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Why Were Dinosaurs so Large?

The following Math Matters article by Dr. Don DeYoung was published in Creation Matters, Vol. 19, No. 3, May/June 2014.

Even though giant dinosaurs get the most publicity, many dinosaurs were smaller. Of the hundreds of known distinct species, the average adult size was that of a dog or sheep. For example the adult Compsognath weighed about 15 pounds, the size of a large cat. The Psittacosaur was smaller yet, similar in size to a squirrel.

With that qualifier, however, the sauropod dinosaurs were truly impressive with massive torsos, pillar-like legs, and long necks. The herbivore Argentinosaur may have been the heaviest beast ever to walk on land. Also found in Argentina, the largest predator may have been Gigantosaur, an economy-sized Tyrannosaur 42 feet long and weighing 8 tons (Schreeve, 1997).

Table 1 gives the weights of several representative animals (DeYoung, 2000). The values given for dinosaurs are estimates based on the size of their bones and on reconstructed models. Uncertainties in actual dinosaur body weights may be as much as 50 percent due to incomplete fossil remains.

Many reptiles grow throughout their lifetimes, gradually tapering off with age. An example is the Nile crocodile, the largest of the living reptiles. Some mammals, such as elephants, also grow slowly throughout their lives. The largest dinosaur fossils, therefore, may be from creatures that had lived for centuries.

Two evidences suggest a rapid early growth rate for the dinosaurs. First, few halfgrown dinosaur fossils have been found. Thus, dinosaurs may have rapidly passed through their juvenile phases. It may also be that specimens having smaller, fragile bones simply did not often survive the burial and fossilization process. Second, dinosaur bone texture, especially for the stegosaurs, shows evidence of rapid youthful growth. Robert Bakker estimates that stegosaurs may have grown from an egg to five tons in just one decade (Bakker, 1986).

The creation view sees purpose and design in nature, including the impressive size of some dinosaurs. The long necks of the sauropods may have allowed them to eat the foliage from tall trees. In this way dinosaur parents could reach a higher food source and avoid competition with their young. However, some paleontologists doubt that sauropods could dinosaur size also provided defense against attack from smaller foes. In the animal world there is a measure of safety in large size. Predators rarely attack something that is big and strong enough to injure them.

There are physical limits to the maximum size a creature can attain because the supporting bones and muscles must increase rapidly as the animal’s weight increases. Consider elephant bones which are so large that the legs themselves must be greatly thickened. Whales require smaller bones because of water buoyancy, but are in danger of fractured ribs if stranded out of water.

Suppose we double the size of an animal in all three dimensions including its length, height, and width. The animal’s volume and weight will then increase 8 times (2^3). However, the bone strength only increases 4 times (2^2). This is because the bones’ supporting strength depends on the cross-sectional area. The numerical comparison made here is between volume and area. If we could make a creature 10 times larger, its weight would increase 1,000 times (10^3) while its bone strength becomes only 100 times greater (10^2). The implication is that, for large animals, bone size must increase disproportionately more than body size. Galileo first wrote about this limitation on large size in 1638, nearly four centuries ago.

Table 2 gives some comparative bone diameters for a variety of relative animal sizes. Animals much larger than the dinosaurs would require bones too bulky to be practical. Notice that bone size must increase substantially out of proportion to animal size. King Kong and Godzilla can exist only in movies—not in the real world. The same area-volume reasoning also limits the possible size of trees. Perhaps you have seen the wide trunk needed to support a 350-foot redwood tree.

References

Bakker, R.T. 1986. The Dinosaur Heresies New York: William Morrow and Company, Inc.

DeYoung, D. 2000. Dinosaurs and Creation: Questions and Answers. Grand Rapids: Baker Books (Kindle edition).

Schreeve, J. 1997. Uncovering Patagonia’s lost world. National Geographic 192(6):120–137

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