NEWLY REPORTED MULTITUBERCULATE MAMMALS FROM THE PALEOCENE OF THE SAN JUAN BASIN OF NEW MEXICO

Multituberculate mammals have long been known from the Paleocene Nacimiento Formation. Only the largest, however, have been well-studied; the surface-collecting techniques first used often miss small taxa, including many multituberculates. A microvertebrate locality on the West Flank of Torreon Wash, NMMNH locality L-312, produces abundant isolated teeth, including those of multituberculates. It is within the Mixodectes pungens zone and has a fauna referable to the latest Torrejonian (To3) Land Mammal Age. In a preliminary publication of L-312, Tsentas mentioned three taxa of multituberculate mammals. Tsentas’ collection was made using relatively coarse screens, however, and is thus biased against the smallest specimens. Recent collecting at the site using 30-mesh screens has resulted in a substantial new collection. Since the To3 interval zone is principally known from the Nacimiento Formation, this represents a significant advance in our knowledge of Multituberculata of this zone.

Study of Williamson’s new collection and reevaluation of Tsentas’ collection indicates the presence of eight multituberculate taxa: Ptilodus mediaevus, Xanclomys n. sp., Ectypodus sp., Parectypodus sp., Krauseia clemensi, cf. Parectypodus, cf. Ectypodus sp., and Neoliotomus sp. Of these, Ptilodus mediaevus and Krauseia clemensi have long been known from the area. Krauseia’s first known appearance is in To3. Neoliotomus, however, is previously described from faunas of the Clarkforkian and Wasatchian Land Mammal Ages, at least 5 Ma younger, so this occurrence probably represents a significant downward stratigraphic range extension. It is also a southward geographic range extension, as Neoliotomus has previously been described from no farther south than Wyoming. Xanclomys has until now been known from only a single locality in the To2 interval zone in Wyoming, Swain Quarry; the species from L-312 is new.