Fassett et al. (2002) undertook geochemical analyses of dinosaur bones from the Ojo Alamo Sandstone and underlying Kirtland Formation. They concluded that there are distinct differences between Ojo Alamo and Kirtland dinosaur bones in the amounts of uranium (U) and rare-earth elements (REE).
Fassett (2009, p. 70) reviewed these results and presented new analyses of additional dinosaur bones to conclude that "the chemically distinct Ojo Alamo Sandstone dinosaur bones were fossilized in place during Ojo Alamo Sandstone (Paleocene) time and cannot be Cretaceous bone reworked from the underlying Kirtland Formation."
We emphasize that the geochemical analyses of
Fassett et al. (2002) and
Fassett (2009) only demonstrate that the chemistry of fossilization and diagenesis in the Ojo Alamo Sandstone is different from that of the underlying Kirtland Formation, not that the Ojo Alamo Sandstone dinosaur fossils are Paleocene. Differing diagenetic pathways for these units are expectable due to the greater permeability of the coarse-grained Ojo Alamo Sandstone.
The geochemical analyses also provide important data not addressed by
Fassett (2009) and evident in his figure 44 – the San Juan River hadrosaur bone has geochemical values that overlap the values of Kirtland Formation bone; indeed, it has virtually identical values to a bone from the Kirtland Formation. By Fassett's assumptions, this provides geochemical evidence that the San Juan River hadrosaur bone was fossilized as part of the Kirtland Formation geochemical system, so its presence in the Ojo Alamo Sandstone could only be due to reworking (see above).
Fassett's (2009, p. 53) claim 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" contradicts the long-standing success of vertebrate biochronologic methods. Vertebrate fossils have been the primary means by which the K/T boundary has been located in the San Juan Basin since
Brown (1910). Furthermore, the Puercan and Torrejonian land-mammal "ages" were defined in the San Juan Basin, and since the 1940s have provided a standard for biochronological correlation of Paleocene mammal faunas in western North America (e.g.,
Lofgren et al. 2004). The dinosaur-dominated assemblages of the Fruitland and Kirtland formations provided the basis for the Kirtlandian land-vertebrate age (Sullivan and Lucas 2003,
2006) and are classic and long-studied late Campanian vertebrate fossil assemblages since the pioneering work of Gilmore and Sternberg in the 1920s. Finally, the dinosaur-dominated vertebrate fossil assemblage of the Naashoibito Member (the Alamo Wash local fauna) has long been recognized as the youngest Cretaceous vertebrate fossil assemblage in the San Juan Basin (see reviews by
Lucas et al. 1987;
Hunt and Lucas 1992;
Lucas and Sullivan 2000; also see bibliography posted at
New Mexico Museum of Natural
History and Science).
Not only is Fassett (2009) dismissive of vertebrate biochronology, but his identification of Paleocene dinosaurs in the San Juan Basin contradicts published data that run counter to his correlations and assessment of the value of vertebrate biochronology in placement of the K/T boundary in the San Juan Basin and elsewhere. Indeed, across the Western Interior, and in particular in Texas and Utah, there are vertebrate fossil assemblages that contain dinosaurs, mammals, turtles and crocodylians that are similar to (or the same taxa as) those found in the Naashoibito Member of the Ojo Alamo Sandstone, and these have long been correlated to the Alamo Wash local fauna (e.g.,
Sullivan and Lucas 2006). Particularly similar to the Alamo Wash local fauna are the dinosaur-dominated assemblages of the North Horn Formation in eastern Utah and the Javelina Formation of the Big Bend region in Texas. Various data from these non-New Mexican sections indicate these are Late Cretaceous vertebrate assemblages (e.g.,
Cifelli et al. 1999;
Difley and Ekdale 1999;
Lehman et al. 2006). So, it strikes us as extraordinary that
Fassett (2009) claimed that only in the San Juan Basin is such a vertebrate fossil assemblage of Paleocene age. Indeed, if the Alamo Wash local fauna is Paleocene, then do we need to rethink the ages of the vertebrate fossil assemblages of units such as the Javelina Formation in Big Bend or the North Horn Formation in Utah?
We also stress that vertebrate biochronology of the K/T boundary in the Western Interior relies heavily on an extensive and detailed mammalian biostratigraphy.
Fassett's (2009) discussion ignored the large body of literature on mammal correlations across the K/T boundary that place the Alamo Wash local fauna in the Cretaceous and the Puercan assemblages, of the overlying Nacimiento Formation, in the early Paleocene. This body of work should have been addressed, because if the Alamo Wash local fauna mammals are Paleocene, then age determinations based on mammals from Alberta to Texas must be revised. We note that the mammal genera of the Alamo Wash local fauna, such as Alphadon, Essonodon and Meniscoessus, are not known elsewhere from Paleocene strata. They are characteristic Late Cretaceous mammals (Cifelli et al. 2004;
Lofgren et al. 2004;
Williamson and Weil 2008).
There is room to argue about the precise age within the Maastrichtian of the Alamo Wash local fauna, and this is a subject of discussion among vertebrate paleontologists (compare
Sullivan et al. 2005a to
Williamson and Weil 2008). However, this in no way diminishes the value of vertebrate biochronology to indicate that the K/T boundary in the San Juan Basin is between the stratigraphically highest in situ dinosaur fossils (in the Naashoibito Member of the Ojo Alamo Sandstone) and the stratigraphically lowest Puercan mammal fossils (in the Nacimiento Formation) (Figure 1).
Finally, there is another questionable issue, discussed by
Fassett (2009, p. 58-60)
— his claim of Paleocene dinosaurs in the Animas Formation. Fassett based this claim on an unpublished "Triceratops" specimen found 11 m above the base of the Animas Formation and a Paleocene flora, published by Knowlton (1924), found "about 60 m above the base of the Animas to near its top more than 500 m above the base of the formation" (Fassett 2009, p. 58). Clearly, the dinosaur specimen is stratigraphically below the Paleocene plant fossils, so how can the plant fossils indicate a Paleocene age for the dinosaur? Furthermore, the dinosaur fossil is stratigraphically below the lowest Paleocene palynomorphs reported from the Animas Formation, which are about 30 m above its base (Newman 1987, p. 158).