托福TPO30 Lecture2

Lecture2-Paleontology Narrator Listen to part of a lecture in a paleontology class. Professor As we’ve discussed, birds are apparently descendants of dinosaurs and shared many commonalities with some dinosaur species, like…um…feathers and maybe even flight and of course egg laid. OK. So, many paleontologists, myself included, have wondered about other similarities between dinosaurs and birds. Since adult dinosaur fossils have sometimes been discovered near or on top of nests, we’ve been looking at the dinosaur parenting behavior. Student Parenting behavior, well, that sounds so gentle and caring. But dinosaurs were ferocious reptiles and reptiles don’t take care of their young, do they? Professor Well, some reptiles incubate their eggs, crocodiles do. And as for popular attitudes towards dinosaurs…well, take the Oviraptor for instance. In the 1920s, a paleontologist discovered the fossil remains of a small dinosaur near a nest containing eggs. He assumed the dinosaur was stealing the eggs, so he named it Oviraptor that means egg thief in Latin, which fueled the generally negative public image of such dinosaurs. But by the 1990s, other experts had convincingly made the case that instead of robbing the nest; the Oviraptor was probably taking care of the eggs. You see, dinosaurs’ closest living relatives – birds and crocodiles – display nesting behavior. And dinosaur fossils have been found in postures that we now believe to indicate brooding behavior, that is, sitting on the eggs until they hatch. So we are curious about the type of care dinosaurs gave to their young. And we’d like to figure out which dinosaur parent, the male or the female gave the care. Student Shouldn’t the behavior of crocodiles and birds give us some clues then? Professor Well, with crocodiles, it is the female who guards the nest, and with birds, it depends on the species, it can be the male or the female that takes care the eggs, or both. In over 90 percent of all bird species, both parents take care of the eggs and the young birds. Student But sometimes it’s just the male? Professor Well, exclusive care by the male parent is much less common, but it does occur. Now, for animals other than birds, the care of young by both parents is pretty unusual in the animal kingdom. Males contribute to parental care in fewer than five percent of all mammalian species. It’s even less frequent among reptiles. And exclusive care by the male is very rare. So researchers have wondered about the evolution of male parenting behavior in birds for quite some time. And now there’s research showing that for some of the birds’ dinosaur relatives, it’s likely that the male parent was also in charge of taking care of the eggs. Student How did they figure that out? Professor Well, first they looked at clutch volume, that’s the number of eggs in the nest of crocodiles, birds and three types of dinosaurs, including Oviraptors that are thought to be closely related to the dinosaur ancestors of birds. So when researchers examined fossilized remains of nests, they found that the dinosaurs had larger clutch volumes, more eggs in the nests that is, than most of the crocodiles and birds that were studied. But, and this is important, their clutch volumes matched those of birds that have only male parental care. You see, bird species in which only the males take care of the nest tend to have the largest clutches of eggs. Student So what’s the connection between bird and dinosaur behavior? Professor Well, researchers now believe, because of this study, that the male parenting behavior of these birds might have its origins in the behavior of dinosaurs. Student Based only on evidence of clutch volume size, the number of eggs? Professor No, there’s more. They also examined the fossilized bones of those three types of dinosaurs that were found on or near nests to determine their sex. You see, adult female birds during egg production produce a layer of spongy bone tissue inside certain long bones. And so did female dinosaurs of the kinds that were investigated. This spongy tissue serves as a source of calcium for eggshell formation. But when the dinosaur fossils were examined, there were no spongy bone deposits. Student Meaning that those dinosaurs on the nests were probably adult males who wouldn’t have needed calcium for making eggshells. Professor Exactly. And then there’s this: birds like the kiwi, the ostrich and the emu; they share certain physical characteristics with these dinosaurs. And interestingly, they also show a consistent pattern of nest care by the male. http://en.wikipedia.org/wiki/Origin_of_birds Are Bird really Dinosaurs? http://www.ucmp.berkeley.edu/diapsids/avians.html http://www.wbu.com/chipperwoods/photos/dinos.htm http://www.dino-web.com/birds.html http://9e.devbio.com/article.php?ch=16&id=161.%22 http://www.enchantedlearning.com/subjects/dinosaurs/Dinobirds.html http://www.sciencedaily.com/releases/2013/04/130418104324.htm http://news.nationalgeographic.com/news/2008/04/080424-trex-mastodon.html http://answers.yahoo.com/question/index?qid=20090410204910AAkxv2n