VERTEBRATE BIOCHRONOLOGY

Lucas et al. (2009) clearly take offense at the statement in Fassett (2009) that "vertebrate paleontology has had limited biochronologic value in determining the age of strata adjacent to the K-T interface in the San Juan Basin." In my opinion, this statement is essentially true, if restricted to uppermost Cretaceous strata in the basin. A detailed discussion of the differing ages assigned to vertebrate fossils over time in uppermost Cretaceous strata in the San Juan Basin is beyond the scope of this response; (see discussion of this topic in Fassett, 2009 and in references therein). Very recent papers (cited in Fassett, 2009) have stated that the vertebrate fossils from the Ojo Alamo Sandstone are Lancian – very latest Cretaceous in age; whereas other very recent papers have declared that the age of this same fossil assemblage is near the Campanian-Maastrichtian boundary. According to the Gradstein et al. (2004) time scale, these placements are as much as 5 m.y. apart. On this basis, I do not find it unreasonable to conclude that "vertebrate paleontology has had limited value" for age determination in Cretaceous strata in the San Juan Basin. Paleocene vertebrates have a much better record for age determinations, thus I confess that my use of "K-T" in the above quote was ill-advised and I should have restricted this comment to Cretaceous strata.

Lucas et al. (2009) thought it "extraordinary" that Fassett (2009) only found Paleocene dinosaurs in the San Juan Basin. The San Juan Basin was my area of study, thus my data had no relevance to ages of vertebrates in other parts of the Western Interior. These authors stated that Fassett (2009) ignored the mammalian fossils from K-T strata in the basin, but that is not true. These fossils are discussed on pages 60-65 of Fassett (2009) and it is concluded therein that the mammals (and dinosaurs) identified from the Paleocene Ojo Alamo are Paleocene in age. As for the value of mammalian fossils as age indicators for K-T rock strata in the Western Interior of North America, Fassett (2009, p. 62) referred to the pointed warning in Clemens and Williamson (2005) that stated that the ages assigned to mammalian fossils from strata adjacent to the K-T boundary were in a state of flux due to limited numbers of collection sites, biogeographic diversity of taxa, and limited knowledge of the evolution and radiation of mammals across the K-T interface. If Lucas et al. disagree with the opinions of Clemens and Williamson (2005) in this regard, they may wish to air those differences in the proper scientific forum; perhaps in Palaeontologia Electronica?