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The recovery and subsequent prolific radiation of mammals in the northern Western Interior of North America following the Cretaceous–Paleogene (K–P) boundary is well documented in rocks attributed to the Puercan Land Mammal Age. The most complete Puercan record is that of the Tullock Formation, which crops out widely in Garfield and McCone counties, eastern Montana. The Tullock Formation overlies the dinosaur-bearing Hell Creek Formation and consists of a stratigraphic series of channel and overbank deposits from which well-preserved Puercan faunas have been collected. These channel deposits are typically bracketed by widespread coal beds. The IrZ- and Z-coals mark the base of the Puercan at the K–P boundary as defined by the highest appearing local occurrences of in situ dinosaur fossils, the highest stratigraphic occurrence of Cretaceous pollen, and an anomalously high concentration of the element iridium (Ir). The IrZ-Coal is overlain sequentially by the Z-, HFZ-, Y-, W-, and U-coals.Fossil vertebrates of earliest Puercan age (Pu0 or Pu1) occur in sandstones of the Hells Hollow Channel between the IrZ-and HFZ-coals in Garfield County and below the Z-Coal in sandstones of the Bug Creek Channels, McCone County. Late Puercan (Pu2/3) fossil vertebrates are abundant between the Y- and W-coals in sandstones of the Garbani Channel in Garfield County, and in sandstones of the Purgatory Hill quarry in McCone County. Earliest Torrejonian (To1?) fossil vertebrates of the Mosquito Gulch localities, Garfield County, occur in sandstones of the Farrand Channel between the W- and U-coals.Magnetostratigraphies developed on sections at Billy Creek, Hauso Flats, Biscuit Butte, Bug Creek, and Purgatory Hill indicate that the IrZ- and Z-coals, the fossil vertebrate-bearing sandstones of the Hells Hollow and Bug Creek channels, and rocks up to midway between the HFZ- and Y-coals are contained in a zone of reversed polarity, correlated here with chron C29r. The Y through W coals, as well as the fossil vertebrate-bearing sandstones of the Garbani Channel and Purgatory Hill, are contained in an overlying zone of normal polarity, correlated with chron C29n. Rocks immediately above the W-Coal through the U-Coal, which include the Farrand Channel, are in a zone of reversed polarity, correlated here with chron C28r. The top of the section, just above the U-Coal, is in the uppermost zone of normal polarity, tentatively correlated here with the base of chron C28n.40Ar/39Ar dating of single crystals of sanidine separated from bentonites in the IrZ-, Z-, HFZ-, W-, and U-coals has yielded high-precision ages with standard errors of 0.1%. Replicate analyses of single crystals of sanidine yield weighted mean ages for bentonites in the following coals: IrZ, 65.16 ± 0.04 Ma; Z, 65.01 ± 0.03 Ma; HFZ, 64.77 ± 0.06 Ma; W, 64.11 ± 0.02 Ma; and U, 63.90 ± 0.04 Ma. These ages provide detailed calibration of Puercan through earliest Torrejonian (PuO, Pu1, Pu2, Pu3, and ?To1) land mammal ages and provide crucial calibration for the nonmarine K–P boundary and the Early Paleogene portion of the geomagnetic polarity time scale.

Six egg-filled depressions discovered in the Upper Cretaceous Anacleto Formation (Campanian) of Patagonia, Argentina, and interpreted as dinosaur nests, provide the only known evidence of titanosaurid sauropod nest construction. These nest trace fossils show truncation of sedimentary structures as well as differences in texture between the host substrate and in-filling sediment. Titanosaurid sauropods excavated and laid eggs in open nests rather than burying clutches in sediment. In addition, this paper establishes criteria for definitive recognition of excavated nests in the stratigraphic record.

Studies of key and newly discovered sections of the Upper Cretaceous Djadokhta Formation along the southern margin of the Ulan Nur Basin allow a new subdivision based on lithology. The formation and its members were mapped at both Bayn Dzak, an area that includes the Flaming Cliffs, and Tugrugyin Shireh, an area about 50 km to the northwest of Bayn Dzak. Stratigraphic sections at both localities were remeasured. The considerably enlarged formation comprises a lower Bayn Dzak Member, dominated by moderate reddish orange sands with subordinate mudstone units, and an upper Tugrugyin Member, composed of pale orange to light gray sands. Investigations of key sections at Tsonzh and Alag Teer demonstrate the presence of transitional mudstone lenses between these members within the Djadokhta Formation. Two distinct, sandy, sedimentologic facies are recognized in both members. Cross-bedded intervals, occasionally exhibiting wind-ripple cross lamination, document the presence). 2 NO. 3498 AMERICAN MUSEUM NOVITATES of a Cretaceous dunefield in the Ulan Nur Basin. Structureless intervals are interpreted to represent wet sandy fluvial deposits and debris flows that moved down the dune faces. In the Bayn Dzak Member, lenses of brownish mudstone are interpreted to represent interdune de-position in shallow ponds by fluvial action. Fluvial action is also represented in the Bayn Dzak Member by beds of caliche, which contain conglomerate at the base but fine upward into limestone. The vertebrate fauna from the Djadokhta Formation is summarized. Although the Bayn Dzak fauna lived somewhat earlier than that from Tugrugyin Shireh based on the superposition of the members, it is not clear how much earlier. The fauna from the Djadokhta Formation has previously been assigned ages from Cenomanian to earliest Maastrichtian. New magne-tostratigraphic data document a sequence of normal and reversed magnetozones through the Bayn Dzak Member up into the basal Tugrugyin Member. The presence of reversed magne-tozones establishes that the sediments containing the faunas were probably deposited after C34n. The quick stratigraphic succession of normal and reversed magnetozones suggests, but does not clearly establish, that the sediments may have been deposited during the rapid sequence of polarity changes in the late part of the Campanian between about 75 to 71 Ma.