来自：查字典高考网 2014-12-25
About nautilus and king nautilus
By 1986, Saunders and I had collected tissue from nautiluses around Fiji, Samoa, Australia, New Guinea, the Philippines, Palau, and New Caledonia. Two independent sequencings of their genes yielded the same remarkable result there are only two distinct groups of nautiluses. One is composed of the king nautilus, which appears to have descended from the chambered nautilus about 15 million years ago; the other group is composed of all the other so-called nautilus species.
If the genetic evidence is accepted, it means that the long-agreed classification of the living species has crumbled. The king nautilus represents a different genus altogether, while the differences in shell morphology of the other species seem to be useless in telling them apart. We had gone from 11 living species belonging to one genus--Nautilus--to two genera, Nautilus and our newly recognized genus, with only two or three species between them. We gave the king nautilus a new scientific name, Allonautilus, which is Latin for other nautilus.
These surprising results made us wonder if nautiloid fossils might also have some secrets to reveal. Unable to study extinct nautiloid DNA, we had to figure out a new way to classify these animals based on their shells alone. Previous studies of nautiloids had classified them on relatively few features, and we hoped that we might find more to examine. Luckily for us, just such a trove of distinctive new characters had been uncovered by Neil Landman of the American Museum of Natural History in New York.
The chambered nautilus hatches at a very large size: it emerges from its egg with seven fully formed chambers and a shell diameter of more than an inch, making it the largest invertebrate at hatching in the world. (Indeed, it may have been this trait that allowed it to survive the great Cretaceous mass extinction, for the nautilus appears to lay its eggs in very deep water, where they take a year to hatch. Juveniles or unhatched eggs might have survived in a deep refuge when the great comet ending the age of dinosaurs, 65 million years ago, turned all the shallower oceanic regions into a toxic, heated cauldron of extinction.) When a living chambered nautilus emerges from its egg, it stops growing temporarily, and this pause leaves a distinct groove in its shell. Since the shell wraps around itself as it grows, these earliest stages are always preserved in the middle. Landman began dissecting fossils to see if similar marks were found in extinct species as well. He discovered that not only did these marks occur, but many other features also.
Saunders and I combined Landmans new characters with the classical ones and then began to study their occurrence in living and extinct nautiloids. Both of us had been taught that present-day nautiluses are the most recently evolved of the 10,000 nautiloids that have swum through the oceans over the past 500 million years. Thus we expected them to have a lot of features that had evolved relatively recently. To our surprise, we found that todays chambered nautilus appears to be extremely primitive--rather than being a descendant of some fairly recently evolved nautiloid, the chambered nautilus evolved much earlier. It may even be the ancestor of most of the nautiloids present on our planet for the last 75 to 100 million years.
【阅读辅导：托福考试阅读背景知识(四十八)】相关文章：
[标签：海外留学,语言考试,托福,,]
你高考成绩高吗？这道题目怎能成立？高难度奥数，能不能把你难住
学渣儿子高考，英语选择题全选B！老师通报成绩的那一刻父亲懵了
爆笑班主任 第一季 第221集 高考结束学生有多疯狂？山东王老师疯狂吐槽
高考阅卷名师给考生的高考作文密训课 第5集 高考作文审题实操方法精讲（三）
广州早晨 2019 山西一高中班主任带学生骑行1800公里去上海
探秘历史 第二季 第233集 考英语用来睡觉，结果仍是高考状元，如今她怎么样了？
创艺第二届：2019届高考录取表彰大会暨“核桃音乐节”合影——你只管努力，剩下的交给创艺
一边扔试卷一边玩摇滚？这个学校的高考减压方式，真是帅到没朋友
视频|2019全国高考今日开考: 语文特级教师评析上海卷高考作文
NBA流言收割机 第6集 神预测？高考数学试题暗示猛龙勇士4
