A survey of Eocene stomach contents illuminates the origins of frugivory and seed dispersal in neornithine (crown group) birds

Stomach contents with seeds in avian fossils from the latest early/earliest middle Eocene Messel oil shale (Germany) are surveyed. 20 bird specimens belonging to 10 species with seeds produced by at least 13 different plant species are reported. The fossils provide the earliest direct evidence for avian frugivory and suggest that seed dispersal by neornithine (crown group) birds occurred earlier and was more extensive than currently thought. For the first time, we report seeds in the stomach content of stem group Trogoniformes, which today are important seed dispersers in the New World tropics. Other birds belong to the Galliformes, Gruiformes (Messelornithidae), Coliiformes, Coraciiformes, and the extinct taxa Halcyornithidae and Zygodactylidae. Extant Coraciiformes are predominantly carnivorous, whereas some Galliformes and Gruiformes as well as the Coliiformes today also consume fruits. All reasonably well-preserved seeds appear to be from angiosperms, including specimens identified as Rutaceae, Mastixiaceae, and Vitaceae. Unidentified seeds represent various other plant groups. Several of the arboreal birds ingested seeds of the Vitaceae. This plant family already evolved in the Late Cretaceous, whereas arboreal Neornithes did not radiate before the early Cenozoic. As such, and at least concerning arboreal birds, these fossils provide evidence for the "recruitment hypothesis", which suggests that early Cenozoic seed dispersers exploited an existing diversity of edible fruits.