Why Are There So Many Skeletons in One Place? Reading a Mass Death

A bonebed is a deposit containing a concentration of fossil bone far above what the surrounding rock holds, and the useful ones contain many individuals of the same species. They are classified by how the bones sit, since bones still articulated in life position indicate little transport, bones disarticulated but associated indicate some disturbance, and bones sorted by size and shape indicate that water moved them and separated them by how easily each piece was carried. The proportion of each skeletal element present is diagnostic, because a natural skeleton has a known number of each bone and a deposit containing far too many of one and none of another has plainly been winnowed. All of that is worked out before anything is said about how the animals died.

Several quite different processes produce a concentration of bone:

• •A single catastrophic event killing a group at once, including flood, drought, volcanic ash or toxic water
• •Gradual accumulation at a place animals returned to, such as a waterhole during repeated dry seasons
• •Hydraulic concentration, where flowing water gathered bones from a wide area into one hollow
• •A predator trap, where carcasses attracted predators that then became trapped themselves
• •Winnowing, where fine sediment was removed and only the bone was left
• •Reworking, where an older deposit was eroded and its bones redeposited with newer ones

A deposit containing many individuals of one species is frequently reported as evidence that the animals lived in groups, and the inference is weaker than it sounds. The animals must be shown to have died together rather than accumulated over time, which requires evidence that the deposit represents a single event, and that evidence is frequently absent. A waterhole visited by solitary animals over centuries produces the same concentration as a herd killed at once. Age distribution helps, since a deposit containing individuals of every size suggests a mixed-age group rather than a single cohort, and several deposits do show that pattern. Trackway evidence showing many animals moving together in the same direction at the same time is stronger and independent. The best cases combine several lines and remain arguments rather than demonstrations.

Excavating a dense bonebed poses practical problems that a single skeleton does not. Bones overlap and interlock, so removing one may require removing several above it, and the order has to be recorded because the arrangement is the evidence. Mapping is done by gridding the surface and plotting every element with its orientation, since alignment of elongated bones records current direction if water moved them. Material is removed in blocks encased in plaster where bones are too closely packed to separate safely in the field, and the separation happens in a laboratory over months or years. Quarries at major sites have been worked for decades across successive field seasons, with maps maintained continuously, and several have produced tens of thousands of specimens. The backlog of unprepared material from such sites is a recognised problem across the discipline.

Regardless of how they formed, these sites are enormously valuable for reasons unconnected to social behaviour. A large sample of one species allows variation within it to be measured, which is the only way to distinguish individual differences from genuine differences between species, and that has led to several supposed species being reidentified as growth stages or variants of another. Growth can be reconstructed by arranging individuals by size and examining bone microstructure. Rare elements missing from isolated skeletons turn up when hundreds of individuals are represented. Disease and injury frequencies can be estimated. And the sheer quantity of material means specimens are available for destructive analysis that a unique skeleton would never permit, which is where a great deal of the chemical and histological work is done.

A bonebed concentrates fossil bone far above background, and how the bones sit tells you how much water moved them before anything is said about how the animals died. Catastrophes, repeated visits to a waterhole and hydraulic gathering all produce the same concentration. Many individuals of one species allows variation to be measured, which has reidentified supposed species as growth stages.