Digestion a thin-walled expanded portion of the alimentary tract used for the storage of food prior to digestion. In some birds it is an expanded, muscular pouch near the gullet or throat. In adult doves and pigeons, the crop can produce crop milk to feed newly hatched birds. Certain insects may have a crop or enlarged esophagus. Herbivores have evolved cecums (or an abomasum in the case of ruminants). Ruminants have a fore-stomach with four chambers. These are the rumen, reticulum, omasum, and abomasum. In the first two chambers, the rumen and the reticulum, the food is mixed with saliva and
The Origin of Birds feature could not have been lost and regained again over the course of evolution. Based on this law, he therefore rejected the possibility of a direct theropod ancestor of birds, though he acknowledged that theropods and birds must have shared a close relationship. Throughout this section, Heilmann examines several groups of possible ancestors in addition to coelurosaurs, including pterosaurs, predentates, and pseudosuchians. Based on his rejection of theropods due to the wishbone issue as well as what Heilmann saw as striking morphological similarities between the skull of "Archaeopteryx", "Aetosaurus" and "Euparkeria", Heilmann concludes that a pseudosuchian origin of birds is