Generally, the adaptations of any animal represents a trade-off. In the case of locomotion, or how an animal moves, there are two general extremes that we can see in land mammals. At one extreme, you have limbs made for supporting a large load. At the other, you have limbs designed for quick movement. There is plenty of variation between these two ends of the spectrum. Limbs adapted to support heavy loads are referred to as graviportal. Graviportal limbs tend to be thick and column-like, pretty much what you'd see on an elephant. This is in contrast to cursorial animals which tend to be smaller like a cat or a deer. Cursorial means running, these animals tend to have longer, lower leg bones and walk on just their toes or even their toenails. Horses are an example of an animal that walks on its toenails. Think of it, a hoof is really just a modified fingernail. Some dinosaurs had graviportal limbs. Some had cursorial limbs and others landed in the gray area in between. Which of these animals do you think had cursorial limbs? Check all of those you believe to be cursorial: A, Argentinosaurus; B, Ornithomimus; or C, Maosspondylus. Remember, there may be multiple, correct answers. Large sauropods had very graviportal limbs, like this Argentinosaurus, while many theropods had legs built for running, like this Ornithomimus. Very early sauropods like Massospondylus, had legs somewhere in between these two extremes. Our cursorial dinosaur is Ornithomimus, so B is the correct answer. Graviportal limbs are deigned to carry heavy loads efficiently, at the expensive of higher speed. On the other hand, cursorial limbs allow an animal to run faster, but be less efficient at carrying a large mass. To demonstrate this, think about your own legs. When you are standing still or walking, your legs are generally held in a similar fashion to that of a graviportal animal. Your legs are relatively straight and the sole of your foot is in contact with the ground. Walking is not terribly fast, but it is very efficient. Compare this to when you are running when you run, you come up on your toes. You can run faster this way. However, this leg position uses much more energy. The way limbs are held under the body represents a trade off between speed and efficiency. This is a strong factor in what drives different groups of animals to either evolve more graviportal limbs or more cursorial limbs. One of the methods we can use to investigate locomotion in dinosaurs is to create muscle reconstructions. I'd like to introduce to you Scott Persons, a PhD student at the University of Alberta who studies the leg and tail muscles of hadrosaurs and tyrannosaurs. >> Let's spend a few minutes talking about how tail muscles help dinosaurs to walk and to run. To understand the relationship between tail muscles and dinosaur locomotion, we begin by dissecting the tails and legs of modern reptiles. We want to examine how tail muscles attach to the reptilian skeleton. On this iguana, I've removed all the soft tissue from the tail except for one muscle. This, is called the caudofemoralis, and it's a tail muscle with a tendon that attaches to the femur. The upper leg bone. When the caudofemoralis retracts, it pulls backwards on the leg. And most lizards and crocodilians rely more on the power of the caudofemoralis than on any other muscle to help them walk and run and the same was true of most dinosaurs. Here is the femur of a hadrosaur or duck billed dinosaur and the spot where the caudofemoralis attaches is easy to identify. It's this tall trochanter or bony crest. Now based on this and many other muscle to bone attachment sites, it's possible to calculate the size of a caudofemoralis in a wide variety of different dinosaur species. To help visual the inner workings of dinosaurs rear end here's a simple clay model. Coto femeralas which is the red clay is located deep within the tail and here you can see the attachment site to the leg. Using digital models, we can create more precise muscle reconstructions. And those results indicate that many dinosaurs, including famous carnivores like Tyrannosaurus and Carnotaurus. Had a caudofemoralis that was, even relatively to their enormous body size, significantly larger than that of modern lizards and crocodiles. And that means that these dinosaurs had more leg power and were probably faster than we previously thought. But size isn't everything and where on the femur the caudofemoralis is also important. In most carnivorous dinosaurs, like tyrannosaurs, it attached high up, near the hips. But on those trabiverous dinosaurs like this Hadrosaur, it attaches in about the middle. Now this means that in the Tyrannosaur, it could swing its leg fast and could probably take longer steps because the muscle does not need to contract as far. But it also means that the tyrannosaur would tire out more quickly. It, because it takes more energy to swing the leg when the lever arm is so short. On the other hand, the hadrosaur was probably not capable of taking long strides and wouldn't have been able to swing its leg as quickly. But it also would not have tired out as fast. So, in a sprint, a tyrannosaur would definitely outpace a hadrosaur. But in a marathon, the slower, but steadier hadrosaur would win the race. That's why understanding dinosaur tails helps us to understand how dinosaurs may have hunted or avoided being hunted. Tails were an important part of every dinosaur's anatomy, and when it comes to locomotion, dinosaurs were packing a lot more than junk inside their trunks. >> Apart from the different stances and moving styles that an animal may have, they may use either two or four legs when they are walking. Animals which walk on two legs are called bipeds. While animals that walk on four legs are called quadrupeds. Before we get into bipedal and quadrupedal dinosaurs lets think again about a group of animals, mainly humans. Would you call a human a biped, a quadruped, or both? Check the correct answer. The answer is both. Virtually all of us have gone through a stage in our lives where we walked on all fours. Mainly as infants. Humans change their stance from quadrupedal babies, to bipedal adults. And during the transitional stage between quadrupedal and bipedal stance, there is plenty of variation. There are plenty of animals that transition between bipedal and quadrupedal, but there are some animals that can only ever walk on two or four legs. For example, an ostrich could never walk on four legs with their very short arms and an elephant can never walk on two legs because of its size. Animals that can only walk on two or four legs are considered obligate, that is, they are obligated or have no choice. They must walk in a certain way. So, an animal that walks primarily on two legs is an obligate biped and an animal that walks mainly on four legs is an obligate quadruped. Now let's take a look at some animals that are not obligated to walk in a single way. Think of a bear, bears typically walk on four legs but are well known to rear up on their hand limbs and can walk some distance on two legs. Bears and all animals that can, but usually don't walk on two legs can be called facultative bipeds. Facultative roughly means optional, so a bear can optionally walk on two legs. Alternately if we look at an animal like a kangaroo. It typically bounds at high speeds on two legs, but will walk on four legs when it is moving more slowly it would be considered a facultative quadruped. Let's think of some of the advantages of being a biped or facultative quadruped. Remember a facultative quadruped is a biped that has the option to move on four limbs, just not all the time. Here's a kangaroo as an example, but this question could apply just as easily to a human, or bird, or any number of other animals. Select the advantages that a biped or facultative quadruped could gave. A, fast runner. B, large body. C, flight. Or D, grasping prey. Check all answers you think apply. Animals that are primarily bipedal are generally small and have their arms freed up to perform a number of functions. They can be fast runners, might be flyers, and might grasp prey with their hands. So the answers A, C, and D are correct. Dinosaurs display an incredible diversity of walking styles and modes. If we think about some of the largest dinosaurs, the sauropods, they were definitely obligate quadrupeds. Their extremely large bodies made it necessary for them to walk around on four legs. Alternately, animals like the carnivorous theropods were entirely bipedal. Some of them, like Tyrannosaurus Rex, had incredibly small front limbs. There's no way that they could have possibly used their arms for walking, making them obligate bipeds. However, there were a lot of dinosaurs that fell in between these two extremes. In fact, all dinosaurs shared a common ancestor at some point and this common ancestor walked on two legs. So, quadrupedal dinosaurs evolved from bipedal ones. At some point, the obligate quadrupeds like the sauropods would have had facultative bipedal ancestors. One group that falls more or less in the middle of the spectrum is the duck-billed Hadrosaurus. Our understanding of the way Hadrosaurus move has changed a lot since they were first discovered. Early on it was thought that Hadrosaurus stood straight up on their hind limbs using their tails for support. Well we don't think that's the case anymore. We'll come back to this group of dinosaurs shortly. Take a look at these two illustrations of tyrannosaurus rex. Here he's standing up right with his tail on the ground. In this one his back is parallel to the ground. Which of these stances do you think is more accurate? The image of tyrannosaurus with his tail off the ground is the most accurate. Dinosaurs most definitely did not drag their tails. In fact, most reptiles do not drag their tails when they move around. But how to we know that dinosaurs didn't drag their tails? We see plenty of preserved dinosaur footprints, but we do not see the impressions of their tails on the ground
