Old movies had a to portray dinosaurs as lumbering, belly dragging beasts. They were typically very stupid. However, most people today understand that dinosaurs were not like this at all. They were active, mobile creatures. Why has our view of dinosaurs changed so much? Specifically, what is the evidence that we can use to back up our modern understanding of dinosaur locomotion. In this module, we'll review a number of different lines of evidence for just how dinosaurs moved and why we think this to be true. >> Understanding the stance and posture of dinosaurs often involves observing modern animals. For each of these two animals, decide whether each has an upright or sprawling stance. The cat has an erect stance because it holds its legs directly underneath it. The lizard has a sprawling stance because it holds its legs out to either side of its body. So, the correct answer for cat is A, while the lizard is B. >> I have a skeleton of a house cat here in front of me. Cats generally walk with their legs held underneath their bodies. And when you look at them from the front, their legs are relatively straight. We refer to their posture, or stance as erect. Compare the stance of that to a lizard or salamander. A lizard holds it's legs out to the side, and has what we call a sprawling stance. All animals with backbones and four legs came from the same common ancestor, who lived about 400 million years ago. These early, fish-like ancestors had limbs more like muscular fins than arms and legs. It wasn't until later they more salamander like creature evolved. This animals developed sprawling stance and began moving around on land. Later still two different lineages of animals developed. One led to birds and a second one led to mammals. Each of these lineages independently evolved a more upright stance. >> Let's look at two creatures of about the same size. We'll use the example of the cat and lizard again. The cat has an erect stance, while the lizard has a sprawling stance. Which animal uses less energy to move around? In general, the upright stance is more efficient. Creatures with an erect stance use less energy to move around. And, using less energy can be a big evolutionary advantage. In our question, the cat uses less energy to move around. So, A is the correct answer. This erect stance has a number of advantages. Think about when you try to do a push up. When you hold your arms bent, at a 90 degree angle, like that of a sprawling animal, it requires a lot of effort just to keep yourself in that position. However if you straighten out your arms, and hold them directly underneath you, in an upright manner. Holding the position becomes much easier. This also means that in general, animals with a sprawling stance do not get as large as those with an upright stance. Remember though, that this is a generalization and there are plenty of cases where having a sprawling stance can be just as good. Or better than being upright. However, an erect stance is more efficient for moving around on land. Figuring out the stance of a living animal is easy. Simply look at its standing and you can say if it has a prowling or erect stance. However with distinct animals like dinosaurs we have to use inference to determine how they might've held their limbs. Luckily the posture of an animal is well imprinted in its bones. By looking at the articulations, that is where two bones are in contact with one another like at this knee joint. We can see how it would've held its legs and how it could've stood. By using comparative morphology that is comparing the shapes of one animal to another, we can use what we know about modern animals and their stance to inform the understanding of dinosaurs. The earliest dinosaurs all had an upright stance. And most dinosaur lineages kept the stance throughout their evolution. That isn't to say that all dinosaurs walked in exactly the same way, but just the dinosaurs walked with their limbs held directly beneath them. Let's spend a moment talking about how mammals have adapted to take advantage of the erect stance. Take a look at these two animals. >> Which of these animals has legs that are more efficient at supporting weight? The elephant or the cheetah? The elephant has limbs that are more efficient for supporting weight. The cheetah's legs are less sufficient but are better for running at high speeds. A, is the correct answer here.
