Did dinosaurs of the Cretaceous suffer from gigantism?

Corvidius

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Of course there doesn't need to be a selective pressure driving every genetic change, or always a reason. A mutation can propagate if it doesn't affect the animals fitness, so long as it is able to pass those genes on.
I think there could also be the case that we are seeing an animal in evolutionary transition. At one point whales still had visible vestigial hind legs, and if their evolution had started a few tens of millions of years later than it did, then we may see whales at that stage now and wonder what use these vestigial limbs were, without the benefit of seeing, as we actually do of course, where evolution led them. Maybe abelisaurs and tyranosaurs were in the process of completely loosing their arms, but due to the extinction we only see them forever frozen at this transitional stage. Just as the potential supreme rulers of the planet, the troodontids, became frozen in time before they invented lasers and such....
 
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Todd Feinman

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Oct 2013
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I think there could also be the case that we are seeing an animal in evolutionary transition. At one point whales still had visible vestigial hind legs, and if their evolution had started a few tens of millions of years later than it did, then we may see whales at that stage now and wonder what use these vestigial limbs were, without the benefit of seeing, as we actually do of course, where evolution led them. Maybe abelisaurs and tyranosaurs were in the process of completely loosing their arms, but due to the extinction we only see them forever frozen at this transitional stage. Just as the potential supreme rulers of the planet, the troodontids, became frozen in time before they invented lasers and such....
That makes the most sense, I think. Birds turned theirs into wings.
 

Bart Dale

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Dec 2009
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Yet the arms of maniraptors increased in size while their tails did not. As to why the arms of tyranosaurs and abelisaurs got smaller, nobody knows, and I've never heard it suggested that it was to reduce the size of the tail. The arms of the alvarezsauridae, like Mononykus, also became extremely small, but their tails did not.
Can you give examples? I saw articles that showed arm bones became smaller in proportion to leg bones with increased body size.

Anytime you are off the center of gravity, you need to compensate. You could increase your tail mass or use muscles to compensate. In general, compare to humans, the arms of bipedal dinosaurs seem smaller in proportion to the leg.

If you look at flighless birds like the ostrich, instead of the having their heads level with the ground like proposed for the bipedal dinosaurs, their heads are held up high. This is in part because the tails of ostriches are small. While an upright head gives better view, it also would increase wind resistance to the neck.
 
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Corvidius

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Therizinosaurs have huge arms and claws, and Megaraptor also has large arms. Having large arms and claws is a major defining character of maniraptors. In fact as some types became ever more bird like, their arms, ever more wing like, got even longer and, those that became birds, gradually reduced the length of their tails until it became just the stump we call a pygostyle.

One of the defining characters of all dinosaurs is having a long "S" shaped neck. On some, like ankylosaurs this is not evident from their external appearance, but it is there. Ostriches look very similar to the non avian ornithomimosaurs which have a long upright "S" shaped neck. T.rex seems not to have an "S" shaped neck, but the "S" is hidden by muscle and skin/scales/feathers. If you look at a drawing of a bird skeleton with the outline of it's appearance overlaying the bones, the skeleton often don't look anything like the life appearance of the bird, unlike us where our skeleton actually looks like us. So the dinosaur neck is nothing to do with bipedalism, it's just something they evolved for reasons that are lost in the past.

All birds, including ostriches, have a more upright stance than non avian dinosaurs, it's only slight, but as all of them fly, or have ancestors that flew, they still needed their long arms for wings, and as they no longer had a long bony tail, their pelvic structure rotated upwards slightly to move the center of mass back, and their femurs rotated to become, in some species, almost horizontal to cantilever the legs and so help in also moving where they balance. Feathers hide this aspect of their anatomy.
 
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Bart Dale

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Therizinosaurs have huge arms and claws, and Megaraptor also has large arms. Having large arms and claws is a major defining character of maniraptors. 
The large claws on the arms of the Therizinosaurs indicates that they were important to their lifestyle and does not invalidate my point. While there was evolutionary pressure to reduce arm size, tnere could be counter evolutionary pressures to keep the arms larger, if they were being used for some critical function. The Therizinosaurs apparently were herbivores, and so might not have need to run as fast - perhaps the claws could have been used for defense.


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In fact as some types became ever more bird like, their arms, ever more wing like, got even longer and, those that became birds, gradually reduced the length of their tails until it became just the stump we call a pygostyle.
This supports my point. The arms were important to keep large for wings, and the tail needed to be reduced for drag, so the posture had to be revised to compensate for center of gravity without an offsetting tail. Birds also lost teeth to reduce weight off the center of gravity. Pterosaurs and bats, not having the same issue, kept their teeth. But in flightless birds, when wings aren't needed, they get reduced.


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One of the defining characters of all dinosaurs is having a long "S" shaped neck. On some, like ankylosaurs this is not evident from their external appearance, but it is there. Ostriches look very similar to the non avian ornithomimosaurs which have a long upright "S" shaped neck. T.rex seems not to have an "S" shaped neck, but the "S" is hidden by muscle and skin/scales/feathers. If you look at a drawing of a bird skeleton with the outline of it's appearance overlaying the bones, the skeleton often don't look anything like the life appearance of the bird, unlike us where our skeleton actually looks like us. So the dinosaur neck is nothing to do with bipedalism, it's just something they evolved for reasons that are lost in the past. 
I think it it is related to avian respiratory system. Birds also have long necks, even birds like robins and sparrows. I recall reading the neck plays a role in breathing, but can't recall exactly what at the moment.

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All birds, including ostriches, have a more upright stance than non avian dinosaurs, it's only slight, but as all of them fly, or have ancestors that flew, they still needed their long arms for wings, and as they no longer had a long bony tail, their pelvic structure rotated upwards slightly to move the center of mass back, and their femurs rotated to become, in some species, almost horizontal to cantilever the legs and so help in also moving where they balance. Feathers hide this aspect of their anatomy.
This is what I was talking about. The birds needed to keep their large arms for their wings, but also needed to reduce the size of the tail, so birds needed to come up with new posture to compensate. I wonder if the differen posture contribute to the fact of why even flightless birds are much smaller than bipedal dinosaurs, a thousand pounds seems to be the max, and birds like the giant moa weren't very fast runners. Ostriches are much smaller than many herd mammals like horses, bison, and not all that much bigger than the equally as fast pronghorn antelopes.
 

Corvidius

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I cannot see what the evolutionary pressure is to reduce arm size in some, and it is only some, large theropods, and even the experts don't know why arms in these species got progressively smaller.

I see what you are saying about the change in bipedal stance in birds, but that is only to do with the loss of the tail, it was not due to a fundamental flaw in basic dinosaur bipedality, and even without a long bony tail, in flightless birds such as terror birds, the synsacrum is almost horizontal, but this is not over all species and there are modern flying birds that have an almost hoizontal synsacrum. Generally, very generally, the larger the bird the more hoizontal it's synsacrum is.

On maniraptorans and their close relatives, I cannot see any reason to do with their horizontal bipedalism for their arms to get longer or shorter. That is just an evolutionary adaptation to what they were doing with their arms. If not much, then shorter, if a lot, like grabbing prey, or climbing trees, then longer, and as already gone over, reduction of the tail is a modification for flight, nothing to do with bipedalism per se.

It seems the case that the dinosaur long neck, and they are only really long in saurischians, is a product of the avian breathing system as all the air drawn in reaches the lungs. If we had very long necks, only part of the air we breath would reach our lungs. Giraffes by the way have a non typical mammalian way of breathing to try to reduce the effects of what is called "dead air", the air that would not reach the lungs in a long necked animal unless it had adaptations. Giraffes simply breath slowly, but it's not a good solution and that is perhaps why there are no other long necked mammals. I already mentioned in another post that the only reason that the sauropod neck could get so long was specifically because of the avian respiratory system. But even in birds like swans or flamingoes, without the avian breathing system I think their necks would need to be shorter, and they wouldn't be able to fly anyway.

If we look at the maximum size of flightless birds, then I would say that two main factors affect it. The first, and most important, is that they are restricted by not having a tail. To get much bigger than about 9 feet is getting into territory where balance becomes a problem when running. I'm sure they could reach T.rex hight if they only ever moved very slowly, but would fall over if they tried move at any speed. They need a tail to provide balance. The second is that they will only grow to match their prey, for instance a terror bird is the right size to run down and kill it's prey, it doesn't need to be as big as a T.rex. My point here is that loss of the tail prevents flightless birds from "reaching their full potential" and they would be better as fully horizontal and tailed bipeds, but that is lost to them, unless Jack Horner and his team makes a breakthrough and we eventually get "Chickenosaurus".
 
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Bart Dale

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Dec 2009
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Thanks. I agree, that without the avian respiratory system, the long necks of the sauropod would not be possible. And I wonder how such a large animal could feed itself without such a long neck? The long neck allowed the huge dinosaurs to reach way down to feed themselves off some bush or plant near the ground. Without a long neck, a really large animal might find it hard to lower its head down far enough to drink from a pool. That all the really huge dinosaurs were lomg necked might not be a coincident.

And I am not really seriously saying that dinosaur bipedalism is inferior or better - imjust wanted to point out that engineering is always a matter of tradeoffs and that is true for biological organism as well a human machines. There is no one "perfect" design that is the best in all circumstances. The dinosaur bipelism allowed for truly large sizes, but it required a large, stiff tail that could be a drawback in some circumstances, such as flight or climbing trees. In the case of bird bipedalism, it probably limited their size, but the overwhelming majority of birds never got large enough that that to matter. The vast majority of birds fly, and flying builds don't approach the limitation in size imposed by avian bipedalism, so they didn't need a form of bipedalism that allowed them to become very large, and eliminating a large tail to reduce drag was important.
 

VHS

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Dec 2015
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Florania
Therizinosaurs have huge arms and claws, and Megaraptor also has large arms. Having large arms and claws is a major defining character of maniraptors. In fact as some types became ever more bird like, their arms, ever more wing like, got even longer and, those that became birds, gradually reduced the length of their tails until it became just the stump we call a pygostyle.

One of the defining characters of all dinosaurs is having a long "S" shaped neck. On some, like ankylosaurs this is not evident from their external appearance, but it is there. Ostriches look very similar to the non avian ornithomimosaurs which have a long upright "S" shaped neck. T.rex seems not to have an "S" shaped neck, but the "S" is hidden by muscle and skin/scales/feathers. If you look at a drawing of a bird skeleton with the outline of it's appearance overlaying the bones, the skeleton often don't look anything like the life appearance of the bird, unlike us where our skeleton actually looks like us. So the dinosaur neck is nothing to do with bipedalism, it's just something they evolved for reasons that are lost in the past.

All birds, including ostriches, have a more upright stance than non avian dinosaurs, it's only slight, but as all of them fly, or have ancestors that flew, they still needed their long arms for wings, and as they no longer had a long bony tail, their pelvic structure rotated upwards slightly to move the center of mass back, and their femurs rotated to become, in some species, almost horizontal to cantilever the legs and so help in also moving where they balance. Feathers hide this aspect of their anatomy.
Should we talk about certain modern birds that are almost 100% dependent on their wings?
Swifts and hummingbirds (also known as hummers for bird fans) are almost 100% dependent on their wings for mobility.
 

Bart Dale

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Dec 2009
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Should we talk about certain modern birds that are almost 100% dependent on their wings?
Swifts and hummingbirds (also known as hummers for bird fans) are almost 100% dependent on their wings for mobility.
While true that some birds have a limited walking ability, many birds are both competent fliers and are are also good awalking. If the hummingbirds had tne need to be able to walk well, I think they could developed such an ability. Bird (and dinosaur0 bidepalism freeded up their arms to become wings, while still retaining the ability to use their legs to get around, either walking or paddling. Birds can readily evolve to a flightless form when conditions are right for it.

In contrast, bats have mostly lost the ability to move around easily on the ground, only the true vampire bats show any agility in moving around on the ground, and no species of bats have evovled to be flightless. I think this because the bats, unlike birds, did not evolve from bipedal ancestors. With bipedalism, birds to have their arms become specialized as wings, but still keep their legs for getting around when not flying.