Frugivory in Sapeornis Zhou & Zhang, 2002b indicates diverse herbivorous trophic habits

Emerging evidence regarding the diet of Early Cretaceous birds from the Jehol Biota suggests that herbivory was the dominant feeding strategy among stem-ward taxa. Evidence suggests two different herbivorous diets. Granivory is inferred based on the presence of disseminules within the esophageal crop and accompanied by gastroliths in the gizzard. Frugivory is inferred when the disseminules are preserved in the abdominal cavity with an absence of gastroliths. Direct evidence indicates that herbivory spans the entire phylogeny of Early Cretaceous birds, with frugivory documented in the long boney-tailed Jeholornis Zhou & Zhang, 2002a, and the enantiornithine Longipteryx Zhang, Zhou, Hou & Gu, 2001, and granivory documented in the basal pygostylian Sapeornis Zhou & Zhang, 2002b, and the non-ornithurine ornithuromorph Eogranivora Zheng, O’Connor, Wang, Wang & Zhou, 2018. Seasonal changes in diet are inferred for Jeholornis based on the presence of a gastric mill in some specimens although not accompanied by direct evidence of ingested food items and suggested for Longipteryx based on the absence of obligate avian frugivores in extant temperate climates. Here we provide the first direct evidence of dietary complexity in a Mesozoic bird. Two specimens of Sapeornis reveal that this taxon also fed on fruits. While this could reflect seasonal differences in diet, the unusual distribution of gastroliths in specimens of Sapeornis may suggest that this taxon was not so specialized. Evidence suggests that phylogenetically disparate members of the Jehol avifauna utilized diverse trophic resources, but ­Sapeornis provides the first direct evidence of this. Dietary complexity and phenotypic plasticity of the digestive tract are likely factors that facilitated the early radiation and success of Aves.