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FIGURE 10.11 Sauropod trackways in the Late Jurassic Morrison Formation near La Junta, Colorado, showing parallelism and indicating probable herding behavior.

FIGURE 10.11 Sauropod trackways in the Late Jurassic Morrison Formation near La Junta, Colorado, showing parallelism and indicating probable herding behavior.

tracks indicated that later individuals stepped on some of the immediately preceding tracks.

Other sauropod trackways from the Upper Jurassic of Portugal show that a group of seven juveniles (based on the small sizes of their footprints) were accompanied by three larger sauropods, strongly suggesting a family structure. Moreover, the similar track sizes of the seven smaller individuals advocate that they represented the same age range and may have been hatched and raised from the same clutch. Such information helps with estimations of juvenile mortality by comparing the number of eggs within sauropod clutches to trackways of probable juveniles. A presumed monospecific track assemblage could also give an independent measure of population structure. However, such interpretations have at least one caveat: the adult tracks preserved on the same horizon may represent stratigraphically younger tracks that penetrated down to the same, older layer as the juvenile tracks (see Fig. 14.8). Nevertheless, trackways left by juvenile sauropods are becoming more recognized in the geologic record, which then help in better definition of sauro-pod populations. Some of the best examples of juvenile sauropod tracks are in Cretaceous rocks of South Korea, where more than 100 such trackways have been discovered.

Of course, just because animals are moving in the same direction does not necessarily qualify as evidence of herding. For example, a linear geographic barrier, such as a shoreline or a cliff, could have restricted the animals' lateral movement over a long period of time. An argument against this critique is that the variation in footprint sizes suggests that the population was composed of both younger and older individuals of the same species. If a geographic barrier equally affected the movement of all animals through an area, tracks of other species might be expected as a source of track variation. Other sauropod trackways that were apparently made by the same type of sauropod at approximately the same time should provide for more testing of herding hypotheses.

Consequently, bone-bed data, nesting grounds, and trackway evidence provide a compelling argument in favor of sauropodomorph sociality, although varying degrees of sociality for each species were likely. Sociality certainly conferred a major advantage in the form of added protection against large, theropod predators, despite already-massive sizes for some individual sauropodomorphs. It also allowed groups to explore for and exploit food resources together. These animals most likely aided each other's survival, thus ensuring reproductive success and thereby passing on their traits to the next generation of sauropodomorphs.

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