by other causes, such as a slowly rising water level during an extended flood. Palaeopathology In 2009, researchers hypothesized that smooth-edged holes found in the fossil jaws of tyrannosaurid dinosaurs such as Albertosaurus were caused by a parasite similar to Trichomonas gallinae, which infects birds. They suggested that tyrannosaurids transmitted the infection by biting each other, and that the infection impaired their ability to eat food. In 2001, Bruce Rothschild and others published a study examining evidence for stress fractures and tendon avulsions in theropod dinosaurs and the implications for their behavior. They found that only one of the 319 Albertosaurus foot bones checked for stress fractures actually had them and none of the four hand bones did. The scientists found that stress fractures were "significantly" less common in Albertosaurus than in the carnosaur Allosaurus. ROM 807, the holotype of A. arctunguis (now referred to A. sarcophagus), had a deep hole in the iliac blade, although the describer of the species did not recognize this as pathological. The specimen also contains some exostosis on the fourth left metatarsal. In 1970, two of the five Albertosaurus sarcophagus specimens with humeri were reported by Dale Russel as having pathological damage to them. In 2010, the health of the Dry Island Albertosaurus assembly was reported upon. Most specimens showed no sign of disease. On three phalanges of the foot strange bony spurs, consisting of abnormal ossifications of the tendons, so-called enthesophytes, were present, their cause unknown. Two ribs and a belly-rib showed signs of breaking and healing. One adult specimen had a left lower jaw showing a puncture wound and both healed and unhealed bite marks. The low number of abnormalities compares favourably with the health condition of a Majungasaurus population of which it in 2007 was established that 19% of individuals showed bone pathologies. Palaeoecology Most fossils of Albertosaurus sarcophagus are known from the upper Horseshoe Canyon Formation in Alberta. These younger units of this geologic formation date to the early Maastrichtian stage of the Late Cretaceous Period, 70 to 68 Ma (million years ago). Immediately below this formation is the Bearpaw Shale, a marine formation representing a section of the Western Interior Seaway. The seaway was receding as the climate cooled and sea levels subsided towards the end of the Cretaceous, exposing land that had previously been underwater. It was not a smooth process, however, and the seaway would periodically rise