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?
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.
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.
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.
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.
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.
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.
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.
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).
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.)
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).
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.
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.