THE BELLY RIVER GROUP
2. The Oldman Formation
The Oldman Formation is divided into three informal units, a lower muddy unit, a middle sandy unit, and an upper muddy unit. The middle Sandy unit is termed the Comrey Sandstone Zone. The sandstones are resistant, and cliff forming, and are responsible for the spectacular Milk River Canyon of the Milk River Conservation area.
All three of these units contain vertebrate microfossil localities, most containing dinosaur eggshell fragments and small clam and snail shells along with the vertebrate microfossils. Peng Jiang-hua sampled about twenty separate sites through this sequence, and was able to trace the changes in the faunal community structure through these beds in relation to environmental changes. Low in section, where the localities were deposited near the coastline, vertebrate microfossil localities are dominated by garfish, the ray Myledaphus, and crocodiles. In the higher localities, which were deposited relatively far from the coastline, these animals, although still present, are relatively rare. The localities are instead dominated by teleost fish (represented primarily by vertebrae), salamanders and frogs. These changes give some indication of the changes in community structure across the coastal plane. In addition, Peng et al. (2001) identified the oldest occurrence of Troodon in the western interior of North America. This small theropod is not present in older localities, but continues up through the rest of the sequence to the end of the Cretaceous. Its distributional pattern is likely a result of an immigration event, with Troodon moving into southern Alberta from Asia, were its relatives are present in much older beds.
Only a few articulated specimens of dinosaurs have been collected in the Oldman Formation in the Manyberries area. No identifiable articulated dinosaur remains have been collected from the Lower muddy unit. The Comrey Sandstone interval has yielded a well-preserved Brachylophosaurus skeleton. The upper Muddy unit has proved to be the most productive for vertebrate macrofossils. A partial, but well as a Saurornitholestes skeleton was collected by the Tyrrell Museum, as was a disarticulated skull of Centrosaurus.
The general composition of the faunal assemblage in the Manyberries area is similar to that in Dinosaur Provincial Park, but differences are present in the relative abundance of the different kinds of animals, particularly turtles. Indeed, some turtles present in Dinosaur Provincial Park are absent from Manyberries area. In this region, the most abundant turtles are trionychids, Adocus and Neurankylus, while in Dinosaur Park, Adocus and Neurankylus are very rare, but Plesiobaena and Boremys are more abundant. Two types of trionychids present in Dinosaur Provincial Park are absent in the Manyberries area, and one turtle, the large macrobaenid, is absent in the Manyberries area.
Since the upper part of the upper muddy unit of the Oldman Formation in the Manyberries area was laid down at the same time as the lower part of the Dinosaur Park Formation in Dinosaur Provincial Park, these differences are best interpreted as difference in distribution of vertebrates across southern Alberta at that time. These differences indicate the presence of differences in climate of the Brooks area compared to the Manyberries area during the Cretaceous. Judging from the sediments, the Brooks area was generally wetter, had larger rivers with more constant flow.
© 2007 The Southern Alberta Dinosaur Research Group.