One of the most common modes of preservation of ammonites in the Upper Cretaceous US Western Interior is in concretions. We examine an accumulation of ammonites from a single concretion in the lower Maastrichtian Pierre Shale of eastern Montana. The concretion is an oblate spheroid 50 cm in length and 26 cm in diameter, with its long axis parallel to the substrate. It contains approximately 90 ammonite specimens representing three species of Hoploscaphites including adults and juveniles. The concretion also contains other fauna, primarily bivalves and gastropods. A total of 33 ammonites, mostly adults, are concentrated in a cluster that spans 71 % of the length of the concretion (called the ''sculpture''). 3-D measurements of the ammonites in the sculpture reveal that (1) the shells dip at all angles, with a significant trend toward more horizontal from west to east; (2) the shells dip with a highly significant bias toward the east, suggesting a current from that direction; and (3) a highly significant number of the shells that are non-vertical face with their left side up. Most of the shells show lethal damage as indicated by missing pieces of body chamber. After settling to the bottom, the shells may have piled up against each other, creating a sediment trap. Other organisms such as scavenging gastropods may have been attracted to the site to feed on the stranded ammonite carcasses. The chambers of the ammonite phragmocones and even some of the body chambers are empty, suggesting relatively rapid burial. Oxygen and carbon isotopic analyses of the ammonite shells reveal that they preserve their original isotopic signature. The values of d 13 C of the carbonate cement in the concretionary matrix are much lighter than those in the ammonite shells and imply that cementation of the concretion occurred in association with the decomposition of organic matter.

The nautilid Eutrephoceras dekayi (Morton 1834) is relatively abundant in the lower Maastrichtian (Upper Cretaceous) Pierre Shale of east-central Montana. We analysed the morphology, size frequency distribution, and isotope composition of a large collection of 220 well-preserved specimens including hatchlings, juveniles and adults. The newly hatched shell is approximately 14 mm in diameter with a body chamber one-third whorl in angular length terminating in the nepionic constriction. Internally, the embryonic shell contains five septa. Juveniles are abundant and comprise twothirds of the sample whereas sub-adults, defined by the incipient flattening of the venter, are rare. Adults comprise approximately one-third of the sample and average 100 mm in diameter. The co-occurrence of newly hatched shells, small juveniles and adults suggests that the eggs were laid in the same area in which the hatchlings developed. Based on the excellent preservation of the juveniles, we conclude that they did not float into the area after death, but lived in the region, implying that this area served as a nursery for young animals. The calculated temperatures of the embryonic shells are similar to those of the post-embryonic shells and generally range from 16 to 18°C. Upon hatching, the nautilids probably followed a demersal mode of life and lived in well-oxygenated water ≤50 m deep. An examination of lethal injuries (puncture holes) suggests that all ontogenetic stages were equally vulnerable to predation. The proximity of the site to the Sheridan Delta suggests that the specimens were smothered by sudden pulses of sediment transported into the area by major storms.

Two species of scaphitid ammonites (Ammonoidea: Ancyloceratina) from the Upper Cretaceous (lower Maastrichtian) of the Western Interior of North America are described. Hoploscaph ites macer, n. sp., is medium size, with coarse ribs on the phragmocone, which become finer on the body chamber, and closely spaced ventrolateral tubercles. It occurs in the upper part of the Baculites baculus Zone and lower part of the overlying B. grandis Zone in the Pierre Shale in Montana, Wyoming, and Colorado, and in the Bearpaw Shale in Montana. Hoploscaphites crip tonodosus (Riccardi, 1983) is larger and more coarsely ornamented, including one or two rows of lateral tubercles on the flanks of the phragmocone. It occurs in the upper part of the Baculites baculus Zone and overlying B. grandis Zone in the Pierre Shale in Montana, Wyoming, Colorado, and possibly South Dakota, and in the Bearpaw Shale in Montana and Saskatchewan, Canada. Both species form part of an evolving lineage of Hoploscaphites that first appears in the Western Interior of North America in the middle Campanian.

FRONTISPIECE. Breaks along the Yellowstone River (background) exposing the Upper Cretaceous Pierre Shale on the Cedar Creek Anticline, Dawson County, Montana. This is one of the classic sites first explored by Ferdinand V. Hayden during the 1850s, and the source of some of the largest species of Hoploscaphites in North America. Photo by T. Linn. BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO.

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The discovery of a specimen of Rhaeboceras coloradoense Cobban, 1987, in the Baculites eliasi zone in east-central Montana is significant for three reasons: 1) it is the most geologically recent occurrence of Rhaeboceras; 2) it connects more closely than previous specimens the lineage between the genus Rhaeboceras Meek and Ponteixites Warren, its apparent smaller descendant; and 3) it significantly extends the geographical range of the species.

A third known specimen of the ammonite Rhaeboceras burkholderi Cobban, 1987, has been discovered in the upper part of the Pierre Shale of early Maastrichtian age (Late Cretaceous) Baculites eliasi zone of Dawson County, Montana. This finding extends both the geographical and stratigraphic ranges of the species. In addition, R. cedarense new species is described from the Baculites baculus/grandis zone of the same region, which extends the stratigraphic range of the genus as currently recognized. The new species is a small form of Rhaeboceras that appears to be closely related to and probably descended from R. burkholderi.

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