The trophic habits of early birds

摘要

Although direct associations that reveal diet are extremely rare in the fossil record, the rich Lower Cretaceous Jehol Lagerstatte has produced dozens of specimens preserving ingested items, which together reveal important information regarding the early evolution of the avian alimentary canal. Direct evidence indicates Jeholornis and Sapeornis ate seeds and like living granivores utilized a gastric mill although only the more derived Sapeornis possessed a crop for food storage. Despite their smaller numbers in the Early Cretaceous, most direct evidence pertains to the Omithuromorpha, indicating a structurally and functionally modern alimentary canal was present in even the earliest members of this Glade. Similar evidence is altogether lacking in the Enantiornithes suggesting this Glade was characterized by a primitive alimentary canal, potentially factoring into the ultimate extinction of this successful Cretaceous lineage. The role of the gizzard in the loss of teeth in Ayes is more complex than in non-avian theropod lineages. As in neornithines, the presence of gastroliths in basal ornithuromorphs may not be entirely indicative of herbivory although this diet does appear to correlate with complete tooth loss in this Glade. Positive selection for specialized tooth morphologies persists throughout the evolution of Ayes and in the Ornithuromorpha appears to be linked to piscivory. Although represented by the greatest numbers, no direct indicator of diet preference is preserved in any confuciusornithiform or enantiornithine suggesting these clades utilized different nutritional strategies. The absence of teeth and gastroliths in all confuciusornithiforms suggest this lineage may have secondarily switched to a non-herbivorous, soft diet. Similarly, the absence of gizzard stones in enantiomithines also suggests a soft diet while their small body size suggests Early Cretaceous species would have fed on invertebrates. The diversity of recognized dental patterns including