we watch "Jurassic" Dino-movies every week. — Michael Ossipoff

Speaking of movies - that excellent sci-fi thriller with Matt Damon trapped alone on Mars - it was pointed out that you couldn't have a gale of the kind that disabled the station on Mars, because the atmosphere only has a third of the density of Earth's.

I'm a bit surprised no one has chimed in that there was no one around in the cretasious, and hence that we cannot know if there even was any gravity...

But my favourite Earth fact is still that the north magnetic pole is a south magnetic pole.

more oxygen allowed insects to grow larger. Nothing to do with gravity.

Less than 1/3, if I remember correctly. And the wind-pressure at a given windspeed is proportional to the air-density. So yes, the wind would be a lot less damaging.

Yes, it does seem a bit odd that everything, even the dragonflies, were bigger in those earlier eras.

I read that the Pterodactyls, especially the big Pteranadon, wouldn't have really been able to fly. They've suggested that Pternatadon, and maybe the other Pterodactyls, soared on a cliff-updraft to hunt fish, and then cllmbed back up to their nest, along the cliff-face.

And i've read that there was a Pterosaur even bigger than pteranadon.

Maybe that's it. The giant dragonflies were probably not bigger than some modern birds known to be able to fly.

Aerodynamicists used to say that a bumblebee couldn't fly, by their calculations, until someone figured out the various tricks that flying insects use. Like the "clap-fling" used also by pidgeons (rock-doves).

In Jurassic Park, of course the Pteranadons fly easily, but that might not be accurate. They fudge things when it suits the story. For example, I read that it's believed that the Pterosaurs were fish-eaters, but theyi're all chasing humans in Jurassic-III and jurassic-IV.

Michael Ossipoff

I looked up Mars' atmospheric density, and it's only about 1% of Earth's. Wind's dynamic pressure is proportional to the air's density and the square of its speed. So Mars' 170 mph winds would only have the dynamic pressure of a 17 mph wind on Earth.

But the 170 mph sand-grains could still do erosion damage.

Michael Ossipoff

Yes, there are some very interesting facts concerning the earth. — Metaphysician Undercover

Yes. Yes there is.

:-|

The equator is not stable, to begin with. The magnetic poles do not line up with the true poles, and are moving. And, the north/south axis flips from time to time, to mention a few, other than the wobble. — Metaphysician Undercover

That's because of the earths core. The magnetic field - which is there to shield us from outside stuff and something visually seen in auroras - is constantly changing or reversing and geomagnetic excursions could occur at any stage; actually, the field itself is decreasing in shield strength.

Earth has anaemia.

I did acknowledge that earlier in the thread. It remains mysterious, however, why the mega-fauna of that age was so much bigger than anything that exists today. Some of the brachiopods weighed as much as today's whales. I was wondering if there is any global change that might explain this disparity. — Wayfarer

So look to the biologists for answers. As a side note, I visited the Alaskan rain forest and also a small patch of woods just outside that zone which we dubbed 'honey I shrunk the kids'. Many plants were recognizable (the same ones I have at home), but about 4x the size I normally see. The dandelions stood about a meter high for instance.

Concerning gravity: Of the eight planets, five of them (including Earth) have pretty similar gravity, but the closest one to us is Saturn, despite massing over 100x as much. So additional mass alone does not necessarily translate to a weight difference. Mars and Mercury weight are almost identical at 3/8 Earth, and Jupiter is in a class by itself at 2.5x.

So look to the biologists for answers. — noAxioms

I did, and they don't appear to offer any. It's still not known.

look to the physicists

One of the problems that MIGHT (I don't know) be in the way of an understanding of T Rex & Company's size is the paucity of fossil evidence. We only have those fossils which we happened to stumble on in weathered stone. We don't strip mine to find fossils. So the sequence of animals from the precambrian up to (pick a date, any date) is fragmentary.

We can't trace the long-term (over millions of years) increase of T Rex & Co.'s size. Then too, we don't have any DNA from T R & Co. Presumably they had genes which enabled them to get that big.

Look at dogs: they range from teacup miniatures to Great Danes and bigger. Only a few genes account for all of the dog differences. A horse that the Vikings used and spread around Europe ambles. It's walk is very smooth and even. Nice to ride. It has the odd habit of picking up it's front feet when it walks--like it was doing an exaggerated prance. 1 gene mutation granted this horse the ability to walk that way. Other horses can't do it.

Maybe T R & C had the good fortune to start out relatively large, bigger than whatever small prey they preyed upon. Maybe there was a lot of food, and they could afford to get bigger. Getting bigger just to starve doesn't have much point, after all.

All well and good, but what is puzzling me is the biomechanics. After all by far and away the biggest things on Earth today are whales, but they're water-borne and so to all intents they don't have to carry their weight. So how is it that critters the size of whales used to be able to do move about? Why didn't their muscles separate from their bones if they turned around too quickly, due to sheer inertia? And it's not only dinosaurs - some of the pterosaurs and pterodactyls were the size of single-engine planes, and ancient dragonflies were the size of today's crows.

I just googled the question Why were dinosaurs so big? - there's quite a few pages, but it's still basically an unknown. So I'm simply guessing that there might have been a global parameter shift between now and then that helps explain it; and as the factor involves mass, then the requisite parameter is gravity. Of course it might be malarky, but from what I'm reading, nobody has any better ideas. X-)

what about plants? They also have to deal with gravity. What was the range in plant size at the time of the dinosaurs?

Also, mammals were not very big at this time -- they were generally quite small. If gravity was less, back then, why wouldn't they be bigger too?

A third: After T R & Co ran out of steam, didn't animals start getting smaller? If gravity was less back then, wouldn't animals have continued to be XXXL?

Fourth: Their bones, muscles, and connecting tissues were proportionate to their size. Why couldn't very big muscles attached to very big bones with big connective tissue (tendons, ligaments, cartilage, etc.)? Maybe there were large ganglions located in the lower back to help coordinate tail and leg movement?

Five: How fast did T Rex & Co move? Presumably they didn't have to catch velociraptors for lunch; they probably looked around for a grazer and simple waddled up to it and bit it's head off.

If you look at alligators and crocodilia, none of them look like they'd be able to move very fast. But alligators manage to eat a few humans and their pets every now and then, some of them captured (by the gators) on dry land. There are alligators swimming around the flood waters of Houston right now, just waiting for some fat, luscious human to come bouncing along... SNAP!

Why couldn't very big muscles attached to very big bones with big connective tissue (tendons, ligaments, cartilage, etc.)? Maybe there were large ganglions located in the lower back to help coordinate tail and leg movement? — Bitter Crank

Perhaps! But the giganticism of the sauropods seems on a different order to anything observed since.

Found a pretty comprehensive article on the problem. Goes on for 5 pages and leaves few stones unturned, including the accretion suggested this thread posits. Also higher Earth spin (shorter day) and all sorts of physical and not necessarily biological explanations. So looking only to the biologists might indeed not be the right thing.

It considers a different gravitational constant G, and doesn't reject it for enough reasons. If G was less back then, the Earth would be cold because it would be much further from the sun. The author seems unaware of that part, but at least is unwilling propose such a fundamental hit to physics.

Some interesting points: The descendents of dinosaurs are birds, most of which are already far less dense than water. Maybe the dinos had that as well and simply didn't mass as much as would an alligator that size.

In the end, a pretty plausible explanation relates to the know small variance in temperature between various latitudes back then. The poles were nearly as warm as anywhere else, suggesting a much more dense atmosphere which would buoy up the large creatures. Maybe Earth was more like Venus back then. What evidence do we have against that? Such a suggestion implies we're losing atmosphere quickly and there won't be much left after not too long. They already predict the oceans will be gone soon, but I thought it more from a warming sun than from just losing it all to space.

Sure is comprehensive! I haven't read that much of it yet, but it is gratifying to see that the issue in the OP is indeed recognised by science:

The exceptionally large size of the terrestrial animals of the Mesozoic era is not a subtle oddity to be dismissed but rather it is a glaring paradox that must be investigated. The essence of science - our belief that we exist in a rational reality - is at stake here.

Interesting site, thanks!

800,000,000,000 tons, or 725,747,792,000,000 kg if my calculation is correct. Dr Google tells me the mass of the earth is 5.972 × 10^24 kg. — Wayfarer

You asked a straight question and I'm going to give you a straight answer.

The space dust that has fallen on Earth is 8 followed by 14 zeroes. In Kilograms. The mass of the earth is 6 followed by 24 zeroes. In Kilograms.This means that the size of earth increased by less than 0.000 000 001 percent. This is not at all significant, despite being more voluminous or heavy than all the buildings put together on manhattan island or all the pyramids in Egypt or the Great Wall of China, or the Great Barrier Reef near Australia, or... there is nothing in Europe I can think of which is similar in magnitude of size. All the cows of Europe.

Thank you! I did eventually come to a similar conclusion after having started this thread, although I do note that the 'giganticism' of dinosaurs is still regarded as a mystery.