NO PALEOCENE DINOSAURS IN THE SAN JUAN BASIN, NEW MEXICO

Until recently, the age of the upper part of the Kirtland Formation (De-na-zin and Naashoibito members) has been somewhat problematic. Although ⁴⁰Ar/³⁹Ar dating has firmly established the age of the De-na-zin Member at 73.37-73.04 Ma (late Campanian), the age of the overlying Naashoibito Member remains contentious. Some dinosaur taxa formerly cited as from the Naashoibito Member (such as “Kritosaurus,” Parasaurolophus and Pentaceratops) are actually from the underlying De-na-zin Member. Few diagnostic dinosaur genera are known from the Naashoibito Member, and some (especially Torosaurus and Tyrannosaurus) rest on problematic identifications. No radioisotopic dates are available, and only three mammal species have been identified from this unit. The possible age ranges from late Campanian to late Maastrichtian, and an early Maastrichtian age is most likely Recent sampling of a lignite at the top of the De-na zin Member at Barrel Springs has yielded conflicting palynomorph data. Fassett maintains that the palynomorphs Momipites tenuipolis and Brevicolporites colpella indicate a Paleocene age for this lignite and the overlying Naashoibito Member, which locally contains numerous (non-reworked) dinosaur fossils. We sampled this same lignite and did not recover any Paleocene palynomorphs, but instead found Proteacidites retusus, P. thalmanni and Tricolpites microreticulatus, taxa that are only known from the Campanian and Maaastrichtian. The presence of Pandaniidites typicus and Umoideipites krempi further restrict the age of the sample to Maastrichtian. No taxa restricted to the Paleocene were recovered, although several species that span the K/T boundary are also present. The suggestion of a Paleocene age for this lignite is also inconsistent with the presence of numerous non-reworked dinosaur remains in overlying strata. A hadrosaur femur from the Ojo Alamo Sandstone near Farmington, apparently stratigraphically above Paleocene palynomorphs, is arguably reworked. We conclude there is no convincing evidence of Paleocene dinosaurs in the San Juan Basin.