STRATIGRAPHIC DISTRIBUTION OF HADROSAURINE DINOSAURS IN THE JUDITH RIVER FORMATION OF NORTHCENTRAL MONTANA

Hadrosaurine dinosaurs are well known from Campanian terrestrial deposits in Montana and Alberta. However, much of their diversity derives from the Two Medicine Formation of western Montana and Belly River Group of Alberta, whereas exposures of the Judith River Formation in central and eastern Montana have generally yielded less diagnostic material. Recent fieldwork has focused on Kennedy Coulee in northcentral Montana, which exposes part of the Judith River Formation corresponding to the upper Foremost Formation and lower Oldman Formation of the Belly River Group of Alberta. The terms for the Albertan members are used here, as members of the Judith River Formation have not been formally defined. The lower exposures of Kennedy Coulee consist of the Taber Coal Zone and Herronton Sandstone Zone of the Foremost Formation, and the upper exposures consist of the lower pale beds of the Oldman Formation. The top of the uppermost major coal of the Taber Coal Zone, the Marker A Coal, is used as a regional correlation surface. All outcrop in Kennedy Coulee is stratigraphically higher than the lowermost Two Medicine Formation, which yields both Gryposaurus and Acristavus, and lower than the Comrey Sandstone Zone of the Oldman Formation, which yields Brachylophosaurus.

Brachylophosaurini (clade including Acristavus, Brachylophosaurus + Maiasaura) material has been recovered throughout Kennedy Coulee, with a dentary in the Herronton Sandstone Zone, and all other material in the lower Oldman Formation. Heights above the Marker A Coal are: isolated dentary 0.4 m; partial skull and skeleton not diagnostic to species level 15 m; fairly complete skull and skeleton of a new species of brachylophosaurin 17.5 m, and an isolated nasal 28 m. The isolated nasal bears similarities to the new Kennedy Coulee species as well as Brachylophosaurus canadensis. Gryposaurus material is found in the lower Oldman Formation of Kennedy Coulee, and includes a monodominant bonebed of at least 8 individuals 26.6 m above the Marker A Coal. Brachylophosaurini material stratigraphically brackets the Gryposaurus material, indicating that these two clades coexisted in the Kennedy Coulee area. There is no evidence for these clades coexisting in any other part of the Judith River Formation or Belly River Group, although they do coexist in the Two Medicine Formation.