There is no indication of such analysis in the referenced article.
“There is no indication of such analysis in the referenced article.”
The PNAS article is behind a paywall, so I don’t have access to read it to determine whether there is or is not such evidence available. I suspect such evidence is unlikely to be available due to the age of the fossil.
Nonetheless, the morphological evidence clearly identifies the animal as a waterfowl in a genus known to predate upon fish in the marine environment. The pseudoteeth of the fossil are very strongly indicative of a fish catching waterfowl. It also cannot be assumed chemical traces of the bird’s prey will not be recovered at some point in the future, so it cannot be concluded or truly said a fossil of this bird cannot tell us what it preyed upon.