Direct evidence of carnivory in the early-diverging Alvarezsaurian Bannykus

A dietary shift from carnivory to insectivory has been proposed to explain the morphological evolution of alvarezsaurians, particularly the adaptive manual digital reduction and body size miniaturization. However, based solely on morphological shifts, this hypothesis lacks direct dietary evidence to support either carnivory or insectivory. Here, we present the first dietary evidence for alvarezsaurians, derived from the intestinal contents of the Early Cretaceous Bannykus wulatensis. Our analysis revealed significantly higher levels of calcium and phosphorus in the intestinal contents compared to the surrounding sandstone. Scanning electron microscopy identified hard tissue debris and possible soft tissues surrounded by phosphatized bacteria and tightly packed hollow microspheres, suggesting that the intestinal contents were strongly pseudomorphed by phosphatized microbes during fossilization. Raman spectroscopy showed characteristic peaks indicative of bone-derived material, consistent with the hard tissue debris in the intestinal contents. Our results suggest that Bannykus had a carnivorous diet with strong chemical digestion, which likely compensated for its delicate cranial structures and small teeth. These results imply that if a dietary shift to insectivory occurred, it likely took place later in alvarezsaurian evolution, probably coinciding with a reduction in body size.