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How Big Were the Dinosaurs?

© Beverly Eschberger

As a paleontologist, one of the questions that I am often asked is "Why were the dinosaurs so big?" Well, the answer is that they were not all large. In fact, many dinosaurs were the size of an ostrich or smaller, but the really big dinosaurs are the ones that always stick in our minds. (Be sure to read my article "Dinosaur Primer" for more information about the different groups of dinosaurs.)

When most people think about dinosaurs, they think about large, herbivorous sauropod dinosaurs such as Seismosaurus, Ultrasauros, Diplodocus, Brachiosaurus and Apatosaurus. Or maybe the carnivorous theropod dinosaurs such as Tyrannosaurus, Giganotosaurus or Carcharodontosaurus.

Not all of the dinosaurs were giants, however. Perhaps you remember the chicken-sized Compsognathus dinosaurs from the movie The Lost World:Jurassic Park." Compsognathus was a theropod dinosaur that lived during the late Jurassic Period (195 to 140 million years ago). There was also the two-foot-tall early theropod Saltopus that lived during the early Triassic Period (230 to 195 million years ago). Microraptor is a recently-discovered crow-sized (about 16 inches long) theropod dinosaur from China.

In the ornithopod group of dinosaurs, Lesothosaurus was a three-foot-tall ornithopod from the early Jurassic Period.

There was also the two-foot-long Wannanosaurus from China. Wannanosaurus was a member of the Pachycephalosaurid group of dinosaurs and lived during the late Cretaceous Period (140 to 65 million years ago).

The smallest dinosaur skeleton to be found to date is the tiny Mussaurus or "mouse lizard," a prosauropod dinosaur from the late Triassic Period. Only the nine- to 16-inch-long hatchling representatives of this species are known, so we do not know how large it grew to as an adult.

As we find more dinosaur skeletons, we will likely find more species of small dinosaurs. We must remember that although among our extant (still living) mammals large, "charismatic megafauna" such as lions, tigers, elephants and whales are exciting to see, the vast majority of mammals on the earth are small species the size of squirrels and even smaller. In fact, last year a new species of shrew was found in South America which is less than an inch in body length. This is due to the fact it is much easier for an ecosystem to support a large population of small animals than it is to support even a small population of large animals.