Why were dinosaurs so big?

Link to Cecil’s Column from 1994.

Roger Benson co-authored a study published in the Proceedings of the Royal Society B in 2012.

“Several aspects of dinosaurian biology may have allowed them to obtain larger maximum sizes than any other land animals,” Benson said.

“For example, in many dinosaurs, parts of the skeleton contained air, and we think they had an efficient bird-like lung. These features helped them to support their weight on land more easily, and made their respiration and heat exchange more effective than in mammals.”

Benson adds that since larger animals can lay more eggs and reproduce more quickly, there may have been a reproductive advantage to being big.*

:confused: Small fish and invertebrates often lay thousands of eggs. Also, surely, large animals generally take longer to reach adulthood (they have more growing to do) than otherwise similar small animals, so they will reproduce more slowly, not more quickly.

Yeah, the principal advantage of being big is you can step on small animals that piss you off.

Actually, it’s thought that the really big sauropods like Brachiosaurus used size as a defense against predators, simply becoming too large to mess with.

I suspect that part of the reason is “because they could”, thanks to higher atmospheric oxygen levels. Of course, mammals didn’t take advantage of that at the time; reasons could include Benson’s, plus niche arguments.

For the plant eating dinos, maybe they were tall because that is where the groceries were. The low hanging fruit had already been eaten.

I just stumbled across this website today and was drawn in by the dinosaur conversation and a terrible need to put in my own thoughts into the matter.

Maybe what we should be asking, is not why the dinosaurs are so big, but why we are so small?

Now, this following babbling has absolutely no standing on anything scientific, but is based on a long trail of extrapolations.
Dinosaurs were primitive, they were some of the first forms of life on our world. Could it be possible that the cells that made them up were larger than what we have now? As time has progressed and species have evolved the information that it took to make that life has been able to be reduced into even smaller forms. Not unlike the evolution of modern electronics.

On a separate tangent. There was far more plant life, and because of this grazers had more food to grow larger, so in turn the meat-eaters do the same, as their slow lumbering prey puts on bulk. There were also very small dinosaurs that balanced this out, but the big guys are certainly more famous. Reduced food supplies regularly show a decrease in size/height. And most species will greatly increase in size when food is abundant.

Welcome to the board, QuickThinkOfAName. And… quick, think of a name!

Dinosaurs were not some of the first forms of life on earth. Life had existed for more than three billion years before they showed up. I don’t think there’s any evidence that cells used to be larger.

Nor are they likely to have been larger. Cells, lacking proper circulatory systems, fall victim to the square/cube law.

Oh, in the Panda’s Thumb Gould mentions the difficulty of maintaining body temperature given a high surface area to volume ratio for small warm blooded dinosaurs. Larger warm blooded beings encounter other problems, like falling causing far greater injury.

actually the k/t mass extinction that killed off the dinosaurs was one of five. the dinosaurs started after the permian mass extinction which killed about 96% of all life on the planet. dinosaurs started out quite small. i think the fact that they were pretty much the only creatures on the planet they quickly filled many empty niches. the easiest evolutionary change for a creature to make is in size so i think that as the prey grew larger, so did the predators. and since the field was relatively open this happened a lot over a long period of time resulting in some of the biggest things to walk, swim and fly that the earth has ever known.

after the demise of the dinosaurs, bird became some major predators. there were some birds (parrot like) that were the size of bears in the early tertiary.

as for the debate regarding warm and cold bloodedness as a factor… we’ll probably never know. since this is a site to end ignorance i think that we would should speak of creatures being ectothermic and endothermic rather than cold and warm blooded respectively. the terms are much better definitions as the ectotherms (cold blooded creatures) use outside heat source and endotherms (warm bloods) make their own heat inside.

one of the weird things to me about dinosaurs is their bipedalism. we humans use our bipedalism as one of the things that make us special. but we’re only special among mammals, for a hundred million years before us bipedalism was almost as common as bilateral symmetry.

Maybe it’s because of the line leading to bipedalism. Human bipedalism is connected with brachiation, whereas dinosaur bipedalism seems to have been connected with height and stride advantage. So we ended up bipedal with long, strong arms, but dinosaurs were simply bipedal. (The notoriously tiny arms of T. rex are only the extreme case of a general trend.) Just a vague speculation at eleven-thirty, though.

There were some pretty big birds right up until humans killed them off – the various moas.

This is the best article I’ve read on this and other questionable physics in evolutionary what-ifs:

The Biology of B-Move Monsters

No. We have the size of osteocytes and skin cells from dinos and can compare them to cell size today. They are the same.

This is better. Dinos were partly warm blooded and the extent of their warm bloodedness varied from Family to Family. It is easier for a larger animal to maintain warm blood in the absence of an insulated covering – feathers or fur. That is because the volume increases by the cube and the surface area by the square. So a larger body has less surface area/volume to lose heat. That dinos were warm blooded and very active is what kept the mammals to the fringe ecological niches of the very small: dinos couldn’t get that small without insulation.

In general, the earth was warmer and the atmosphere had more oxygen duing the Mesozoic, which means more plant life and more oxygen for metabolism. So yes, herbivores could find enough to eat. And being larger than predators is a very good defense. It works well for mammals like elephants and rhinos today.

But yes, dino bones are more like bird bones. So the lighter (but just as strong) bones were already there before birds evolved from dinos. This makes a dino weigh less than a similarly sized mammal. Thus, it helps in growing larger.

BTW, dinos grew up VERY fast, within 10 years. They had growth rates equal or greater to modern mammals. This is one bit of evidence that dinos had a high metabolism and were thus warm blooded.

Bill Nye the Science Guy and Jose Canseco Weigh In