There seems to be a sense among the general public that dinosaurs were failures. After all, they’re all dead and long gone, and isn’t that a sure sign of failure? Th e real fact is that extinction is the fate of all animal groups. Despite that inevitability, dinosaurs survived a major extinction at the end of the Triassic that eliminated a wide assortment of other large, fierce, and exotic reptiles. And, as is now widely accepted, dinosaurs are not truly gone — they persist in the form of birds today. However, in the remainder of this essay, the word ‘dinosaur’ will refer to the original, not-including-birds concept of dinosaurs.
One way of demonstrating that dinosaurs were not failures is that they were around for roughly 166 million years, first appearing as body fossils in the Middle Triassic about 232 million years ago. Compare that to something like the almost-as-famous, non-dinosaur Dimetrodon and its close relatives, which inhabited the planet for about 25 million years. There are tantalizing hints, in the form of distinctive three-toed fossil tracks, that dinosaurs may have been present in the earliest Triassic, not long aft er the most severe mass-extinction known — the end-Permian — implying that dinosaurs may have existed 250 million years ago. To give another sense for just how long dinosaurs existed, it is interesting to note that we are closer in time to Tyrannosaurus from the latest Cretaceous at 66 Ma than Tyrannosaurus was to Allosaurus from the Late Jurassic at 155 Ma. A time gap of 89 million years separates these two well-known theropods.
Another indication of the success of dinosaurs is the huge range of body sizes that they evolved. They ranged in size from thrush- and chicken-sized forms weighing just a few tens of grams to the absolutely immense mid-Cretaceous, South American sauropods such as Argentinosaurus that are estimated to have weighed 80–100 tonnes. Excluding the aquatic whales which don’t have to support their bodies against gravity, the largest land mammals known to have existed are creatures such as Paraceratherium from the Oligocene of Asia, with a body mass estimated to be 16 tonnes. While a very large animal, it weighed less than a quarter that of Argentinosaurus.
Another common misconception is that dinosaurs had ridiculously small brains for their size, thus predisposing them to be failures and go extinct. It has been well established by experimental studies over the past 70 years that brain size in all animals does not increase at the same rate as body size. In fact, reptiles have smaller brain volumes than mammals for a given body size, so dinosaurs were just following the reptile trend. Furthermore, large animals have proportionally smaller brains than do small animals: the ratio of brain to body mass in an elephant is less than that for a mouse.
No animal group can be considered to be perfect, and dinosaurs were no exception. Although attaining immense body sizes, existing for many tens of millions of years, and having their remains found on all the continents, there were some things that dinosaurs never did accomplish. Classical dinosaurs never evolved any aquatic forms. This may be due to the fact that Jurassic and Cretaceous seas were already home to other aquatic reptiles such as the plesiosaurs, ichthyosaurs, and turtles. Alternatively, it may be that the spines, hips, and legs of dinosaurs were so well adapted for supporting great body weights on land, that they could not be modified by natural selection to become effective for living in water. In contrast, aquatic mammals in the form of early whales began to appear within 20 million years of the extinction of the dinosaurs. Lastly, despite existing for at least 166 million years, dinosaurs never did evolve a tool-using culture. Mammals, in the form of early hominids, accomplished this within 64 million years of dinosaur extinction, a little more than a third of total dinosaur time.
In summary, we can see that dinosaurs were not failures, but just animals that were dealt a bad hand by the solar system in the form of an asteroid impact at the end of the Cretaceous.