Bug Creek Faunas Contain Reworked Dinosaurs/Lancian Mammals

Evidence suggesting that association of dinosaurs and Lancian mammals in Bug Creek assemblages at McGuire Creek is the result of reworking is outlined below:

1. Bug Creek assemblages from McGuire Creek are restricted to the basal lags of channels which were incised into Lancian strata (up to 20 m) (see chapter above on Geological and Palynological Correlations). This correlation of channel incision with the presence of Lancian mammals and dinosaurs in Bug Creek assemblages is not coincidental. Such a particular mixture of faunal components was caused by reworking. Similar arguments were proposed by Smit and Van der Kaars, 1984; Bryant et al., 1986; Retallack et al., 1987; and Bryant, 1989, based on the depositional setting of the original Bug Creek sites.

2. The reworking hypothesis can be tested with analyses of Facies G strata that overlie (or are laterally equivalent to) channel fills containing Bug Creek assemblages in the upper Hell Creek Formation at McGuire Creek. Facies G strata represent a low-energy, floodplain depositional setting where the possibility of reworking can be virtually eliminated. Unfortunately, this depositional setting usually is not conducive to producing microvertebrate concentrations. However, dinosaur remains commonly are present in floodplain deposits, and presumably would be present in Facies G floodplain deposits that are equivalent in age to channel facies that contain dinosaur remains. However, dinosaurs were not recovered from Facies G deposits. The absence of dinosaur remains in Facies G floodplains (which yield other vertebrates) is especially significant for laterally accreted channel facies (Facies E), where floodplain deposits and contemporaneous channel facies are juxtaposed (Figure 10). One would expect to find dinosaur remains in floodplain sediments (Facies G) if dinosaurs had been extant during the deposition of directly adjacent channel fills (Facies E). Because dinosaur remains occur in the basal lags of channels but are absent in age-equivalent floodplain deposits, their presence in these channels is probably the result of reworking.

3. Channel fills with nearly identical assemblages (those taxa not marked by * in

Figure 10
Taphonomic model adapted from Behrensmeyer, 1982, for occurrence of dinosaur remains in Paleocene
(or Pu0-Pu1) Facies E channel fills at McGuire Creek. The three possible sources of the dinosaur remains
(or Lancian mammals) are: (X) reworked from older strata through bank erosion, (Y) overland transport
from the floodplain surface into the channel, (Z) individuals that died in the channel. Dinosaur remains
are found in the basal lags of Facies E channels but not in age-equivalent floodplain deposits. The
absence of dinosaur remains in floodplain deposits which aggraded juxtaposed to laterally accreting
channels yielding dinosaurs suggests that the source of the remains is pathway X. If dinosaur bones
and teeth were entering the channel via pathways Y or Z, their remains would be expected to occur in
both the channel and floodplain deposits. Therefore, it is unlikely that dinosaurs lived during the
formation of these channel fills.

Table 4) sometimes produce dinosaur remains and Lancian mammals (BG, SR, UU) and sometimes do not (HE, ZL). Based on taxa which make their first appearance in the Puercan, it was concluded that these channel fills are approximately the same age but differ primarily in their Lancian components. Lancian taxa are found only in those channels that cut Lancian strata (Table 4, compare BG, SR, and UC with HE or ZL). These data indicate that the Lancian fossils were reworked.

The question then becomes one of deciding which argument is more compelling: reworked or not reworked? I suggest that the reworking argument is more compelling for the following reasons: (1) deep incision of channels (with Bug Creek assemblages) resulted in the subsequent erosion of a tremendous volume of Lancian strata; (2) exclusive association of Bug Creek assemblages with channels that demonstrably cut Lancian strata; (3) the presence of dinosaur remains in the basal lags of channels but their absence in age-equivalent floodplain deposits that yield other vertebrates; (4) the lack of dinosaurs in the stratigraphically lowest Paleocene floodplain deposits identified palynologically; and (5) the unequivocal occurrence of Cretaceous dinosaur remains in a Paleocene channel fill (Lofgren et al., 1990).

These data are hard to dismiss, especially considering the enormous potential for reworking in this depositional and stratigraphic setting. The burden then rests on those who argue for faunal contemporanity of dinosaurs and Lancian-Puercan mammals in Bug Creek assemblages, or "Paleocene" dinosaurs, to demonstrate that the fossils in question are not reworked.

To this end, a standard for recognition of "Paleocene" dinosaurs, consisting of four criteria, was proposed: (1) articulated dinosaur remains from a Paleocene channel fill; (2) in situ dinosaur from a Paleocene floodplain; (3) abundant dinosaur remains from a channel fill that did not scour Cretaceous sediments; and (4) dinosaur remains demonstrably reworked from Paleocene sediments (Lofgren et al., 1990). These criteria provide a standard for assessing the significance of Lancian mammal and dinosaur fossils in Paleocene rocks. Until at least one of these criteria is met, the reworking hypothesis should be dismissed as unsubstantiated.

In conclusion, the presence of dinosaurs and Lancian mammals in Bug Creek assemblages (and Paleocene channels) from the upper Hell Creek Formation at McGuire Creek is a result of reworking. Similarly, the claim of dinosaur survival into the Paleocene is rejected.