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.
I stand corrected.
There have been really big birds in relatively recent times: Phorusrhacidae - Wikipedia
The very biggest dinosaurs were a small portion of the dinosaur population- most were not giants, but more moderately sized.
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.
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.
[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]
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Colibri
General Questions Moderator
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.
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
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 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.
Obligatory comic:
Since an extinct species represents the end of an evolutionary line, “cousins” will continue to evolve independently (till they too get extinct.)
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.
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 is that climate change with increasing seasonality was largely the cause of megafauna extinction. (In contrast to the human over-hunting hypothesis.)
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.
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.
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:
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.
With the talk of an annual freeze cycle, it sounds like a really garbled understanding of varves.
Interesting goalpost moving there.
remotely approached the dinos in size> still not in the range of the megadinosaurs.:rolleyes:
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.
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 compared to 50 to over 100 tons) and the largest ever mammalian carniviore 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.
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.