A dinosaur graveyard represents a compelling mystery in paleontology. These sites are dense concentrations of fossilized remains, suggesting a mass mortality that brought together dozens, hundreds, or even thousands of individuals in a single location. The volume of bones prompts scientists to investigate the ancient environmental forces powerful enough to cause such widespread death and burial. Understanding how these collections formed is fundamental to unlocking the secrets of dinosaur behavior and the dramatic events of the Mesozoic Era.
Defining the Dinosaur Bonebed
The scientific community refers to a dinosaur graveyard as a “bonebed,” defined by the high concentration of fossils originating from multiple individuals within a relatively small area. Paleontologists apply specific criteria, often requiring a minimum number of individuals (MNI) or a quantifiable bone density per square meter, to classify a site as a true bonebed. For example, some sites exhibit densities ranging from 100 to 300 bones per square meter across a broad area. A bonebed provides a snapshot of an ancient population, contrasting sharply with the more common discovery of a single, scattered skeleton. The characteristics of the bonebed, such as the condition of the bones and the surrounding sediment, offer clues about the event that brought these creatures to rest.
Formation by Catastrophic Events
One primary mechanism for bonebed formation involves rapid, catastrophic events that cause mass mortality followed by immediate burial. These scenarios often involve sudden natural disasters that overwhelm large populations, leaving little time for the remains to be scattered or consumed by scavengers. The Centrosaurus bonebed in Alberta, Canada, is a classic example, where thousands of horned dinosaurs likely perished simultaneously in a flash flood or a tropical storm surge. Such events typically result in bones that are relatively well-preserved, showing minimal evidence of weathering or extensive scavenging, though they may be disarticulated due to the force of the water. The debris flows or sudden inundations that trapped the herds provided the massive influx of sediment necessary for quick burial, which is the first step in the fossilization process.
Formation by Attritional Accumulation
Bonebeds can also form through a slower process known as attritional accumulation, where individual remains gather over long periods rather than in a single, sudden event. This formation often occurs in environments that naturally concentrate carcasses, such as river bends, lake margins, or watering holes that become deathtraps during prolonged droughts. The remains of animals that died naturally over decades or centuries are steadily introduced into the same sedimentary layer. These slower accumulations are identifiable by specific taphonomic features, which are the physical changes that occurred to the bones between death and final burial. Bones in attritional sites often show significant signs of weathering, abrasion from being rolled in water, and heavy fragmentation. The resulting bonebed typically contains a mix of species and age groups, with the bones highly disarticulated and scattered.
Scientific Insights from Mass Burial Sites
The value of mass burial sites lies in the unique insights they offer into the biology and ecology of extinct species. Unlike isolated skeletons, bonebeds provide a large sample size of a single population, allowing paleontologists to study demographic data like age and sex ratios. This collective data is essential for reconstructing the social behavior of dinosaurs, such as confirming the existence of large migratory herds in species like Pachyrhinosaurus. Discoveries like the multiple mass tyrannosaur graves in North America provide evidence that these apex predators may have been social, rather than solitary, hunters. By analyzing the mix of species and the surrounding geological context, scientists gain a comprehensive understanding of the ancient ecosystem and the environmental forces that shaped it.