ScienceSunday: What Makes a Dinosaur?
Ask most kids, and they’ll tell you dinosaurs are some of the biggest, scariest, and coolest creatures to ever walk the planet. Most have a favorite: Tyrannosaurus, Stegosaurus, and Triceratops are among the most popular. Anyone can name a dinosaur or three, but what actually makes something a dinosaur? To understand this better, we need to delve into deep time and look at the evolutionary history of the dinosaurs.
Back in the Permian, between 299 and 251 million years ago, a group of creatures called the archosaurs split off from other reptiles. Think of it a bit like a family tree. From the parent group of “scaly things” came two daughters: the archosaurs and the lepidosaurs. The lepidosaurs are things like lizards and snakes. The archosaurs are the crocodiles, the dinosaurs, marine reptiles, and one other set of animals (we’ll get to that later). These groups had different circulatory systems and respiratory systems, along with other genetic and morphological differences that made them distinct. To understand what makes a dinosaur, however, we need to look at the split within the archosaurs.
The feature that makes dinosaurs distinct from crocodiles and other archosaurs is their legs. Think of your average crocodile or alligator: its legs sprawl out to the side, forming nearly right angles at the first joint. Dinosaurs, however, stood with their legs directly under them, the same way mammals like us do. Technically, the hip joint changed, which is what really allowed the legs to be upright rather than sprawling. Additionally, the way they walked changed. Many early dinosaurs were bipedal, walking around on two legs. Even those that weren’t used a much different kind of walking. Crocodiles tend to move their bodies in a sort of slithering, s-shaped motion as they walk. Four-legged dinosaurs moved much more like mammals, such as the elephant, where the torso and body stays mostly straight as the legs alternate. These innovations allowed dinosaurs to move much faster than most of their crocodylian relatives, making them both better at capturing prey and at avoiding becoming dinner themselves.
The flying reptiles, the Pterosaurs, are not dinosaurs, despite the fact that a pterodactyl comes in every child’s tube of dinosaurs. They are archosaurs, like the dinosaurs and crocodiles. They split off from the other groups early in their evolutionary history, however, and are not closely related to the dinosaurs. They are maybe second cousins, to use our family tree analogy. Pterosaurs are also not birds… their wing structure is more bat-like than bird-like, they had hair-like filaments instead of feathers, and they are cousins of, not descendants of, the dinosaurs.
Dinosaurs are also distinct from marine reptiles, such as mosasaurs, ichthyosaurs, and pleisiosaurs. In fact, these groups aren’t closely related at all… the last common ancestor would have been before the archosaurs split from other reptiles. To say these groups are relatives would be like saying two kids whose great-great-great-great-great-great-great-great-great-great-grandmothers lived in the same village are related. The marine reptiles, or Sauropterygians, specialized into a completely different environment, the Mesozoic oceans, contributing to an evolutionary event referred to as the Mesozoic Marine Revolution. But that’s a story for another day.
Now, everyone knows that the dinosaurs went extinct when the asteroid struck at the end of the Cretaceous, right? Well… not exactly. The non-avian dinosaurs went extinct, but a small group of raptor dinosaurs, the avians, closely related to theropods such as Velociraptor, made it through, and have been quite successful. This group of dinosaurs is very familiar to us… you may have even eaten one over the holidays. The birds are the living legacy of the dinosaurs. They’ve changed, but many characteristics stay the same: the scales, the upright legs, the fast metabolism (a characteristic of later theropod dinosaurs). And recent genetic analyses, combined with fossil evidence, makes the connection even stronger. If you want to know what makes a dinosaur, look at your museum, then look at the birds flying about outside, and know that the dinosaurs are alive and well in the modern world.