ADDITIONS TO THE EARLIEST PALEOCENE MULTITUBERCULATE MAMMALS OF GARFIELD COUNTY, MONTANA

Over 1000 new specimens of multituberculate mammals from the earliest Paleocene of Garfield County in Eastern Montana provide taxonomically modest additions to the known early Puercan (Pu1) diversity of this area, yet add significantly to our understanding of survivorship of the end-Cretaceous extinction event. The specimens are from three UCMP localities low in the Tullock Member of the Fort Union Formation: V74111, V72210, and V77087. They do not belong to the Bug Creek faunal facies and there is no evidence of reworking of fossils from Cretaceous sediments in these sites. Previously published (Archibald, 1982) samples from these sites were scanty; we can now confirm with greater confidence that the K-P boundary represents a significant extinction among multituberculates in this section and that new taxa appear suddenly at the base of the Paleocene.

Among the taxa not previously described from these localities are Kimbetohia? mziae, for which this is a northward geographic range extension, and a new, small neoplagiaulacid. There are also two unusual teeth that suggest the presence of rare and undescribed taxa in V77087. If these do not represent aberrant individuals, they indicate more morphological diversity than previously recognized among Pu1 multituberculates.

The large samples also elucidate already-described taxa. There are molars of a microcosmodontid probably referable to Acheronodon garbanii, which was previously known only from a damaged P/4. In V74111, which has the largest sample, two readily distinguished and significantly different (p < .05) size classes of the eucosmodontid Stygimys are present. This finding is in sharp contrast to that reported from the Bug Creek faunal facies (Lofgren, 1995), and may reflect less mixing of temporally separated populations by reworking. Whether it indicates two species is arguable; apart from size the teeth are not morphologically divergent, and they may represent sexual dimorphism in a single species.