary tract of the new enantiornithine resembled that of extant avians in having efficient antiperistalsis and a two-chambered stomach with a muscular gizzard capable of compacting indigestible matter into a cohesive pellet. The inferred occurrence of these advanced features in an enantiornithine implies that they were widespread in Cretaceous birds and likely facilitated dietary diversification within both Enantiornithes and Ornithuromorpha.