Reason for massive size of Dinosaurs etc.

Could anybody explain to me, why dinosaurs and even other species hundreds of millions of years ago were so incredibly large? Whatever paleontologists dig out these days seems to be several times bigger than today's 'equivalent' mammal or bird for example. What's more confusing, todays animals would probably not be able to move or live if they reached sizes close to dinosaurs. Very naively asked, could it be that the fossil size was just an artifact of let's say 2-fold or 3-fold 'dilution'? The 'bones' found would still indicate that the extinct animal was very large, but maybe not THAT MUCH? Are there any plausible explanations, why we have to trust the size of fossils and that they couldn't possibly have expanded over time (=100 million years)? I read about the theory of a lower earth gravity, too. Does this make more scientific sense? Are there any indications or maybe even proofs that for example a T-Rex was truly 4 meters (13 feet) tall and not just 2 meters (6.5 feet)? 6-7 feet would still be tall, wouldn't it? I am obviously not a paleontologist but just curious to know what makes scientists so sure to trust what they dig out?

Thanks, Dan

the process of permineralization, which is the method of preservation of most dinosaur bones, does not cause them to change scale. The original bone is buried and then mineralized water seeps through, replacing small cavities of the original material washed away with crystallized minerals.

In many cases, in fact, the process of fossilization tends to reduce scale, such as an originally three-dimensional trilobite squished nearly flat between layers of shale. I know that in one case, with Elrathia kingi trilobites in Utah, the calcite crystals that grew in the cavities left by the original trilos actually did expand the razor-thin carapace molds to quite a bit thicker than they were originally. The increase in thickness, however, is on the order of millimeters, and were it applied to a dinosaur bone, you would end up with at most the same amount of thickening on the outside of the bone

There were land mammals which rivalled the dinosaurs in size also a large bird used to live on Madagascar.

The simple answer to your first query is that sometimes an animal is better able to survive the bigger it is. So Evolution and Natural Selection will see to it that this happens. So certain animals become

Well, close enough. Odonata emerged in the Upper Carboniferous. When did Meganeuron (which I believe had a wingspan of about one foot)

If mammals had as much time to evolve as dinos ( I'm not counting the precenozoic period, in which mammals weren't able to grow big because of the rivaly of dinos), the would have probably exceed bigger sisez. What I mean is, let's wait a couple of milion of years and there would arise some really big species of mammals. Sorry for my English. Cheirs.

Yes, you are right. Mammals have been able to dominate the large-size ecological niches for the last 65 million years. In that time there have been a number of very large land mammals (Indricotherium for example), all of which are now extinct. Don't forget the largest animal in the whole history of the Earth is the blue whale and this species is

Before I posted my question here, I found this link: http://microlnx.com/dinosaurs/size_distribution/ Hotton.jpg , where the median weight for mammals is estimated to be 631 grams and for dinosaurs 1.99 tons. So, this huge difference in median size between dinosaurs and mammals is only an effect of the longer time period that dinosaurs 'ruled' the world? Nothing else? According to you, the average size of mammals will approximate towards that of dinosaurs over the next millions of years, right? Is there any evidence? Comparable trends over time or such? I have to admit that it's still a little hard for me to believe, that there's really no difference between the sizes of most dinosaurs and the sizes of most mammals throughout the eras, (at least if we only focus on land animals).

We really don't know. Dinos were all egg layers. Mammals aren't. The biggest dinos had teeth to bit off food and 'chewed' it in a gizard. All they did all day long was bit and swallow. The climate was different. Part of the time you had a super continent then two super continents when the big one came apart. Then these came apart. These most likely did affect things. How they affected things is at best a semi educated guess.

I think I know the explanation for the large difference between the median weights of the two different groups. It is all to do with the long power struggle for domination of the large sizes. The mammals were defeated for a very long time because the dinosaurs became bigger, faster than the mammals. The only niches left for the mammals were the rodent sized ones. The battle began some 245 million years ago after a sudden mass extinction event. The dinosaurs were better suited to exploit the larger sizes at that time, the mammals couldn't compete. During the Mesozoic age the dinosaur group never needed to become smaller to survive, so on the whole they stayed large. (However the birds are an exception here.) Whereas the mammals stayed relatively small for a very long time indeed. It seems that even to this day many mammalian species still dominate the smaller sized ecological niches simply because they can. So this explains the large difference in average weight between the two groups.

These two numbers are not directly comparable. In the modern fauna mammals occupy both high body weight and low body niches. In the Mesozoic fauna dinosaurs occupied the high body weight niches, and were (mostly) absent from low body weight niches.

You also want to be concerned about the bias introduced by the megafaunal extinctions of the last 100,000 years - adding Mastodon, Mammuthus and Megatherium would shift the mammal average significantly - and about the effects of taphonomic biases.

The median weight of mammals is as I assume was estimated only on living animals and dinosaur one on all dinosaurs ever living. If we would count only dinosaurs that are, let's say, 167.8 mya the result would be less than 1,99 t and if we count all the mammals in the history of their evolution the result would be more than 631 grams.