# 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:** 4
**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:** [August 14, 2013, 1:24pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/61 "2013-08-14T13:24:23Z")
> [@Alessan](#): > > My guess? Hollow bones. Once you’ve evolved those, it’s hard to evolve back. > [@aNewLeaf](#): > > Dinosaurs had hollow bones. Yes, as I tried to point out earlier, scientists think hollow bones were one of the advantages that enabled the really big dinosaurs to _become_ so big. So hollow bones do not rule out large size. ---
**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, 1:49pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/62 "2013-08-14T13:49:17Z")
I stand corrected. ---
**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, 1:55pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/63 "2013-08-14T13:55:30Z")
> [@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? There have been really big birds in relatively recent times: [Phorusrhacidae - Wikipedia](http://en.wikipedia.org/wiki/Phorusrhacidae) The very biggest dinosaurs were a small portion of the dinosaur population- most were not giants, but more moderately sized. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [August 14, 2013, 1:57pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/64 "2013-08-14T13:57:08Z")
> [@the\_diego](#): > > 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. Mammoths started out as relatively small creatures that evolved into something much larger. Towards the end of their existence, they evolved back into small creatures in some areas (and died out in others) as their food supply and other resources dwindled. It’s not quite A-B-A since the pygmy mammoths weren’t exactly the original species that the mammoths evolved from, but it’s kinda close and it’s interesting as well. ---
**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, 2:24pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/65 "2013-08-14T14:24:09Z")
I wouldn’t say that mammoths evolved back to smaller sizes, as that makes it sound as if the entire population shrank. It is more correct to say that the last known surviving population of mammoths (on Wrangel Island) were a dwarf variety. Dwarfism is very common in isolated island populations. Oddly, so is gigantism. ---
**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, 2:43pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/66 "2013-08-14T14:43:13Z")
[QUOTE=breakneck] 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. [/QUOTE] > [@breakneck](#): > > 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 **Moderator Note** **breakneck** , I am a moderator and I _do_ get to tell you what to do. When people have the good grace to give you well-thought out answers to your questions, don’t respond by mocking them. This is an adult forum. If you don’t make a minimal effort with the spelling, grammar, and punctuation of your posts you are unlikely to get a positive response in the future. Any more posts of the kind above may get you a warning. Please put more thought into what you post in General Questions in the future or I will consider closing your threads. Colibri General Questions Moderator ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [August 14, 2013, 2:51pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/67 "2013-08-14T14:51:29Z")
> [@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? This is simple mechanics. The structures which allow flight also have a large surface to volume ratio and lifting surface to weight ratio, and the scaling is not linear, plus the required joint strength at the wing grows out of proportion, which requires more musculature. Proportionally, a 90 kg person would have to have a wingspan in excess of 8 m (~27 ft) just to generate enough lift to glide, never mind actually taking off from the ground. There is just little evolutionary pressure for birds to get considerably larger, and it is worth nothing that the largest birds in existence today are also either flightless or endangered. > [@engineer\_comp\_geek](#): > > It’s not quite A-B-A since the pygmy mammoths weren’t exactly the original species that the mammoths evolved from, but it’s kinda close and it’s interesting as well. Nothing can literally “evolve backwards”; the genome just doesn’t accept direction to return to a previous state. It does contain genes for phenotypes of predecessor species–hence, why human fetuses developed both gills and a prehensile tail–but while those are often expressed as part of embryogenesis, the genetic and epigenetic instructions for developing these phenotypes to a functional state are lost or modified. A species might evolve to return to a condition similar to a previous state, such as from small to large to small, but this is just straight adaptation, not regression to a prior state (although by having certain prior features it may be easier to redevelop them). Stranger ---
**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, 3:20pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/68 "2013-08-14T15:20:05Z")
> [@DSeid](#): > > Why _aren’t_ there any birds that have become even very large mammal sized, let alone megadinosaur sized? Modern birds are descended from carnivorous dinosaurs. The largest dinosaurs and mammals are herbivores (since a lot more plant food is available than prey). Few modern birds have become pure herbivores. Flying birds are limited by the fact that eating vegetation relies on eating a large volume of food and a prolonged digestive process to break down cellulose. They can’t afford retaining a large amount of food in the gut or the added weight of the complicated digestive apparatus itself. Very large flightless herbivorous birds have only evolved where ruminant mammals were absent, that is, the elephant birds of Madagascar and moas of New Zealand. On continents they may have problems competing with herbivorous mammals with digestive systems that are better adapted to break down cellulose. In competition with large mammalian carnivores, non-flying birds may be limited by the lack of functional forelimbs to help catch prey. (Admittedly, this did not stop tyrannosaurs.) Also they may be limited to the role of cursorial predator by their body plan rather than stealth predator. The one place that very large non-flying predatory birds evolved was South America before it became connected to North America when the isthmus of Panama formed, where the [phorusrhacids](http://en.wikipedia.org/wiki/Phorusrhacidae) were among the largest predators. However, their main competitors were marsupial predators, which may have been limited by having a lower basal metabolism than placental predators. ---
**Author:** ![Malthus](https://avatars.discourse-cdn.com/v4/letter/m/85f322/32.png) [@Malthus](https://boards.straightdope.com/u/Malthus)\ **Post date:** [August 14, 2013, 9:01pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/69 "2013-08-14T21:01:37Z")
Obligatory comic: > **[Birds and Dinosaurs](https://xkcd.com/1211/)** > > Sure, T. rex is closer in height to Stegosaurus than a sparrow. But that doesn't tell you much; 'Dinosaur Comics' author Ryan North is closer in height to certain dinosaurs than to the average human. ---
**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, 11:54pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/70 "2013-08-14T23:54:46Z")
> [@engineer\_comp\_geek](#): > > Mammoths started out as relatively small creatures that evolved into something much larger. Towards the end of their existence, they evolved back into small creatures in some areas (and died out in others) as their food supply and other resources dwindled. > > It’s not quite A-B-A since the pygmy mammoths weren’t exactly the original species that the mammoths evolved from, but it’s kinda close and it’s interesting as well. Since an extinct species represents the end of an evolutionary line, “cousins” will continue to evolve independently (till they too get extinct.) ---
**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 15, 2013, 2:11am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/71 "2013-08-15T02:11:54Z")
Thanks for the several proffered hypotheses. **aNewLeaf** , Again, no argument that most dinosaurs were not huge. But the huge land animal niche reached sizes with dinosaurs that have not been even remotely approached by land mammals or birds since. The question is why. **Stranger on a Train** , While I am presuming a flightless bird filling the mega niche, it must be noted that flighted birds are not even reaching pterosaur proportions. The metabolic demands of flight certainly does not apply to flightless birds and does not explain why none even pterosaur sized. **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. OTOH that agile flight trick was something for a predator to exploit, a niche with little competition in it. It is very noteworthy that your linked phorusrhacid was unable to successfully compete with large placental mammal predators. And in those cases in which there was no mammalian ruminant to compete with avian prey gigantism was contstrained by needing to only stay large enough to discourage attack by large mammalian predators … and since land mammal gigantisim is more constrained the size arms race stopped relatively early on. Is there _any_ example that you know of in which the result of a marsupial and a placental predator being introduced into one or the other’s range resulted in the marsupial being the dominant predator? Thanks. ---
**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 15, 2013, 3:00am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/72 "2013-08-15T03:00:16Z")
Also offered up for consideration - has the post dinosaur eras just had too much seasonality and ongoing climate shifts to support that level of gigantism? Note that even for mammalian megafauna (and here we are talking rhino, not apatasaurus sized) [a major prevailing hypothesis](http://www.stanford.edu/group/journal/cgi-bin/wordpress/wp-content/uploads/2012/09/Gibbons_NatSci_2004.pdf) is that climate change with increasing seasonality was largely the cause of megafauna extinction. (In contrast to the human over-hunting hypothesis.) > [@](#): > > Because > of their large size, competition affected > the megafauna more than the smaller > existing species. Larger animals generally require a larger primary habitat, > more forage, and more water to maintain themselves than a smaller species. > Therefore, there are usually only a few of > them in a given area. As Grayson says, > “the inverse relationship between body > size and population size plays a powerful > role in increasing the risk of extinction > faced by larger animals” (Grayson 1991: > 214). Following Guilday’s logic, shrinking habitats forced large animals into > cohabitating smaller areas, containing > insufficient resources to support them > all. The result was the elimination of > some species due to competition. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [August 15, 2013, 3:49am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/73 "2013-08-15T03:49:28Z")
> [@DSeid](#): > > Thanks for the several proffered hypotheses. > Again, no argument that most dinosaurs were not huge. But the huge land animal niche reached sizes with dinosaurs that have not been even remotely approached by land mammals or birds since. The question is why. I showed you the Baluchatherium. Colibri showed you the Phorusrhacids. These animals certainly _remotely approached_ the dinos in size. Yes, the very biggest dinos were bigger, but Mammals and Aves have done fairly well. Not to mention reptiles. ---
**Author:** ![Darth\_Ayebaw](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darth_ayebaw/32/9870_2.png) [@Darth\_Ayebaw](https://boards.straightdope.com/u/Darth_Ayebaw)\ **Post date:** [August 15, 2013, 7:04am UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/74 "2013-08-15T07:04:52Z")
Would lack of teeth be too out there of a suggestion? I’ve read a fair number of articles and essays about how teeth structures contributes or detracts from the survival of species. One example would be the medium sized mammalian predators in the first 10M years after the K-T impacts, which were remarkably successful prototypes, but supposedly eventually doomed to be tread under by carnivora, as their teeth were better. Or so I’ve read. Either way, once birds lost teeth, it would seem almost impossible to get them back, even if crude substitutes have since been developed. Big herbivores need teeth, so it’d seem that route was cut off. Most land predators, and all over a certain size, have also had teeth, while lacking functional forelimbs was by no means a requirement. Just an idea, I’d be very interested in hearing from someone more knowledgeable than I on the subject. As for the megafauna, the climate had been rapidly shifting back and forth for several million years, sometimes in timespans as short as a few kiloyears. Dozens, if not hundreds of swings, and the megafauna grew/shed coats, migrated, adapted. Even the fact that their extinction coincided with our emergence on the scene could be written off as interesting coincidence, if not for the fact that the way it coincided with our migration patterns worldwide is too much to write off. There may have been other factors, it may have been that we were merely stressing an already overtaxed system, but I have yet to see a theory that takes humans out of the equation entirely and still has something new happening in the last 100K years that hadn’t happened repeatedly in the past 2-7M years. ---
**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 15, 2013, 12:58pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/75 "2013-08-15T12:58:06Z")
> [@DrDeth](#): > > I showed you the Baluchatherium. Colibri showed you the Phorusrhacids. These animals certainly _remotely approached_ the dinos in size. Yes, the very biggest dinos were bigger, but Mammals and Aves have done fairly well. Not to mention reptiles. No, the phoruurhacids did not remotely approach the size of the megadinosaurs. Most specimens found have been under 3 ft tall with one found that may have reached 3 m … that one basically ostrich sized but a predator. Impressive, surely something I’d not want to meet, but not remotely mega. Its significance is how it failed to successfully compete with large placental mammal predators. The baluchatherium got closer as the largest land mammal but still not in the range of the megadinosaurs. Interestingly relevant to the paragraph below, it existed during the Oligocene epoch, a fairly stable coolish climate period when most of Erasia was large open fields of plants and deciduous forests. **Darth** , read the linked article for a review of the arguments. [Here’s another recent one](http://www.sciencedaily.com/releases/2013/05/130506181711.htm): > [@](#): > > May 6, 2013 — Most species of gigantic animals that once roamed Australia had disappeared by the time people arrived, a major review of the available evidence has concluded … “The interpretation that humans drove the extinction rests on assumptions that increasingly have been shown to be incorrect. Humans may have played some role in the loss of those species that were still surviving when people arrived about 45,000 to 50,000 years ago – but this also needs to be demonstrated,” said Associate Professor Stephen Wroe, from UNSW, the lead author of the study. … “It is now increasingly clear that the disappearance of the megafauna of Sahul took place over tens, if not hundreds, of millennia under the influence of inexorable, albeit erratic, climatic deterioration,” What I am floating out is that the megacondition species are a poor match for changing and fragmenting environments. The bigger, longer lived, fewer progeny species are at greatest risk from erratic (from an evolutionary time scale) environment, needing large stable foraging areas and unable to adapt quickly enough. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [August 15, 2013, 1:22pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/76 "2013-08-15T13:22:00Z")
> [@Maastricht](#): > > Your scenario of big floods every half year has never been found anywhere. With the talk of an annual freeze cycle, it sounds like a really garbled understanding of [varves](http://en.wikipedia.org/wiki/Varve#Formation). ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [August 15, 2013, 3:44pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/77 "2013-08-15T15:44:38Z")
> [@DSeid](#): > > The baluchatherium got closer as the largest land mammal but still not in the range of the megadinosaurs.. Interesting goalpost moving there. _remotely approached the dinos in size\> still not in the range of the megadinosaurs_.:rolleyes: ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [August 15, 2013, 3:49pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/78 "2013-08-15T15:49:09Z")
Just depends on what you mean. An animal that weighed 10-15 tons ‘approaches’ the size of an animal weighing 25 tons. But I had a sudden insight this morning about evolution in the other direction: Hummingbirds are very tiny dinosaurs. ---
**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 15, 2013, 4:51pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/79 "2013-08-15T16:51:22Z")
If it makes you happy I will withdraw the word “remotely.” The point remans that what passes for megafauna among mammals would be puny for the megadinosaurs with the largest example of a mammalian herbiviore being a fraction of the size of the larger dinosaurian herbiviores ([18 tons](http://en.wikipedia.org/wiki/Paraceratherium) compared to 50 to over 100 tons) and the [largest ever mammalian carniviore](http://dinosaurs.about.com/od/mesozoicmammals/p/andrewsarchus.htm) weighing in at 1/2 to 1 ton compared to say T. Rex which weighed about 7 tons. That largest mammalian herbiviore instructs some by the conditions that allowed for/induced it. But sure, if you want to say that at most a third of the size is more than “remotely” I’ll not argue. The question of why no birds larger (with the largest carnivorous bird being large ostrich sized) given the same structural advantages for gigantism however remains. ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [August 15, 2013, 5:06pm UTC](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138/80 "2013-08-15T17:06:44Z")
1/3 size is in the same order of magnitude, if that makes you feel better. I think the answer to the question about gigantism really brings out the idea of ‘directed’ evolution. Evolution just happens, and the random results compete against other random results. Gigantism is useful in some situations but not others, and after the K-T event, there was no starting pool of giants. Since all the animals started out small, no species had an initial size advantage that later got magnified. [Previous page](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138.md?page=3) [Next page](https://boards.straightdope.com/t/why-did-the-dinosaurs-not-evolve-back/666138.md?page=5)