About 230 million years ago, a new group of reptiles appeared on the Earth. They were gigantic and they were small, ranging in size from a 737 airliner to a chicken. They were plant-eaters, they were meat-eaters, and the last of them disappeared about 65 million years ago. That’s about all that paleontologists can agree upon. Everything we know about these extinct creatures, scientists deduce from their fossil remains – at least the ones that have been uncovered. So, that leaves a lot up for heated debate and new interpretations. And much disagreement.
But we can make a few conclusions. The earliest dinosaurs, Herrerasaurus and Eoraptor, found in Argentina, have been dated to be 227.8 million years old. “The fact that Herrarasaurus was already a fully developed dinosaur indicates that there must have been dinosaurs before that,” says Hans-Dieter Sues, vice-president of collections and research at the Royal Ontario Museum in Toronto. The earliest dinosaurs were also bipedal and walked upright. As a group, the dinosaurs grew bigger over time. “Almost all of the really big dinosaurs are either Late Jurassic or Cretaceous in age,” explains Sues.
The dinosaurs were land-dwelling reptiles that lived through three time periods during the Mesozoic Era: Triassic (248 to 206 million years ago); Jurassic (206 to 144 mya); and Cretaceous (144 to 65 mya). Different species lived and died during different periods while others may have spanned the entire Mesozoic Era. Were they endothermic or exothermic? That is, did they generate their body heat internally like most birds and mammals, or did their body temperatures fluctuate dependent on the environment, like most living reptiles? That depends on the dinosaur and perhaps even on a given situation – were they resting or hunting? “The ones that evolved into birds are most certainly warm-blooded in a strict sense,” explains Sues. “But if the really large ones, like sauropod dinosaurs, had been warm-blooded in a mammalian or bird-like fashion, they would have had a huge physiological problem of getting rid of excess body heat because they were living in a warm climate. In those animals, the mass-to-surface ratio is so unfavourable, that they just couldn’t dump all the heat.”
Many, if not the majority of dinosaurs, says Sues, had fairly advanced thermal regulation systems. “We know that they grew very quickly from the histology (microscopic study) of dinosaur bones. And that’s something you can only do if they are more advanced. Present-day cold-blooded reptiles like crocodiles grow slowly.”
And the name dinosaur? That came from the Greek words deinos and sauros, which meant ‘fearfully great lizard’.