Abelisaurs and Tyrannosaurs

Tyrannosaurus, by far the most famous Tyrannosaur

At the end of the Cretaceous, two types of large predatory dinosaur rose to prominence. Although vastly different, they may have had some similar hunting strategies, specializations, and advantages over the previous lords of the land, so they will be discussed in this one page. In this case, we will not start with the first alphabetically, because the Abelisaurs are not nearly as well known, so we will begin with the most famous Predatory Dinosaurs ever, the Tyrannosaurs.

## Tyrannosaurs

There are some traits that set the Tyrannosaurs apart from other large predatory dinosaurs. For example, the massive Carcharodontosaurs are actually known for having very weak bites comparatively. Although the largest Carcharodontosaurs were larger than Tyrannosaurus, Tyrannosaurus' bite was more powerful by far. The hunting strategy of the Carcharodontosaurs revolves around causing blood loss in their victims, which makes sense when observing their prey. The huge Sauropods and Titanosaurs like Argentinosaurus would have such huge bones that it would be nearly impossible for a predator to fit its mouth around the leg bones. If it did so, crushing through them would be impossible. So bone crushing would not be possible for the Carcharodontosaurs. This explains why they relied on bleeding. On the other hand, a more powerful bite is much more effective when taking out smaller prey, because they can be disposed of instantly without having a huge thrashing fight.

Another thing that makes the Tyrannosaurs different is the position of their eyes. The way that they are placed on the head of the animal allows for stereoscopic vision, and analysis makes researchers think that the vision of dinosaurs like Tyrannosaurus was up to six times better than that of a human. On the other hand, most other large predatory dinosaurs had eyes on the side of their head, which would obviously not allow for very good forward vision, but would more significantly mean no 3-D vision. 3-D vision happens in what is called the stereoscopic arc, where the vision of both eyes cross. One can witness this very well by placing their hand right in front of their eyes. When very close to the head, it is hard to see the hand very well, but at a certain point, the hand suddenly becomes clear. This is where the arc is, where the vision of both eyes cross. Clearly, a predator with eyes on the sides of their head could not have this 3-D vision.

Probably the most significant difference between Tyrannosaurs and the other large predatory dinosaurs is lineage. For a long time, it was suspected that Allosaurus, a Jurassic theropod, evolved into the Tyrannosaurs, although this is based off no evidence other than the fact that they are both large predatory dinosaurs. Just by comparing their silhouettes, there are obvious differences. Overwhelming evidence now suggests that Tyrannosaurs have Coelurosaurian ancestors, which would make them much more closely related to birds then the Allosaurs. Since birds can also be classified as Coelurosaurs, it is clear that these dinosaurs were completely unrelated to the Carnosaurs (i.e. Allosaurs, Carcharodontosaurs, and Ceratosaurs).

Many other mistakes are made based on the fact that both Tyrannosaurus and Allosaurus were large predatory dinosaurs. Often Allosaurus is described as being a "Tyrannosaurus rex with horns", although Allosaurus and Tyrannosaurus are entirely unrelated. Using that logic, Allosaurus would just as easily be a "Velociraptor with horns", or a human would be a "cat that walked on two legs". It should also be pointed out that the size difference between Tyrannosaurus and Allosaurus is quite monumental. Since the size of Allosaurus is estimated at 1,500 kg, it is only a fourth as large Tyrannosaurus at 6,000 kg.

## Tyrannosaur descent

For a long time, there was debate as to which continent the Tyrannosaurs originated. Although the largest form, Tyrannosaurus, came from North America, the older Eotyrannus came from Europe. There were also many early Tyrannosaur forms from Asia. The oldest to date was found in China in 2006. This crested Tyrannosaur called Guanlong dates back to the Jurassic, showing that the Tyrannosaurs evolved nearly a hundred million years before the end of the Cretaceous. The more direct ancestors to Tyrannosaurus, however, come from Canada.

## Abelisaurs

During the Cretaceous, sea levels were much higher than they are today, and there were clearly northern and southern continents, and there was typically no way to cross over between the two (although a hadrosaur found in South America suggests that possibly at some point there was a bridge between the two). The Tyrannosaurs lived only on the northern continents. During the Cretaceous, the world's fauna was changing. Many of the larger Sauropods were disappearing, and the Carcharodontosaurs that preyed on them were also dying out. Newer animal forms were taking over the land. It seems that the first continents to make the transitions were North America and Asia, and South America was behind these. In continents such as South America and Africa, the dominant predators were Abelisaurs. Unlike the Tyrannosaurs, Abelisaurs were not Coelurosaurs. Instead, they were Carnosaurs, more closely related to the Allosaurs. Evidence suggests that the Abelisaurs evolved from Ceratosaurs, a type of large Jurassic predatory dinosaur.

Most Carnosaurs such as Carcharodontosaurs and Allosaurs relied on relatively weak bites, but what is interesting is that some Abelisaurs and Ceratosaurs had fused skulls, a trait that is seen in predators with more powerful bites, like the Tyrannosaurs for example. What is also odd is that not all Abelisaurs and Ceratosaurs had fused skulls. Carnotaurus, the most famous abelisaur, is actually thought to have had a relatively weak bite. This is because the skull does not appear to be designed to handle a bone-crushing bite. Like Allosaurus, Some supposed to have used its skull like a 'hatchet' and smacked the prey with the upper jaw while the lower jaw hung down without getting in the way. This theory, however, is still open to debate. Another theory about Carnotaurus is that it lived on some sort of flatland, plateau, floodplain, etc, because of its highly specialized tail vertebrae. These vertebrae are designed to house muscles on both sides, which would leave little room for flexibility. However, this would increase speed, so this theory has not received much opposition.

Another advantage that the Abelisaurs would have over other carnosaurs would be the position of their eyes. Like the Tyrannosaurs, Abelisaurs had forward-facing eyes. This would give them also excellent 3-D vision. At any rate, the Abelisaurs outcompeted the Carcharodontosaurs, and although evidence is mounting to suggest that in some isolated places Carcharodontosaurs did survive until the end of the Cretaceous, they had been marginalized.

Much of the Abelisaur body plan is known from Aucasaurus, an Abelisaur that was found only missing the tip of its tail. One thing noted about the Abelisaurs was that they had even smaller arms than the Tyrannosaurs. It is perhaps possible that these arms would have disappeared if Abelisaurs and Tyrannosaurs survived another few million years.

## Cannibals and Scavengers

Recently, the Abelisaur Majungasaurus has experienced a surge in popularity.This Abelisaur from Madagascar was shown to be a cannibal when evidence of Majungasaurus killed by their own kind was produced. Other predatory dinosaurs such as Allosaurus and Tyrannosaurus could also have been cannibals.

Tyrannosaurus, in particular, has been accused of being a scavenger. One reason for this is actually the bone-crushing bite. This would be advantageous for a scavenger, although that does not prove that it was a scavenger. It certainly does not seem likely that a predator with so many specializations would simply be a scavenger. Obviously, most predators will be part-time scavengers, but that does not mean that Tyrannosaurus' primary food source was carrion. That certainly seems unlikely based on the specializations and advantages of this predator.

## Abelisaur (especially Carnotaurus) Musculature

The Abelisaurs were incredibly muscular predators, with some amazing specializations that tell researchers much about their lifestyle. Unlike the Tyrannosaurs, some Abelisaurs such as Carnotaurus had relatively weak bites. This can be inferred because the skull of these predators does not seem to be strong enough to handle a very powerful bite. However, many other features of these Abelisaurs appear to point towards them being fairly muscular. For example, the tail of Carnotaurus did not only act as stabilization, but it contained a massive muscle. This muscle would help pull the legs which would increase the speed of the animal. The drawback of having such a huge muscle situated here would be that there would be less space for the muscles that would keep the tail erect, although this is partially solved by the highly specialized shape of Carnotaurus' tail vertebrae. Still, this animal would not have been very flexible and would have relied on acceleration and its incredible speed to run down prey. Because of its lack of flexibility, Carnotaurus probably did not live in a dense forest, but rather a more flat land.

Even the neck of Carnotaurus was highly muscular. This adds to the 'hatchet' theory because the Carnotaurus could perhaps use its neck to wield the 'hatchet-like' head with more power. The mouth of the Carnotaurus also has an upward curve to it that would allow the gape to be bigger and would make it easier to use its head like a hatchet without the lower jaw getting in the way.

## Hunting Strategies

For a while, it was assumed that Tyrannosaurus could run around 30 km/h. For this to be true, however, about 80% of its muscle mass would have to be in its legs. Since this is obviously not possible, it probably ran much slower at a little over 15 km/h. This means that it, and probably its other Tyrannosaur relatives, were probably not pursuit predators. However, it is still important to point out that although Tyrannosaurus was one of the very bulky Tyrannosaurs, not all genre were as robust comparative to their body size. For example, genre such as Albertosaurus seem to be more gracile.

On the other hand, the Abelisaurs were almost certainly pursuit predators. Based on the incredible muscles that would operate the legs of predators such as Carnotaurus, these hunters could most likely run over 55 km/h. This would make them even faster then Allosaurus. Although this would certainly compromise their maneuverability, if their environment was ,in fact, more of a plain, they could still run down prey without needing to be particularly agile.