Exceptionally preserved plant parenchyma in the digestive tract indicates a herbivorous diet in the Middle Eocene bird Strigogyps sapea (Ameghinornithidae)

The holotype skeleton of the avian species Strigogyps sapea from the Middle Eocene of Messel preserves remains of the digestive tract that have not been studied so far. The crop content of the specimen includes numerous densely packed, tiny spherules containing large cavities. These peculiar structures match well with previously described fossil plant parenchyma cells with starch grain moulds. We hypothesize that the spherule layer represents plant parenchyma ingested by the bird, and that the spherules themselves are casts of cytoplasm with storage organelles. Our study provides a further example of exceptional soft tissue preservation in Messel vertebrates and shows that Strigogyps was at least facultatively herbivorous. A herbivorous diet of Strigogyps is also indicated by the fact that the crop content includes seed remains, and it constitutes further evidence against previously hypothesized close affinities between the taxon and the carnivorous Phorusrhacidae.