m**d
发帖数: 21441 | | w********r
发帖数: 14958 | 2 看不到图。
回答标题提出的问题， 因为史前地球氧气含量比现在高得多。 35%妥妥的 | m******m
发帖数: 308 | 3 没人吃，不怕目标明显。
【在 m**d 的大作中提到】
| m*****d
发帖数: 13718 | 4 问题不对，是巨大的史前生物基本都被淘汰了
【在 m**d 的大作中提到】
| m***y
发帖数: 14763 | 5 "都”不对吧，三叶虫也是屎前的，不比老汉一坨屎大。 | H********g
发帖数: 43926 | 6 好像一些动物一辈子只要不死就会一直长大。鱼啊什么的。据说袋鼠也是。可能恐龙普
遍还没演化出停止生长的基因。 | H********g
发帖数: 43926 | 7 The skeletons of most mammals reach a certain size and then stop growing.
However, many animals, including some mammals, keep growing throughout their
lives. Kangaroos, for example, just keep growing and growing until they die
. Most fish, amphibians, lizards, and snakes are also indeterminate growers.
Until something, disease, a predator, or old age-takes them down, these
animals know no bounds when it comes to size.
What about that goldfish in your home aquarium? With plenty of food, no
predators, and a disease free environment, what’s to stop it from growing
right out of the tank? Only its innards. The fish’s skeleton may keep
growing, but its organs can only keep things going until the body gets too
big for its own good. The size of the body is constrained by the ability of
the organs to support it.
【在 H********g 的大作中提到】
: 好像一些动物一辈子只要不死就会一直长大。鱼啊什么的。据说袋鼠也是。可能恐龙普
: 遍还没演化出停止生长的基因。
| b***u
发帖数: 12010 | | p*******5
发帖数: 6446 | 9 终于明白了，博导是史前生物
【在 m**d 的大作中提到】
| h*******u
发帖数: 15326 | 10 跟你屎一样大的虫，也是巨虫
【在 m***y 的大作中提到】
: "都”不对吧，三叶虫也是屎前的，不比老汉一坨屎大。
| R***a
发帖数: 41892 | 11 博导只有部分是史前的
【在 p*******5 的大作中提到】
: 终于明白了，博导是史前生物
| k**l
发帖数: 2966 | 12 my precious...
【在 R***a 的大作中提到】
: 博导只有部分是史前的
| O*******d
发帖数: 20343 | 13 The ancestors of cetaceans are believed to have been the semiaquatic
pakicetids, no larger than wolves, of about 53 million years (Ma) ago.[11]
By 40 Ma ago, cetaceans had attained a length of 20 m or more in
Basilosaurus, an elongated, serpentine whale that differed from modern
whales in many respects and was not ancestral to them. Following this, the
evolution of large body size in cetaceans appears to have come to a
temporary halt, and then to have backtracked, although the available fossil
records are limited. However, in the period from 31 Ma ago (in the Oligocene
) to the present, cetaceans underwent a significantly more rapid sustained
increase in body mass (a rate of increase in body mass0.259 of a factor of 3
.2 per million years) than achieved by any group of terrestrial mammals.[5]
This trend led to the largest animal of all time, the modern blue whale.
Several reasons for the more rapid evolution of large body size in cetaceans
are possible. Fewer biomechanical constraints on increases in body size may
be associated with suspension in water as opposed to standing against the
force of gravity, and with swimming movements as opposed to terrestrial
locomotion. Also, the greater heat capacity and thermal conductivity of
water compared to air may increase the thermoregulatory advantage of large
body size in marine endotherms, although diminishing returns apply.[5]
Cetaceans are not the only marine mammals to reach unprecedented size in the
modern era. The largest carnivoran of all time is the mostly aquatic modern
southern elephant seal.
http://en.wikipedia.org/wiki/Megafauna |
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