Titanosaurid trackways from the Upper Cretaceous of Bolivia: evidence for large manus, wide-gauge locomotion and gregarious behaviour

A preliminary study of the Humaca dinosaur tracksite, in the Departamiento Chuquisaca (Bolivia), has revealed 11 parallel trackways of subadult sauropods travelling to the WNW as a group. The nearby Cal Orcko site preserves more variable trackway orientations produced by larger individuals. The Humaca trackways are interpreted as evidence of social behaviour among small titanosaurids. Together with the Toro Toro site it represents one of the few Late Cretaceous examples of evidence of sauropod herding. All three Bolivian sites provide useful evidence on the morphology of the titanosaurid manus and pes, and on trackway gauge. When considered in conjunction with trackway data from another Maastrichtian titanosaur tracksite, near Fumanya (Spain), it appears that titanosaurids were slightly to moderately wide-gauged as adults, with a large manus (manus-pes ratio, or heteropody about 1:2). Small individuals (sub-adults) may have been narrow-gauge in some cases, becoming wider-gauged as adults. The newly described Humaca and Cal Orcko sites also reveal an abundance of theropod tracks of various sizes, suggesting a theropod-sauropod, or ‘saurischian’ dominated assemblage as also recorded at Toro Toro. The detailed context and preservation of the sauropod and theropod tracks at all three sites are different and require further study, but at the Cal Orcko site indicate a well-developed perennial lake basin with a rich aquatic fauna. Other tracksites are known in the region, which has rich ichnological potential.