Was weaker gravity responsible for large dinosaur size?

A piece of a Supersaurus vertebra, with paleontologist glamor for reference. While no other part of Supersaurus has been found yet, the shape of the bone identifies it as a sauropod, and the size of the fragment (that's not the whole vertebra!) marks it probably as the largest land animal that ever lived. [Credit: Sauropod Vertebra Picture of the Week/Mike Taylor]
Most of a Supersaurus vertebra, with paleontologist glamor shot for reference. The shape of the bone identifies it as a sauropod, and its size of the fragment marks it as one of the largest land animals that ever lived. [Credit: Sauropod Vertebra Picture of the Week/Mike Taylor; caption corrected from previous version]
How did the sauropods—the long-necked dinosaurs like Apatosaurus, Brachiosaurus, and Diplodocus that grew to be the largest land animals that ever lived—get so huge? The full answer to that question is complicated and interesting, involving a lot of science I don’t know. Come to think of it, the true answer involves a lot of physics I don’t know: calculating the compressive strength of bones, the fluid dynamics involved in getting blood from the heart up to the head, and so forth. So, I’ll leave unabashed sauropod snuggler Brian Switek to talk about the biological and reproductive aspects of the big dinosaurs, and address another hypothesis: that Earth’s gravity was noticeably weaker in the Mesozoic Era, the Age of Dinosaurs.

I first read about that hypothesis in Brian’s book, My Beloved Brontosaurus, but despite my intentions, I didn’t follow up until a series of tweets from baseball player Jose Canseco received widespread mockery last week. Here’s a sample of what Canseco posted to Twitter:

It’s easy to mock Canseco for saying things like that, but let’s face it: every report, every column, every mockery only made fun of him without explaining what’s wrong with what he wrote. (Think of this post as my personal atonement for having done that.) While Canseco, based on his hostile response to Bill Nye, doesn’t seem to understand that scientific theories require evidence to back them up, some scientists had proposed weaker gravity as an explanation for big dinosaurs. Knowing why those scientists—and Canseco—are wrong is important. The question “how do we know Earth’s gravity didn’t change radically over 100 million years?” is a variation on the most important question we can ask about science: how do we know what we know?

Some preliminaries

I’m not sure exactly how much weaker gravity must be in either Canseco’s or the other hypotheses. We don’t need to be as extreme as Canseco thinks, though: he states the weight of Supersaurus to be about 200 tons. Current estimates place it closer to 40 tons: it was a pretty slender beast compared to its length! (In fairness, 40 tons is hardly skinny: the record-setting male African elephant was about 11 tons. Sauropods were still big animals.) To quote Brian, “Sauropods were weird from snout to tail.”

For the sake of argument, however, let’s assume the largest land mammal was as big as any animal can get: the 20-ton Paraceratherium, which lived in the Oligocene (about 23-34 million years ago). This is consistent with some earlier estimates, which means that gravity in the Jurassic Period (when Supersaurus lived) must have been about half what it is today, or something else must have been going on.

The weight of evidence (ha!) is that sauropods could and did get that big, not because of lower gravity or higher oxygen content, but because of their reproductive strategy. They were lighter than earlier scientists thought because they were very bird-like in bone structure and respiratory system, allowing them to survive under normal gravity and oxygen levels. Having started the discussion of weaker gravity, though, let’s carry it through to the end.

(Note: I still haven’t determined the source where weaker gravity is proposed, so I’m inferring its arguments from later papers. It’s evidently discussed in a later paper on the theoretical maximum size any mammal could be, but this one is unavailable to me without the appropriate academic credentials. However, here’s the citation: Economos, AC. The largest land mammal. Journal of Theoretical Biology. 1981; 89:211–215 .)

The gravity of the situation

Earth’s gravity—as with any planet, star, moon, asteroid, etc.—is determined primary by its mass and size. Mass is (roughly speaking) the amount of matter in the planet, and that’s something hard to change drastically: if you wanted to make gravity noticeably stronger, you’d have to add the equivalent mass of another planet or moon, something that can’t just happen spontaneously. While Earth is constantly being bombarded by tiny asteroid fragments and dust grains, and is also losing small amounts of its atmosphere to space, neither of those effects is very big. To my knowledge, nobody has proposed the idea that Earth was lighter in the past as a solution to dinosaur size anyway, so let’s leave it alone.

The relationship of gravity to size: if two objects have the same mass, but one is smaller than the other, the smaller object will have correspondingly higher surface gravity.
The relationship of gravity to size: if two objects have the same mass, but one is smaller than the other, the smaller object will have correspondingly higher surface gravity.

A somewhat more reasonable idea is that Earth shrank. (Again, Canseco didn’t propose this idea, so please don’t take this as a strawman argument. I’m covering many possibilities to be thorough!)  When Earth formed, its was molten rock, a completely different planet than it became subsequently. As it cooled and solidified, it would have contracted, shrinking by a noticeable degree. We see effects like that on the Moon and Mars, where the dramatic canyon Valles Marineris may have formed when the crust fractured, then grew bigger via erosion.

However, there are two strikes against that as an explanation. First, while the 65 million years since the last dinosaur is a long time by human standards, Earth has been around 4.5 billion years. The cooling-down period ended long before the colonization of land by animals, which itself happened long before the first dinosaur. The second problem is that, to double Earth’s gravity between Supersaurus and today, Earth would have had 1.4 times the diameter in the Jurassic. While that doesn’t sound like much, it translates to twice the surface area and nearly three times the volume of modern Earth. That’s a much bigger planet! (For the details of this calculation, see the note at the end of the blog post. For more about gravity, see my post on Le Petit Prince and the inverse square law.)

Blue plate tectonic special

Earth isn’t a perfectly smooth sphere, and its composition varies somewhat from place to place. That means both the density of rock and the gravitational force vary slightly around the planet. It’s not a huge variation, but it’s measurable and important if you work in a field where knowing precisely what direction is “down” is important. (To wit: if you’re trying to design a water system for a city, you’d better know how gravity will affect the flow of water, or else you might end up with stagnation or low pressure.)

Eastern and western hemisphere geoids: variations of the Earth's gravitational influence, as measured by the GRACE satellites. The lumps and colors both indicate deviations from the average, with blue indicating slightly stronger gravity, and red indicating slightly weaker. [Credit: NASA/EOS]
Eastern and western hemisphere geoids: variations of the Earth’s gravitational influence, as measured by the GRACE satellites. The lumps and colors both indicate deviations from the average, with red indicating slightly stronger gravity, and blue indicating slightly weaker. [Credit: NASA/EOS]
A pair of satellites called GRACE (Gravity Recovery And Climate Experiment) flew in tandem around Earth, mapping tiny variations in Earth’s gravity. The result is a diagram called a geoid, one version of which is shown at right. In some places—such as the Himalayas, Andes, and ridges in the north Atlantic—the concentration of rock is much higher than normal, making the gravitational force that much stronger. Notice, though, that these spots don’t correspond exactly to continents, though they do correlate strongly to young mountain ranges (whether above or below water). Despite the appearance of the geoid, though, these fluctuations are pretty small, measured in tens of “milligals” or “milligalileos”. The average gravitational strength is 981 gals, so 50 milligals (0.05 gals) is relatively tiny.

The continents during the Jurassic Period, the age when the biggest sauropods lived. The supercontinent of Pangaea had broken up into two smaller pieces, but most of Earth’s landmass was still concentrated on one side of the globe.

During the Jurassic period, the continents had just broken apart from Pangaea, the supercontinent encompassing most of the land in the entire world. That meant that most of the landmass on Earth was still concentrated in one hemisphere. In his tweets, Canseco conjectured that this land concentration actually shifted the position of Earth’s core in compensation. A moment’s thought lets us dismiss the “compensation” argument: if the location of the continents made such a difference, wouldn’t it result in increased gravity rather than reduced? However, let’s take the hypothetical results at face value.

Earth’s interior is strongly differentiated, meaning that below the surface (the crust) there are distinct layers. The mantle is a region of rock heated until it behaves like plastic: mostly solid, but capable of flowing like a liquid under high pressure. The core consists of two regions: the molten outer core (made largely of iron and nickel) and the solid inner core. The temperatures are highest at Earth’s center, but because the pressures are also highest, the inner core stays solid, much like you can heat water far beyond its boiling point in a pressure cooker. To move that core around would require rearranging Earth’s interior pretty drastically. The crust is no more than 50 kilometers (30 miles) thick at most, which sounds like a lot until you realize that Earth’s average radius is 6371 kilometers. (I say “average” because Earth isn’t a perfect sphere.) Even with a relatively high concentration of crust on one side of the globe, you couldn’t shift the core very much: the forces aren’t strong enough.

The truth is that Earth’s tectonic plates, on which the continents rest, are always in motion, rearranging themselves very slowly over tens of millions of years. Yet the Moon reliably orbits, which wouldn’t be true if plate tectonics made a huge difference to Earth’s gravity. In fact, there’s another sign Earth’s gravity hasn’t changed much in the last 100 million years: the Moon is actually moving away from Earth, albeit very slowly. If Earth’s gravity had doubled since the time of the sauropods, we would expect the opposite effect.


Admittedly, this may seem like using a sledgehammer to crush a gnat. I’ve expended a lot of words and diagrams to combat a few short tweets from a baseball player, which he may or may not have thought very carefully about. However, as with many far-out “what if?” ideas, the real answers are known by science, and can be tested. Canseco, to his credit, did hypothesize something that’s testable; that his ideas are wrong doesn’t make him any worse than many others who have postulated such things over the centuries. While it’s doubtful he’ll read either my post or Brian’s companion post on sauropod anatomy, the answers are out there, if he wants to know. Inquisitiveness is part of science; being open to new knowledge (and I certainly learned a lot writing this post!) allows us to move from naive speculation to a deeper understanding of our world.

Notes on the physics

The strength of gravity at the surface of Earth, which tells how much a falling object accelerates, is determined by Newton’s law of gravity:law_of_gravitywhere g is the acceleration, G is Newton’s constant (just a number that tells us the strength of gravity), and R is Earth’s radius. If we assume Earth’s mass stays the same, for the reasons I mentioned earlier, then changing gravity is a matter of changing Earth’s size and/or its shape.

Let’s consider a change just in size first, so Earth maintains its spherical shape. What we care about is the ratio of gravity now to gravity then, and what that means for the change in size:ratioIf gravity now is twice what gravity was then, the ratio is 2, and the radius then would have to be √2 = 1.41 times larger than Earth’s radius today. Earth’s surface area goes like the square of the radius, sosurface_areaIn words: double the gravity now implies double the surface area of the planet then.

If Earth’s mass isn’t distributed evenly, as in the Canseco conjecture, we can still use Newton’s law of gravity, adding up the effect of all the bits of mass to get the net effect at a position on the surface. (That’s a large part of the construction of the geoid I mentioned earlier.)


56 responses to “Was weaker gravity responsible for large dinosaur size?”

  1. Hi, Matthew, good to see this discussion. Just wanted to point up a potentially misleading statement in the caption to the Supersaurus-vert illustration.

    You say “The shape of the bone identifies it as a sauropod, and the size of the fragment (that’s not the whole vertebra!) marks it as one of the largest land animals that ever lived.”

    You’re right that the vertebra is not quite complete; but it very nearly is. The only significant part that’s missing is the prezygapohyses — bony protrusions that overhang the front of the vertebra. You can see several nice examples in the second illustration at http://svpow.com/2009/09/06/bifid-brachiosaurs-batman/ The missing part would have been pretty small.

    Sorry to disappoint :-)

    1. Thanks – I was under the impression the fragment was missing more than it really is. I’ve corrected the caption.

    1. I think I read about 20 of the posts at SVPOW to prep for my post (so many sauropods!), but I decided against talking about Amphicoelias. I’m a mere physicist, unqualified to talk about what little evidence we have about it!

  2. Apparently the expanding earth theory is quite popular in some circles. http://www.youtube.com/watch?v=_f6hcGJbjL0

    1. I don’t think a YouTube video series counts as something being “popular”. Not only is there zero evidence for it, there’s a lot of strong evidence against it.

      1. Given the number of views and similar videos I would say its popular enough. Certainly not in scientific circles, but then again nor are conspiracy theories. The fact that this video even exists and has been watched more than the odd numberphile video is a sad commentary on our society.

  3. […] Earth’s environment that drove dinosaurs to gargantuan sizes. My friend and fellow science writer Matthew Francis covers the gravity angle over at his blog – there’s no evidence that the Earth’s gravity was weaker during the heyday of the dinosaurs. […]

  4. […] And over at Galileo’s Pendulum, Matthew Francis debunks the ancient gravity myth: […]

  5. Reduced gravity on an Expanding Earth is discussed here:

    [YouTube link removed by blog owner]

    1. Sorry, I will not allow YouTube links to dubious hypotheses on my blog. As both my post and Brian’s state, there is no need for different gravity in ancient times, and no physical mechanism by which it could be reduced.

      1. No discussion allowed then?

      2. Posting a 15-minute YouTube video is not “discussion”.

  6. […] astronomer Matthew Francis elucidated in a piece coordinated with mine, gravity hasn’t significantly hanged since the heyday of enormous dinosaurs. And in my own contribution, I outlined how unique aspects of dinosaur biology – such as air sacs […]

  7. How about a link to a technical paper on reduced gravity. Is that allowed?


    1. An interesting paper, I liked the discussion of the scale of life under various strengths of gravity. (It makes me wonder about the scale of life in water, where gravity is offset by buoyancy, and especially in very saline environments.) but I am not sure that I can follow that line of logic to ancient life. (I would consider it possible that weaker gravity could be responsible, but it does not by far exclude other possibilities.) I have never been a fan of the expanding earth hypothesis.

      1. That is hardly a technical paper, and would not pass peer review in any reputable journal because it has no scientific merit. There is no reason to believe Earth’s gravity was weaker in previous times. Dinosaurs could stand up under their own weight and the large flying reptiles had no trouble getting airborne. Additionally, there’s no evidence Earth changed size for reasons I wrote extensively about in the original blog post, and there’s certainly no way it could change mass.

        I am terminating this discussion now. You can write about your ideas in your own space, but this blog is devoted to science.

  8. Vilnis Krumins Avatar
    Vilnis Krumins

    Have you considered the effect on the weight (i.e., not mass) of dinos that would accompany a radical change in the rotational frequency of the earth (e.g., a change caused by the earth “toppling” its axis and loosing a lot of spin while its axis was aligned with its orbital plane)?

    1. This would be negligible.

    2. The mass of all life put together is tiny compared to the mass of nonliving matter on Earth – the rocks in the interior, the core, the mantle, etc. Weight in this case is proportional to mass, so it’s not a separate consideration.

      (This reminds me that I should blog at some point about what “weight” actually means. Memo to myself!)

  9. […] and “ancient gravity”. Everyone mocked him in the usual smug and smarmy way. Brian Switek and Matthew Francis actually bother to explain something, and make a fan out of Canseco. DO YOU THINK THERE MIGHT BE A […]

  10. Hi Matthew,

    As you suggested I’ve written up my comments about your blog and put them here:


    Let me know your comments on your blog, my website or email, twitter etc.

    Also Brian Switek was telling me last night on twitter that his new book has a small bit about gravity in it. Do you discuss it anywhere else?

    All the best


  11. I hadn’t realized that Canseco’s idea was linked to an expanding Earth. I went through a lot of the problems with that concept here:

    But was only scratching the surface of the problems.

    1. I think Stephen is misrepresenting Canseco if he thinks he’s an ally to the “expanding Earth” hypothesis. I had a long Twitter conversation with Canseco on Friday night (blog post to come), and he’s genuinely interested in the science.

      1. I wasn’t trying to imply that. Apologies I if gave that impression. The EE link is interesting though – always interested in augments against it as for it.

    2. Hi John,

      Just tried to reply on your article but it didn’t seem to want to play, so perhaps I’ll just reply here…

      [remainder of comment excised for being a response to another post]

      1. Kindly do not leave comments intended for another post on this one.

  12. […] Then I stepped in it: I wrote back to Ed, saying something unfairly derogatory about the publication that mocked Canseco (which I subsequently deleted, with an apology). During the resulting back-and-forth exchange, my friend Brian Switek proposed each writing a blog post, focusing on our own areas of expertise. Brian’s two posts discussed dinosaur reproduction and blood pressure; mine discussed gravity. […]

  13. Allow me to reiterate a point I’ve made before: this blog is not a forum for advertising your own personal theories on ancient gravity, dark matter (or lack thereof), why quantum mechanics is wrong, aquatic apes, or similar. I will automatically delete comments that consist only of advertisements for such work, just as I would for commercial products.

    1. My comment is not an advertisement. It supports the subject of this thread, i.e., Mr. Canseco’s assertion that surface gravity was lower during the dinosaur era.

  14. Mr. Francis,

    Your steadfast opposition to posters who have different views, than yours, about the possibility of lower surface gravity in the distant past gives one the opinion that you have an agenda which you are not revealing. Are you threatened, economically or otherwise, if it turns out that it is scientifically established that surface gravity was, in fact, lower in the past?

    1. Or maybe he just doesn’t have an obligation to give equal space to pseudoscience on his blog?

      1. Well….one day his comments on the reason for dinosaur gigantism will, in retrospect, be considered pseudoscience.

    2. That’s Dr. Francis to you, sir.

  15. Read the blog which addresses Jose Canseco’s comments and an explanation for dinosaur gigantism:

    [link removed – this blog is not a place to expound your personal theories]

  16. Spencer Erwin Avatar
    Spencer Erwin

    The expanding earth theory (and even the hollow earth theory) is just as idiotic as the theory that the Sun was the center of the solar system 500 years ago.

  17. I will remind all commenters that I do have a commenting policy: https://galileospendulum.org/commenting-policy/
    Please abide by these rules, or run the risk of having your comment deleted. Crying “censorship” is foolish in any case: I’m not shutting you up in your own space, just in mine. This goes double if you insult my intelligence and that of my readers. Thank you.

  18. No, I don’t see any sort of ‘dashboard’ all I can see the the ability to cancel a reply. It may be my script blocker, I have to use a rather severe one to keep my somewhat outdated PC safe. Feel free to remove the comment.

  19. Yet another comment deleted for expounding a theory of changing gravity. Please read my commenting policy (https://galileospendulum.org/commenting-policy/) , especially item 3:
    “On a similar note, the comments are not a space for you to expound on your own theories, promote your own ideas, or tout conspiracy theories. I reserve the right to edit or delete comments of that sort summarily. You can start your own blog and write whatever you want there (within limits, of course), but you don’t have the right to expect me to provide free advertising space for you. If you feel the need to compare yourself to Copernicus, Einstein, or Galileo, then you’re probably going to find yourself edited or deleted.”

    It’s particularly ironic that people think they’ll find a sympathetic forum for a theory of changing Earth gravity in a post where I argue *in detail* that such theories aren’t viable or even necessary.

  20. The size of the Earth is an equilibrium between internal pressure due to compression and weight due to gravity. It can’t easily be smaller because then it wouldn’t be in equilibrium.

    The only way I can see for Earth’s gravity to change is if the Earth’s mass changed because the mass of the proton changed. That’s new physics to put it mildly but it’s more possible than any of the other ideas I’ve seen on this page. (I’m discounting the gravitational constant changing because that’s basically meaningless – it’s 1 in fundamental units, which the proton mass isn’t.) A smaller proton mass would mean a lower gravity, which would mean a larger Earth, which also means lower gravity.

    The smaller Earth surface area today would have required recycling of ocean floors but if it happened over tens of millions of years then maybe the continents could survive. I’d guess we have the evidence to rule out that much recent shrinkage but I don’t know for sure. (My background is physics, paleontology is a hobby.)

    One downside of lower gravity, from a pterosaur’s point of view, is that the atmosphere thins out. That would take away some of the benefit of the lower gravity. Probably no big deal for a sauropod, though – thanks to those handy air sacs.

    In fact those air sacs are such an advantage it’s a wonder birds didn’t take over the world (even more than they have). My guess is that it’s because mammal blood balances avian lungs – denucleated erythrocytes means small capillaries which is also a huge metabolic advantage. Has much work been done on this?

  21. Excellent demonstration! Could you do the same to show that higher air pressure could not have sufficiently reduced the weight by means of buoyancy?

  22. If not a change in the fairly secure physics of gravity, what are your views on what else could have been responsible for the large size of dinosaurs. In particular, how could pterosaurs ‘fly’ when it has been proved impossible in a modern atmosphere. A fifty-ton whale can ‘fly’ in water. Any views on an atmosphere which might support the mass of a dinosaur/pterosaur?

    1. Kindly reread both my post and Brian Switek’s post(s). There’s nothing impossible either about large dinosaur size (as I emphasized in my post, and as Brian has noted many times) or about pterosaur flight. There’s no evidence that Earth’s atmosphere was markedly thicker 65 million years ago, either, based on analysis of the sediments and plants of the era.

      Earth’s atmosphere did have a much higher oxygen content in the Carboniferous period (the era of the huge flying insects), but that was long before the dinosaurs.

      In our posts, Brian and I took pains to point out that, while dinosaurs are phenomenal in many ways, the mystery of their large size has been solved without the need to hypothesize crazy physics or geological phenomena. Yet you commenters still demand that I/we address theories that aren’t necessary.

      1. Dear Mr. Francis..

        I am struck by your extremely strong statements that are in complete disagreement with the evidence. Let’s just look at just two of your most misleading statements. “There’s nothing impossible either about large dinosaur size (as I emphasized in my post, and as Brian has noted many times) or about pterosaur flight.” and “… the mystery of their large size has been solved ” It is common knowledge that when the paleontology had an RC model of a pterosaur built that despite all of their cheating it still could not fly. When you say that the problem is solved who believes you beside yourself and the handful of people that submitted this nonsense hypothesis?

      2. Remote-controlled models are neither here nor there. (Has anyone made a realistic mechanical model of a bird or bat that flies exactly the same way, from take-off to landing? Yet we know how birds fly very well, even the weird ones like albatrosses.)

        Try the books reviewed at this link:
        and this article at Scientific American for a start:

  23. Dear Dr Francis,

    “There’s nothing impossible either about large dinosaur size (as I emphasized in my post, and as Brian has noted many times) or about pterosaur flight.”

    Your post from which I have quoted above is, if I may say so, very un-scientific. Your sweeping statements that ‘pterosaur size and flight’ is all known about is plainly ridiculous. This is supposed to be a scientific forum, not the bar in the local pub!

    I have no ideas about what made dinosaurs grow large – that is well outside my knowledge or interest. However, as an aerodynamicist pterosaur flight most certainly is.

    Whenever I read items about flight from palaeontologists, they invariably give away their degree of innocence concerning both the theory an practice of aerodynamics. Let me give you some examples:

    First: ‘There are three types of flight – gliding, soaring and flapping’ Wrong. There are only two. Soaring IS gliding – it is merely done in a block of air which is rising (either dynamically or thermally) faster than the ‘glider’ is descending.

    Second; The idea that Quetzelcoatas n. could launch itself in single bound is so ridiculous it brings to mind Batman and Spiderman with their improbable feats. How can a creature the size of a giraffe, with a wing loading greater needing a speed of over 30mph to generate lift, get to that speed in a single bound (with its wings still folded by the way).

    Then we have Chatterjee and his ‘biplane’ theory!

    I came into the field of bird flight 40 years ago as a Cadet at the RAF College and the topic comprised my graduation thesis. Since then I have been flying model aircraft (including model birds) and I have also had access to RAF Wind Tunnels, to run tests on recently killed bird wings.

    I have tried all the various hypotheses including flight in ground-effect; Chatterjee’s sailboat (which, since it isn’t flight, doesn’t count); the idea that Q.n could only take off from a high slope and a stiff breeze; and several more that challenge the flat-earth theory for stupidity.

    Since you are so confident that Q.n could fly in an atmosphere like the present, let’s see something more scientific please. A few numbers and demonstrations might help.

    1. Yes, those articles I linked are non-technical pieces aimed at non-specialists, but they point to decades of published research by experts. If you follow that trail, you’ll see figures and numbers; whether you’re satisfied with that research or not isn’t my call. I am happy to defer to experts when I’m out of my field, just as I hope they would defer to my expertise when appropriate (e.g., gravitation and cosmology). My point is merely that the research exists, and that paleontologists are not stupid people: they aren’t working on models of pterosaur flight by guessing.

  24. Dear Dr Francis,
    Thanks for your reply, but I’m afraid it was a real cop-out. Your paleontologist colleagues indeed ARE working on models of pterosaur flight by guessing! Do they really know what a snap-stall is? Or the function of the alula feather?

    Your point that the “research exists” is lost on me I’m afraid. I haven’t been able to find a single article or paper that remotely convinces me that a beast the size of Q.n could fly.

    For example, which paleontologist would put his/her reputation on the line over the real weight of Q.n? 200Kg? 2000Kg?

    Pennycuick? Rayner? Habib? Chatterjee? Witton? Hone? Conway? Elgin? Wellnhofer?

    For Q.n I use 700 Kg / 12m span / 1.7m mean chord / 35Kg/m2 wing loading in my calculations. The result is a creature with the flying ability of a house-brick! Either some of these have got to be reconsidered radically, or the formulae for modern aerodynamic theory needs to change instead. Air density is about the only one we can alter. Thick Atmosphere?

    Give us the numbers (weights, lengths, masses etc) and we can pin down the physics. That is all we aerodynamicists need to work on.

    But with the physical characteristics given to us by the paleontologists, I would put my reputation (such as it is) on Q.n being unable to fly. (Lets stick to simple gliding for the moment, we can look at takeoff and flapping flight later).

    I put it to you, Dr Francis, that Q.n could not glide, even if it took off from a huge cliff. By glide I mean that it could not sustain controlled descending flight below about 70mph. That is not to say that it couldn’t crash-land at the bottom (rather like my own hang glider landings) with a snap stall flare. But then what does it do? How does it get airborne again, having assumed it has landed like a vulture to eat. With a stall speed of 70mph, it would need Dr Habib himself to give it a powerful push to get it up to the 70mph launch speed in the single bound he has allowed it!

    People think that the Wandering Albatross has difficulty taking off. It does, but only from land. With webbed feet, constant high winds and waves that can push it up into the airstream, Diomeda Exulans gets airborne much more easily from the sea than from the land. I know because I have watched them from the Falkland Islands. Incidentally did you know that D.E and it’s cousin the Giant Petrel fly more than twice as fast as the largest northern hemisphere gulls. These birds fly really fast – a function which cannot be appreciated on film or TV. I have stood on the harbour wall at Port Stanley. As an RC Glider enthusiast, I can tell you that these birds were whistling past me at twice the speed of my models. This means that their stalling speed would be nearer to 50mph than a gull’s 30mph. Wing loading again.

    So give me your vital statistics for Quetzalcoatlas and we can work on the aerodynamics between us. You give me what you think they are, and I will tell you they don’t work!

    1. Quetzalcoatlus northropi is the smoking gun for reduced surface gravity during the Mesozoic. Reasonable people realize that a flying animal, whether a reptile or not, that is frequently illustrated as being as tall as a giraffe, could not possibly fly if it existed today.

      Robert T. Bakker, in his book ‘The Dinosaur Heresies’, observed the following:
      The experts divide all pterodactyls into two groups: short-tailed and long-tailed. He notes that most of the long-tailed pterodactyls were wiped out at the Jurassic-Cretaceous extinctions, leaving the short-tailed pterodactyls to prosper but doesn’t venture an explanation for this. He does offer a characteristic of the short-tailed pterodactyls that, I believe, helps to explain this. He notes that the short-tailed pterodactyls had longer forearms.

      If surface gravity increased during the late Mesozoic, pterodactyls that were able to evolve an increase in their wing area to mass ratio would be better suited to survive. And, as surface gravity continued to increase, pterodactyls with larger and larger wings should evolve. This is exactly what we see. The longer forearms of the short-tail pterodactyls may have given them the advantage in increasing the wing area/mass ratio. A rapid increase in surface gravity at the end of the Cretaceous along with a limit to the wing area to mass ratio would doom all pterodactyls.

      1. Dear Staten-John,
        I wrote a fairly comprehensive reply to your blog.

        It has been deleted.

        It clearly doesn’t accord with Dr Francis opinions. Sorry. If you want to continue the discussion, we’ll have to find another blog.

  25. Incidentally, a propos the gravity theory; just assuming we could prove that there was increased gravity in the Mesozoic. How did it disappear again?
    At least with the ‘thick atmosphere’ hypothesis, it is feasible to imagine the atmosphere being ‘sucked’ away by a passing event (one of the newly discovered rogue planets perhaps?)

    Phil Parsons

    1. Phil,
      Most people who think about how the Earth’s surface gravity could change believe there are only two possible causes:

      1. An expanding Earth.
      2. A contracting Earth.

      There is a third explanation. What would happen if the Earth’s cores and densest part of the lower mantle moved off-center (remaining in the equatorial plane)? Surface gravity would lower on one part of the surface and commensurately lower antipodally, i.e., a gravitational gradient around the globe. And, if the 3 parts periodically moved back toward Earth-centricity and away again, there would be corresponding rises and falls of surface gravity.

      There is a logical explanation for how the above described movement could happen.

      1. The fourth option is actually the correct one: there’s no plausible mechanism for changing Earth’s gravity as drastically as you want, and there’s no reason it’s even necessary. Please take this discussion to your own websites. We stick to real physics here.

  26. In azdharchid pterosaurs, there is a very specific, constrained size relationship between the delto-pectoral crest, the coracoid flange, and the torso size. Since all three are preserved in Quetzalcoatlus species and the Quetzalcoatlus northropi delto-pectoral crest is also preserved intact, the size of the northropi torso can be approximated to a reasonable degree of confidence. The average distance from notarium socket to acetabulum in Q species is just over 12 inches. So, in Q northropi it is roughly about 25 or 26 inches. In short, the animal is nowhere near the volume of a giraffe and the weight is nowhere near 700 kg. 150 kg is a more probable approximation with fat animals (fueled up prior to travel) perhaps reaching 200 kg. I love Mark Witton’s sketch of a big azdharchid, himself, and a giraffe standing together, but his illustrated azdarchid torso has about twice the linear dimensions that the living animal did, and perhaps 6 or 7 times the volume. One should not base a mass estimate on that illustration — Mark didn’t.

    The quad launch was first described during a talk that Paul MacCready and I gave at a Conference in late February, 1999. Launch required an acceleration of roughly 4.5 g’s, well within the capability of the front legs with the animal achieving flight speed before the front feet (hands) left the ground.

    1. Dear Jim Cunningham,
      I’m afraid your conclusions are from fairyland. Lets take them one at a time:
      1. All your references to the Q.n torso indicate a much smaller creature than has hitherto been explained. The one figure you don’t mention is the 34ft wingspan. There is simply not enough volume in your torso to handle the massive musculature which flapping such a huge wing would need. 20% of body mass is the usual rough figure for flight muscles. 20% of 200Kg is 40Kg. Your torso is apparently the size of a small dog. How to you wrap 40Kg of muscle around a Labrador’s rib cage?
      (A hang glider has about the same span/area as Q.n is supposed to have. I can tell you that the muscle mass to move that lot through the air is eyewateringly huge. I know, I spent years flying them.) Even 40Kg of primary flight muscles just wouldn’t cut it.

      2. If Mark Witton didn’t base his drawings on the calculations of mass, why was he drawing pictures in a scientific journal instead of for Walt Disney? Mark too has this idea that the Qn torso was only 65cm from Delto-pectoral crest to acetabulum (why don’t you just say from shoulder to hip? Or doesn’t that impress people quite so much.) That is less than the chord of the wing itself.

      3. The quad-launch hypothesis is just not feasible. (I thought Chatterjee invented it?). For anyone brought up with flying machines of any sort, the idea is just silly. This is where it is easy to fire off a load of maths and make it sound like science. We are talking here of a 200Kg creature with a 34ft wingspan. With a wing loading over 25Kg/m2 it would have a stall speed of about 30mph. Stall is also the minimum flight speed less the intermittent thrust vector from a flapping wing. So you are saying that 4.5g launch thrust gets a 210Kg creature from 0-30mph in twice the length of its humerus – about 4.6m? OK – an Olympic sprinter using starting blocks (to stop his feet skidding backwards) can achieve 1G initial acceleration averaging to about 0.57G over 5m (his second leg needs time to move forward and down ready to give its own push once the first leg has finished pushing). How did Q.n’s toes and fingers allow even 1g into the ground surface without skidding and scrabbling?
      So, Qn needs to move 210Kg of mass through 4.6m at about 45 degrees to the horizontal and attain sufficient airspeed to allow it to unfurl its wings, spread them out above itself against the airflow, and then start to push against the air in order to start the first downstroke. My maths tells me that it would need a push force of about 2g horizontally and 2g vertically. It is just possible to imagine a modern bird doing this, but most of it’s thrust would be coming from its (already flapping) wings, not from its legs. So the whole idea is totally preposterous and does nothing for science – although it’s great for cartoon animations. Superman and Spiderman could launch like that, I know because I too have seen the films, so why not Q.n?

      Phil Parsons

      PS. Did you know that the maximum jump-height recorded for a Red Kangaroo is 2.5m. Clearly Qn could beat this by over 100% with its wings shut.

  27. I am permanently closing comments on this post, because the commenters are refusing to abide by my commenting policy. I have said repeatedly that this blog is no place for debating your various “theories”, and things have veered very far off-topic from the original post. (Refresher: the post was about why we can be very sure Earth’s gravity didn’t change drastically over the past 65 billion years. It was not about mechanisms for changing gravity, or about pterosaur flight, or anything of that nature.) Go continue your petty and rude arguments on another website.

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