I'm standing in front of Camarasaurus, a giant Sauropod that could have weighed as much as 10 tons. However, this Camarasaurus like all vertebrates would have started out life as a single cell, a fertilized egg. So how did it get to a single cell to something the size of a house? And how did two house size animals manage to conceive and fertilize more eggs? In this lesson, we'll look at the birth, growth, and reproduction of dinosaurs. >> Which came first? The chicken or the egg? Paleontologists would say the egg. Long before chickens evolved, land dwelling animals developed the ability to lay hard-shelled eggs. This was very different from the fish and amphibians that had dominated the planet up to that point in time. Both of these two groups laid eggs without a protective shell. Can you think of the possible benefits of laying an egg with a hard shell? We can postulate a few advantages for this adaptation. There's water retention and protection from predators, to name a couple. But a disadvantage would be the impact of a hard shell on oxygen exchange. All living cells need oxygen. The first eggshells likely developed because land is so dry. Eggs which are laid in water, like those of a fish or a frog, are surrounded by water, and so will not dry out. Harve, an egg on land will dry out very quickly, hence the need for an eggshell. As animals became terrestrial, spending more and more time out of the water, those animals which could lay hard shelled eggs, had an advantage. They could live father from water, and they didn't have to return to water, to lay their eggs. Dinosaur inherited this hard shelled egg laying ability. Even so, hard shells had to remain relatively thin in order to facilitate oxygen exchange with the air. So even hard shelled eggs remained susceptible to predators. Even today, when you crack eggs to your breakfast, you realize, it's very easy to get into a hard shelled egg. Take a guess at which of these dinosaur families laid the biggest eggs that we know of. A ceratopsians, b stegosaurs, c sauropods, or d theropods. The answer is actually D, the theropods. The largest known egg belonged to an oviraptorosaurid from the Henan and Hubei regions in China. The eggs from this dinosaur were half a meter long. Although sauropods were undoubtedly bigger, they did not produce the largest eggs that we currently know of Although many types of eggs have hard shells they are not a sealed system. Rather than being like a piece of sealed Tupperware where nothing gets in or out, eggs are more like sievs. Where certain small things can pass in or out. For example, while an embryo is inside the egg, its cells are using up oxygen and producing carbon dioxide. If that egg was impermeable, the embryo would use up all of the oxygen inside the egg and suffocate. Instead, an eggshell has very small pores that allow oxygen to come in. And carbon dioxide to go out. You can think of an eggshell as a sort of lung allowing the exchange of oxygen and carbon dioxide. However as eggs get bigger, the inside volume gets larger much faster than the surface area. This principles called the cube-square law. This law has already been mentioned, in this lesson, and we'll see it come up again in a few other places. Basically, the surface area of the egg, or the eggshell, and the inside volume of the egg, have a relationship. As the egg increases in size, the surface area gets relatively smaller in comparison to the volume. As an egg gets bigger, it is essentially outgrowing its lungs. If the lungs are too small, the egg can't breath properly and it will suffocate. Because of this, there's essentially a maximize size that an egg can get, no matter how big the dinosaur might be that laid it.
