نطق سوم

Lecture 3 - Sauropod

Narrator: Listen to part of a lecture in a zoology class.

Professor: Your reading for today touched on dinosaur fossils from the Mesozoic era, which ended about 65 million years ago. Today we will be discussing the sauropods.I think our discussion of sauropods will illustrate what we can learn by comparing the fossil record to modern animals. By fossils, we mean traces of prehistoric animals such as bones, which become mineralized, or impressions of bones or organs that are left in stone.

Now sauropods were among the largest animals to exist ever! They were larger than blue whales, which are the largest animals alive today. They weigh up to one hundred tons, twenty times as much as elephants. Also, they were an extremely successful kind of dinosaur. There’s evidence of sauropods in the fossil record for an unusually long time, over one hundred million years. So, why were sauropods so successful?

Biologically speaking, sauropods shouldn’t have been successful. Large animals like elephants, say, they require much more food and energy and have fewer offspring than smaller animals. This makes maintaining a population harder. The largest animals today don’t live on land.

But in the ocean where food is easier to find, a blue whale, for instance, can eat up to 8,000 pounds of food a day. And they give birth only once every few years. We also know that body heat, that… well, large animals can’t easily get rid of excess body heat. But for an oceangoing whale, that’s not a problem. For a 100-ton land animal, it can be.

For years, we have assumed it was the abundant plant life of the Mesozoic that allowed these giants to thrive. However, we now know that since oxygen levels were much lower in the Mesozoic than we assumed, there was much less plant life for sauropods to eat than we thought.

So now, well, we are looking at other… we are, we are trying to understand the biology of sauropods, comparing their fossils to the anatomy of modern animals to get a better idea of how they lived. What we’ve found is that sauropods were experts at conserving energy.

They had enormous stomach capacity, the ability to digest food over a long period, converting it to energy at a slower pace, saving it for later. For animals with small stomachs, it takes lots of energy to constantly look for food and then digest it.With larger stomachs and slower digestion, you don’t need as much energy. Joseph?

Student: Does… do scientists actually know about sauropods from looking at… I mean, how much can we actually learn looking at some ancient bones compared to all we can learn from modern animals? And, comparisons between animals that lived millions of years apart? Well, it just seems… more like guessing.

Professor: There’s always some guesswork when studying extinct animals. But that’s exactly what leads to discoveries, a hypothesis, a type of guess is made. We guess the hypothesis by looking for evidence to support it. Then some questions are answered, which may lead to new questions. For example, let’s look at one of these comparisons.

We know sauropods couldn’t chew food. Their skulls show they had no chewing muscles. Lots of modern animals, like birds and reptiles, also can’t chew food.

They need to swallow it whole. But modern animals have an interesting aid for digesting food.

They swallow stones, stones that are used to help grind up the food before it’s actually digested in the stomach. These stones are called gastroliths. Gastroliths make food easier to digest, essentially smashing food up, just as we do when we chew.Over time, gastroliths inside the animal are ground down and become smooth and rounded.

Now, sauropod fossils are commonly found with smooth stones. For years we thought these were gastroliths. They look just like gastroliths and were found in the area of the sauropods’ stomachs. A recent study measured the gastroliths in modern animals, in ostriches. And the study showed that ostriches need to ingest about one percent of their total body weight in gastroliths.But we have been able to determine that the stones found with sauropods totaled much less proportionally, less than a tenth of one percent of their body weight.

So now we are not quite sure what these sauropods’ stones were used for. It could be they were accidently ingested as the sauropods foraged for food, that they served no real purpose. Other researchers speculate that sauropods ingested these stones as a source of some the minerals they needed, such as calcium.

So the original hypothesis that the stones found with sauropods were gastroliths, even though it hasn’t been supported, has helped us to make new hypotheses, which may eventually lead to the answer.