Metatherian mammals from the Naashoibito Member, Kirtland Formation, San Juan Basin, New Mexico and their biochronologic and paleobiogeographic significance

Abstract

ABSTRACT Isolated teeth representing several taxa of metatherian mammals were recovered from NMMNH locality L-4005 in the Naashoibito Member, Kirtland Formation, San Juan Basin, northwestern New Mexico. These include fragments of upper and lower molars and a fragmentary premolar that are referred to indeterminate “pediomyids,” an isolated p2 or p3 that is referred to an indeterminate peradectid, and two lower molars, an m1 or m2 and an m4 that are referable to the hatcheriforme metatherian Glasbius cf. G. intricatus. Two upper molar fragments are also tentatively referred to Glasbius. These taxa are part of the Alamo Wash local fauna. Glasbius has been proposed as a potential first appearance datum for the Lancian North American Land Mammal ‘age’ (NALMA). The presence of both Glasbius and the multituberculate Essonodon within the Naashoibito Member support an age correlative with other formations producing Lancian faunas. A Lancian age for the Naashoibito Member suggests that there is a significant disconformity between the top of the late Campanian De-na-zin Member, which is constrained by radiometric dates to be about 73 Ma, and the base of the Naashoibito Member. Terrestrial faunas of Lancian age show differences that have been attributed to provinciality. The widespread distribution of mammalian index taxa indicates some degree of faunal homogeneity during this interval, but the refined stratigraphic correlation of the Alamo Wash local fauna allows us to recognize other taxa as geographically restricted, as opposed to asynchronous.