The Nautiloid eutrephoceras Sp. From the Cretaceous of New Mexico Likely Represents Multiple Species

Abstract

Compared to ammonites, Cretaceous nautiloids are relatively rare in New Mexico, and Eutrephoceras sp. is the most common nautiloid of that age in the state. The genus Eutrephoceras ranges from the Late Jurassic to the Middle Miocene worldwide. In New Mexico, Eutrephoceras sp. has been reported from the Turonian and Campanian (e.g. Scott et al., 1986), Santonian and Campanian (Sealey and Lucas, 2019, 2022), E. alcesense Reeside has been reported from the Santonian, and E. montanaensis (Meek) from the lowermost Maastrichtian (Sealey and Lucas, 2019, 2022). Morphological features of many New Mexico specimens of Eutrephoceras sp. are similar to Eutrephoceras dekayi . However, Landman et al. (2018) stated that the lowest occurrence of E. montanaensis from the Cedar Creek Anticline in Montana—the uppermost Campanian Baculites eliasi Zone or the lower part of the lowermost Maastrichtian B. baculus Zone—is below that of E. dekayi, and that their ranges may overlap. Given that the stratigraphically highest known specimen of Eutrephoceras sp. in New Mexico is from the middle upper Campanian B. compressus Zone, specimens from the state are best not assigned to E. dekayi . Furthermore, E. dekayi has been a “wastebasket” taxon for North American Cretaceous nautiloids (Neal Larson, written communication, 2018). Eutrephoceras sp. is present in the D-Cross Member of the Mancos Shale and Gallup Sandstone in Socorro County, the Satan Tongue of the Mancos Shale in Sandoval County, the Lewis Shale in Rio Arriba and San Juan counties and the Fort Hays Limestone Member of the Niobrara Formation and the Pierre Shale in Colfax County. In New Mexico, Eutrephoceras sp. has been found in the upper Turonian Scaphites ferronensis and Prionocyclus quadratus zones, the lower Campanian S. hippocrepis II Zone, the middle Campanian Baculites gregoryensis Zone, the upper Campanian Didymoceras nebrascense , Exiteloceras jenneyi , D. cheyennense and B. compressus zones and unknown zones. The Satan Tongue occurrences are Santonian in age. These Turonian, Santonian and Campanian occurrences of Eutrephoceras sp. from many different ammonite zones in New Mexico likely represent multiple species, and we propose that further study is needed