# why did the dinosaurs not evolve back **URL:** \ **Category:** Factual Questions\ **Created:** [August 13, 2013, 3:34pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138 "2013-08-13T15:34:17Z")\ **Posts on this page:** 20\ **Page:** 5
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [August 16, 2013, 2:36am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/81 "2013-08-16T02:36:48Z")
> [@aNewLeaf](#): > > … Gigantism is useful in some situations but not others … Well that is the point of the question, sort of. Not just useful in some situations and not others, but possible is some and not others as well. No goal directed evolution involved; just curiosity about why a particular niche either no longer existed or was no longer able to be filled. The question is precisely what made that level of gigantism both useful and possible among some dinosaurs and not among any other lines hence, including and in particular their descendents, the birds? What were the critical features that were different about the situations? ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [August 16, 2013, 1:51pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/82 "2013-08-16T13:51:41Z")
Sometimes there’s no critical feature, it’s just a cusp. A coin balanced on an edge is stable. Bump it, and it will fall down. It will fall heads up or tails up, but there’s no ‘why’ to it. If, after, the K-T event, some species somewhere had an advantage due to size, that might have been passed down and expanded. But intelligence became the operative track, the arms race was about brains and not claws. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [August 16, 2013, 4:29pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/83 "2013-08-16T16:29:04Z")
A coin rarely just balances on edge for no reason; it settles into one stable state or the other (an attractor basin if you are familiar with the concept). If you find a coin on its edge for a prolonged period of time there is a reason it is doing that then as opposed to any other time. Mega-size was not a cusp; it was a stable state given the features of dinosaurs, the environments they existed in, and the competition for niches towards the mega state extant at the time. Again, I accept that one sufficient answer may be that the competition for large predator from placental mammals was too tough once they had filled the niche (whether that was because of intelligence or other features), and you don’t get to mega without going through large. I accept that any bird aiming for large herbiviore also had tough competition from then extant ruminants. I also accept as sufficient that fragmenting and relatively erratic ecosystems put mega at an adaptive disadvantage. I do not accept “just because” or “its time was past” or “there is no ‘why’ to it” as sufficient explanations however. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [August 16, 2013, 6:49pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/84 "2013-08-16T18:49:31Z")
> [@Darth\_Ayebaw](#): > > Big herbivores need teeth, so it’d seem that route was cut off. This is not true, as the sauropods demonstrate. Many sauropods had very small weak teeth useful mainly for clipping vegetation. They did not have grinding teeth like mammalian herbivores, but did most processing in the gizzard. A toothless beak would have served them just as well. In fact, the lack of heavy duty teeth and big chewing muscles is what allowed them to have tiny heads at the end of long. necks, and the long neck was a very efficient way to gather food over a large area while minimizing movement. This is thought to be one of the factors that allowed big sauropods to such extreme sizes. ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [August 16, 2013, 6:58pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/85 "2013-08-16T18:58:12Z")
The sauropods were anomalous lizard-hipped dinosaurs, as most of the lizard-hipped were carnivorous and the sauropods were herbivores. All the bird-hipped dinos (as far as I recall) were herbivores. ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [August 16, 2013, 7:00pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/86 "2013-08-16T19:00:15Z")
> [@DSeid](#): > > I do not accept "… “there is no ‘why’ to it” as sufficient explanations however. This gets us back to directed evolution, or possibly intelligent design. Evolution does not have a ‘why’ component, that’s fundamental to the theory. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [August 17, 2013, 8:09pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/87 "2013-08-17T20:09:59Z")
You are using “why” in an odd way for discussions of science. Indeed evolutionary mechanisms are evoked all the time for “why” questions. Asking why is a basic tool of science. Answering why involves formulating hypothesis, making predictions, forming models, and testing them. “Why” does not involve intelligent design or directed evolution. There is a why, for example, marsupial predators were dominant in certain regions and then mostly went extinct. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [August 17, 2013, 8:25pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/88 "2013-08-17T20:25:21Z")
> [@aNewLeaf](#): > > Evolution does not have a ‘why’ component, that’s fundamental to the theory. Of course it does. Scientists ask why a feature evolved all the time. [QUOTE=aNewLeaf] Sometimes there’s no critical feature, it’s just a cusp. A coin balanced on an edge is stable. Bump it, and it will fall down. It will fall heads up or tails up, but there’s no ‘why’ to it. If, after, the K-T event, some species somewhere had an advantage due to size, that might have been passed down and expanded. [/quote] There is always a reason why particular features are found in organisms. They may not be _adaptive_ reasons, based on selection for those features. They may be due to developmental constraints, or because they are linked to some other feature that is adaptive. It is extremely unlikely that differences in size between organisms are due to no particular reason. > [@](#): > > But intelligence became the operative track, the arms race was about brains and not claws. This is just wrong. ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [January 13, 2015, 4:09pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/89 "2015-01-13T16:09:56Z")
bumping because[there was just a great lecture that addresses some of this](https://www.youtube.com/watch?v=6HqiOrZEyMQ&feature=youtu.be) - specifically, dinosaur size. Dinosaurs could get so big, compared to what mammals can do, for a few reasons: [ul] [li]they had more efficient lighter skeletons than mammals (they were air-filled but just as strong)[/li][li]their metabolism wasn’t as high as mammals[/li][li]bearing live young puts a governor on size - the bigger the animal, the longer the gestation period, which limits total offspring[/li][li]quality of good (leafy) vegetation. The biggest land animals are always herbivores, and plants back then were almost all leafy, not grassy.[/li][/ul] Of course, the video lecture I just linked to is a lot better than my list. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [January 13, 2015, 5:11pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/90 "2015-01-13T17:11:26Z")
Zombie Dinosaurs! Aarrrgh! (sorry, had to) ---
**Author:** ![Kamino\_Neko](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kamino_neko/32/34_2.png) [@Kamino\_Neko](https://boards.straightdope.com/u/Kamino_Neko)\ **Post date:** [January 13, 2015, 6:01pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/91 "2015-01-13T18:01:17Z")
> [@DrDeth](#): > > In fact really, we shouldn’t be lumping the two together as “dinosaurs”', just a mistake by the guy who coined the name. Not actually true. Dinosauria are a perfectly valid clade, at least by current theories - Ornithischia and Saurischia are (believed to be) more closely related to each other than to other Archosaurs (such as pterosaurs and crocodiles). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 13, 2015, 6:30pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/92 "2015-01-13T18:30:47Z")
> [@](#): > > Not actually true. Dinosauria are a perfectly valid clade, at least by current theories… As long as you include birds. ---
**Author:** ![Kamino\_Neko](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kamino_neko/32/34_2.png) [@Kamino\_Neko](https://boards.straightdope.com/u/Kamino_Neko)\ **Post date:** [January 13, 2015, 7:14pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/93 "2015-01-13T19:14:23Z")
> [@Chronos](#): > > As long as you include birds. Oh, definitely. I wouldn’t even think of excluding them…‘bird’ just automatically gets filed under ‘dinosaur’ for me, these days. Which is, ironically, why didn’t specifically include them…it just didn’t occur to me it might be confusing otherwise. :smack: (Hell, half the reason I chose crocs as the other Archosaur example was it amused me that of living groups, they’re more closely related to birds than to lizards - even if it’s a fairly distant relation.) ---
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [January 15, 2015, 3:33pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/94 "2015-01-15T15:33:57Z")
> [@DSeid](#): > > **Colibri** , your explanation seems most apt. The first birds were smallish predators and the next larger size up niche was filled up fast by larger mammal predators who could outcompete them. The path to predator gigantism was blocked realtively early on. Sorry for the late reply, but this struck my eye. It’s my understanding that “true” birds were around for many millions of years before the end of the Cretaceous. Mammals were small and unable to radiate out into the large-animal niches occupied by dinosaurs (avian and nonavian). I don’t think it makes sense to posit that large mammals outcompeted the first birds–there were no large mammals until birds were well-established. ---
**Author:** ![bup](https://avatars.discourse-cdn.com/v4/letter/b/6bbea6/32.png) [@bup](https://boards.straightdope.com/u/bup)\ **Post date:** [January 15, 2015, 4:11pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/95 "2015-01-15T16:11:18Z")
> [@Sailboat](#): > > Sorry for the late reply, but this struck my eye. It’s my understanding that “true” birds were around for many millions of years before the end of the Cretaceous. Mammals were small and unable to radiate out into the large-animal niches occupied by dinosaurs (avian and nonavian). I don’t think it makes sense to posit that large mammals outcompeted the first birds–there were no large mammals until birds were well-established. It depends what you mean by ‘large,’ and in this instance, where ‘large’ means the next size up from small bird predators, it’s not far-fetched. The artists’ depictions of the Cretaceous always have tiny shrew things in the corner looking fearfully up from under a plant, but there were mammals up to 20 kilos (according to the lecture I linked above). [Here’s a 14 kg / 30 lb. predator](http://en.wikipedia.org/wiki/Repenomamus) from the early Cretaceous. ---
**Author:** ![breakneck](https://avatars.discourse-cdn.com/v4/letter/b/e99b99/32.png) [@breakneck](https://boards.straightdope.com/u/breakneck)\ **Post date:** [January 10, 2018, 11:29am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/96 "2018-01-10T11:29:10Z")
> [@Great\_Antibob](#): > > What do you mean by “evolve back”? The descendants of (some of) the dinosaurs are still around and kicking. > > You may as well ask why human beings don’t go back to being unicellular life forms. > > Most weren’t. Most mammals aren’t elephant sized, either. > > Look at mammal fossils from 10 million years ago. We tend to put the larger specimens in museums and those are the ones we remember. It doesn’t mean 10 MYa animals were all large. > > No, and why would they have to be? Do cows eat larger plants than sheep? Do cows eat larger plants than squirrels? Or do they just eat more than their smaller counterparts? > > Also, some dinosaurs (just like some modern animals) didn’t feed on plants. > > Ask the same question for meat-eaters. Do large meat-eating animals always eat larger prey? > > The largest single creature to ever exist on planet earth is the blue whale. It’s much larger than any other creature that’s ever existed. And it exists right now and feeds on some of the smallest animals on earth: krill. > > Study up on physics and biology and get back when you have developed it further. Right now, your theory doesn’t hold up to basic facts. well what about girafes? why do they have long necks and so tall? Anyways i was not talking about mammals and how they evolved i was talking about how dinosaurs evolved . Then they are wiped out with a mass extinction . Why did they not evolve again . Like they did the first time around? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [January 10, 2018, 11:51am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/97 "2018-01-10T11:51:37Z")
> [@breakneck](#): > > well what about girafes? why do they have long necks and so tall? Anyways i was not talking about mammals and how they evolved i was talking about how dinosaurs evolved . Then they are wiped out with a mass extinction . Why did they not evolve again . Like they did the first time around? The thread already contains the answers to your question. Exact repetition of anything that happened once is incredibly unlikely. Superficial repetition (different things repeating some of the same general forms, because those forms fit in niches) is more likely - and it’s what we see. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [January 10, 2018, 12:10pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/98 "2018-01-10T12:10:18Z")
Hey, the OP’s question evolved back! ---
**Author:** ![RickJay](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@RickJay](https://boards.straightdope.com/u/RickJay)\ **Post date:** [January 10, 2018, 12:29pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/99 "2018-01-10T12:29:56Z")
Dinosaurs didn’t evolve again for the same reason the Chicago Cubs didn’t win the World Series again in 2017; whatever the conditions, there’s a random element to evolution. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [January 10, 2018, 12:45pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/100 "2018-01-10T12:45:39Z")
> [@Darren\_Garrison](#): > > Hey, the OP’s question evolved back! If zombie evolution is real, why aren’t there any dinosaur zombies!? [Previous page](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138.md?page=4) [Next page](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138.md?page=6)