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It's very costly to be a huge animal. Your mass grows in cube when you scale up, but you still only have two/four legs to support the same weight. This increases the pressure that your body needs to cope with.(this is easy to see if you compare an ant with an elephant. The elephants legs are much thicker and strudier in comparison to it's body)

Looking at a T-rex for example, speciemns have been found that are believed to weight more than 9 tonnes, compared to an elephants 10 tonnes. T-rex has ofcorse has only two legs. The heaviest dionsaur is believed to have weight 80 tonnes. That is the weight of about 20 cars on each of their feet.

How could they support such massive weights?

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I've heard that the greater amount of oxygen in the atmosphere was a factor, at least in the large size of the dinosaurs. –  shigeta Feb 13 '12 at 18:21
    
@shigeta Actually atmospheric oxygen was lower then present day throughout the time of the dinosaurs. –  kmm Feb 14 '12 at 16:21

3 Answers 3

up vote 10 down vote accepted

Assuming that gravity was essentially the same (other answers to this question notwithstanding), very large dinosaurs were dealing with the same forces that they would today. There are two clades of dinosaurs in which gigantism evolved, Sauropoda (quadrupdeal sauropods) and Theropoda (including T. rex). Each "solved" the problem of large size in different (but also somewhat similar) ways.

The main reason why large size was not a problem was that, if posture changed to align the forces between the animal and the ground, the bones are compressed. Bone is very strong in compression.

Theropoda

Theropod essential operate as a see-saw, with a large muscular tail balancing a large head. As such, they did not likely use much active muscular force to balance. The analogy is a human standing. Just standing, you don't need much muscle force to balance.

Hutchinson and Garcia (2002) showed that, because of a lack of plausibly large leg musculature, T. rex could not run. For a range of postures, they estimated how much muscle would be required to balance the animal and found that running behavior was unlikely.

Hutchinson, J.R. and M. Garcia. 2002. Tyrannosaurus was not a fast runner. Nature 415:1018-1021.

Sauropoda

Sauropods show many similar adaptations as elephants, the largest extant land mammals. Their limbs were held upright (erect), which requires less energy for balance. Some sauropods had air-filled bones, which would also lighten the skeleton. Wilson and Carrano (1999) document the evolution of posture through sauropod evolution from a biomechanical perspective.

Wilson, J.A. and M.T. Carrano. 1999. Titanosaurs and the origin of “wide-gauge” trackways: a biomechanical and systematic perspective on sauropod locomotion. Paleobiology 25:252–267.

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I love your answer! I have been pondering this for year! Do you have any knowledge regarding their ability to survive a fall (I'm mostly thinking of the two-legged giants) –  Kristoffer Nolgren Feb 14 '12 at 16:48
    
I think this paper (jstor.org/pss/4523665) deals with falling T. rex. –  kmm Feb 15 '12 at 1:11
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This is the best day of my life! –  Kristoffer Nolgren Feb 15 '12 at 9:04
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Ornithopods got just as large as Theropods, right? –  Noah Snyder Jun 9 '12 at 1:59

The reason that there were huge dinosaurs during the Mesozoic Era was that the surface gravity on the Earth was much lower than it is today. A new theory explains how this happened. See www.dinoextinct.com*, click on 'The Gravity Theory of Mass Extinction.'

*Please be aware of a personal association with this resource.

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Hi =) It's always a good idea to summarise the article you are liking to within the question in case it is not available in the future. Furthermore if you're promoting your own website we do ask that you mention that fact in the question (see faq). This may explain the downvotes you are receiving. –  Rory M Feb 14 '12 at 12:25
    
Could the change in gravity were really a significant effects? –  Marta Cz-C Feb 15 '12 at 9:33
    
Current understanding of the Earth's formation suggests that this is unlikely. –  Poshpaws Feb 22 '12 at 15:59

Weight is propotional to volume while strength is proportional to cross section of bones and muscle, i.e. you have a classical square/cube problem. Extrapolating the strogest athletes to the size at which stading becomes a maximal effort indicates a theoretical max (present) size for the planet of around 20,000 lbs while the largest elephants are around 3/4 of that i.e. around 14000 - 15000 lbs. A sauropod dinosaur would be crushed by his own weight in our present world; he'd collapse in a heap and suffocate. Lesser gravity in past ages is the only solution.

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