The lower shale member of the Aguja Formation is one of only a few terrestrial stratigraphic units of early Campanian age in North America. Apart from a prolific microfossil site that yielded the Lowerverse local fauna, the lower shale is sparsely fossiliferous, and very little is known about its larger vertebrate fauna. Most of the specimens collected thus far are fragmentary, but among these are a hybodontid shark, an amiid fish (Melvius sp.), and bothremydid, trionychid, nanhsiungchelyid (Basilemys sp.), and baenid (cf. Denazinemys sp.) turtles. An isolated osteoderm is referable to the alligatoroid crocodylian Deinosuchus sp. A partial skull and skeleton of a goniopholidid crocodyliform is the most complete specimen recovered thus far, and likely represents a new genus and species. The most abundant dinosaur in the fauna is a new genus and species of hadrosaur. A partial pelvis and metatarsus of an ornithomimid dinosaur also likely represents a new taxon. Indeterminate tyrannosaurid, dromaeosaurid, ceratopsid, and nodosaurid dinosaurs are represented. The lower Two Medicine Formation in Montana, lower Wahweap Formation in Utah, and Menefee Formation in New Mexico bear the only other terrestrial vertebrate faunas of broadly comparable age in North America. The Aguja lower shale fauna is allied with ‘southern’ faunas in New Mexico and Utah in sharing the amiid fish Melvius, the baenid turtle Denazinemys, bothremydid turtles, and goniopholidid crocodyliforms. These taxa are absent in para-contemporaneous faunas of Montana and Alberta. Non-marine strata of early Campanian age do not appear to be present farther south in Mexico, and so the lower Aguja fauna is significant in documenting latitudinal variation in early Campanian continental environments.
- Western Interior Basin