Colossal dinosaurs: discoveries and palaeobiology of south American titanosaurs

Abstract

The number of named titanosaurs has increased dramatically in recent years, with roughly 50 valid species from South America alone. Preservation of relatively complete skeletons in huge sauropods is difficult because its bones suffer a strong disarticulation processes before the burial. This is a consequence of both extrinsic or environmental and intrinsic or biological factors. For instance, in southern Mendoza Province (Argentina), three partially skeletons of an unnamed new titanosaur species show exceptional preservation andare related with both type of factors. Specimen UNCUYO-LD 303 has almost complete axial skeleton (from the first cervical to the last caudalvertebrae, but do not have skull). In a second specimen UNCUYO LD 309, a partially disarticulated skeleton has a relative complete skull whichis preserved below a huge sacrum that protects it and avoids the fluvial transport. In the specimen UNCUYO LD 302, fragments of skull are relatedwith an articulated cervical series, and disarticulated dorsal vertebrae and appendicular bones. These specimens come from the same Plottier Formation (upper Coniacian?lower Santonian) and are separated each other around 50 to 60 meters,but were recovered in different sedimentary facies of the same meandering fluvial system with ephemeral episodes. These and other taphonomicquarries of huge sauropods can allow describing at least two important intrinsic factors related with the bone anatomy: ?structural fragility? and ?weakpoints of disarticulation?. The first one refers to those bones that destroy more easily, such as the case of the characteristic weak and small skull ofthese dinosaurs. The second factor refers to vulnerable joints where the skeleton frequently is disarticulated. This is the case of the atlas-skull, thetarsal-metatarsals and the last caudal centra. From phylogenetic viewpoint, the results of four recent, largely independent analyses support the existenceof a derived titanosaurian lineage distinct from the ?Saltasaurinae line,? which is termed Colossosauria. At present, this clade includes severalgigantic species like Patagotitan, Argentinosaurus, Puertasaurus and Notocolossus. From paleobiological viewpoint, giant sauropods were recorded in different moments of their evolutionary history (like diplodocids in the Jurassic and titanosaurs in the Cretaceous). Unfortunately, our understandingof the gigantism of these animals is at initial stage. Recently, some studies on sauropods indicates that abiotical factors (environmental parameters) cannot easily correlate with their gigantism. In contrast, a ?biological evolutionary cascade model? has been proposed to better explain the huge size. This model can explain the main biological changes of sauropods to reach large bodies, but the coexistence of relatively small and large titanosaur sauropods do not have satisfactory explanation until more accurate morphofunctional and behavioral studies are made. Gregarious behaviors inferred based on titanosaur tracks (e.g. Titanopodus) and nests from Auca Mahuevo (Patagonia) are important adaptions to evaluate in this palaeobiological framework. Finally, in titanosaurs we are in a stage of discovery of new species every years, which is extremely attractive and include an unsuspectedbiological diversity.