Why were the Dinosaurs so Big?

By G. Lee Zimmerman


Often the first thing that you notice when you come face to face with a dinosaur fossil is how big it is. It is estimated that the largest land dinosaur was six times the size of the largest land animal of today. Its skeleton and teeth suggest that some of these massive dinosaurs were capable of moving at great speed, leaping and tearing prey to bits. The evidence left behind by these terrible lizards does not bring to mind the slow, lumbering, elegant movements of elephants.[1]

Even more perplexing, today the largest flying birds, such as the albatross, have a wingspan a little over 11 feet and can barely get off the ground. In 1972, in Texas, fossilized bones were found belonging to a giant pteranodon: Quetzalcoatlus. This beast was probably the largest flying animal that had ever lived. It had a conservatively estimated wingspan of about 50 feet – half the wingspan of a Boeing 737. [2]

Were the dinosaurs just very efficient at growing large? Did the dinosaurs have super strength bones and muscles that allowed them to move their bulk with great agility or flap impossibly large wings? Did the dinosaurs have extra lightweight bones, organs, and muscles, unlike what is seen in animals today, so their great size did not indicate a great heavy bulk? What created this sharp size divide between the animals of today and the dinosaurs? And more importantly why do scientist from a variety of related fields studying prehistoric times shy away from this question?

There is a hypothesis, rarely stated, which offers a solution to these seemingly unresolvable questions. What if the dinosaurs lived on an earth where they felt less effective weight?

Less effective weight would explain the great size of the land dinosaurs relative to land animals today. It would explain birds with wings that muscles couldn't lift today. In the oceans, where the effective weight is more a factor of density than mass, there exists today beasts that are two to three times the size of the largest dinosaurs. The blue whale, at 160 tons, seems to be the largest animal that has ever lived. If effective weight plays a role in maximum animal size, then less effective weight means larger animals. This seems to be true.

If we assume the hypothesis is true and the dinosaurs felt less effective weight than we do today, what could account for less effective weight?

There are two elements to the weight that we feel on land: gravity (which is determined by the mass of the earth) and spin (rotational velocity leads to less weight around the equator). A faster spinning, smaller earth could explain the great size and agility of the dinosaurs.

Consequences of a Faster Spinning Planet

If the earth were spinning 15 times its speed today, animals at the equator would only feel about 20% of the weight that they would feel at the poles (this assumes the earth has the same mass and size). There would be a continuously changing effective weight with the least being at the equator, the greatest at either pole. You would expect find the largest animals living in a band around the equator. The largest animals in an area would get smaller as they approach the poles. (Here is a link to the physics)

Even more intriguing, since night and day would pass more quickly with a faster spinning earth, we would expect to find less temperature variations due to solar radiation. Less night to cool off. Less day to heat up. This might make ideal conditions for animals to evolve that don't need to regulate their body heat. If you develop large ferocious cold-blooded beasts around the equator, you would leave room for smaller animals, able to regulate their own body heat, to develop in the colder (heavier) climate of the poles - hiding where the terrible lizards couldn't go.

How Do We Get To Here From There?

If you mention to someone studying dinosaurs, that a faster spinning, smaller earth would address some of these big issues, often the scientific dialogue ends. It ends because this hypothesis brings with it a scientifically uncomfortable idea. A smaller, faster spinning earth of the dinosaurs implies a tremendous celestial impact. An impact that could bring new mass to the planet and slow its rotation. It is hard to think, that an impact of this scale, would not destroy the earth completely or at a minimum wipe out all living things.

To truly assess this fear, we would need to examine the impact. We would need to take into account, the material of the impacting object, the speed, position, and angle of impact as well as the movements of the planet, the brittleness of its crust and gooeyness of the mantle and core. Until we examine the various parameters necessary in a collision like this, we cannot know what the limits are for damage. For all we know, the necessary change in rotational velocity to the earth might be produced by a glancing blow.

There is already evidence that the earth has been hit by something celestial that has changed it on a planetary scale. The fact that the earth's rotational axis is not perpendicular with respect to its path around the sun is proof of an impact in the past.

Maybe even more important to scientists, an impact at this scale would shake the tools of prehistoric science and geology to its core. These sciences fundamentally assume that the planet was essentially the way it is now - spinning at the speed it does today, with little mass added from space, sitting at the same tilt and orbit. The proposed impact would change the measurement of time itself.

Something happened 60 to 70 million years ago, that exterminated this class of giants. It was their immense size that allowed them to dominate the planet for 200 million years. It is also possible that their immense size killed them. Because we are afraid to imagine, we may be blinding ourselves to this fact.


[1]  Wikipedia – Largest Organism

[2] Lawson, D. A., 1975, Pterosaur from the Latest Cretaceous of West Texas. Discovery of the Largest Flying Creature. Science, 187, pp. 947-948.