A new important record of earliest Cenozoic mammalian history :geologic setting, Multituberculata, and Peradectia

Abstract

The type Ferris Formation of south-central Wyoming is thick, comparatively undeformed, and relatively fossiliferous. We documented more than 100 vertebrate-bearing, stratigraphically superposed fossil localities that span roughly 3,000 ft ( c. 900 m) of continental strata of Lancian (latest Cretaceous) and Puercan (earliest Paleocene) age. Fossil mammals were recovered from 39 of the localities, 32 or 33 of which represent Puercan time. The mammalian fossils allowed a detailed biostratigraphic zonation of the Puercan section, which is thicker, by nearly an order of magnitude, than any other known of that age. Preserved in a 1,763 ft- (537 m-) thick section are mammalian assemblages that represent all three Puercan Interval-zones ( i. e. , Pu1–Pu3), originally defined elsewhere from principally non-superposed strata. The local strata underwent only minor deformation, and that occurred late in the regional Laramide orogeny, not before the late Paleocene. On the basis of mammalian faunas, we place the Lancian-Puercan boundary at approximately 2,050 ft (625 m) above the base of the type Ferris Formation; remains of dinosaurs occur to just above that level, in absence of Puercan mammals. The lowest stratigraphic occurrence of Protungulatum donnae , a placental mammal diagnostic elsewhere of the earliest Puercan, exists at the 2,075 ft (632 m) level. Taxonomic composition of palynological samples is compatible with our placement of the Lancian-Puercan boundary. Previous workers assumed that advent of locally derived clasts in the Hanna Formation could be used to distinguish its outcrops from those of the underlying Ferris Formation. However, diverse pebbles from local sources also occur in the type Ferris Formation, even within its dinosaur-bearing parts. We have been unable to determine any combination of lithologic criteria that can be used reliably in the field to distinguish between outcrops of Ferris and Hanna Formations. We summarize important variations in depositional regime within Lancian-Puercan parts of the type Ferris Formation. We provide systematic description and discussion of multituberculate and peradectian components of the mammalian fauna. All reported taxa represent new records for the Hanna Basin and southern Wyoming in general, and the faunas help fill distributional gaps between species known to the north and south of central Wyoming. At least one species of multituberculate is recognized as new. Geographic range extensions include: (1) most southerly records of Cimolodon nitidus, Alphadon lulli, Mesodma ambigua, M. hensleighi, M. sp. cf. M. garfieldensis , and Catopsalis joyneri ; and (2) most northerly records of Ptilodus sp. cf. P. tsosiensis and Taeniolabis taoensis . Within the Hanna Basin, no genera of multituberculates or peradectians from the Ferris Formation have been documented in strata both of Lancian and Puercan age; several examples of pseudoextinction, however, may exist through taxonomic artifact. Temporal range extensions include first: (1) Puercan records of Mesodma hensleighi and Ectypodus spp.; (2) records within Puercan Interval-zone Pu3 of Ptilodus sp. cf. P. tsosiensis ; and (3) record in Puercan Interval-zone Pu2 of Catopsalis joyneri . In general, the Lancian multituberculate and peradectian faunas of the type Ferris Formation are similar to, although not nearly so diverse as, those from the type Lance Formation; the lower diversity almost certainly is an artifact of paucity of specimens available for study.