How did dinos get to be so big ... and so little?
There is much to marvel about dinosaurs — their diversity (well over a thousand species), their adaptability (swamps to deserts, mountains to shorelines), the length of their reign as top terrestrial vertebrates (130 million years) — and the sheer size of some.
That is certainly true of the sauropods, the four-legged, long-necked, long-tailed “brontosaurs.” When brontosaurus was announced in 1879, its length of about 70 feet was thought to be enormous.
Other discoveries followed, with larger and larger sauropods being found, such as brachiosaurus and diplodocus.
Later discoveries showed sauropods could get even larger, such as argentinosaurus at an estimated 115 feet, and it seems that every few years another contender for largest sauropod is discovered.
Such is not the case, however, for the theropods, the two-legged, mainly carnivorous dinosaurs. Tyrannosaurus, named in 1906, was a mere 40 feet long.
Theropods discovered more recently, such as South America’s giganotosaurus and Africa’s spinosaurus, may have been larger, but the skeletons are incomplete, so estimates of their size are unreliable.
In almost 200 years of collecting, no theropod based on reliable data was much longer than 40 feet. There seemed to be a limit to how big two-legged animals could get.
Some recent research looked at that question by analyzing skeletons of 14 theropods, from one to 40 feet long. Only species for which we have complete skeletons were used. The research focused on the size of the pelvis and cross-sectional area of the tail.
Being two-legged, all the muscles for balance, movement, and turning, were anchored on the pelvis and the tail. For example, one of the main muscles, the caudifemoralis, runs from the back of the thigh bone (the femur) to the base of the tail. When it contracts, the leg is pulled back, propelling the animal forward.
The power of a muscle is controlled by its cross-sectional thickness. To get stronger, a muscle must get thicker. The area on the skeleton it attaches to, in this case, the pelvis and tail, must also become larger.
The longer the theropod, the more it would have weighed, but the two are not proportional — as length increased, weight increased by a factor of 3.5. The heavier the animal, the more muscle it would have taken to move it.
A limit was reached as to how big the animal could get and still be propelled by the muscles of its legs. The largest theropods had less than 10% the acceleration capacity of the smaller ones.
Also, the longer the animal, the more rotational inertia it would have had, requiring more muscle to turn or to stop it from turning once turning started. The largest theropods had significantly reduced agility.
It appears that a 40-foot-long theropod is likely the largest we will probably ever find.
If you are interested in dinosaurs, or other fossils or rocks or minerals, the place to be today is the Central Ohio Mineral, Fossil, Gem & Jewelry Show, from 11 to 5 at the Rhodes Center of the Ohio Expo Center, where there will be dealers from around the world, exhibits, and educational activities and free samples for kids. I will be there — stop by and say hello.
Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University. Contact him at gnidovec.1@osu.edu