Why aren’t animals the size of Dinosaurs alive today?

There is no question that dinosaurs once inhabited the Earth.  This is an undeniable fact, and if you do not concur, you should simply move on to the next article.

Something that has always puzzled me is that life once evolved on this planet such that animals of enormous size were prevalent; whereas today this is not the case.  The largest animal on Earth today is a whale; however it is not a land locked animal.  This is an important fact to note, as living it’s life in the ocean a whale doesn’t have to succumb to the problems that would inherently be involved with getting around outside of the ocean.  Imagine how difficult it would be for an animal the size of a sperm whale to walk (assuming it had legs).  The weight of it’s own body would prohibit its motion entirely, unless the leg muscles of this imaginary beast were stronger than any other that currently lives today.  Even our largest land locked animals today; animals such as elephants and rhinos are not typically anywhere near the size of the dinosaurs, nor are they very quick.  Rhinos can be quick in short bursts; however they spend much of their lives wading in water in an effort to eliminate the stresses of their own weight.

Our planet exerts a force of gravity that is equal to 9.78 m/s squared.  Our sole astral satellite, the moon, exerts a force of gravity that is equal to 1.622 m/s squared.  Well enough, but what does that mean?  It means that while the Earth is pulling objects in space towards it, so is the moon.  Since the moon orbits the Earth, the force of gravity on the Earth is directly affected by the counteracting force of gravity on the moon.  Again, where am I going with all of this?  Through careful study, scientists have been able to determine that the moon (for reasons not relevant to this discussion) is moving further away from the Earth day.  In fact, the total distance that the Moon moves away from the Earth is equal to 3.8 cm per year (not a lot).  However, 65 million years ago (approximately when the dinosaurs were wiped out by an asteroid or comet), the Moon would have been 247 million centimeters closer than it is today.  To put that into perspective, the Moon is currently about  384,000 km from the Earth at any given time.  Using our math from above, we can calculate that the moon (65 million years ago) would have been 2,470 km closer to the Earth, meaning it would be 381,530 km from Earth.  

That’s not a huge number; however it could be enough to account for other factors, such as the force of gravity that was being exerted on the Earth 65 million years ago.  Almost certainly, during the past 65 million years, and even long before that, the Earth was being bombarded with debris from space, resulting in the mass of the Earth to increase.  This increase in mass has a measurable and direct affect on the force of gravity.

It is my theory that Dinosaurs were so much larger than animals today for one simple reason.  That reason is that they could be.  The force of gravity was much lower 65 million years ago, and even many years prior to that.  This allowed animals to evolve; however those animals evolved to the size that the force of gravity would allow without those animals being crushed by their own size.  It is my further belief that if we were to clone a large dinosaur today, it would likely not be able to survive due to the force of gravity being so much stronger today than it was many millions of years ago.

Then again, I could be wrong.  🙂



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  1. Read http://www.dinoextinct.com/page13.pdf which explains why surface gravity was lower during the era of dinosaurs.

    • Thanks. And this helps to provide more evidence to support my theory. The fact that less gravity was present equated to much larger animals being able to grow and survive.

    • And you may have missed the point of my article. It is not simply that the Moon is the reason dinosaurs don’t or couldn’t survive given today’s force of gravity. It is SPECIFICALLY that the force of gravity today is not the same as it was at the time the dinosaurs roamed the Earth.

      My theory is one, just like the article you quote is ONE. However proof of either takes a lot more than we are capable as human beings. There are certainly a great number of variables that I took “on faith” for lack of a better term. Such as the 3.8 cm per year. That probably has not remained constant throughout the past 65 million years; however I assumed it to be for the sake of argument.

      The same leaps of faith on many variables (Pangea being one) are being made by your article. The inner and outer core movements, and how they affect surface gravity are hypothesis at best, and can’t be proven, at least not until we find another planet with similar conditions as those proposed by your article.

      Again, there are no certainties in cases like this, only educated guesses.

  2. @Staten-John – It would be an undeniable fact that the Moon’s being closer to us would counter act the pull of the force of gravity, if only a little. The same science that you use to explain why surface gravity being lower would also prevail in my opinion as well. You can’t have one without the other. So, yes, the moon did and to this day DOES have an influence. Point of fact, the moon’s gravity causes the Earth to bulge and consequently causes the tides. If the Moon didn’t affect our gravity, we wouldn’t even have tides, at least not for the reason that we do.

  3. pccoder,

    This is my final post on this subject.
    The Moon’s gravitational effect on life on the Earth is insignificant due to the great distance of the Moon from the Earth.

    If you think about it, on a daily basis, the change in distance from a point on the Earth to the Moon can be up to the diameter of the Earth, due to the Earth’s rotation. The Earth’s diameter is 12,742 km. This far exceeds your estimate of 2470 km for the difference between 65 mya and present, of average distance from the Earth to the moon. Yet, your weight doesn’t change from hour to hour during the day, or month. Therefore, this change in distance from the Earth to the Moon is insignificant.

    Previously, others have suggested the same scenario that you have and have not been able to support their theory based on the above explanation I have given.

    Yes, I agree that surface gravity on the Earth (specifically on Pangea) was less when dinosaurs existed. The only theory I believe is viable is the one I provided a link to.

    Good luck.

    • It’s your choice to believe the “only theory….you provided”, but that’s awful arrogant. I guess you’re so smart that nobody else could possibly have any other “viable” theories. Must be a nice feeling knowing you’re right all the time. Good luck to you too.

  4. It’s because of oxygen. There is not enough oxygen present in the atmosphere today to sustain many animals of that size. Back then the atmosphere was way more oxygen dense. Evoulution forced most organisms to have to be a much smaller size so their bodies would require less oxygen to be alive

  5. The force of gravity was less?? The force of gravity is the same everywhere and always has been – that’s basic science. Local gravity depends on the mass of the planet you are on. Unless Earth was considerably smaller back then, the gravity experienced by animals would have been the same as it is now. There is about a 120 ton limit on weight for any land creature, and that is the same now as it is now. The huge size of some dinosaurs (most were actually quite small) is simply an evolutionary quirk.

    • I guess I needed to be more specific. The “force of gravity” from the Earth was most certainly the same, assuming it was the same size. However the moon was undoubtedly much closer. The offset of gravity from both the Earth and moon would allow for the “relative feel” from the gravity on both to be much less. This is simply a hypothesis, and can’t be proven. Neither can any guess made by you. For all I know the moon may have been REALLY close to the Earth, so much that it was only 1/4 the distance it is now or even an 1/8th? Since none of us was around back then, we can only guess.

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