of the fish indicates it was about to be regurgitated as a pellet, or that it was stored in the crop. No other fish remains are present in the slab. Though it is unknown how common fish were in the diet of Confuciusornis, the finding did not support a herbivorous diet, and the researchers pointed out that no specimens have been found with gastroliths (stomach stones), which are swallowed by birds to help digest plant fibers. Instead, they suggested it would have been omnivorous, similar to for example crows. Andrei Zinoviev assumed it caught fish on the wing. The skull was relatively immobile, incapable of the kinesis of modern birds that can raise the snout relative to the back of the skull. This immobility was caused by the presence of a triradiate postorbital separating the eye socket from the lower temporal opening, as with more basal theropod dinosaurs, and the premaxillae of the snout reaching all the way to the frontals, forcing the nasals to the sides of the snout. Paleoenvironment and paleoecology Confuciusornis was discovered in the Yixian and Jiufotang Formations and is a member of the Jehol Biota. Tuff makes up a considerable amount of the rock composition in both due to frequent volcanic eruptions, which were slightly more frequent in the Yixian Formation. Shale and mudstone also are major components of the formations. The tuff has allowed detailed dating of the formations by using 40Ar-39Ar isotopes. This results in an age of approximately 125 to 120 million years ago for the Yixian formation and approximately 120.3 million years ago for the Jiufotang Formation. The fossils were buried as a result of flooding and volcanic debris. This method of preservation resulted in fossils that are very flat, almost two-dimensional. The volcanic strata have allowed the preservation of various soft tissues, such as detailed feather impressions. Using oxygen isotopes in reptile bones found in the formation, a 2010 study determined that many formations from East Asia, including the Yixian, had a cool temperate climate. The mean air temperature of the Yixian Formation was estimated at 10 °C ± 4 °C. Fossils of Xenoxylon, a type of wood known from temperate areas of the time, have been found throughout the region. Additionally, reptiles needing heat, such as crocodilians, are absent. The majority of Jehol flora has been discovered in the lower Yixian Formation. This flora includes most groups of Mesozoic