The fossil record of P. pervetus includes many complete skeletons from Green River Formation sites (Early Eocene), suggesting that the birds nested in colonies and possibly were liable to succumb to botulism, similar to many colony-nesting waterfowl or shorebirds today. P. isoni is known from the Late Paleocene Aquia Formation (Maryland, 61-62 MYA) humerus (USNM 294116) and a fingerbone (USNM 294117) that were initially described, as well as from the humeri that were initially believed to be from Headonornis (BMNH PAL 3686, 5105, 6240). As these are Late Eocene or even Early Oligocene (BMNH PAL 5105, Bembridge Marls) in age, they possibly belong to a distinct taxon.