Stratigraphy, sedimentology, and taphonomy of the albertosaurus bonebed (upper Horseshoe Canyon Formation; Maastrichtian), southern Alberta, Canada

摘要

The Albertosaurus bonebed provides evidence for the mass-mortality of at least 12 Albertosaurus. Albertosaurus, other vertebrates, and trees are concentrated on upper point bar surfaces of a small meandering paleochannel. Throughout the field area, Albertosaurus, other vertebrates, and coalified plants are also preserved patchily in overbank mudstones at the same horizon. Although the bonebed is dominated by Albertosaurus, there are also sparse macrofossil remains of Hypacrosaurus and Albertonykus. The basal lag yields vertebrate microfossils containing at least 19 additional taxa. Freshwater invertebrates are preserved at and below the base of the paleochannel. Large elements are over represented at the site, whereas smaller elements are under represented, suggesting hydraulic sorting. The taphonomic signature of the site suggests that death, disarticulation, and burial took place within one year. We conclude that one or more storms of large intensity downed trees and temporarily drowned the paleolandscape, ultimately killing the tyrannosaurids directly or indirectly. Logs and carcasses were reworked on a point bar during the waning stages of the storm and over subsequent years. Scavenging and breakage of remains by large carnivores was probably inhibited by the mixed log and bone jam. Remains of smaller scavengers (small theropods and snails) are plentiful suggesting these forms were more successful at accessing remains. Juvenile Albertosaurus are under represented at the site suggesting the possibility of age segregation. Because the group of Albertosaurus may have been forced together by rising floodwaters, it is not possible to assess the degree to which the taxon was gregarious from these data alone.