为什么恐龙的体型会这么大?在进化过程中，是什么使它们拥有如此巨大的体型? 为什么今天没有这么大的生物?
Why were dinosaurs so large in size? What in the evolutionary process caused them to be of such large size? Why are there no living organisms of such size today?
译文简介
数字可能非常具有误导性。
正文翻译
Why were dinosaurs so large in size? What in the evolutionary process caused them to be of such large size? Why are there no living organisms of such size today?
为什么恐龙的体型会这么大?在进化过程中，是什么使它们拥有如此巨大的体型? 为什么今天没有这么大的生物?
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Before we start, what you need to realise is that dinosaurs were definitely large, but not so large. You probably know the numbers: the largest land mammals ever are around 6–8 meters long, while the largest dinosaurs were… is it 40 meters?
Damn, what a number!
However, numbers can be veeeery misleading. Below are the second-largest land mammal ever, Indricotherium, and one of the largest dinosaurs, Brachiosaurus:
The difference seems to be incomparable…
However…
Those are two entirely different body shapes: most of the brachiosaur’s length is used up by its enormous neck and tail. To make it fair, I want you to use your two thumbs: place one over the dinosaur’s neck, and the other over the tail (hopefully, you are not reading this from a touchscreen).
And suddenly, enormous becomes quite… normous. Obviously, Brachiosaurus is still larger than Indricotherium, but it’s not four times larger like the numbers would suggest. The real, fair difference between the two is roughly the same as the difference between an elephant and a hippo:
Untorne Nislav, 冷血的生物学家
在我们开始之前，你需要意识到的是恐龙的确很大，但也没那么大。
你可能知道这些数字:最大的陆地哺乳动物大约有6-8米长，而最大的恐龙大概40米。
天啊，好大啊。
然而，数字可能非常具有误导性。下面是有史以来第二大陆地哺乳动物，巨犀(史前生物)，以及最大的恐龙之一，腕龙:
这样的差异似乎大到毫无可比性。然而……
这是两种完全不同的体型: 腕龙的长度大部分都与它巨大的脖子和尾巴有关。为了公平起见，我希望你用你的两个拇指:把一个放在恐龙的脖子上，另一个放在恐龙的尾巴上(希望你不是用触摸屏来阅读的这篇文章)。
突然间，巨大变得更加…巨大了。很明显，腕龙仍然比巨犀大，但并不是数字显示的四倍那么大。两者之间真正的、公平的区别大致上就像大象和河马之间的区别:
Moral of the story: don’t let the body shape mislead you.
So here’s the answer to the “so large” part of your question: because they weren’t.
However, there is still some “true” difference in size to account for. And at least two factors could’ve contributed to it:
1) Different rules of herbivory
In the age of mammals, the most effective strategy of herbivory is grazing.
Tree foliage doesn’t grow like grasses, yet still there’s usually a considerable amount of it per area unit, because it overlaps vertically many times.
Dinosaurs that fed from canopies could afford to grow large: for thermoregulation or defense from predators — usual reasons.
However…
Any animal that grows too big inevitably experiences difficulties with food. At present, any herbivore that became too large would likely just move onto grasses. But dinosaurs couldn’t. Hence, the only solution that they had was to grow necks even longer to get even more foliage. But if you grow a larger neck, you also need a larger tail (for balance). Then, you also need broader and thicker bones for all those muscles to attach, stronger legs to support the extra tonnes of weight, and so on and so on.
Effectively, it was a dead loop: dinosaurs became large, then they grew longer necks to support the growing need for food, which in turn made them become even larger, which in turn further increased their need for food. Browsing herbivory was likely the driving force of sauropod size, and in the end, the only limiting factor was probably the height of the highest canopy.
这个故事的寓意是:不要让身型误导你。
所以这就是你问题中“如此大”部分的答案:因为它们没那么大。然而，在规模上仍然有一些“真实”的差异需要解释。至少有两个因素导致了这一结果:
1. 不同的食草规则
在哺乳动物的时代，最有效的食草策略是放牧。
草原非常有效率。两种最高效的哺乳动物主导的生态系统，是热带大草原和(现已消失的)猛犸象大草原:两者都能养活数量庞大的大型哺乳动物。草类在世界各地以疯狂的速度生长，地球上没有其他食物来源能承载如此大的哺乳动物生物量。
这个故事的寓意:如果你想长得高高大大的，吃草吧。
然而，情况并不总是这样。在恐龙时代，草是不存在的。因此，体型最大的动物被迫采取仅次于食草的策略:植食（browsing）。
树的叶子不像草一样生长，但每个单位面积上仍然有相当数量的叶子，因为它们垂直重叠了很多次。以树冠为食的恐龙能够长得更大: 通常的原因是为了调节体温或防御捕食者。
然而，任何长得太大的动物都不可避免地会在食物上遇到困难。目前，任何变得太大的草食性动物都只能（进一步）锁定在草类上，但是恐龙不能（因为当时没有草）。
所以，如果你的脖子变大了，你也需要一条更大的尾巴(为了平衡)。然后，你还需要更宽更厚的骨头来连接所有的肌肉，更强壮的腿来支撑额外的重量，等等等等。
实际上，这是一个死循环:恐龙变得越来越大，然后它们的脖子变长，以满足对食物日益增长的需求，这反过来又使它们变得更大，反过来又进一步增加了它们对食物的需求。植食性可能是蜥脚类恐龙体型的驱动力，最后，唯一的限制因素可能是最高树冠的高度。
（译注：蜥脚龙是最大型的恐龙，也是地球上出现过的最大的陆生动物。所有的蜥脚龙均有相似的身体结构︰头小，颈极长；躯体庞大而长，粗大的胃肠道；腿粗壮，柱状，用以支撑硕大的身躯；尾颇长，从基部至末端渐细，鞭状。——百度百科）
This one doesn’t really answer the “why sauropods were large?” question, but rather the “why mammals aren’t that large?” question.
A typical sauropod was, effectively, a reproductive frog. It laid dozens if not hundreds of small eggs that hatched into very small babies that had little to do with adults: they occupied very different niches and fed on different food. For sauropods, it killed two problems: firstly, it made pregnancies easy and unnoticeable — which is a factor when you weighed 60 tonnes —, and secondly, it removed competition for food between adults and babies.
In other words, sauropods could afford to become as large as necessary without worrying much about how it would affect their pregnancy and reproduction.
On the contrary, being a pregnant 60-tonne mammal is a nightmare — of a real and deadly kind.
All (placental) mammals bear relatively large offspring. However, if you weighed 60 tonnes, that would be… what, 2 tonnes heavy offspring? Carrying extra 10 kg of weight at the peak of pregnancy is difficult enough for humans, but having to carry 10 extra tonnes is just impossible, unless you are a whale and swim.
Not to mention that it would be a very long pregnancy.
Not to mention that pregnant females require even more food.
Not to mention that the young must be fed, only to grow up to compete with you for the same food later.
Moral of the story: children are expensive… unless you are a frog or a sauropod.
2.生殖限制
这并没有真正回答“为什么蜥脚类动物体型庞大?为什么哺乳动物没有那么大?”。（
典型的蜥脚类动物实际上是一种繁殖蛙（a reproductive frog）。它会产下数十甚至数百枚小蛋，孵出非常小的婴儿，这些婴儿与成年龙几乎没有什么关系:它们占据着非常不同的生态位，以不同的食物为食。
对于蜥脚类动物来说，它解决了两个问题:首先，它让怀孕变得容易而不引人注意。当你重达60吨时，怀孕是一个重要因素；其次，它消除了成年恐龙和幼龙之间的食物竞争。
换句话说，蜥脚类动物有足够的能力变得足够大，而不用太担心这会影响它们的怀孕和繁殖。相反，怀孕的60吨重的哺乳动物是一场噩梦——一场真实而致命的噩梦。
所有(有胎盘的)哺乳动物都生育相对较大的后代。然而，如果你重达60吨，那你的后代应该……有2吨重吧? 在怀孕末期携带额外的10公斤体重对人类来说已经够困难的了，而要携带额外的10吨是不可能的，除非你是一只会游泳的鲸鱼。
这还不提怀孕的时间很漫长。
这还不提怀孕的雌性需要更多的食物。
这还不提必须喂养幼崽，长大后会和你争夺同样的食物。
这个故事的寓意是:养孩子是很昂贵的，除非你是一只青蛙或者蜥脚类动物。
There are two problems with this. Firstly, it just doesn’t happen. There are relatively small newborns in some placental mammals, but nothing like the difference between sauropod adults and babies.
Secondly, if babies are too small, then they become unavailable for social interactions: in fact, they are better to stay away from parents immediately to avoid being stomped on. Social behaviour and learning are the backbone of mammalian success. Trying to get rid of it just isn’t worth it.
So in the end, dinosaurs - that weren’t really all that large - were large because they bred like frogs and because their kitchen was… a little underrepresented.
生小一点的宝宝怎么样?
这有两个问题。首先，它不会发生。一些胎盘哺乳动物的新生儿相对较小，但与蜥脚类动物成年和幼崽的区别完全不可同日而语。
其次，如果婴儿太小，他们就无法进行社交活动:事实上，他们最好马上离开父母，以免被践踏。然而，社会行为和学习能力是哺乳动物成功的支柱，试图摆脱它是不值得的。
最后，恐龙，其实也没那么大。之所以会这么大，是因为它们像青蛙一样繁殖，而且它们的厨房（食物来源），有点被人所忽视了。
Gotta note that parenting strategies, or lack thereof, in sauropods are entirely speculative. We certainly don’t have evidence of whether or not sauropod young were guarded by their parents. We know they were mobile when they hatched and didn’t require feeding, but lots of reptiles guard their young. Sauropods may have as well.
需要注意的是，在蜥脚类动物中，是否有育儿策略完全是推测。
我们当然是没有证据证明，蜥脚类动物的幼崽是否被它们的父母保护着。我们知道它们在孵化时很灵活，不需要喂养，但很多爬行动物会保护它们的幼崽。蜥脚类动物可能也有。
Possibly — sure; but extremely unlikely. The size difference between the babies and the adults is too big. They wouldn’t’ve been effective at guarding against the predators that could’ve targeted the young (small ones), not to mention the risk of stomping.
Many reptiles guard their young, and many dinosaurs probably did, but sauropods specifically are among the last on the list of possible candidates.
If we suddenly found out that sauropods did guard their young regardless, that would’ve been one of the weirdest things ever…
确实有这可能性，但小得可以忽略不计。
婴儿和成年龙的体型差异太大了。它们不能有效地防范那些可能以幼崽为目标的掠食者，更不用说踩扁它们的风险了。
许多爬行动物会保护它们的幼崽，也许很多恐龙也是如此，但蜥脚类动物是最后一个可能的候选者。
如果我们突然发现蜥脚类动物确实会保护它们的幼崽，那将是有史以来最奇怪的事情之一……
I like your answer. But your descxtion of evolution sounds a bit too purposeful.
我喜欢楼主的答复。
但你对进化的描述听起来有点太有目的性了。
It’s just an abstraction. One could hardly account for every minute detail of evolution and preserve the readability at the same time.
这只是概述。
人们很难在解释进化的每一分细节的同时，又保持可读性。
You know that. And I know that.
It's a handy tool. But many people in the general public never get to read any account of an evolutionary process that doesn't take this shortcut.
The result is that the impression of purposeful evolution is very widespread.
你知道这点，我也知道。
这种方法很方便。但是许多普通大众从来没有读到过任何关于进化过程的描述，后者没这种捷径。
结果是，有目的性的进化的印象非常普遍。
If you grow a long neck, you need a long tail for balance —- yet giraffes have pretty short tails, down to their hocks, nothing like a Diplodocus. Why?
如果你长出长长的脖子，你就需要长尾巴来保持平衡。
然而长颈鹿的尾巴很短，也一点也不像梁龙。为什么?
Giraffe neck is not that long really. They are very tall, but half of it is thanks to legs. Their neck is nothing like in sauropods.
Giraffe neck grows upright, and is supported by elastic ligament that keeps it that way. A fair question then is why dinosaurs didn’t do the same? Well, some tried something similar (Brachiosaurus, for example, which had relatively short tail compared to Diplodocus, for example). But overall, firstly, they just evolved long necks differently (giraffes didn’t have fleshy tails in the first place); and secondly, giraffe’s stiff, ligament-suppported neck wouldn’t be very useful to sauropods. Giraffe is actually a very slender and light animal, so it moves around and uses its height to get the canopy right above itself. Sauropods were massive. They depended on very flexible, muscular neck to pick up as many foliage as possible everywhere around them from where they were standing: because moving 60 tonnes around is… well, expensive.
1.长颈鹿的脖子其实没那么长。它们非常高，但一半要归功于腿。它们的脖子和蜥脚类动物的不可同日而语。
2. 长颈鹿的脖子直立生长，由弹性韧带支撑，使其保持直立。一个合理的问题是，为什么恐龙没有这样做呢? 一些恐龙尝试了类似的方法，例如，腕龙的尾巴与梁龙相比相对较短。但总的来说，首先，它们只是进化出了不同的长脖子(长颈鹿本来就没有显而易见的尾巴); 其次，长颈鹿僵硬的、由韧带支撑的脖子对蜥脚类动物来说用处不大。
长颈鹿实际上是一种非常纤细轻盈的动物，所以它四处走动，利用自身的高度来获得头顶上的树冠。
蜥脚类恐龙是庞然大物。它们依靠非常灵活、肌肉发达的脖子，从它们站立的地方尽可能多地采摘周围的树叶:因为移动60吨重的身体真的……很费力。
Nice answer. You should add in response to his last question that there is a mammal today that is larger than any known dinosaur, and indeed, is the largest creature ever known to live. That is, of course, the blue whale. While it's only 30 meters in length as compared to your mentioned dinosaur's 40, it does weigh a massive 173 tonnes.
不错的答复。但你应该回答提问者的最后一个问题，现在有一种哺乳动物比任何已知的恐龙都要大，而且确实是已知最大的生物。
那就是，蓝鲸。
虽然它只有30米长，而你提到的恐龙有40米长，但前者重达173吨。
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I just thought that aquatic mammals is not what the OP had in mind.
Interestingly though, blue whale is indeed fairly much bigger than sauropods: not long neck, no long tail.
我只是觉得水生哺乳动物不是提问者想要的。
有趣的是，蓝鲸确实比蜥脚类动物大得多:脖子不长，尾巴也不长。
Do we know for sure that there weren't larger whales at the time of dinosaurs?
我们能确定恐龙时代没有更大的鲸鱼吗?
We don't, and can never, know for certain. There may very well have been animals with bones of pure cartilage and no teeth that grew to even greater sizes. However, there is no evidence to suggest it.
That's the great thing about science. If new evidence comes into play, what we know can be changed or dexed entirely. Science never picks and chooses evidence; rather, it incorporates it all.
What we know right now says blue whales are the largest creatures to ever exist on earth, but a new find tomorrow may tell us of an ancient sea animal that grew to 100 meters and 200,000 tonnes. We would, of course, have to reconsider what we know about animals and size limitations if that were the case.
我们不能，也永远不可能确切知道。
很可能有些动物的骨头是纯软骨，没有长得更大的牙齿。然而，并没有证据表明这一点。
这就是科学的伟大之处。如果有新的证据出现，我们所知道的可以被完全改变或过时。科学从不挑选证据。相反，它包含了这一切。
据我们目前所知，蓝鲸是地球上有史以来最大的生物，但明天的一项新发现可能会告诉我们，一种古代海洋动物能长到100米和20万吨。
当然，如果是这样的话，我们将不得不重新考虑我们对动物和大小限制的了解。
I love how there is grass in the picture of the Sauropod.
我喜欢蜥脚类动物照片上的草。
Well yeah. Magic, I suppose:)
Something did grow on the ground in the age of dinosaurs for sure. But nothing like modern grasses that could withstand ruthless grazing.
是啊，我想这是奇迹吧/笑
恐龙时代的地面上确实长了一些东西。但无法像现代草那样能经受得住无情的放牧。
I think it’s more that people cannot even imagine a world without grass.
我认为更重要的是，人们甚至无法想象没有草的世界。
Then, when did appear grass on Earth? Is it a modern plant?
那么，草是什么时候出现在地球上的呢?这是一种现代植物吗?
Grass first began appearing during the Cretaceous period, though was not the dominate plant it is today that took a few million more years
草最早出现在白垩纪时期，虽然不是当时的主要植物，但也花了几百万年才出现。
Then it became one the most successful plants in the history of Earth. Almost everywhere.
那它就是地球历史上最成功的植物之一。
几乎无处不在。
Isaac Asimov said aliens viewing Earth may assume grasses are the dominate form of life on the planet. The grasses have enslaved creatures that constantly tend grasses and expand their ranges. [humans being the slaves
艾萨克·阿西莫夫说，外星人看到地球可能会认为草是地球上主要的生命形式。
草奴役了生物，使其不停地播种和扩张草场范围。人类也被其奴役了。
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What I don’t see in your answer, is the composition of air back then. Lots more oxygen, more fuel for bigger engines…
在楼主的回答中，我没有看到当时空气的成分。
动物变得更大的动力是，需要更多的氧气和食物……
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Oxygen concentration is not the size-limiting factor in vertebrates. It’s a popular misconception.
It’s an insects’ problem, not dinosaurs’ or mammals’.
氧浓度并不是限制脊椎动物体型的因素。这是一个普遍的误解。
这是昆虫的问题，不是恐龙或哺乳动物的问题。
No, I actually meant the opposite, I assumed that more oxygen means more fuel available, therefore bigger engines /animals possible. So not limiting, but expanding.
不，我认为恰恰相反，我认为更多的氧气意味着更多的食物，因此更大的动物成为可能。
所以没了限制，自然体型就变大了。
I got it — just my poor choice of words. Not affecting factor then. Vertebrates are very effective at delivering oxygen to cells virtually regardless of their size and oxygen concentration available. You can’t limit vertebrate size by cutting oxygen in atmosphere, or vice versa. Our respiration just doesn’t work like that. Unlike insects, where there’s a direct correlation.
It, of course, might not be a pure coincidence: for example, trees might’ve been taller and more numerous on average at that time, which allowed sauropods to grow larger and also caused the oxygen spike.
我懂你的意思，我用词不当，但氧气浓度不是影响因素。
脊椎动物在向细胞输送氧气方面非常有效，无关细胞的大小和氧气浓度如何。你不能通过减少大气中的氧气来限制脊椎动物的体型，反之亦然。我们的呼吸不是这样工作的。不像昆虫，它们之间有直接的联系。
当然，这可能是一个纯粹的巧合:例如，树木在当时可能更高，平均数量更多，这让蜥脚类动物长得更大，也导致了氧气达到峰值。
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Actually it’s more complicated than “higher oxygen = bigger insects” so more research is still currently going on. I would say it’s one of the contributing factors, but not the decisive one. High oxygen level does not account for the large monstrosities like the titanopterans (imagine a cross between a grasshopper and a mantis), which lived just right after the P-T extinction (see that big drop in the line chart between 300 and 200 million years ago?). The Mesozoic - age of dinosaurs - also had only moderately-sized insects, despite the massive oxygen level.
实际上，这比“高氧含量=大昆虫”要复杂得多，所以目前还在进行更多的研究。
我认为这是其中一个因素，但不是决定性因素。高氧气含量并不能解释像泰坦翼龙这样的大型怪物(想象一下蝗虫和螳螂的杂交)，它们生活在P-T大灭绝之后。中生代，恐龙时代，也只有中等大小的昆虫，尽管氧气含量很高。
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The first reason is quite simple. Plant eating dinosaurs grew bigger to out compete each other and avoid predators. The bigger they got the less predators they have. Just look at Sauropods who are practically invincible. But this still does not explain why they got much larger than their modern day counterparts. why weren’t most sauropods elephant sized? Why weren’t theropods the size of bears or large cats? I will now explain this in more depth.
Scientists once thought that high oxygen content allowed dinosaurs to grow so large but recently it has been discovered the oxygen was actually lower than present day.
I believe the answer to why dinosaurs grew so big rest on the shoulders of the sauropods or long necks.
The first dinosaurs of the Triassic were actually very small. One group of dinosaurs evolved to be larger so that they could avoid predators. These were the pro-sauropods which includes the famous Plateosaurus. As these dinosaurs got bigger they had relatively few predators and naturally the carnivorous theropods had to get bigger so that they could hunt them.
Joshua Walters史前野生动物终身粉丝
第一个原因，很简单。
植食恐龙长得更大以在竞争中取胜，并避免捕食者。体型越大，捕食者就越少。只要看看几乎无敌的蜥脚类动物就知道了。
但这仍然不能解释为什么它们比现代的同类大得多。为什么大多数蜥脚类动物不能只长到大象那么大?为什么兽脚亚目食肉恐龙不能只长到熊或大型猫科动物那么大?我现在将更深入地解释这一点。
科学家们曾经认为高氧含量是恐龙能够长到如此巨大的原因，但最近发现氧气含量实际上比现在要低。
我相信恐龙长得这么大的原因在于蜥脚类动物的肩膀和长脖子。
三叠纪的第一批恐龙实际上非常小。有一群恐龙进化成体型更大的恐龙以躲避掠食者。这些是蜥脚类动物的近亲，包括著名的板龙。随着这些恐龙长大，它们的掠食者相对较少，自然兽脚亚目食肉恐龙必须变大才能捕食它们。
Once the Cretaceous hit more of the herbivores evolved with weaponry and stayed around elephant sized. Ceratopsians, ankylosaurs and the fast hadrosaurs were the primary large herbivores. Sauropods declined a little but the titanosaurs, which include the largest dinosaur Argentinosaurus ruled. Some predators of the early Cretaceous like the carcharodontosaurids grew very large to hunt the titanosaurs while the tyrannosaurs evolved with devastating bites to hunt armored prey. Many of these dinosaurs had to keep their large size to deal with their large predators but like the Jurassic there were many smaller dinosaurs that adapted to fit different niches than their larger cousins.
One problem many have with large dinosaurs like sauropods is the fact they needed so much food to support their bodies. it has been recently suggested instead of chewing their food these behemoths swallowed it whole. They could store vast amounts of food in their stomachs where they broke it down. Without the need to chew their heads could become smaller and lighter and thus their necks could get longer to reach higher vegetation and avoid competition from other herbivores. Their size forced predators to get bigger and with bigger predators other herbivores needed to become larger and develop weaponry to deal with these predators.
在侏罗纪晚期，我们看到更大的兽脚亚目恐龙，如异龙和蛮龙，出现来捕食这些更大的猎物。食草动物也发生了变化。随着蜥脚类恐龙越来越大，一些更特殊的恐龙，比如剑龙，获得了阻止掠食者的武器。也就是说，一些植食恐龙体型很小，这样它们就可以快速隐蔽地躲避掠食者。同样地，许多兽脚亚目食肉恐龙为了捕食这些速度更快的猎物，体型也变小了。
自白垩纪以来，更多的食草动物进化出了武器，保持了大象的体型。角龙、甲龙和动作迅速的鸭嘴龙是主要的大型食草动物。蜥脚类动物的数量减少了一些，但是雷龙，包括阿根廷龙统治了世界。白垩纪早期的一些掠食者，比如鲨齿龙长得非常大以捕食雷龙，而暴龙则进化出毁灭性的咬合力来捕食带甲的猎物。许多恐龙必须保持体型庞大以应对大型掠食者，但像侏罗纪时代一样，许多体型较小的恐龙比体型较大的表亲适应了不同的生存环境。
像蜥脚类恐龙这样的大型恐龙都有一个问题，那就是它们需要太多的食物来支撑身体。最近有人提出，这些巨兽不是咀嚼食物，而是整个吞下。它们可以在胃里储存大量的食物并将其分解。由于不需要咀嚼，它们的头会变得更小更轻，因此它们的脖子会变长，以够到更高的植物，避免来自其他食草动物的竞争。它们的体型迫使掠食者变得更大，而面对更大的掠食者，其他食草动物也需要变得更大，并发展出对付这些掠食者的武器。
So to sum it up carnivorous dinosaurs were fast bipedal predators. The slower quadruped herbivores had to get bigger to scare off these predators and eventually became the sauropods. As a result the carnivores got bigger as well and the cycle continued. other herbivores developed armor so they did not have to compete with the sauropods for food but were still safe from predators. While this was happening some dinosaurs stayed small to fill the gap left by their bigger relatives.
Hope this helps!
为什么今天没有类似的大型动物呢?好吧，看看大象，它是不可战胜的海盗。它为什么要变大呢?我能想到的原因只有一个，人类。即使这些动物有机会进化成更大的体型，我们仍然会杀死它们，因为我们开发了它们永远无法匹敌的技术。另外，由于世界上的污染，较小的尺寸实际上是有利的。许多较小的动物仍能生存在这个世界上，而像鲸鱼这样的大型动物却很难改变。
综上所述，食肉恐龙是迅捷的两足食肉动物。速度较慢的四足食草动物必须变大才能吓跑这些掠食者，最终成为了蜥脚类动物。结果，食肉动物也变大了，如此循环。其他食草动物也进化出了甲胄，所以它们不必与蜥脚类动物争夺食物，但仍然可以免受捕食者的伤害。与此同时，一些恐龙保持体型较小，以填补其体型较大的亲戚留下的空缺。