Eggshells assemblages and stable isotope composition in the Upper Cretaceous (Campanian) El Gallo Formation of Baja California, Mexico: Paleoenvironmental inferences

Fossil eggshells were recovered from the El Gallo Formation, Baja California, northwest Mexico, is one of the few North American Campanian continental outcrops outside the Western Interior Basin. Based on microstructure, the eggshells are referred to ornithopod (ootaxon Spheroolithus), theropods (ootaxa Prismatoolithus, two Tubercuoolithidae eggshells, Continuoolithus, Tristraguloolithus, Disperituberoolithus, Pseudogeckoolithus, Laevisoolithidae? eggshells and Enantiornithine indet.), and one crocodylomorph (cf. Neokrokolithes). While some of these ootaxa were associated with the modest, but diverse osteological record preserved there; others reveal the presence of small theropods, including as the first evidence of enantiornithine birds in the formation. Because the stable isotopic compositions of the eggshells differ from those of pedogenic carbonate nodules, we infer minimal diagenetic influence. We thus propose that the C3 plants had δ13C compositions between −27‰ and −29‰ and that warm evaporative conditions led to enriched δ18O in ingested water compared to meteoric water at least during the short period of dinosaur eggshell formation. •The diversity of eggshells is greater than the osteological record.•Eggshell's carbon isotopic values suggest humid conditions.•Estimated temperature from δ18O values is congruent with the paleolatitude position.