The view that sauropodomorphs were gregarious animals, and that they moved about together in at least family units, if not herds, has gained popularity recently, exemplified by artistic depictions of great migrating herds of sauropods. In this case, a popular idea about dinosaurs also may be correct. Most of the data supporting the idea of herding behavior in these animals, which as individuals could have
outweighed a family unit of elephants, is based on trackway evidence. However, taphonomic analyses of sauropodomorph assemblages and the previously-mentioned nesting horizons of titanosaurs are independent lines of evidence that also support the probability of sociality for some sauropodomorphs. Taken collectively, the data indicate that sauropodomorphs were more likely to have lived in groups than not. This assertion can be tested in individual cases but is now generally accepted by many paleontologists who study sauropods.
The aforementioned nesting horizons of titanosaurs argue strongly for these sauropods having participated in social groups for their brooding of young, similar to what has been hypothesized for some ornithopods (Chapter 11). The close proximity of the nests, the sheer number of eggs, and their abundance on different horizons in the same small area reflect its revisiting in different nesting seasons by large numbers of sauropods of the same species. Although only one locality so far has body fossil evidence to corroborate a sauropodomorph nesting ground, this example provides a model for testing in other places, whether their reproductive and brooding behavior was:
1 more typically solitary;
2 based on family units; or
3 involved more than one family unit living in the same area at the same time.
Taphonomy also gives an insight into sauropod sociality through the examination of monospecific bone beds. Just as in some theropod species, such as Coelophysis baurii (Chapter 9), a monospecific assemblage of sauropods provokes the question, "How did only bones from many individuals of the same species end up in the same place at the same time?" For example, a bone deposit of the Middle Jurassic cetiosaurid Patagosaurus fariasi of Argentina is composed of bones representing five individuals, including both juveniles and adults. The current answer to this question is that the assemblage may represent a family that died and was buried together during a sudden event such as a river flood. Although adherence to only one hypothesis is risky, all other answers would be far more complicated than this scenario. As a result, it is the one that has been conditionally accepted for the cetiosaurid deposit and similar monospecific dinosaur bone beds. Two adult Camarasaurus skeletons associated with a juvenile of the same species, in the Upper Jurassic of Wyoming, provide another case of a possible family structure, and assemblages composed of only Alamosaurus bones from the Upper Cretaceous are also cited as evidence of gregariousness in sauropods.
Of course, the large number of sauropodomorph trackways documented worldwide constitutes excellent additional evidence for their social behavior. Multiple trackways on the same horizon are good clues as to how some species of sauropods traveled together or otherwise had preferred directions of travel. For example, one trackway horizon in Upper Jurassic rocks of southeastern Colorado shows five equally-spaced and parallel sauropod trackways, which is strong evidence supporting the hypothesis that they were traveling together as a herd (Fig. 10.11). This hypothesis is held up by the coincident direction of the trackways, which varied in harmony with one another, and the regular spacing between the trackways. Placing a minimum distance between one another when traveling in groups is a behavior observed in large modern animals, such as elephants. The preservational condition of the tracks also is similar, indicating that they were all formed within a short span of time before the character of the substrate changed. Another trackway from the Lower Cretaceous of Texas, discovered by Roland Bird (Chapter 3), shows that as many as 30 sauropods, seemingly of the same type (species), were moving in the same direction over a relatively narrow area. Some overlap of the
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