Informal Biochrons, bk1-bk2-bk3, of the Pu0 Interval Zone
Archibald and Lofgren (1990) subdivided the Pu0 Protungulatum/Peradectes interval zone into three informal biochrons defined by first appearances of the "condylarths" Protungulatum (bk1), Mimatuta (bk2), and Oxyprimus (bk3). The type localities for the three informal biochrons are the original Bug Creek sites: Bug Creek Anthills (bk1), Bug Creek West (bk2), and Harbicht Hill (bk3). The three biochrons and correlations based on them were tentative because of the lack of stratigraphic control on the three local faunas, and the differences in faunal composition between the localities could be a factor of either inadequate sample size or lack of recent taxonomic reviews. Therefore, Archibald and Lofgren (1990) chose to propose the biochrons informally until additional faunal data from these sites is available. Analyses of these faunas is presently underway by J. K. Rigby, Jr., and it is hoped that the results of this work will be available soon.
With the uncertainties surrounding the type localities of the bk1-bk2-bk3 biochrons, it might be better to abandon hope of recognizing different faunal levels within the Pu0 interval zone. However, the Bug Creek Anthills Local Fauna is especially significant in this regard because it is older than other Pu0 sites in the upper Hell Creek Formation. Thousands of mammal teeth and jaws have been collected from Bug Creek Anthills and all systematists who have studied these mammalian samples agree that the larger-sized "condylarths" Ragnarok spp. or Protungulatum gorgun present at other Pu0 sites in the upper Hell Creek Formation are not present at Bug Creek Anthills (Tables 4,7,9). Given the huge sample size and similarity in facies between Bug Creek Anthills and other Pu0 sites in the upper Hell Creek Formation, the lack of Ragnarok spp. and P. gorgun at Bug Creek Anthills indicates that this local fauna is older.
However, analysis of faunal differences between other Pu0 sites is complicated by the stratigraphic, taxonomic, and sampling uncertainties discussed below.
Lack of Stratigraphic Control
Stratigraphic relationships among the type localities for biochrons bk1 (Bug Creek Anthills), bk2 (Bug Creek West), and bk3 (Harbicht Hill) are uncertain. This is clearly the case with the Bug Creek Anthills locality, whose stratigraphic position has been debated at length (Smit and Van der Kaars, 1984; Fastovsky and Dott, 1986; Smit et al., 1987).
In spite of this, Sloan (1987:174) refers to the three original Bug Creek faunas as occurring within channel fills that are in superposition. In actuality, the Bug Creek West and Bug Creek Anthills local faunas occur within channel fills 2 km apart on opposite sides of the Bug Creek drainage and superposition of these channel fillings is not demonstrable (Fastovsky and Dott, 1986; Smit et al., 1987: figure 4; Rigby et al., 1987: figure 3). Also, the Harbicht Hill site is located over 20 km north of Bug Creek West and Bug Creek Anthills, and it is impossible to physically trace individual channel fills over 20 km in complex fluvial deposits such as those that comprise the upper Hell Creek Formation (Fastovsky and Dott, 1986; Fastovsky, 1987; this report).
As with the type localities, there is a general lack of superpositional or cross-cutting relationships among sites referable to the Pu0 interval zone. However, a series of localities in the Bug Creek drainage provides support for a bk1-bk2-bk3 succession. Tentative channel-fill relationships (Rigby et al., 1987) and preliminary faunal data (Rigby, 1989) indicate that the Big Bugger Channel yields Oxyprimus (bk3) and overlies the Bug Creek Anthills Channel, which yields only a single "condylarth," Protungulatum (bk1) (but see Luo, 1989, 1991). Also, the By George Channel yields Oxyprimus (bk3) and cuts the Scmenge Point Channel, which contains Protungulatum and Mimatuta but apparently not Oxyprimus (sensu bk2) (Rigby et al., 1987: figure 3; Rigby, 1989: figure 5). Therefore, channel fills with bk3 faunas overlie or cut channel fills with bk1 or bk2 faunas. The preliminary reports by Rigby et al. (1987) and Rigby (1989) are the only available biostratigraphic data which support a bk1-bk2-bk3 succession of informal biochrons.
Sampling Uncertainties
Biochronologic zonations based on small faunal differences are particularly susceptible to sampling uncertainties. Successive first appearances of the "condylarths" Protungulatum, Mimatuta , and Oxyprimus define the respective biochrons bk1-bk2-bk3 of the Pu0 interval zone. McGuire Creek faunal data (mainly Pu1 sites) indicate that Protungulatum, Mimatuta , and Oxyprimus are not rare and are represented in roughly approximate numbers of specimens (Table 10). It is assumed that collection of a large sample would yield these "condylarth" genera if they were extant in the immediate area at the time the deposit was formed. The sample size from the Bug Creek Anthills locality is again critical in this regard, because the absence of certain taxa which are
known to occur in other local faunas in the upper Hell Creek Formation strongly suggests that Bug Creek Anthills is older than other Pu0 sites.
However, the problem of insufficient sample size has been overlooked in previous biostratigraphic-biochronologic studies on the Bug Creek faunas. For example, a survey of the UCMP collection of mammals (n=300) from Harbicht Hill indicates that the marsupials Alphadon (UCMP 137313) and Glasbius (UCMP 137314) are present, as is the eutherian Gypsonictops (UCMP 137304) (Table 4). These taxa were thought to be extinct by the time the Harbicht Hill channel was filled (Sloan and Van Valen, 1965; Sloan, 1976). Similarly, the very small UCMP sample of mammals (n=10) from Bug Creek West yields an upper molar of Didelphodon vorax (Appendix 1). This is the first reported occurrence of Didelphodon from Bug Creek West. According to previous work, "Didelphodon vorax . . . is quite unknown from any of the Paleocene localities in either Garfield or McCone counties" (sensu Bug Creek West and Harbicht Hill; Sloan and Rigby, 1986:1174). The former lack of records probably reflects small sample size because D. vorax and Glasbius are easily identified.
Last appearances of Lancian taxa such as Didelphodon, Pediomys, Glasbius, Alphadon , and others, and first appearances of certain "condylarths" from the Bug Creek West and Harbicht Hill sites have been accorded temporal significance (Sloan and Van Valen, 1965; Sloan, 1976; Van Valen and Sloan, 1977; Archibald, 1981, 1982, 1986, 1987c). The sizes of the samples from which these interpretations were made are not clear, although Sloan and Van Valen (1965) indicate that the minimum number of individuals (MNI) in their samples was 108 at Bug Creek West and 54 at Harbicht Hill. These samples are not large enough to include many of the rarer taxa. The sampling uncertainty is compounded by the fact that Bug Creek West and Harbicht Hill are channel-fill local faunas in which reworked fossils can occur. For example, samples from McGuire Creek indicate that virtually all Lancian taxa occur in Pu1 faunas (Table 4). However, many of these taxa are not found at Bug Creek West and Harbicht Hill which are Pu0 in age and presumably older than Pu1 sites. Is the presence of these taxa in McGuire Creek sites the result of reworking, or were Bug Creek West and Harbicht Hill not sufficiently sampled?
Based on the new faunal data presented above, the answer to the second question is yes. The reworking possibility was investigated earlier (see chapter above on Reworking of Fossils). In either case, it is apparent that the last appearance of Lancian taxa has limited value for biochronologic zonation. Also, the model of stepwise extinction of Lancian mammals (Van Valen and Sloan, 1977; Archibald, 1986, 1987c) within the Pu0 interval zone (Bug Creek Anthills-bk1, Bug Creek West-bk2, Harbicht Hill-bk3) is suspect because of their occurrence within the Pu1 interval zone (Tables 8, 9). Accordingly, Archibald and Lofgren (1990) abandoned last appearances of Lancian taxa as a basis for proposing or recognizing successive biochrons within the Pu0 interval zone.
Based on the examples discussed above, the available faunal lists of local faunas from the type localities of bk2 (Bug Creek West) and bk3 (Harbicht Hill) are far from complete, and what additional taxa these sites might eventually yield is unknown. The insufficient database from these localities raises doubt concerning the separability of the bk2 and bk3 biochrons.
Taxonomic Uncertainties
In 1965, announcement of the discovery and a brief description of mammalian components of the original Bug Creek faunas (Bug Creek Anthills, Bug Creek West, and Harbicht Hill) were published (Sloan and Van Valen, 1965). Other than "condylarths" from Bug Creek Anthills (Archibald, 1982; Luo, 1989, 1991), detailed systematic treatments of the mammalian faunas from the Bug Creek sites even now have yet to appear. Partly because of this lack of recent taxonomic review, proposal of a series of biochrons based on the mammal faunas from these localities was tentative (Archibald and Lofgren, 1990). Formal biochronologic zonations should be based on faunas that are well sampled, thoroughly described, and systematically analyzed. Therefore, mammals from the original Bug Creek sites require detailed description before informal biochrons (bk1-bk2-bk3) of the Pu0 interval zone can be either formally proposed or abandoned. Additional faunal data outlined below, based on UCMP collections support this contention.
Bug Creek Anthills: A survey of the multituberculates indicates that Cimexomys gratus (formerly C. hausoi ) is represented by a few isolated teeth (see Appendix 1). Previously, only the smaller species of Cimexomys, C. minor , was thought to occur at Bug Creek Anthills (Sloan and Van Valen, 1965; Archibald, 1982).
Luo (1989, 1991) argued that Mimatuta morgoth and Oxyprimus erikseni are both present at Bug Creek Anthills, though rare. If his views are accepted, the bk2 and bk3 biochrons will require redefinition. In that case, the successive biochrons should be redefined as the Protungulatum donnae/Protungulatum gorgun bk1, the Protungulatum gorgun/Ragnarok bk2, and the Ragnarok/Peradectes bk3 (pending complete descriptions of the Bug Creek West and Harbicht Hill local faunas).
Bug Creek West: The UCMP sample of mammals is small (n=10) and adds no new data beyond the occurrence of Didelphodon vorax discussed above.
Harbicht Hill: The UCMP mammal sample (n=300) yields a few specimens of Cimexomys gratus and a single specimen of Catopsalis which is referable to C. alexanderi (Appendix 1). In the original description of the Harbicht Hill fauna, Sloan and Van Valen (1965) list only the smaller species of Catopsalis, C. joyneri . Either both species are present at Harbicht Hill or specimens identified as C. joyneri in 1965 are now referable to C. alexanderi , a taxon erected later by Middleton (1982). Analysis of samples of Ragnarok nordicum from Mantua Lentil and R. harbichti from Harbicht Hill indicates synonymy of these species, with R. nordicum having priority (Appendix 1).
In summary, the mammalian faunas from the type localities of the bk1-bk2-bk3 biochrons of the Pu0 interval zone are insufficiently described and sampled. Results of ongoing studies by J.K. Rigby, Jr., are required before these biochrons can be formally adopted or abandoned.