What Is a Bone Bed? Hundreds of Animals in a Single Layer

Several quite different processes produce a dense accumulation, and distinguishing between them is the central question at any such site. A mass death event kills many animals at once, through drought around a shrinking waterhole, a flood, a volcanic ash fall, a toxic bloom or entrapment in mud, and produces a layer where the individuals died together. An attritional accumulation builds up over a long period at a place where animals die repeatedly or where carcasses collect, such as a river bend, a cave entrance or a predator trap, and the animals in it never met each other. A hydraulic concentration happens where moving water sorts and gathers bones that were already skeletal, depositing them together according to size and shape regardless of where the animals died. Each leaves different evidence, and reading it is the work of taphonomy, the study of what happens between death and discovery, which has become a substantial discipline precisely because the answer changes what the site means.

Excavators record a specific set of observations before removing anything, because the arrangement carries more information than the bones alone:

• •Articulation, meaning whether bones remain in life position, which indicates rapid burial and little transport
• •Orientation, since bones aligned in a common direction indicate water flow and randomly oriented ones do not
• •Sorting by size and density, which is the signature of hydraulic concentration rather than of animals dying together
• •Age profile, where a range from juveniles to adults matching a living population suggests catastrophic death and a profile weighted to the very young and very old suggests gradual attrition
• •Weathering stage, since bones exposed for years before burial crack and flake in a documented sequence, and a mixture of stages indicates accumulation over time
• •Tooth marks, breakage patterns and shed predator teeth, which indicate scavenging and can identify who was feeding at the site

Certain sites have shaped how dinosaurs are understood. Monospecific bone beds containing many individuals of one horned dinosaur species in Alberta and Montana are the main evidence for herding behaviour in those animals, since attritional explanations struggle to account for so many of one species and nothing else, and a flood or drowning interpretation is generally favoured. Dinosaur Provincial Park in Alberta contains many such beds within a small area and has supplied a large fraction of known specimens from its period. The Cleveland-Lloyd Quarry in Utah is unusual for containing a great excess of predators over prey, which has been explained as a predator trap where animals became mired attempting to reach trapped carcasses, though the interpretation has been repeatedly revised. Hadrosaur nesting sites in Montana produced evidence about growth and parental care from the remains of nests, eggs and juveniles together. Each of these required the arrangement of the bones to be recorded before excavation, which is why modern digs map every element in three dimensions.

A dense single-species accumulation is scientifically valuable in ways that isolated specimens are not. It provides a population sample, allowing variation within a species to be measured, which has repeatedly shown that features previously used to separate species are individual or age variation, and several named dinosaur species have been sunk on that basis. It permits growth series to be assembled, showing how an animal changed from hatchling to adult, which produced the substantial revision of several familiar dinosaurs when juveniles turned out to have been named as separate species. It supports statistical work on sexual dimorphism, disease and injury frequency. And it supplies context, since the associated sediments, plants, pollen and other animals describe the environment in which the population lived. The cost is that excavation is slow and enormous, that most material remains unprepared in storage for decades, and that a site can be destroyed by careless digging in a way that no amount of later laboratory work can recover.

A dense layer of bones can mean animals that died together, animals that accumulated one at a time over centuries, or bones gathered by moving water, and the arrangement rather than the bones distinguishes them. Articulation, common orientation, sorting by size, age profile and weathering stages each point one way. Single-species beds are the main evidence for herding and supply the population samples that collapse over-named species.