Yanornis () is an extinct genus of fish-eating Early Cretaceous birds. Two species have been described, both from Liaoning province, China: Yanornis martini, based on several fossils found in the 120-million-year-old Jiufotang Formation at Chaoyang, and Yanornis guozhangi, from the 124-million-year-old Yixian Formation. Description Y. martini was the size of a chicken, had a long skull with about 10 teeth in the upper jaw and 20 teeth in the lower jaw, and was both able to fly and walk well, having a well-developed U-shaped furcula (wishbone). The absence of the prefrontal bone and the non-diapsid skull allows Yanornis to be classified as an ornithuromorph, a member of a group of stem-birds which also includes the common ancestor of living birds. Similarly, its scapula and coracoid had evolved the basic shape and layout as in modern birds, enabling Yanornis to lift its wings far above its back for an efficient upstroke. It was probably a more efficient flyer compared to Enantiornithes (which have the modern condition in a less well-developed form), and especially compared to Confuciusornis and Archaeopteryx, which were only marginally able to perform upstrokes. To allow for the necessarily large flight muscles, the sternum of Yanornis was longer than it was wide, again representing an essentially modern condition. Diet Several fossil specimens of Y. martini preserve the remains of fish in the stomach and crop, suggesting that these birds were primarily fish-eaters. Their fish-eating and associated adaptations show convergent evolution with the unrelated enantiornithine Longipteryx. One specimen preserved large amounts of supposed gastroliths ("stomach stones") in the gizzard region. Large numbers of small gastroliths are usually associated with species that need to grind tough plant material, such as seeds, after swallowing them. This led some scientists to suggest that Yanornis was capable of "diet switching", perhaps seasonally, between fish and seeds. However, later studies cast doubt on the diet-switching hypothesis. Further study of the specimen found that the supposed stomach stones were not massed around a single region corresponding with the gizzard, as in other fossils with such stones, including specimens of Archaeorhynchus and Hongshanornis. Rather, the stones were spread throughout the body cavity in a front-to-back arrangement. This has been suggested to correspond more closely with the intestines, and may represent impacted sand. In modern birds, sand is often swallowed accidentally during feeding (particularly when feeding on dead fish), and, due to some obstruction, may eventually become impacted