Titanosaur Armor and Theropod Evolution

In the latter Cretaceous, the predominant sauropod groups were armored. Osteoderms covered titanosaurs extensively, conferring a protection unknown in Jurassic taxa. Lithostrotians proliferated, and were armored.

This defense had a profound effect on theropods. Osteoderms rendered the carcharodontosaur hunting strategy obsolete. Bladelike teeth could not slash a reinforced hide. As armor became the norm among sauropods, carchs waned, replaced by other theropods with new hunting strategies.

Although titanosaurs remained potential prey, most theropods couldn't cope with osteoderms, hence evolved other approaches. With the exception of Chenanisaurus, abelisaurs may have focused on juvenile titanosaurs lacking armor. Avoiding the armored back or flanks, Megaraptor probably killed by slashing the underbelly or throat. If the latter was frequently targeted, it may explain the proliferation of robust-necked lithostrotians.

Unlike other theropods, tyrannosaurs eventually overcame armor instead of avoiding it. Facing the armored Alamosaurus, Tyrannosaurus evolved extremely powerful jaws and teeth. These conferred an ability to penetrate osteoderms. In contrast, Tarbosaurus was less powerful, even though it also faced a large titanosaur, Nemegtosaurus. This suggests Nemegtosaurus(=Opisthocoelicaudia) was not as well armored as Alamosaurus. Even though Nemegt sauropods are known from much material, (including isolated or fragmentary elements such as teeth) no Nemegtosaurus osteoderms have been reported. Given the widespread appearance of osteoderms in titanosaurs, the absence of Nemegtosaurus armor is probably a taphonomic artefact. However, lack of evidence does suggest minimal protection. Less extensive or less thick armor may explain why Tarbosaurus did not possess the massive jaw power of Tyrannosaurus. Tarbosaurus was, however, more derived than its predecessors and anticipated the archpredator. Its intermediate condition reflects a gradual increase in tyrannosaur prowess, or an "arms race" in which both hunter and hunted enhanced their means of survival.