PALYNOLOGY

Fassett (2009) claimed that Paleocene palynomorphs from the Ojo Alamo Sandstone demonstrate its dinosaur fossils are Paleocene in age. Indeed, palynology is the principal database used to support his claim. Yet, despite Fassett's recitation of (mostly) previously published palynostratigraphy, only two localities of Paleocene palynomorphs are claimed by him to be stratigraphically below dinosaur fossils. This is an important point, because as Anderson (1960), Baltz et al. (1966) and others have demonstrated, there are undisputed Paleocene palynomorphs at various localities in the Kimbeto Member of the Ojo Alamo Sandstone, but the dinosaur fossils in the Kimbeto Member are fragmentary and isolated bones, all of which may have been reworked from underlying Cretaceous strata (see Fassett et al. 1987 for a detailed review of most Kimbeto Member dinosaur fossils). No palynomorphs have been reported from the Naashoibito Member, which has yielded an extensive, dinosaur-dominated vertebrate fossil assemblage (the Alamo Wash local fauna of Lehman 1981; Figure 1).

There are two published records of Paleocene palynomorphs stratigraphically below dinosaur bones in the San Juan Basin. Along the San Juan River near Farmington, an essentially complete, and well-preserved, hadrosaur femur was recovered from the Kimbeto Member of the Ojo Alamo Sandstone (Fassett et al. 1987; Fassett and Lucas 2000; Fassett 2009, figure 37.1). The occurrence of Paleocene palynomorphs stratigraphically below this femur has been used to argue that the femur came from a dinosaur that lived during the Paleocene (Fassett et al. 1987, 2002; Fassett and Lucas 2000; Fassett 2009). However, despite the well-preserved nature of the bone, Sullivan et al. (2005b) concluded that the bone has been reworked but not transported any significant distance, thereby preserving the integrity of the bone's outer surface. Indeed, the isolated occurrence of the femur indicates transportation of the bone must have occurred following decomposition, disarticulation and dismemberment of the hadrosaur. The well-preserved nature of the element does not preclude reworking. There are many examples of reworked fossils, including upper Paleozoic brachiopods found among the pebbles of the Ojo Alamo Sandstone (pers obser.), which preserve shell morphology in great detail, and pristine but reworked dinosaur teeth in Paleocene channel bottoms in Montana (Argast et al. 1987; Lofgren et al. 1990; Lofgren 1995).

The second example of supposed Paleocene palynomorphs below dinosaur fossils in the San Juan Basin is a carbonaceous shale ("lignite") bed in the uppermost De-na-zin Member of the Kirtland Formation at Barrel Springs (De-na-zin Wash) in the southwestern part of the basin that Fassett et al. (2002) claimed contains Paleocene palynomorphs, a claim repeated by Fassett (2009). However, two of us (RMS and SGL) sampled that bed in 2000 and reported palynomorphs that included Maastrichtian species, such as "Proteacidites" (=Tschudypollis) retusus and "P." (=Tschudypollis) thalmanni, as well as Pandaniidites typicus and Ulmoideipites krempi; no palynomorphs indicative of a Paleocene age were recovered (Sullivan et al. 2005b). Upon being informed of the results, Fassett (personal commun., 2001) claimed that RMS and SGL had actually not sampled the correct stratigraphic level, which according to him, is about 1 m higher in the same bed (Fassett 2009, figure 63). Accordingly, in 2001, RMS and SGL sampled stratigraphically higher in the carbonaceous shale bed, but processing of several samples yielded no identifiable palynomorphs (Sullivan et al. 2005b). Indeed, Fassett et al. (2002, p. 319) reported a late Campanian to early Maastrichtian assemblage of palynomorphs from the lower part of the carbonaceous shale bed, an assemblage that is essentially identical to that reported by Sullivan et al. (2005b). Thus, because RMS and SGL were unable to replicate the palynological results of Fassett et al. (2002) through repeated sampling, Sullivan et al. (2005b) considered them invalid.

Fassett et al. (2002) and Fassett (2009) attached great significance to a 1985 sample from this carbonaceous shale bed in the uppermost part of the Kirtland Formation, which they concluded indicates a Paleocene age. Based on this age, Fassett et al. (2002) and Fassett (2009, figure 63) inferred that there must be an unconformity within the carbonaceous shale bed in the uppermost Kirtland Formation, representing a hiatus of about eight million years.

However, the age of the De-na-zin Member is late Campanian, about 73 Ma, as is demonstrated by biostratigraphy and by the ash dates documented by Fassett and Steiner (1997) (Sullivan et al. 2005b; Sullivan and Lucas 2006). These dates are consistent with the magnetostratigraphy (Figure 1). There is no physical evidence of an unconformity such as internal scour surfaces or paleosols in the carbonaceous shale bed high in the De-na-zin Member.