Tyrannosaurus rex is perhaps the most famous and popular dinosaur.
When people think of theropod dinosaurs, or even dinosaurs in general, T-rex is usually what comes to mind.
In many ways Tyrannosaurus was expectational compared to most other theropods.
While large predatory theropods were present in most dinosaur , very few even approached the same size as Tyrannosaurus.
So, why was Tyrannosaurus rex able to grow so large? And why didn’t most other Mesozoic ecosystems contain similarly sized apex predators?
This video aims to answer these questions.
Understanding how Tyrannosaurus was able to grow to such an exceptional size first requires understanding the normal size constraints faced by large predators face.
The amount of prey needed to support even just one carnivore is massive,
meaning that predators are much rarer than prey. Larger body size requires even more energy,
providing an incentive to be no larger than necessary. However, larger size allows for a predator to hunt larger animals
and nature abhors a vacant niche. But after a point,
the benefits of larger size are outweighed by the larger energy costs.
If a larger predator is not able to kill more prey than a smaller one, or the additional herbivores it can kill are too few to make up for the costs,
it will be less completive and become extinct. Therefore, the top size of predators in an ecosystem is determined by the size of the herbivores.
But a single predator being able to find enough food for itself is not the only problem.
In general, large herbivores are far less common than smaller ones. To make up for this,
larger predators have larger territories than smaller predators. The larger the predator, the larger the range needed to support it,
resulting in a total lower population for that species.
With fewer individuals, less has to go wrong for them to become extinct.
. Fewer individuals also means there is less genetic diversity in the population, reducing their ability to quickly evolve in the face of changing conditions or disease.
However, Tyrannosaurus doesn’t appear to have been a mere fluke. It lived for at least two million years
and appears to have only gone extinct as a result of everyone's least favorite meteor.
There was also a linage of carcharodontosaurids of similar size as Tyrannosaurus in South America and Africa. These existed for even longer than Tyrannosaurus.
Spinosaurus, who was a contemporary of one of these mega-carcharodontosaurids, also reached Tyrannosaurus-like size
Therefore, it is clear that there was a way around the problems faced by mega-predators that Tyrannosaurus exploited.
As predators are shaped by the environment they live in, understanding how Tyrannosaurus was able to grow so large
first requires understanding how the environment it lived was different from that of its ancestors.
Before Tyrannosaurus evolved, the apex predators of Late Cretaceous North America were all members of Tyrannosauridae, the same clade as Tyrannosaurus
But while large, the earlier tyrannosaurids did not measure up to the later Tyrant King.
While Tyrannosaurus’s lower body estimate is around 8.4 metric tons, most other tyrannosaurids did exceed a third of that.
Most of the prey of these tyrannosaurids were hadrosaurids or ceratopsians.
These were large and abundant enough to support predators larger than any alive today, as well as large enough to require massive size to take them down.
However, they were still too small make a predator the size of Tyrannosaurus completive.
Most of the earlier tyrannosaurids were also much faster than Tyrannosaurus and were thus able to hunt smaller, faster prey such as ornithomimids.
On the other hand, Tyrannosaurus’s massive size made it too slow to be able to catch such prey, leaving it with a less flexible diet.
However, the fauna Tyrannosaurus coexisted with 68 to 66 million years ago was very different from those from the past 25 million years.
In general, the large herbivores that coexisted with Tyrannosaurus were much larger than their ancestors.
Many, such as the ankylosaur Ankylosaurus and the ceratopsians such as Triceratops, were the largest of their respective clades.
Edmontosaurus, while not the largest known hadrosaurid, was still much larger than most of its predecessors.
Taking these giants down would have been a challenge for the earlier tyrannosaurids.
Therefore, it would seem that Tyrannosaurus needed to be larger in order to hunt the its larger prey.
However, while much larger than the prey typically available to most other tyrannosaurids,
the Jurassic and Cretaceous periods were full of herbivorous dinosaurs much larger than coexisted with Tyrannosaurus.
But few of these ecosystems with larger terrestrial herbivores had predators the size of Tyrannosaurus. So why was this?
he answer would seem to lie in differences between the primary large herbivores in Late Cretaceous North America
and those seen in most other Jurassic and Cretaceous ecosystems.
Instead of ceratopsians and hadrosaurs, the large herbivore in most of these ecosystems were instead sauropods, who had a very interesting population dynamic.
It is thought that there were very few adult sauropods, compared to juveniles.
This small number of adults would be mostly safe from predators and continuously lay new eggs.
Not many of these would survive, and for each sauropod that did make it to adulthood, countless more perished.
In the case of the larger sauropod species, there would have been too few adults in a given area for the top predators to easily specialize in hunting them.
While killing these adults may have been possible, it have been very dangerous, and there have been to few of them for the local predators to do so regularly.
Instead, the contemporary theropods mostly focused on the more abundant and less threating juveniles
Therefore, the largest herbivores present in sauropod dominated ecosystems were less reflective
of the maximum size of the top predators than in Late Cretaceous North America.
Most of Tyrannosaurus’s prey were largely not sauropods,
and while they had a higher number of young comparted to large mammals, most the adults made up more of the population and total biomass,
This allowed Tyrannosaurus to grow large enough to specialize in regularly hunting the adults most of the largest species.
It would also seem that theropod who specializing in hunting sauropods didn’t need to be as large as their prey in order to take it down.
The razor-sharp teeth of the carcharodontosaurids would have allowed them to easily slice through the flesh of their whale sized prey,
allowing them to slowly weaken them from blood loss.
Abelisaurids seem to have instead specialized in going for the weakest part of a sauropod, its neck, by biting it and holding it until the sauropod suffocated.
The Asian tyrannosaurid Tarbosaurus, one of the largest tyrannosaurids after Tyrannosaurus,
seems to have used a similar strategy, enabled by a more rigid skull than most other tyrannosaurids.
The end result was theropods in sauropod dominated ecosystems were only as large as Tyrannosaurus in the ecosystems with the largest sauropods,
as with Mapusaurus and Argentinosaurus.
Tyrannosaurus lacked these traits, instead relying on its powerful, bone crunching bite, which had the largest bite force of any terrestrial animal in Earth’s history.
But as powerful as it was, with larger prey like sauropods, there would have been fewer opportunity to actually crush bone.
Unfortunately for Tyrannosaurus, the shape of its teeth was less effective at slicing flesh than those of the carcharodontosaurids,
meaning it would struggle to hunt animals significantly larger than itself.
As a side note, this likely meant Tyrannosaurus was unable to hunt adult Alamosaurus,
a large sauropod that had immigrated to parts of western North America from South America.
On the other hand, subadults, would still have been vulnerable.
While the relative abundance of large, slow prey may have made Tyrannosaurus sized individuals viable,
and its hunting method would have pressured it to be larger,
this still doesn’t explain how the Tyrannosaurus species overcame the normal problems of a heavily reduced population as a result of the massive territory required by each individual.
Part of the answer is Tyrannosaurus’s total range differed from those of previous North American tyrannosaurids.
For much of the Late Cretaceous, North America was not homogenous. The Western Interior Seaway split the continent in two parts
The Western Interior Seaway split the continent in two parts. The western half, Laramidia, was were the ancestors of Tyrannosaurus rex evolved.
. At any given time, Laramidia seems to have been divided in anywhere from two to three ecosystems, each with a largely different, but closely related, fauna.
Each of these had a tyrannosaurid as the apex predator, and perhaps even more than more than one in some cases.
With such a limited protentional range, the total size of these carnivores would have been restricted so as to maintain a viable population.
Then, towards the end of the Cretaceous, sea levels dropped, both increasing the total landmass and reconnecting the eastern and western halves of North America.
However, western North America was still divided into three ecosystems.
For instance, in one, the most common large herbivores were Triceratops and Edmontosaurus.
In another it was instead Torosaurus and Alamosaurus
But unlike the previous 25 million years, Tyrannosaurus was present in the all three of these ecosystems.
Tyrannosaurus may have also been present in Eastern North America, but the fossils these are too few to be sure.
Regardless, the was result was Tyrannosaurus rex possessed a much a larger range than the previous tyrannosaurids.
While each individual Tyrannosaurus would require more territory than its ancestors, there was more territory to go around.
In turn, this allowed for both a higher total population and degree of genetic diversity than would have been possible for a Tyrannosaurus sized predator during the prior 25 million years.
Within these ecosystems, Tyrannosaurus would have also had very little competition.
The largest predatory dinosaur that coexisted with Tyrannosaurus was the 5.5-meter long Dakotaraptor.
It wasn’t just other dinosaurs Tyrannosaurus didn’t need to worry about.
Whereas some earlier tyrannosaurids had to compete the massive crocodilian Deinosuchus for prey, Deinosuchus was already extinct by the time Tyrannosaurus evolved.
But the opportunities created by the environment it lived in weren’t the only reasons Tyrannosaurus was able to maintain enough genetic diversity.
Another factor was that at any given time, most of the Tyrannosaurus alive were not actually supported by the population of large herbivores.
This is because of the differences between Tyrannosaurus adults and juveniles.
Juvenile Tyrannosaurus rex were not just mini-versions of the adults. Instead, they had a more slender build that allowed for greater speed.
While it has been proposed that Tyrannosaurus worked in packs of mixed ages, with juveniles catching prey
and the larger adults killing it, there was really no need for such behavior in Tyrannosaurus. Any prey that only a juvenile was fast enough to catch wouldn’t require an adult to finish off.
Likewise, the species that would require an adult to kill, like Edmontosaurus, were even slower than an adult Tyrannosaurus.
Given that each growth stage was better suited to hunting a particular type of prey, it seems that juveniles lived separately from the adults, each relying on a different set of species.
Therefore, the large herbivores were only needed to support the adults, with the rest of the population living off smaller prey.
However, an ecosystem can support many more medium carnivores than top predators.
As immature Tyrannosaurus were the only medium sized predators in the ecosystem, there was likely a much larger population of juveniles than would be necessary to replace the adults.
. As juvenile Tyrannosaurus fossils are much rarer than the adults, this suggests they had relatively low mortality rate,
and most that made it to the medium carnivore stage also made it to adulthood.
The ecosystem could not support that many adult Tyrannosaurus, so what happened to them?
The answer seems to be that adult Tyrannosaurus didn’t live that long. While it took eighteen years for a Tyrannosaurus
to reach sexual maturity, it only lived an average of only six years after that.
Even the oldest two Tyrannosaurus specimens, Trix and Scotty, were only 30 years old when they died.
Therefore, even the oldest Tyrannosaurus individuals spent most of their lives as juveniles.
In other species, low juvenile mortality followed by a high adult mortality is most commonly the result of the stress related
to reproduction,. This can range from taking care of offspring or fierce competition for mates.
Additionally, while Tyrannosaurus rex was perhaps the most dangerous predators that ever evolved,
hunting was still very risky since its primary prey were dangerous themselves.
Like most large predators, Tyrannosaurus would have mostly weaker individuals,
like the young, old, or sick, though even this carried the possibly of death.
Whatever the cause of the high mortality rates among adult Tyrannosaurus, while certainly horrible for individuals,
it would have had an actual advantage at the species level.
An average of six years was brief enough to allow most of the more numerous juveniles to become adults
without overtaxing the ecosystem. It was also long enough for them to have a chance to reproduce before dying.
This allowed for Tyrannosaurus to possess the genetic diversity than would otherwise be possible for such a large predator
Overall, there were many factors that contributed to Tyrannosaurus’s gargantuan size.
These included its increased range,
larger prey,
its hunting method favoring large size,
the population dynamics of its prey compared to those in sauropod dominated ecosystems,
and Tyrannosaurus’s own population dynamics.
Some factors, such juveniles serving as the ecosystem’s medium sized predators,
were already present in some other theropods,
while others, such as its hunting method favoring larger size,
were otherwise only present in its close relatives who lived in an environment too small for mega-predators.
Once that environment changed, the result was one of the most fearsome predators the world has ever seen.
Thank you for watching. I hope you learned something interesting.
Have a great day, and, if you enjoyed the video, please remember to hit the like button.