Predators or Scavengers: Locomotion and Speed

In Jurassic Park, John Hammond said the park T-rex could run at 32mph,
but is that even possible?

My new post on the Predator or Scavenger debate! This time the topic is about T-rex and related Tyrannosaurids' different forms, leg ratios, musculature, and maneuverability to find out the overall speed estimates of different species of Tyrannosaurids. Were Tyrannosaurs fast enough to catch their meals? Or were they the tortoises on the race track? Lets find out:
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Forms

As I pointed out earlier in the comments of my last Predators or Scavengers topic, there seems to be two body forms among different species of Tyrannosaurids. One form is robust, stocky, and heavy, while the other form is gracile, long-legged, and light (as far as large Theropods go). You can fit all tyrannosaurids into one of these two categories:

Robust Forms:

• Daspletosaurus
• Tarbosaurus
• Tyrannosaurus (Shown Right)
• Zhuchengtyrannus

Gracile Forms:

• Albertosaurus
• Alectrosaurus*
• Alioramus*
• Gorgosaurus (Shown Right)
• Teratophoneus

* Indicates that the creature is known only from juvenile or immature remains

Many paleontologists think that these two forms represent different lifestyles among the Tyrannosauridae species, but there are also differences in development when it comes to different forms of Tyrannosaurs. Young Tyrannosaurs typically are more gracile and have longer legs than adults. Juvenile T-rex for example have longer legs and far more gracile builds than adult T-rex.

Even on an individual basis, Tyrannosaurids show robust and gracile forms. Specimens of T-rex for example show differences of both a robust and gracile form, which is considered a gender difference by some, with females often being characterized by the robust form. The discovery of reproductive bone tissue inside the leg of a robust form Tyrannosaurus named B-rex seems to provide evidence to this idea. Others think the morphs might instead represent differences in population and geography, or possibly age with older animals being part of the robust morph.
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Leg Ratios

The legs of Tyrannosaurs tend to, again, vary from species to species. While gracile and young Tyrannosaurids tend to have very long legs and long strides, adults of the robust form tend to have shorter, stockier legs and much lower speeds. Among modern day bipedal animals, you can normally be able to judge an animal's speed by measuring the ratio between its femur and tibia. If the ratio is higher than 1, it normally hints at an animal that can't move very fast, but if it is lower than 1, it typically means the animal was capable of fast movement.

T-rex leg

When looking at the legs of most gracile form Tyrannosaurids, such as Albertosaurus and Gorgosaurus, the ratio tends to be lower and suggests that they were capable of faster movement. However, the legs in robust form Tyrannosaurids, such as T-rex and Daspletosaurus, the ratio is higher than 1, and suggests an animal of slow speeds.

These robust forms likely had very low speeds, possibly too low to be capable of actively hunting prey. However, Dr. Thomas R. Holtz, a paleontologist at the University of Maryland pointed out in 1998, that all Tyrannosaurids have a tightly interlocking metatarsal (foot bones) structure which could help transmit energy from the foot to the ground more efficiently than in other dinosaurs.

Despite this, the legs of at least the robust form Tyrannosaurids still don't seem to be built for very fast movement. Other factors like the robust sizes and thick bones don't suggest a fast movement speed either. Some have even suggested that these Tyrannosaurids likely were incapable of running at all, but instead walked almost all the time.
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Musculature

How muscular were the Tyrannosaurs? They were actually quite heavily built, and jam packed with powerful muscles all throughout the body. Could a T-rex or other robust form Tyrannosaurid use these powerful muscles to move fast enough to catch prey? Or did their overall bulk render them quite ineffective hunters? There are some recent discoveries for both sides of the argument.

Recreation of a T-rex's Musculature

The first find was made earlier last year by University of Alberta scientists Scott Persons and head of Paleontology Philip J. Currie, and found that T-rex had a little extra spring in its step due to a large muscle in its tail. Called the caudofemoralis muscles, it's found in modern day Archosaurs and a few species of mammals. The caudofemoralis muscles in crocodilians and birds provide the powerful leg stroke allowing fast forward movement, but T-rex's legs had a surprise.

Like the other animals seen, T-rex had a form of rib bones attached to the vertebrae in its tail. These rib-like extensions would've attached to the upper part of the caudofemoralis muscles, but T-rex's rib extensions were higher up on the tail than in any other Archosaur seen before. This would've allowed more area for the the caudofemoralis muscles to bulk-up and expand and put more power in its step.

This provides some good evidence for the T-rex hunter debate, as this would increase T-rex and other theropod speed estimates much higher than previously thought. However, another study found out something interesting about the famous T-rex at The Field Museum of Natural History in Chicago.

This study focused on T-rex's overall weight and musculature, by using a technique never used before. They used a high tech laser scanner, normally used by police and detectives to scan crime scenes for investigations, on Sue, the world's most complete T-rex and formerly the largest ever discovered (we have since discovered larger T-rex.) Normally, scientists used scale- up models to estimate the weight of dinosaurs, but this is the first time we actually were able to weigh an entire skeleton. Pieces that were too large to scan, such as Sue's massive head, were sent to Ford Motors to use one of their massive engine scanners.

Recent studies have shown that Sue tipped the scale at 9 tons

Once the skeleton was mapped, the scientists overlaid the digital skin onto the models in order to get a body volume. The team then modeled separate body parts to make the model more accurate. Finally, the mass was calculated after taking into account the fact that the lungs and bones of Tyrannosaurs were hollow, and made a range of mass calculations ranging from undernourished to obese. They ended up applying this to at least five different T-rex specimens including Sue and came up with some surprising results.

They found that the T-rex weighed were far more heavy than previously thought. While most of the T-rex scanned ranged between 5.5 and 7 tonnes, Sue was in a league all of her own. She weighed in at a whopping 9 tons. This is about 30% more than originally expected, and gives T-rex a massive weight almost twice that of a typical Asian Elephant.

Along with the overall weight discovery, they found some contradicting data on Currie and Persons' claim of T-rex's large caudofemoralis muscles. They found in the study that the caudofemoralis muscles that drove Sue and the other Tyrannosaurus weren't as large as Scott Persons' claim. Instead, the caudofemoralis muscle was much smaller than they inferred, instead only using 25% tail segment mass (although it did have the same amount of area to expand as their study), more similar to earlier estimates. Not just that, the animal's overall musculature seemed to be bulking up towards the upper part of its torso with growth, and actually losing much of the musculature in the tail.

Another interesting statement that Currie and Persons made was that the caudofemoralis muscle stayed the same size throughout growth. But they stated this after using a juvenile Gorgosaurus specimen and an adult Tyrannosaurus. They never found the size of an adult Gorgosaurus caudofemoralis and never looked into the idea that the muscle's size could range from species to species. This study instead used an actual juvenile Tyrannosaurus specimen (Jane) and five other Tyrannosaurus to show that the caudofemoralis muscle actually decreased in size with age. Adults had a caudofemoralis muscle that was about 1.95–3.9% body mass for all the different size models, while Jane had a caudofemoralis proportionately larger, using about 3.4–3.8% of her body mass.

Along with the huge weight increase, it seems the the musculature in Tyrannosaurus actually worked against them. Being so heavily muscled means that T-rex weighed more than we expected, decreasing its top speed.

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Maneuverability

Now for another interesting question, how maneuverable were Tyrannosaurs? Were they fast turners or slow plodders? For a long time it was thought that Tyrannosaurs had a fast turning speed, capable of using their long tails to turn on a dime in order to cut off escape routes for prey. However, recent evidence has hindered this idea.

T-rex's tail was originally thought to be used in turning,
recent evidence has shown that this is unlikely.

You see, when looking at modern day fast moving animals like cheetahs, which can turn at a split second to catch up to fast moving prey, they have long tails that help with quick turns. For a long time it was thought Tyrannosaurs could use its tail similarly, as both a rudder and a counterbalance for its massive head.

That was until 2007 when a discovery using computer models showed that T-rex would have had a difficult time turning as fast as previously thought. Lead by biomechanics expert John Hutchinson while at Stanford University in California, the team studied the rotational inertia of 12 theropod dinosaurs to see exactly how fast could negotiate a turn. They found that unlike originally believed, T-rex was actually slower than we thought, taking about 2 seconds to pivot 45 degrees, something that humans and many other animals can do in a fraction of a second.

The reason why is because unlike a cheetah's tail which is thin and flexible, the tail of T-rex and other Tyrannosaurs was thick and rod-like, making it great as a counterbalance, but horrible as a rudder for changing directions. However, the study did show that if T-rex could reduce the average time it took turning by arching its back, tail, and by pulling its head and forelimbs close to its body, it could have reduced its rotational inertia, rather like how an ice skater pulls in his arms close to his body in order to twirl faster, though in T-rex's case it wouldn't have made a big difference.
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Speed Estimates

Now for the moment you've probably all been waiting for:  How fast was each species of Tyrannosaur? I'd first like to point out that some of these estimates are based on actual scientific analysis, while others are educated guesses based on what is known of their anatomy (most likely because they have no scientific analysis behind them). I'll also provide reasons for my conclusions. So, lets finally sink our teeth into Tyrannosaur speed:

Alectrosaurus: 27 - 30 mph
Why: Alectrosaurus is known only from partial remains, and it's possible that they may have come from a juvenile specimen. But from what we know of it, it seems to be similar in speed to other young Tyrannosaurs.

Alioramus: 27 - 30 mph

Why: Like Alectrosaurus, Alioramus is also known from partial remains, and the known material may come from a juvenile specimen. It was probably similar in speed to Alectrosaurus, but since it's a bit more gracile, it might be a little bit faster.

Teratophoneus: ?

Why: Again, this guy is too new of a discovery to find an accurate speed estimate, and almost nothing has been described of it yet.

Albertosaurus: 25 - 30mph

Why: Albertosaurus possibly has the longest legs compared to body size of all large-bodied Tyrannosaurids, if that title isn't taken by Gorgosaurus. Along with the long legs, it also has a relatively low ratio of femur-to-tibia, suggesting a fast movement speed despite its large size. It's gracile build also suggests a fast hunter.

Gorgosaurus: 25 - 30mph

Why: Very similar to Albertosaurus. Possibly has the longest legs compared to body size of all large-bodied Tyrannosaurids. It also has a relatively low ratio of femur-to-tibia, suggesting a fast movement speed despite its large size. The gracile build also suggests a fast predator.

Daspletosaurus: 15mph

Why: The complete opposite of Albertosaurus and Gorgosaurus, Daspletosaurus' legs were unusually short among the Tyrannosaurs. Along with that, it also has the shortest tibia compared to body size of the Tyrannosaurs, and the femur-to-tibia ratio is very high. So it seems likely that Daspletosaurus is a relatively slow mover.

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Tarbosaurus: 16 - 18mph

Why: Another robust form Tyrannosaurid with short legs, Tarbosaurus seems to have a speed somewhere in between that of Tyrannosaurus and Daspletosaurus. Juveniles have longer legs and more gracile builds, suggesting a much faster movement possibly similar to Alectrosaurus.

Zhuchengtyrannus: ?
Why: Again, a recent discovery so speed is so far unknown. But it might be similar to Tarbosaurus' speed seeing how they're supposedly close relatives.

Tyrannosaurus: 18mph

Why: The most recent analysis of T-rex's speed was in 2007, and used computer models to find that  T-rex could run at 18mph. The study, like the one used on Sue to find her weight, used actual fossils instead of scaled-up models of living animals to find their speed, and is arguably the most accurate estimate of dinosaur speed ever made. Sue-sized Tyrannosaurus or larger were probably much slower animals, and Jane-sized Tyrannosaurus were probably far faster than an adult.

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Speed of Prey

So we finally know how fast Tyrannosaurs were, but that doesn't mean anything without knowing the speed of their prey. A predator can be any speed, even less than 2mph; as long as it's as fast or faster than its local prey items, it can be a top carnivore. For some species of Tyrannosaurs this is good news despite their really slow speeds, while for others it spells bad news. But before I compare Tyrannosaurs to their prey, there are some things that need to be sorted out.

One way that many scientists support the active hunter hypothesis is by asserting that Tyrannosaurs might have been slow, but faster than their local prey items of Ceratopsians and Hadrosaurs. However, I find this to be a very narrow-minded approach to the subject, especially since recently it has been proven to be wrong.

Chart comparing modern day Rhinoceros to Ceratopsians
(Chart from Gregory S. Paul and Per Christiansen's paper)

Many analysis of Hadrosaurs like Edmontosaurus have shown that they were capable of moving at 28mph, that's 10mph faster than the most acurate speed for T-rex. I have also found it odd that many people state that Triceratops was slow like a rhino, yet rhinoceros can run over 25mph, with the fastest species moving at about 34 mph. An analysis by Gregory S. Paul and Per Christiansen from March 2000 seemed to confirm my suspicions; their analysis found that at least the more advanced Ceratopsians like Triceratops could move at speeds faster than an elephant and similar to a rhinoceros. (However, not all people agree with this study) Some recent studies on ankylosaurs even suggest that these armored behemoths were faster than some Tyrannosaurs.

Now, to finally learn whether Tyrannosaurs could catch up to their prey, and when looking, it seems like we have many examples on ecosystems. To start, Alectrosaurus lived within in an environment dominated by dinosaurs smaller than itself, such as the small Archaeornithomimus, Oviraptorids, and Protoceratopsians. Most of these animals were fast, but Alectrosaurus seems to be just as fast as the smaller dinosaurs that lived around it, thus it could still be an effective hunter. If Alioramus is in fact an adult animal, it also seems to have lived alongside small fast prey like Pachycephalosaurs and Ornithomimids, which have been discovered from the same bonebeds as Alioramus, so the predatory argument seems to be a safe assumption.

Now, onto discussion of some of the more famous Tyrannosaurs like Albertosaurus and Gorgosaurus. Albertosaurus lived in the lower part of the Horseshoe Canyon formation alongside similar-sized Hadrosaurids, Lambeosaurids, Chasmosaurids, and Centrosaurids, and seems to have been the top predator, using its large size and fast speed to catch these said prey items. Gorgosaurus was a top predator as well, but it shared that niche with Daspletosaurus for millions of years. When you have two large carnivores in one region it typically means that each is specializing in specific types of prey, or one is a hunter while the other scavenges.

Two Daspletosaurus hunting an Edmontonia.
Looks like it isn't giving up without a fight.

There is a large problem for Daspletosaurus to be an active predator, seeing as it moved so slow and most of its local prey items were close to twice as fast as it was. Maybe that would explain it's shared nich with Gorgosaurus, and the fact we find evidence of it scavenging on hadrosaurs. Of course, like modern day predators, it could've used certain methods, like ambushing, to try and catch Hadrosaurs or Ceratopsians off gaurd. I wouldn't think that a crocodile could catch a wildebeest if it didn't use it's ambushing method.

Onto another tyrannosaur slowpoke, Tarbosaurus also seems to have short legs compared to its body size (longer than Daspleto, but shorter than T-rex). However, unlike many of the other robust form Tyrannosaurs, he instead lived in an environment dominated by slower animals. Titanosaurid sauropods, Ankylosaurids, and Saurolophus angustirostris (which was about 3 times as massive as some of the biggest North American Hadrosaurs) dominated much of the paleofauna. Tarbosaurus didn't need much speed to hunt these giant but slow prey animals, thus, it could have easily been an efficent predator.

And finally we have the king himself. Tyrannosaurus rex was also a robust form Tyrannosaurid like Tarbo and Daspleto, but could its speed help it catch its local prey items? Well, the answer is somewhat of a yes, and a no.

You see, T-rex's range went all the way through the United States. From Alberta to Texas you can find T-rex specimens, and throughout these areas T-rex ranged over a number of habitats, from swamps, to woodlands, dry basins, and coastlines. Living in so many habitats means that different T-rex lived alongside different animals that lived in those areas.

In the northern areas of T-rex's range were large woodland forests filled with Hadrosaurs such as Edmontosaurus, and Ceratopsians like Triceratops. Many of these animals were faster than T-rex, and would've made it hard for a T-rex to actively take down prey. Ankylosaurs are also seen in the environment, but they were far too rare to be considered a stable food source for the much more common Tyrannosaurus. There were also small herbivores like Thescelosaurus and Pachycephalosaurus, and omnivores like Ornithomimus, but they were all fast animals that I have no doubt could outmaneuver a T-rex. I guess like Daspleto, T-rex could have been an ambush predator, but it still seems that T-rex was a rather slow king in the north.

T-rex preying on an Alamosaurus

However, the southern areas of T-rex's range included dry open basins with Ceratopsians, including Triceratops and its possible adult form Torosaurus (I'll probably talk about that in a future post), and some unidentified Hadrosaur remains. There were no ankylosaurs though, but instead, their seemed to be the Sauropod Alamosaurus. Originally thought to be a small Sauropod about 65ft long, recent finds show that the original remains were of juvenile and sub-adult animals and that adults were as large, or larger than Argentinosaurus, who currently holds the title as one of the largest land animals. These Sauropods were slow and powerful, but it isn't impossible to think that Tyrannosaurs could at least sometimes hunt down one of these giant herbivores, seeing how powerful and muscular T-rex is. In fact, of the recent Alamosaurus remains found, one specimen is surrounded by shed Rex teeth, suggesting that it did feed on these animals. It seemed in the south at least, T-rex might have been a more effective predator of giants.

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Conclusion

Looking over the many different species of Tyrannosaurids, it seems that many of them were capable of being active predators. The gracile forms like Albertosaurus and Gorgosaurus show the most promise, especially since they are lightly built and can run very fast. Some of the robust forms like Tarbosaurus lived alongside slow prey, and could have easily been top predators. But T-rex, Daspletosaurus, and some other robust form Tyrannosaurs were much slower than their local prey, and likely would've had a harder time securing a meal.

However, as I stated above, all Tyrannosaurs could have applied types of hunting methods on prey, such as ambushing on game trails or near places where animals gathered. Though how a predator could catch an animal almost twice as fast as itself seems rather difficult, even with ambush. But crocodiles can do it, and they're a lot slower than T-rex. 

Looking over younger Tyrannosaurs though, it shows that they were likely almost twice as fast as adults. This might indicate that younger Tyrannosaurs were fast runners that took down prey, while larger adults might've been scavenged more with age. Modern day animals are known to scavenge more with age, as their larger sizes make it easier to scare competing predators off kills. This behavior is seen in modern day bears, lions, crocodiles, eagles, and Komodo dragons. Why couldn't Tyrannosaurs have practiced it?

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Links to my Sources:

Sue's New Weight and Tyrannosaur Musculature: http://www.ploscollections.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0026037;jsessionid=83E2CCE3577DC1ADBCA6DD2FCF37163E

T-rex Menuverability and Rotational Inertia: http://news.nationalgeographic.com/news/2007/06/070607-trex-dinosaur.html

Details on Tyrannosaurid Skeletal Anatomy: The Princeton Feild Guide to Dinosaurs: by Gregory S. Paul (p.102-109)

T-rex and other Theropod Speeds: http://www.isgtw.org/feature/how-fast-could-t-rex-run

Hadrosaur Speed Estimates: http://palaeo-electronica.org/2009_3/180/index.html

Ceratopsian Speed Implications: http://www.bioone.org/doi/abs/10.1666/0094-8373%282000%29026%3C0450%3AFPINDI%3E2.0.CO%3B2