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.
posted by starman at 12:36 AM
12 Comments:
Sounds like the Cretaceous was an exciting time for sauropods with their new found defense hide. So the arm race between predators and preys were upped another level. Both became evolutionarily more formidable and probably continued so had it not for the death blow from the heavens above those 66 million years ago
Yes the arms race would've continued had the big, nonavian dinosaurs lasted longer.
Notice, btw, the post didn't mention abelisaurs, which may not appear consistent with this view. They co occur with some armored titanosaurs such as Saltasaurus but most don't seem very formidable. Perhaps abelisaurs preyed mainly on young titanosaurs in which armor wasn't yet expressed, or ornithopods. However, at least one abelisaur, Chenanisarus, may have converged with Tyrannosaurus.
January 3, 2023
A large sauropod was a huge source of meat for a carnivorous theropod, for example, an abelisaur or a tyrannosaurus. One kill would have provided an excellent meal that would enable the theropod to be satisfied for several days. In some cases, more than one theropod may have been involved in attacking a titanosaur.
I agree Neal, sauropods were lucrative quarry so the selection pressure for means to overcome them was often great. And pack hunting was probable among sauropod hunters.
January 4, 2023
I think that when theropods attacked sauropods, they often went for the neck, which was a good target.
Yes indeed, the neck was a good target. It is possible that lithostrotians evolved robust necks to better withstand theropod attacks.
January 5, 2023
I also think that some lithostrotians developed strong necks. A robust neck could have effectively withstood attacks from some theropods. Tyrannosaurs would have been better able to penetrate the neck than some other theropods, since the teeth of tyrannosaurs were quite large.
Hi Neal. Interestingly, a Tyrannosaurus tooth was found in association with an Alamosaurus cervical vertebrae. The Tyrannosaurus could've been just scavenging, but the neck would've been a prime target and, as you say, Tyrannosaurus teeth were large and strong. They could've penetrated the neck or, if the titanosaur turned to keep its neck away, the hindlimb, flank or belly. Osteoderms didn't necessarily mean invulnerability; Tyrannosaurus (imperator?) was coadapted.
January 6, 2023
A bite from a Tyrannosaurs would have penetrated deeply into the flesh. That would have seriously weakened the sauropod. Then the Tyrannosaurs could have delivered additional bites. Just one bite on the windpipe would have been fatal to the sauropod.
Many theropods, including carcharodontosaurs, probaby used the slash and let bleed tactic. Titanosaur armor, however, rendered this tactic obsolete. Ordinary, bladelike theropod teeth couldn't cut through osteoderms. In order to penetrate "deeply into the flesh," Tyrannosaurus had to be able to smash through armor, hence evolved extraordinary jaw power and teeth.
January 11, 2023
Btw it was quite likely titanosaur was hot blooded right? Also there were many other herbivorous dinosaur species available at the time for big bad tyrannosaurus to prey on...
Also wish to ask what type of plants and trees titanosaurs most likely chewed on?
Thanks very much for your input here Martin. :) Yes titanosaurs, like other dinosaurs, were almost certainly endothermic, with elevated metabolic rates. And yes, tyrannosaurs had other potential prey besides titanosaurs; indeed in some environments, such as the Hell Creek, sauropods appear absent. Hadrosaurs were probably the most common prey of most tyrannosaurs although ankylosaurs, therizinosaurs, ceratopsians, pachycephalosaurs, deinocheirids etc also came under attack.
Large titanosaurs probably browsed on coniferous trees. Smaller ones ate lower growing food including angiosperms. Btw sauropods didn't chew as their teeth evolved to pluck food.
January 11, 2023
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