What Is a Bonebed? Many Animals in One Layer of Rock

The term covers any sedimentary layer containing a concentration of bones from more than one individual, and the range runs from a handful of animals in a small area to deposits with thousands of individuals traceable across kilometres. Some contain a single species and some contain a mixture. Some hold articulated skeletons still in anatomical order and some hold disarticulated material sorted by size and shape. Those differences are the evidence, since the condition of the bones and their arrangement in the sediment record what happened between death and burial. A monospecific bed with articulated skeletons suggests a group of animals dying together and being buried quickly. A mixed bed of broken and abraded fragments suggests material accumulated over a long period and transported some distance. Reading the difference is the work, and the interpretation is never automatic.

Several distinct processes produce concentrations and each leaves characteristic signatures:

• •Mass mortality, where a group dies together in a flood, a drought, a volcanic ashfall or a toxic event, which produces single-species beds of similar preservation
• •Attritional accumulation, where animals die individually over years at the same place, typically a waterhole or a river bend, producing mixed ages and species
• •Hydraulic concentration, where moving water gathers bones into a channel or a bar and sorts them by density and shape
• •Predator or scavenger accumulation, where a den or a nesting site collects prey remains, recognisable by tooth marks and by the size range of the material
• •Trap sites, including tar seeps, fissures and sinkholes, where animals become stuck and further animals are drawn in
• •Reworking, where an older deposit is eroded and its bones redeposited, which mixes material of different ages into one layer and is a real hazard for dating

The reason these sites are valuable beyond sheer quantity is that they supply information a single skeleton cannot. A bed containing many individuals of one species allows variation within a population to be measured, which separates genuine species differences from individual variation and has repeatedly caused named species to be recognised as growth stages or as variants of one another. Growth series become available, since juveniles through adults appear together, and much of what is known about how dinosaurs changed as they grew comes from such sites. Sex differences can be investigated where they exist. Population structure emerges from the age distribution, and a bed dominated by juveniles implies something different from one with a normal spread. And where mass mortality of a single species is well supported, the site is evidence that the animals were together in life, which is one of the few routes to inferring social behaviour from fossils.

Excavating a dense accumulation is a different operation from recovering a single skeleton and it takes years rather than weeks. The standard approach maps every element in place before removal, recording position, orientation and the depth at which it sits, because that spatial information is the evidence for how the deposit formed and is destroyed the moment a bone is lifted. Grids are laid over the exposure and the site is worked systematically rather than towards the most attractive specimens, which is a discipline that early collecting conspicuously lacked and which is why some historically important sites yield less information than their size suggests. Large sites are worked over many field seasons by successive teams, with the exposure protected between them. The volume of material creates a long-term problem of its own, since a productive bed can supply more specimens than any institution can prepare, catalogue and store, and unprepared material from major excavations sits in collections for decades.

The inference from a monospecific bed to a herd is the most contested step in the field and it is not secure on its own. Animals can be concentrated after death by processes that never had them together in life, since a river carrying carcasses will sort and accumulate them, and a drying waterhole will draw solitary animals to one place over weeks. Repeated use of the same site over many seasons produces a single layer containing animals that never met. Distinguishing these requires careful sedimentology, an assessment of how much transport the bones show, evidence about how quickly burial happened and attention to whether the ages present match a living group or a sequence of separate deaths. Several famous sites have been reinterpreted on exactly these grounds, with claimed herds becoming accumulations, and the more cautious current practice treats social behaviour as a conclusion requiring several independent lines of support rather than one that follows from abundance.

A layer holding many individuals forms by mass death, by slow accumulation at one spot, by water sorting, by predators or by trapping, and the condition and arrangement of the bones distinguish them. Such sites supply growth series and population variation that single skeletons cannot, which has collapsed named species into growth stages. Inferring a herd from a single-species bed is the contested step, and several claimed herds have been reinterpreted.