Fossil mammals from the "Mesaverde" Formation (late Cretaceous, Judithian) of the Bighorn and Wind River basins, Wyoming : with definitions of late Cretaceous North American land-mammal "ages". American Museum novitates ; no. 2840

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Date
1986
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New York, N.Y. : American Museum of Natural History
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Abstract
"Mammalian faunas are documented for the first time from the 'Mesaverde' Formation (Late Cretaceous) of Wyoming. Nonmarine fossils from the Bighorn and Wind River basins indicate a Judithian (revised definition) 'age' for the assemblages through comparisons with approximately contemporaneous faunas of the Judith River (Montana) and Oldman (Alberta) formations. Three previously unknown genera are recognized, although not named herein, from the 'Mesaverde' Formation (Multituberculata, new genus and species, unidentified; Dryolestidae, new genus and species, unidentified; and Falepetrus barwini). Three new species of previously described genera (Alphadon sahnii, A. attaragos, and Paranyctoides megakeros) are named. All species-level taxa (16 total) except Alphadon lulli are reported from Wyoming for the first time, and new records involve both geographic and geologic range extensions. The temporal record of the dryolestid appears relictual, being previously unknown from post-Jurassic strata in North America. Taxonomic comparisons suggest that the Judithian mammalian fauna of what was then coastal parts of the western interior was essentially homogeneous geographically, at least from southern Alberta to central Wyoming. Nonmarine mammalian assemblages from the Oldman, Judith River, and 'Mesaverde' formations correlate temporally with more easterly marine rock units that lie within (or within four zones above) the Baculites gregoryensis (cephalopod) Zone, part of the standard zonation of Upper Cretaceous rocks of the North American western interior. The upper part of the Red Bird Silty Member of the Pierre Shale at Redbird, Wyoming, holds the largely endemic invertebrate macrofaunal assemblage characteristic of the B. gregoryensis Zone as well as a newly described planktonic foraminiferal assemblage. The microfossils allow correlation to the upper Taylorian and/or lower Navarroan foraminiferal stages of the Gulf Coast. This, in turn, correlates approximately to the Campanian-Maastrichtian stage boundary at Gubbio, Italy, and European stratotypic sections. Judithian mammal faunas of the Rocky Mountains, therefore, must be younger in age (i.e., late Campanian and/or early Maastrichtian) in terms of the European stages than usually is considered on the basis of molluscan zonations within the North American western interior (e.g., well within the Campanian). Judithian mammals from the Rockies probably lived about 74-76 million years ago during the late part of geomagnetic Polarity Chron 33 or the early part of Polarity Chron 32, during the regressive phases of the Claggett cyclothem as recognized for the western shoreline of the Western Interior Seaway. They correlate stratigraphically with the lower part of the Aquilapollenites quadrilobus palynomorph Interval Zone of the northern Rockies. Aquilan (oldest), Judithian, and Lancian (youngest) provincial North American land-mammal 'ages' are redefined for the Late Cretaceous nonmarine sequence of the western interior from an older stage concept; the 'ages,' based upon species-level mammalian assemblages, are modeled after the system used successfully for nonmarine Cenozoic faunas of North America. An Edmontonian 'age' (previously used as a stage term, chronologically intermediate between the Judithian and Lancian) is probably identifiable as a discrete interval of geologic time, but is not yet defensible on the basis of mammalian assemblages. Therefore, it is not redefined as a land-mammal 'age.' We concur with the interpretation that the Djadokhta Formation of southern Mongolia and the Judith River and Oldman faunas of North America are essentially of the same age. This compresses the faunas of the Barun Goyot and Nemegt formations of southern Mongolia, plus that of the poorly known, nearby Bugeen Tsav locality, into an interval of time equivalent to the Judithian and/or Lancian North American land mammal 'ages' and, most probably, to the Maastrichtian stage as typified in western Europe"--P. [1]-2.
Description
68 p. : ill., maps ; 26 cm.
Includes bibliographical references (p. 58-68).
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