Search

A magnetostratigraphic study in the northeast part of the Crazy Mountains Basin of Montana sampled strata that includes both the Cretaceous-Paleogene boundary sequence as well as numerous, well-documented fossil mammal and invertebrate localities. Ninety-five horizons were sampled from nine separate sections, with a composite stratigraphic range from the upper part of the Hell Creek Formation, through the Bear and Lebo Members of the Fort Union Formation, and into the lower part of the Melville Member of the Fort Union. Data show that the (1) Hell Creek Formation-Bear Member contact lies within a magnetozone of reverse polarity (chron C29r); (2) Simpson Quarry and the Bear Member-Lebo Member contact occurs within the overlying normal polarity magnetozone (chron C29n); (3) magnetozones defining chron C28 fall completely within the Lebo Member; and (4) Gidley and Silberling quarries and the Lebo Member-Melville Member contact occurs within a reverse polarity magnetozone (chron C27r). The resulting high-resolution chronostratigraphic framework--incorporating both biostratigraphic and magnetostratigraphic data--places the Crazy Mountains Basin strata and fossil localities into a more well-defined context with other depositional basins in the Western Interior of North America. [ABSTRACT FROM AUTHOR]

Ursolestes perpetior gen. et sp. nov. is a large purgatoriid plesiadapiform primate from the middle/late Puercan (earliest Paleocene) Simpson Quarry, Montana, USA. It differs from Purgatorius Van Valen and Sloan, 1965, the only other known purgatoriid and the oldest discovered primate, in its substantially larger body size as inferred from the dimensions of the known parts of its dentition, while resembling Purgatorius in possessing a large lower canine, unreduced p1, and in the upper molars, a protocone twist and weakly developed postprotocone fold; moreover, the m3 of U. perpetior possesses an expanded, plesiadapiform-like posterior lobe of the talonid that supports the hypoconulid. The large lower canine and the greater development of trenchant molar crests suggest that U. perpetior was more insectivorous, differing appreciably from the omnivorous niche long hypothesized for Purgatorius and purportedly marking the insectivore-primate transition. Hence, U. perpetior adds significant new knowledge regarding the breadth of the adaptive zone occupied by purgatoriids in their position at the very beginnings of primate evolution. [ABSTRACT FROM AUTHOR]

Discusses the discovery of mammalian primatomorph Purgatorius from the Paleocene Bear Formation, Crazy Mountain Basin in south-central Montana. Significance of postprotocingulum and mesial inflection of molar trigonids; Indication of `plesiadapiform' affinities; Comparison of features to P. unio.