Fossil trackways have shown paleontologists that some sauropod dinosaurs moved together in herds. But how were their herds organized? Were they made up only of particular age groups or were individuals of different ages all mixed together? In a new paper in Palaeogeography, Palaeoclimatology, Palaeoecology, scientists Timothy Myers and Anthony Fiorillo discuss two different sites that suggest that at least some sauropods segregated their herds by age.
Before discussing the fossil sites in detail, Myers and Fiorillo review some of the problems in inferring behavior from fossil trackways alone. A photo included in the paper, for instance, shows the tracks of a human next to those of a grizzly bear. Was this person walking alongside gentle Ben? No, the tracks had been made hours apart. The same principle holds for fossil tracks. The presence of tracks made by two individuals in the same place does not necessarily mean they were there at the same time. Further evidence would be required to show this was true.
There can be difficulties with evidence from bone beds, too. The fossils from Mother's Day Quarry in Montana are from a herd of sauropod dinosaurs that may have died during a drought. What is strange, however, is that nearly all the bones are from juvenile and sub-adult animals. Immature animals typically suffer higher death rates than adults during droughts, but the question was whether this site represents a herd of immature animals or simply the immature portion of a larger herd. The lack of adults and the fact that the bones had not been transported after the animals died led Myers and Fiorillo to suggest that the Mother's Day Quarry site represents an actual herd of immature animals separate from adults.
The Big Bend site in Texas differs in that it consists of three juvenile Alamosaurus that died and were buried together. Like the Montana site, this bone bed represents a single event rather than the accumulation of multiple skeletons over time. The fact that no adult bones are found and that no accumulations of multiple Alamosaurus adults are known suggests that these dinosaurs herded together when young but became more solitary as they became mature.
So what do these two sites mean? Factors that might potentially bias the formation of bone beds must be kept in mind, but they appear to suggest that, in at least some sauropods, juvenile individuals formed groups separate from herds of mature individuals. This may have to do with size. The adults were much, much larger than immature individuals and may have had different dietary needs. This may have segregated herds by age with the younger animals grouping together for protection. This type of age segregation was probably not present in all sauropods, but it may have been prevalent among some of the largest species.