Nearly everyone you meet has at least a passing familiarity with dinosaurs. Children can rattle off the names of dozens of species at the drop of a hat, and even most adults know at least a couple of the A-listers- Tyrannosaurus, Triceratops, Brontosaurus, or Stegosaurus, for example. Each of these are wondrous animals in their own right, but it doesn’t take a paleontologist to realize that Velociraptor and Brachiosaurus are very different animals- so why do we group them together? What makes a dinosaur a dinosaur?
First, a quick history lesson: people have been finding dinosaur bones long before “paleontology” was a profession. In the early 1800s, European scientists were becoming increasingly perplexed over the discovery of fossils belonging to truly gigantic reptiles, unlike anything that was alive today. Early discoveries included the jaw of the fearsome Megalosaurus- the first genus of dinosaur to be named- or the teeth of Iguanodon, named for the resemblance they had to the teeth of the modern iguana (although a great deal larger!).
The word “dinosaur” was coined by a naturalist named Sir Richard Owen in 1841. The word comes from the Greek words “deinos,” meaning fearfully great, and “sauros,” meaning lizard or reptile. Thus, the word dinosaur means “fearfully great reptile.” That might be a little different than what you’re used to hearing. In popular culture, the translation is often shortened to “terrible lizard,” but I feel this translation disregards Owen’s true intent of conveying the awe these creatures invoked.
But...what are they? Simply put, dinosaurs are a specific type of archosaur. Archosauria is the group of reptiles that includes crocodilians, pterosaurs, birds, and of course, dinosaurs. Fun fact: because they are in the same group, crocodiles and alligators are more closely related to birds than they are to any other reptile!
Dinosaurs have many traits that set them apart from other archosaurs. Most of these are steeped in technical terms and deal with proportions of certain bones to others- things that can be hard to pick up on for people who don’t study this stuff for a living. However, dinosaurs do have a few traits that anyone can observe by looking at a skeleton in a museum. You might notice that the skulls of dinosaurs seem to have an awful lot of holes. These holes, which scientists call fenestrae, would have served as sites for muscle attachment as well as helping the animal regulate the temperature of its head. Between their eye socket and nostril, they have a large hole called an antorbital fenestra. This is a trait possessed by archosaurs as a whole, and can be seen in modern birds (crocs have lost theirs). Dinosaurs, unlike most other archosaurs, also hold their limbs directly underneath their body, like mammals, as opposed to the sprawling gait of a crocodile or pterosaur.
Dinosaurs are traditionally classified based on their hips. The hip, or the pelvis, is made up of three bones: the ilium, the pubis, and the ischium. Early paleontologists noticed that there appeared to be two major groups of dinosaurs based on the structure of their hip bones: those with a pubis that points forward and down, and those where the pubis points backward, parallel with the ischium. Those with a downward-pointing pubis are called saurischians (pronounced “saw-RIS-kee-uhns”), or “lizard-hipped,” due to their superficial similarity to...well, lizard hips. This group includes theropods (bipedal, carnivorous dinosaurs such as Velociraptor and Tyrannosaurus) and sauropods (long-necked dinosaurs). Dinosaurs with a backwards-pointing pubis are called ornithischians (pronounced “awr-nuh-THIS-kee-uhns”), or “bird-hipped,” because their hips resemble those of (you guessed it!) birds. This group contains ceratopsians (horned dinosaurs), hadrosaurs (duck-billed dinosaurs), and ankylosaurs (armored dinosaurs). Ornithischians also have an extra bone on the tip of their lower jaw called the predentary, which served as an attachment point for a keratinous beak. Next time you see a dinosaur skeleton in a museum, see if you can figure out whether it is a saurischian or an ornithischian!
There is one more feature of the dinosaur hip that makes them unique: a perforated acetabulum. The acetabulum is the hip socket where the head of the femur sits. In dinosaurs, this feature is not a deep socket as seen in in birds and reptiles. Instead it is open, or perforated. To put it simply, it’s a hole in the hip.
Each of these characteristics we've looked at are synapomorphies: traits that are possessed by all members of a certain group. For example, fur and milk production are synapomorphies of mammals, and scales are a synapomorphy of reptiles. So if you see a skeleton in a museum that 1) holds its legs directly underneath its body, 2) has a large hole between the eye socket and nostril, and 3) has a hole in the hip, you can be sure you’re looking at a dinosaur.
There are lots of fascinating creatures you're probably familiar with that you might think of as dinosaurs, for example, pterosaurs, Dimetrodon, and marine reptiles like mosasaurs, plesiosaurs, and ichthyosaurs. But contrary to popular belief, none of these animals are dinosaurs! Some of these, like pterosaurs, are close cousins of dinosaurs, but not dinosaurs themselves. Marine reptiles represent multiple groups, but each of them are generally more closely related to non-dinosaurian reptiles. Even more surprisingly, Dimetrodon isn't a reptile at all. In fact, it's a precursor to mammals, making it the least dinosaur-like of any of these critters!
Did you know that you can see living dinosaurs today? No, I’m not talking about sketchy reports of prehistoric monsters hiding in impenetrable jungles. I’m talking about the dinosaurs that you probably see every time you step outside. Some of you may have already figured out what I’m referring to…
That's right! Birds are, in fact, a type of dinosaur. Because we've been looking at several dinosaur relatives today, let me be clear: Birds are not just similar to dinosaurs, and not just related to dinosaurs, but actually dinosaurs themselves! Specifically, they are a type of theropod- the same group of dinosaurs containing Velociraptor, Allosaurus, and Tyrannosaurus rex. Thus, dinosaurs can be divided into avian dinosaurs, or birds, and non-avian dinosaurs, or all dinosaurs other than birds. In a future post, we will discuss in more detail the relationships between birds and dinosaurs and the remarkable similarities dinosaurs like Velociraptor share with living birds.