Abstract:
A crop adapted for an herbivorous diet of seeds has previously been documented in the Early Cretaceous birds Sapeornis
and Hongshanornis. Here we report on several specimens of Yanornis that preserve a crop containing fish. One specimen
preserves two whole fish in the oesophagus, indicating that Early Cretaceous birds shared trophic specializations with
Neornithes for the increased energetic demands of flight – namely the storing of food for later consumption when the
stomach is full. Whole fish also indicate that despite their presence, teeth were not used to orally process food, suggesting
the hypertrophied dentition in this taxon were utilized in prey capture. The presence of macerated fish bones in the crop of
other specimens indicates the highly efficient advanced muscular system of peristalsis responsible for moving ingested
items between different segments of the alimentary canal was also in place. Despite the fact many features of the modern
avian alimentary canal are inferred to compensate for the absence of teeth in birds (expandable oesophagus, grinding
gizzard), the derived alimentary canal was apparently present in toothed Cretaceous birds. Although Yanornis was
considered to have switched their diet from piscivorous to herbivorous, based on position and morphology we reinterpret
the gastroliths reported in one specimen as sand impacted in the intestines, and reconstruct the taxon as primarily
piscivorous. This is a novel interpretation for fossilized gastroliths, and the first documentation of this condition in the fossil
record.
