A. Comparisons between land animals and ocean-going animals of the Mesozoic era
B. Comparisons between sauropods and modern animals
C. Possible reasons why sauropods became extinct
D. New theories about the climate of the Mesozoic era
NARRATOR:Listen to part of a lecture in a zoology class.
FEMALE PROFESSOR:Your reading for today touched on dinosaur fossils from the Mesozoic era... which ended about 65 million years ago.Today, we'll 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're 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 weighed up to 100 tons-20 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 100 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's 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... uh, well... large animals can't easily get rid of excess body heat... but for an ocean-going whale, that's not a problem; for a 100-ton land animal, it can be.
For years we've 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'd assumed, there was much less plant life for sauropods to eat than we'd thought.So now... well, we're looking at other-we... we're-we're 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?
MALE STUDENT:[respectful, but unconvinced] Does... mm, do scientists actually know about sauropods, from looking-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-w-well, it just seems...more like guessing.
FEMALE 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 "test" 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're 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 smoothed stones... for years, we thought these were gastroliths-they looked 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 1% of their total body weight in gastroliths.But we've been able to determine that the stones found with sauropods total much less proportionally... less than a tenth of 1% of their body weight.
So now, we're not quite sure what these sauropod stones were used for.It could be they were accidentally 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 of 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.
原文定位：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.
文章一开始就明确了文中接下来说的事情是sauropods 并且表明了用和modern animal compare的方法。