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]

Greater size meant fewer competitors could harm them physically.

Which only helped until T-Rex came along!

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. :cup:

[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. :cup:

[InfiniteNow]

... 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 :cup: )?

[Turtle]

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.:hyper:

http://www.livescience.com/animalworld/061011_giant_insects.html

[hallenrm]

To me the large size of the dinosaurs was a statistical aberration, like in any dissipative open system with large population, (a la ilya Prigogine. In such a system it has been shown that there can be several metastable states, each far from equilibrium and hence susceptible to change. For example, in a gaseous system, while, on the average, most molecules would have a certain velocity, there are molecules with exceptionally greater velocity. Such molecules become 'extinct' as the gas approaches equilibrium.:hyper:

[HydrogenBond]

Isn't there a gland in the brain that controls size, i.e, pituatary gland? The movement to large size back to smaller size would reflect this gland becoming very active and then gradually deactive.

[Buffy]

Isn't there a gland in the brain that controls size, i.e, pituatary gland?

Yes, but it "just doesn't scale." Pituitary activity just affects growth rates, but its abilities are bounded by structural limits on the current physical design of the parts. The design of the parts have to actually evolve to support order-of-magnitude plus sized changes a la dinos.

 

Way big,

Buffy

[ronthepon]

For example, when the gland hyper-secretes in humans, they don't live to be healthy people.

 

If you scale up a human about two times, the skeleton will not be able to hold the weight, inspite of the fact that even the skeleton has grown. The bones will have to get stronger, or the framework will have to be changed.

 

Overall size depends on many, many factors, not just one or two bodily growth stimulants.

 

Mmmm... ever heard of the guy who grew to eight feet eleven? The world's tallest guy? He died quite early, about twenty four, I think. He suffered from hypersecretion of Human Growth Hormone.

[alxian]

i thought it had more to do with the size of their overall ecosystem/biomes.

 

where a species, as in a group of animals where invidual creatures interbreed, grows larger because they have more land to roam and live longer, they will become much bigger.

 

i wonder how long sauropod live spans were. given they were reptiles that would point to very long life spans indeed.

 

also on tiny islands you get pigmy creatures yet on massive continents you get super large creatures like the sauropods. no to mention the largest living creatures are ocean borne. because its such a huge ecosystem.

 

i never knew that rainfall had anything to do with it other than benefitting plants and thus boosting the populations of producer species and grazers thus benefiting all other creatures that feed on the producers directly and their byproducts.. the higher up those benefits go the more complex the higher order animals will get so long as the numbers hold out before the population crash occurs..

[somebody]

i dont know if any organism prey on dinos. But if dinos were the top predators during their time and no one would hunt them then isn't it possible that they grew larger and larger as time went on. After so many thousand years thru evolution their genetic makeup changed giving them their large physical shape. (This is an immature though)

[InfiniteNow]

i dont know if any organism prey on dinos.

Some dinos went after one another. Also, I'd be led to believe that viral and bacterial organisms did some preying... :cup: