# 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:** 3
**Author:** ![breakneck](https://avatars.discourse-cdn.com/v4/letter/b/e99b99/32.png) [@breakneck](https://boards.straightdope.com/u/breakneck)\ **Post date:** [August 14, 2013, 12:11am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/41 "2013-08-14T00:11:16Z")
> [@Lemur866](#): > > The long-necked sauropods–that is, brontosaur type creatures–were only one type of dinosaur. And we think that only some of them used their long necks to feed on trees. Some of them used their long necks to stand still and eat acres of ground cover while only moving their heads. > > And have you ever been to a museum to see dinosaur skeletons? They were really big, but not hundreds of feet. > > We used to have the idea that the big sauropods were aquatic or semiaquatic like hippos, but now we know that they lived on dry land and ate land plants. > > As for the notion of yearly global floods, think about it. How does a polar ice cap form? When snow falls in the winter, and doesn’t melt in the summer. If the snow melts in the summer you don’t have an ice cap, you have winter snow. So even if there’s a lot of snow in one winter, you’re not going to have enough snow to lower global sea levels, and cause floods when that winter’s snow melts. > > And besides, the current evidence is that the world was generally much warmer in the age of Dinosaurs than it is today. That means that there were no polar ice caps–even at the north and south poles you’d have bare ground (or sea) in the summer. Dinosaurs lived in Antarctica, which was forested at the time. Sea levels were higher because of the absence of polar ice caps, what is now the American great plains was a shallow inland sea during the Cretaceous. So why were there no ice caps at the poles then? maybe because the earths axis was of one of much different as it is now? ---
**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 14, 2013, 12:26am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/42 "2013-08-14T00:26:15Z")
> [@breakneck](#): > > So why were there no ice caps at the poles then? maybe because the earths axis was of one of much different as it is now? No, the axis was the same. The continents were in a different configuration, though, and the atmosphere had more greenhouse gasses to hold heat. Really, consider why we have polar ice caps now. At the North, we have a basically circular ocean with limited exchange with the rest of the seas. This allows cold water to build up in the north. In the south, we have a basically round continent which allows snow to build up into ice sheets. This configuration was not present during the Mesozoic. ---
**Author:** ![Great\_Antibob](https://avatars.discourse-cdn.com/v4/letter/g/e47c2d/32.png) [@Great\_Antibob](https://boards.straightdope.com/u/Great_Antibob)\ **Post date:** [August 14, 2013, 12:26am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/43 "2013-08-14T00:26:45Z")
> [@breakneck](#): > > So why were there no ice caps at the poles then? maybe because the earths axis was of one of much different as it is now? No. Temperature alone explains it pretty nicely. Venus has an almost vertical axial tilt yet no polar ice caps. The massive temperatures have something to do with that. Likewise, the amount of ice at Earth’s poles don’t directly depend on its axial tilt. Look: you’re trying to create something that merely _sounds_ plausible. That’s nice and all, but you should try looking at actual facts instead of sticking stubbornly to an idea that doesn’t fit the data. For one, the amount of tilt in the Earth’s axis has been studied at length. While it has varied a small bit, there’s no evidence it has varied nearly as much as you suggest. Worse, a meteor impact that could kill the dinosaurs without killing all life on earth would simply be too small to affect earth’s tilt. And as noted above, theories about absurdly large plants and flooding is pretty easily debunked by the geologic record. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [August 14, 2013, 12:40am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/44 "2013-08-14T00:40:52Z")
> [@AndrewL](#): > > You are reading that correctly. Birds evolved from “lizard-hipped dinosaurs”. I suspect that the two branches of dinosaurs were named before it was known for sure that birds had evolved from dinosaurs, and the resemblance of the hips of “bird-hipped dinosaurs” to the hips of actual birds was coincidental or a case of convergent evolution. Long before. In fact really, we shouldn’t be lumping the two together as “dinosaurs”', just a mistake by the guy who coined the name. ---
**Author:** ![Wendell\_Wagner](https://avatars.discourse-cdn.com/v4/letter/w/8491ac/32.png) [@Wendell\_Wagner](https://boards.straightdope.com/u/Wendell_Wagner)\ **Post date:** [August 14, 2013, 2:00am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/45 "2013-08-14T02:00:43Z")
Are there some basic books that we can recommend that breakneck read about evolution, dinosaurs, mass extinctions, the organisms and geology of the K-T period, and the K-T mass extinction? I think that breakneck is going to need to learn a lot more than we can explain in one thread. Also, breakneck, I’d like to suggest that you take a course on writing. I mean ordinary nonfiction expository writing, not fiction writing. You need to do a better job at expressing yourself. If you’re out of school, see if you can find some place that will teach adults such classes. ---
**Author:** ![snowthx](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snowthx/32/10660_2.png) [@snowthx](https://boards.straightdope.com/u/snowthx)\ **Post date:** [August 14, 2013, 2:06am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/46 "2013-08-14T02:06:38Z")
> [@Wendell\_Wagner](#): > > Are there some basic books that we can recommend that breakneck read about evolution, dinosaurs, mass extinctions, the organisms and geology of the K-T period, and the K-T mass extinction? I think that breakneck is going to need to learn a lot more than we can explain in one thread. Also, breakneck, I’d like to suggest that you take a course on writing. I mean ordinary nonfiction expository writing, not fiction writing. You need to do a better job at expressing yourself. If you’re out of school, see if you can find some place that will teach adults such classes. Never mind reading some books; a scan thru the very high-level Wikipedia article on dinosaurs should at least provide a foundation and launching point to some of the core concepts the smart dopers here have already provided. Even that, tho, will take _some_ effort. ---
**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 14, 2013, 2:17am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/47 "2013-08-14T02:17:33Z")
> [@DrFidelius](#): > > No, the axis was the same. The continents were in a different configuration, though, and the atmosphere had more greenhouse gasses to hold heat. Yes. The presence of ice at the poles has much more to do with the configuration of continents, how ocean currents transfer heat around the globe, and the level of greenhouse gases in the atmosphere than it does with axial tilt. Since the K-T event the climate has varied from much warmer than today in the Eocene to much colder during the Pleistocene all with essentially the same axial tilt. ---
**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 14, 2013, 2:36am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/48 "2013-08-14T02:36:06Z")
Okay, after the large dinosaurs were taken out of the picture mammals evolved to fill the niche of large animal in both the prey and predator groups. The largest land mammals have been dwarves compared the giants of the dinosaurs. So question one is what allowed the large land dinosaurs to become so much larger than any land mammal and what factor(s) drove that as an adaptive niche to fill? [This article](http://www.wired.com/wiredscience/2011/01/jumbo-shrimps-why-mega-mammals-still-looked-puny-next-to-the-biggest-dinosaurs/) offers the speculation that the air sacs that birds still have were the key in allowing some dinosurs to become so large, allowing for greater cooling and from an engineering standpoint, lightening the load, that and long necks: > [@](#): > > air sacs like those seen in modern birds. Modified from simpler versions present in the last common ancestor of the sauropod and theropod dinosaur lineages, way back down the evolutionary tree, these air sacs would have been beneficial to the giant dinosaurs in several ways. In addition to allowing the dinosaurs to breathe much more efficiently than a mammal of comparable size, they may have also acted as an internal cooling system that would prevent the large sauropods from overheating. These physiological boons may have allowed the sauropods to have more active metabolisms than previously thought. > > From a structural point of view, though, the air sacs significantly lightened the skeletons of these dinosaurs without sacrificing strength. The intrusions of the air sacs reduced the density of sauropod skeletons and gave them enough of an engineering edge to push the limits of body size. Mammals lacked this advantage, as well as the physiological benefits the air sacs brought with them, and this may explain why the biggest mammals of all time were still small compared to the largest dinosaurs. > > Air sacs are not the entire picture, though. As argued in a recent, comprehensive review by paleontologist Martin Sander and colleagues, one of the key, early features of the sauropod dinosaurs was a long neck. Rather than extensively chewing food as mammals do – something that requires a big, heavy head and therefore limits possible neck length – sauropods just horked down plant food and processed it inside their digestive systems. By vacuuming up plant food, the heads of sauropods remained small and their necks were able to become longer and longer to reach plants that could not be plucked by other herbivores. In turn, this affected the evolution of air sacs within the body and opened up evolutionary options not available to large mammals. The basic construction of dinosaurs made all the difference. Fine. It doesn’t answer the second part though. What advantage was there to being that large? That I suspect has a decent WAG out there. The next question though is more interesting, is then informed by the first two, and is close to what I think our op is asking. Got it that the larger animal niche was mostly filled by some _relatively_ mega-mammals. But birds still have that structural advantage and some birds have indeed gotten a bit big in eras since … but emu and ostrich big, not much bigger. Birds have also gotten smart. [Corvids are freaky smart](http://www.nytimes.com/2011/02/01/science/01angier.html?_r=2&hpw&), especially given the size of their brains. So given that birds have same structural advantages that allowed for mega size that their dinosaur ancestors had, and that they have the capacity to develop significant intelligence out of a brain that does not need to even weigh that much, why in all this time, throughout all the environments and ecological systems that birds have existed in, have no birds evolved to fill the very large animal niche? Why _aren’t_ there any birds that have become even very large mammal sized, let alone megadinosaur sized? ---
**Author:** ![shijinn](https://avatars.discourse-cdn.com/v4/letter/s/cc9497/32.png) [@shijinn](https://boards.straightdope.com/u/shijinn)\ **Post date:** [August 14, 2013, 2:36am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/49 "2013-08-14T02:36:19Z")
> [@](#): > > … Baby sauropods did not start out large. They emerged from eggs smaller than a soccer ball and grew at astonishing rates of up to 12 pounds (5.4 kg) per day, according to Michael Novacek, a senior vice president at the museum. … how did they arrive at the growth rate? surely fossil records aren’t accurate to the day? ---
**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 14, 2013, 3:41am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/50 "2013-08-14T03:41:07Z")
[Here’s one speculation](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC64714/) as to what was special for the dinosaurs that birds have not had: > [@](#): > > the top carnivorous dinosaurs were still 12 times heavier, and the herbivorous dinosaurs 1.5–3 times heavier, than predicted from the regression lines from Fig. ​Fig.11A for extant ectotherms; therefore, the never-since-surpassed size of the largest dinosaurs remains unexplained. Perhaps the answer has to do with high net primary productivity during the dinosaur era: atmospheric CO2 levels then were up to 10 times the present levels (37) and elevated CO2 stimulates productivity of some plants (38). Not sure I buy that though. ---
**Author:** ![breakneck](https://avatars.discourse-cdn.com/v4/letter/b/e99b99/32.png) [@breakneck](https://boards.straightdope.com/u/breakneck)\ **Post date:** [August 14, 2013, 5:16am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/51 "2013-08-14T05:16:29Z")
> [@Wendell\_Wagner](#): > > Are there some basic books that we can recommend that breakneck read about evolution, dinosaurs, mass extinctions, the organisms and geology of the K-T period, and the K-T mass extinction? I think that breakneck is going to need to learn a lot more than we can explain in one thread. Also, breakneck, I’d like to suggest that you take a course on writing. I mean ordinary nonfiction expository writing, not fiction writing. You need to do a better job at expressing yourself. If you’re out of school, see if you can find some place that will teach adults such classes. Well Wendell, thanks for the advice. It s a freakin forum , not an essay . Really thanks hahahahaha these means s\*it to me mearly entertainment . Like I’m going to spend time on proofread this crap. ---
**Author:** ![Wendell\_Wagner](https://avatars.discourse-cdn.com/v4/letter/w/8491ac/32.png) [@Wendell\_Wagner](https://boards.straightdope.com/u/Wendell_Wagner)\ **Post date:** [August 14, 2013, 9:12am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/52 "2013-08-14T09:12:16Z")
So you think that you can write any nonsense you feel like, but we have to give you well-thought-out replies? I think you’re one of those posters who think that because you write very fast that we can read your posts very fast. We can’t. Your posts are hard to understand. Furthermore, most of us don’t spend very much time on things like grammar and spelling because we have an enormous amount of practice in writing. We can do a quick check of our posts just before we hit “Submit Reply” to correct any small mistakes. You clearly need more practice in writing. I don’t think that asking breakneck to read the Wikipedia entries on the appropriate subjects is going to be much use. Those entries assume some basic knowledge that he doesn’t have. He needs to read some lower-level books on these subjects. I don’t think, for instance, that Stephen Gould’s books are going to be the best place to start. ---
**Author:** ![the\_diego](https://avatars.discourse-cdn.com/v4/letter/t/7bcc69/32.png) [@the\_diego](https://boards.straightdope.com/u/the_diego)\ **Post date:** [August 14, 2013, 9:55am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/53 "2013-08-14T09:55:03Z")
I don’t recall any A-B-A or A-B-C-A successions on the fossil record. That would have been worth studying. But I might be wrong. ---
**Author:** ![breakneck](https://avatars.discourse-cdn.com/v4/letter/b/e99b99/32.png) [@breakneck](https://boards.straightdope.com/u/breakneck)\ **Post date:** [August 14, 2013, 11:13am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/54 "2013-08-14T11:13:57Z")
> [@wendell wagner](#): > > so you think that you can write any nonsense you feel like, but we have to give you well-thought-out replies? I think you’re one of those posters who think that because you write very fast that we can read your posts very fast. We can’t. Your posts are hard to understand. Furthermore, most of us don’t spend very much time on things like grammar and spelling because we have an enormous amount of practice in writing. We can do a quick check of our posts just before we hit “submit reply” to correct any small mistakes. You clearly need more practice in writing. > > I don’t think that asking breakneck to read the wikipedia entries on the appropriate subjects is going to be much use. Those entries assume some basic knowledge that he doesn’t have. He needs to read some lower-level books on these subjects. I don’t think, for instance, that stephen gould’s books are going to be the best place to start. well check it out wendell. The caps are probally making your skin curl. You sit there and tell me these things i need to do or that i am not . I got only one thing to tell you to do … That is you dont need to read my posts . Plain and in simple english hahahaaha true i dont have enormous amounts of practice in writing . But what i do for everyday doesnt allow me the luxury to brush up. Like i said entertainment ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [August 14, 2013, 11:33am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/55 "2013-08-14T11:33:05Z")
> [@](#): > > . I got only one thing to tell you to do … That is you dont need to read my posts [Agreed](http://boards.straightdope.com/sdmb/profile.php?do=ignorelist). ---
**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 14, 2013, 11:59am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/56 "2013-08-14T11:59:45Z")
Excuse me. Between **breakneck** ’s poor writing skills, lack of consideration for the needs of others who might be trying to read his/her posts, etc., and the snark directed at him, there IS still a set of open questions that his post minimally evoked. I can guess what the advantage to very large might be - large can be a defense as much as fast or armored can be, especially if foraging is abundant. And large prey drives adaptive increase in size from predators to neutralize that defense. But WHY have birds so rarely returned to large sizes and then to large sizes that are not even very large by large land mammal sizes, let alone by the bar their ancestors set? ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [August 14, 2013, 12:12pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/57 "2013-08-14T12:12:58Z")
Differing competition. After the K-T event, big brains became the fashion rather than big bodies. ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [August 14, 2013, 12:52pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/58 "2013-08-14T12:52:53Z")
> [@DSeid](#): > > But WHY have birds so rarely returned to large sizes and then to large sizes that are not even very large by large land mammal sizes, let alone by the bar their ancestors set? My guess? Hollow bones. Once you’ve evolved those, it’s hard to evolve back. ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [August 14, 2013, 12:56pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/59 "2013-08-14T12:56:01Z")
Dinosaurs had hollow bones. ---
**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 14, 2013, 1:14pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/60 "2013-08-14T13:14:10Z")
Even the largest terrestrial birds today are descended from flying species. I would not be surprised if there are any number of developmental pathways needed for flight which constrain the possibilities for future adaptations. [Previous page](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138.md?page=2) [Next page](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138.md?page=4)