David Esker has a dinosaur theory. He thinks they grew as large as they did due to a buoyant atmosphere. I don’t think he’s right in this, but he defends his theory well. Esker points out all sorts of facts that fit his conclusion. One such fact is that Venus has an atmosphere 91 times more dense than here on Earth. He concludes from this that Earth might well have had an atmosphere that was several hundred times what it is today back in the days of the dinosaurs. A thick atmosphere would also explain why the climate was more uniform across the globe back then than it is today.
Another interesting fact has to do with the shape of dinosaurs. The fast ones had large hind legs. Their tails were also large, and Esker makes the argument that this is due to a thick atmosphere. Dinosaurs were half swimming, half walking, through Earth’s thick atmosphere, and an aerodynamic shape helped in this. He suggests that their long tails helped them propel their bodies forward. However, all of this can be explained quite differently.
If our planet was half the size it is today, as suggested by geologists like James Maxlow, Earth’s climate would’ve been different too, with a more uniform climate the most likely consequence.
When it comes to the shape of dinosaurs, it’s true that Darwin’s law dictates that form follows function. No animal develop into a shape for no reason. When we look at dinosaur skeletons, we know from their shape whether the animals were fast or slow, whether they could fly or not, and so on. However, the shape of fast dinosaurs can be explained just as well with low gravity as with atmospheric buoyancy.
Walking and running involves a use of gravity in which moving land animals lean forward. We are constantly falling forward, and walking is mostly a matter of preventing ourselves from falling over by putting our feet forward.
Running involves more forward propulsion, and it’s the hind legs that does this most effectively. Hence, fast animals always end up with stronger hind legs than front legs. However, the extent to which this is required depends on the strength of the force pulling us down. The stronger that force is, the less we need to lean forward. In cases where the force is very light, leaning forward is no longer effective. Jumping becomes the most effective way to move, as was quickly discovered by the astronauts who visited the moon.
All we can say from the shape of fast dinosaurs is that they must have leaned farther forward when running than is presently required. The force pulling them down was not as strong as it is today. As for their tails, they’re useful for other things than propulsion.
A big tail functions as a rudder, giving a fast moving dinosaur stability as well as the ability to turn on a dime. When going in a straight line, the tail steadies the motion. Flipping the tail to the side allows the animal to change direction even at high speeds. It’s therefore natural for fast animals to develop tails that are as heavy as gravity will allow them to be. Fast land animals have relatively long tails, even today. It’s the slower animals that drop their tails. However, this is only because of gravity.
A light gravity environment with low atmospheric density would allow all animals to develop a big tail for defense. Such a tail could develop a club at its end, as happened with the Ankylosaurus, indicating that this dinosaur existed in a low gravity and low atmospheric density environment.
A big tail would also function as a good counterweight for dinosaurs with long necks. Such tail can also be used as an extra hind leg for animals to lean back on, as Kangaroos do in Australia.
Another problem for Esker’s theory is the fact that some animals have grown smaller without changing their shape. They have not changed their aerodynamic properties, and we can therefore assume that air resistance has remained fairly unchanged. Only the pull of our planet has changed.
However, this is not to say that I think Earth’s atmosphere has undergone no change at all since the time of the dinosaurs. It may have changed quite a lot, and Esker provides plenty of evidence for this. However, I doubt that it ever was much thicker than what we have on Venus, and an atmosphere 91 times thicker than what we have today would only reduce our weight by ten percent, not the fifty to eighty percent required to explain the dinosaurs. For that to happen, we would need an atmosphere that’s 500 to 800 times thicker than what we have.
On a final note, it’s worth noting that large animals have gone extinct quite recently. There’s a pattern in which large animals go extinct before smaller ones of the same type. Large Mammoth’s went extinct before smaller ones. Lions were larger in Roman times than they are today. Sabre tooth tigers are long gone, but not their smaller relatives. If this is due to a change in how heavy things are, it’s either due to a big change in our atmosphere or a subtle change in gravity.
For things to have been lighter by a factor of one in a thousand in Roman times, our atmosphere must have been 2g per liter instead of 1g per liter as it is today. That’s twice as dense as it currently is. However, gravity needs only to have increased by a factor of one in a thousand to have achieved the same effect.