Why were dinosaurs so large?

[HydrogenBond]

If we look at the evolution of animal life on earth, animal life started small, got very large during the time of the dinosaurs and then got smaller again. There are still a few large critters but even their average size has decreased, i.e, mammoths to elephants, etc.

One possible explanation has to do with the evolution of the brain. The large body mass helped set the potential for the early brain to grow. In other words, the brain not only sends signals to activate the muscles, but a feedback current flows back into the brain. The large bulk of muscles may have been used for adding extra neural potential to the brain, helping to increase the size of the brain.

As the brain got stronger, its contribution to cellular differentiation control caused cells to stop replicating sooner and sooner, resulting in the gradual shrinking of the average animal size.

[InfiniteNow]

It's an interesting thought HB, but I really don't think there's too much merit there. Brain size really has nothing to do with brain abilities. It tends to be the organization of the brain that's important. While it's nifty to compare the brain to a computer, the analogy is no where near one to one. Also, following your logic, the smaller the animal the smarter it is, and I'm pretty sure you'd agree that this is not the case.

In response to your thread title, I'd say most dinosaurs were large because it provided them an evolutionary advantage. Greater size meant fewer competitors could harm them physically. Also, greater surface area allows for better heat release (cooling), much like the huge ears on an elephant.

[pgrmdave]

I've always thought it seemed like an arms race - a few creatures found it evolutionarily beneficial to be larger, however, because they were bigger, the other creatures now had a stronger evolutionary pressure to become larger. The larger the animals got, the more pressure to be even larger.

[ughaibu]

Hydrogen Bond: Size is also associated with rainfall, there have been periods of giant mammals.

[ughaibu]

(I really wish that we could get rid of the idea that humans are some kind of pinnacle of evolution, as if evolution is directed, such thinking is pretty much the same as any other creationist myth)

[Boerseun]

Originally Posted by InfiniteNow

Greater size meant fewer competitors could harm them physically.

Which only helped until T-Rex came along!

Originally Posted by InfiniteNow

Also, greater surface area allows for better heat release (cooling), much like the huge ears on an elephant.

Not really. For the volume and size, the bigger the body, the better and more economical heat retention works.

From what I've read up to date, it seems that the dinos grew to such proportions simply because of their diet. The herbivorous dinos dined on leaves and twigs, which are very low in nutrients. In order to garner as much as possible from their diet, they had to eat an enourmous amount, which needed a big vessel in which to ferment to release the usable nutrients. Hence, diplodocus and his kin.

Problem, of course, for the carnivoures, is that the bigger the prey became, the bigger they had to get themselves, in order to ensure dinner. And from there you ended up with T-Rex and his buddies.

Mammals could never grow to the same size, because they are warm-blooded. They need even more energy, if only to sustain their body temperature. It's been calculated that a cold-blooded reptile need only 10% of the energy a mammal needs (for the same size, of course) because it don't need to keep itself warm. So, if a diplodocus needs a ton of food a day, for the same body size a mammal would need ten tons. And ten tons of twigs could mean a lot to a reptile, but not to a mammal.

After the dino period ended, the remnants of the dino line could have refilled the niches left by their ancestors, but by that time it was filled by their hairy, warm-blooded cousins. So we don't have five-story high leaf-eaters anymore.

[InfiniteNow]

Thanks for the clarification on points B.

[Mercedes Benzene]

In the animal world, bigger is often better. Bigger means more strength and more intimidation.
I think that in a future "evolutionary cycle", animals will start to get bigger again.
I would like to point out however that many dinosaurs were not big. When we think of dinosaurs, we think of the big ones because they're... well... big. Those seem appealing to little kids because of their size and majesty. There were lots of small dinosaurs though.
Just something to keep in mind.

[InfiniteNow]

Originally Posted by Mercedes Benzene

... well... big. Those seem appealing to little kids because of their size and majesty. There were lots of small dinosaurs though.

Excellent observation MB. Also, perhaps the big ones seem to stand out in greater frequency due to the greater ease of finding fossil records for large dinos as opposed to tiny dinos.

You go digging in the backyard, you think you're more likely to find a Boeing 747 or a hot wheels (if they are both there that is )?

[Turtle]

Originally Posted by HydrogenBond

If we look at the evolution of animal life on earth, animal life started small, got very large during the time of the dinosaurs and then got smaller again. There are still a few large critters but even their average size has decreased, i.e, mammoths to elephants, etc.

Rather than wild speculation and no substantiation for it, I propose looking at a scientific basis for large animals. In this case, it seems an oxygen rich atmosphere affects animal growth at least in the case of insects. Similarly, dinosaurs may have simply taken advantage of the extra oxygen by growing large.
http://www.livescience.com/animalwor...t_insects.html