Rivers of bone
In the badlands of Alberta, the Red Deer River cuts through layers of rock that hold evidence of ancient cataclysms. These are the Red Deer River bonebeds, vast deposits of dinosaur skeletons concentrated by the hundreds or thousands. Many of these sites are "monodominant," meaning they consist almost entirely of one species. These mass graves record herd life and catastrophic death during the Late Cretaceous Period.
The fossils are primarily found within the Dinosaur Park Formation, a layer of rock dating from 76.5 to 74.8 million years ago, and the younger Horseshoe Canyon Formation. At that time, this region was a low-lying coastal plain bordering the Western Interior Seaway, a vast inland sea. The environment was prone to seasonal tropical storms, similar to modern hurricanes, which caused widespread flooding. The leading hypothesis is that entire herds of dinosaurs, particularly horned dinosaurs (ceratopsians), were drowned while attempting to cross swollen rivers. The floodwaters then carried the carcasses downstream, eventually depositing them together in dense logjams of bone.
One of the most significant sites is the Hilda mega-bonebed, which is composed of at least 14 separate bonebeds of the dinosaur Centrosaurus apertus. This site, located west of Hilda, Alberta, covers an area of at least 2.3 square kilometers and is considered one of the largest dinosaur bonebeds in the world. Researchers estimate it contains the remains of thousands of individuals from a single massive herd that perished in a large-scale flood.
A graveyard of giants
The sheer density of fossils in these bonebeds is difficult to comprehend. In Dinosaur Provincial Park, a UNESCO World Heritage Site, seventeen Centrosaurus bonebeds and two Styracosaurus bonebeds have been documented. These sites have provided the first major evidence that some dinosaurs were gregarious, living in large herds. The composition of the herds often includes individuals of all age groups, from juveniles to adults, further supporting the idea of social groups being wiped out simultaneously.
While ceratopsians like Centrosaurus, Styracosaurus, and Pachyrhinosaurus are the most common inhabitants of these mass graves, they are not the only species. The Horseshoe Canyon Formation contains a famous bonebed dominated by the tyrannosaurid Albertosaurus sarcophagus. This suggests that apex predators may have congregated in groups. Evidence of scavenging is also present; shed teeth from tyrannosaurids are often found among the ceratopsian bones, indicating that predators feasted on the carcasses after the floodwaters receded. The orientation of the long bones in many bonebeds aligns with the direction of the ancient water flow, showing the taphonomic process of their watery demise.