Why were dinosaurs so big?

Cecil,

Back in 1994 someone asked if gravity might have been a contributing factor in the mammoth size of some dinosaurs and you responded;

Cecil replies:
We have two questions here, Andrew–the one you asked, and the one you would have asked if the drugs hadn’t kicked in first. The best that can be said for question #1 is that if you mention it to a stranger on the bus you’re guaranteed to get the seat to yourself. Question #2 is more interesting: why were dinosaurs so big? Some theories:

If I read you right, you were chiding Andrew for asking if gravity was a factor. I am a 44 year old man and after learning that astronauts can gain a full inch of height when in space, the same idea occurred to me. I then set to pondering this question and I realized that some anthropologists have asserted that average height does vary over great amounts of time and if early man was shorter or taller on average than modern man, perhaps minute differences in gravities pull could indeed be a factor in how large an animal tends to grow.

A good living example of this hypothesis would be the sea. This is where the worlds largest creatures exist today and thanks to the greater density of ocean water, gravities effects are dramatically reduced. This is why NASA trained astronauts in water tanks. To prepare them for the diminished gravity on the moon.

So, as I read your response to Andrew that you wrote over 14 years ago, I thought you might want to revisit this idea with a bit less bravado and perhaps a touch of explanation.

Maybe you could tell us WHY this hypothesis is worthless rather than accuse us of being stoned and then blabbering on about the ideas that have been hypothesized to date.

You mean you want an explanation why idea that gravity can randomly change without sending whole orbital mechanic of solar system into total disarray is ridiculous?

Well, beside being contradictory to our knowledge about physics it’s also internally inconsistent and illogical.

Is this the column you are referring to?

It’s true that the Earth was spinning a little bit faster back in the day (say, 100 million years ago). How much faster, and how much that would affect weight at different latitudes, I don’t know. (There’d be two significant factors: greater centrifugal force, and greater bulging of the Earth.)

Didn’t we have a whole thread about that once? I don’t remember the conclusion, but on the face of it I doubt if there’d be that much difference.

But really, emeryemery (why do I find that name so abrasive? :wink: ), I’ll try doing this with less flippancy as you asked, though that’s asking a lot around these parts, as you may have noticed.

As stolen from here: http://en.allexperts.com/q/Physics-1358/Gravity-Centrifugal-Force-Earth.htm

And which is a bit of an understatement, since the current gravitational force is 292 times the centrifugal force. But what happens if we double the Earth’s rotational velocity?

F = (75 kg * (0.926 km/s)^2 )/ 6,375 km = 10.09 N

Sure, that makes the centrifugal force four times greater, but that just takes its relationship to the gravity acting on us from “imperceptible” to “not much at all.” It is not nearly enough to make a 500kg cow grow into a 10,000kg megacow.

Oh, and the reason astronauts “grow” is because their joints aren’t compressed so much in low gravity. They squish back to their normal height not long after returning to Earth.

Thank your for some facts. That makes the idea that gravity played a role in animal growth much more difficult to prove or fit in to our current understanding of the mechanics of earth’s mass and rotation.

That said, what do you say to scientists who have compiled data that supports this idea and in some ways, challenges our current understanding of gravity and the history of our planet?

Again, thank you very much for the technical explanation of mass and gravity. It was a great help in my personal understanding of this very complex idea.

Not a scientist. Scientists publish their discoveries and hypotheses in scientific papers in refereed journals, not in e-books aimed at the general public. The table of contents alone practically screams “paranoid loon”.

Even to consider dinosaurs alone, dinosaur skeletons are proportioned properly for their weight. If gravity had really been less, their bones would be lighter than they are, just as a mouse’s bones are much lighter than the bones of an imaginary elephant shrunken to the size of a mouse.

I hate to take the Dark Side in this, but how do we know they are proportionate? Were someone to dig up my skeleton there would be few clues I’m the lardass I am.

The thickness of your thighbones would make it pretty darn clear that you weighed more then 50 pounds.

Heck. I’ve met dropzone. His ego weighs more than 50 pounds! :slight_smile: I’m kidding…

sorta. :slight_smile:

John, of course that’s true and it’s one of those things I should’ve thought of before posting. I try to [del]cover up[/del] allow for my habit of posting half-cocked (not to mention half-crocked) by leaving in weaselly words like “few,” as in “there would be few clues.”

Sam, think of my ego as being proportional to the rest of me. :smiley:

I haven’t read (or even heard of before now) that book, but a quick scan suggests that his premise is that the Earth’s gravity has decreased significantly over long periods of time, and his proof is the size of animals as preserved in the fossil record.

Leaving aside the physics, he needs a lot more support to the gravity change hypothesis than just plant and animal size changes. I don’t see it.

No, he believes gravity increased because the Earth got bigger and increased gravity made huge animals impossible. Some people hold that the Earth puffed up like a kernal of popcorn because of internal heat. They then conclude that an expanded Earth with the same mass had greater gravity, which is directly opposite of the truth that the strength of a gravitational field depends on the mass of the object and the distance from the center that field is being measured.

Since that book is called “Dinosaurs and the Expanded Earth” Hurrell may be one of those people or he may believe the Earth is bigger because of the cosmic debris that it attracts. I remember being told in grade school that the Earth grows larger by an inch a year because of all the dust and meteorites that land on it. If you take that factoid and assume it has continued for the 65M years since the dinos died out, it says that the diameter of the Earth is some 1650 kilometers larger than it was.

Actually, there have been some hypotheses in physics which would call for gravity to change over long spans of time, and some of the strongest evidence against those hypotheses has come from the fossil record. Among other effects, a change in the strength of the gravitational force would result in a change in both the orbit of the Earth and in the brightness of the Sun, which would together result in significant changes to the temperature of the Earth. We find fossils from the appropriate time of critters which couldn’t survive at those temperatures, so we know those hypotheses are incorrect.

Well, in Hurrell’s introduction he states that he was, “particularly interested in calculations that the bones of the larger dinosaurs were too weak to support their own body weight,” so I don’t have great hopes that his science is cutting-edge. I recall statements like that decades ago, but subsequent studies show that to not be so and the only people you still find making that claim are ones who have bought into the increasing gravity notion. Perhaps they, like I, have memories from 4th grade science class they can’t shake.

IANAPhysics Major, but it seems to me that unless you are postulating that the force of gravity has changed universally over time, the only factors that contribute are mass, distance from mass center, and counter-forces determined by body spin rate. What others would there be?

That’s exactly the idea – that Big G has changed. But, although it would answer some questions, attempts to confirm it directly have failed.

I think it involves dark energy squishing the strings that make up matter closer together which causes a temporal disturbance and the flask of poison reassembles and the dead cat jumps out of the box through a wormhole and is alive and kicking.

Damn, I miss our old Newtonian universe. If nothing else, the math was easier and I suck at math.

So you mean the investment plan I’m in that plans on moving Angus cows to Quinto may not have the rate of return I was expecting?

I think this is the clinching evidence. If we ever discovered lifeforms on planets with less mass and thus less gravity, we’d observe that their bone size would be appropriate to support the reduced weight of the creature, and that a creature could support a larger wieght with the same bone structure required for less weight here. Thus the size of whales.