On Avialan Theropod Dinosaurs
I saw my favorite species of dinosaur, yesterday. Don’t let those science books fool you! Contrary to popular belief, dinosaurs were not wiped out in a massive extinction event, 65 Million years ago. They have lived side-by-side with humanity throughout our entire history, and even today, they’re still around. I saw one, yesterday, and it was beautiful. As I was driving over to the gym, preparing to teach the five-o’-clock Kids’ Jiu-Jitsu class, I saw it pacing along the side of the road as it stared at the carcass of a recently roadkilled deer. As soon as the traffic had cleared, enough, I saw the raptor bound towards its meal on two scaly, talon-footed legs which tensed with powerful muscles. Diving face-first into the fresh meat, the creature tore away chunks of flesh, gobbling them down with efficiency and zeal. However, as my car approached, the magnificent beast suddenly became apprehensive. Spreading its glossy black wings, the dinosaur leapt into the air and flew away.
My favorite species of dinosaur is Corvus corax, the common raven.
At this point, you might be asking whether I have gone crazy. A raven is a bird, you might say, not a dinosaur! Some among you might have heard the popular science reports that birds evolved from dinosaurs, but you might protest that they aren’t dinosaurs anymore. However, this betrays a common misunderstanding of exactly what it is that biological evolution does. Wholly new types of creatures do not just pop into existence, and the traits of an organism’s ancestry persist with modification in descendant populations. Birds are dinosaurs in exactly the same way that humans are mammals.
Surely, you might then contend, I must be using some peculiar and overly-broad definition of “dinosaur!” After all, a raven looks nothing like a triceratops! On the contrary, I completely assure you that I am utilizing the standard scientific definition of a dinosaur. A dinosaur is any organism classified within the Series Amniota, Class Sauropsida, Subclass Diapsida, Infraclass Archosauromorpha, Division Archosauria, Subdivision Avemetatarsalia, Infradivision Ornithodira, and Superorder Dinosauria. That means that dinosaurs all have the following traits:
- They are vertebrates
- Their young develop in amniotic fluid
- They have antorbital and mandibular fenestrae (that is, holes) in their skulls
- Their legs support their weight from directly beneath their bodies
This, of course, is not an exhaustive list of dinosaur base traits. If you want a more exhaustive list, check out each of the links, above. However, morphologically speaking, there is no defining characteristic of a dinosaur which is not also present in birds.
Still, you might not find that convincing. After all, dinosaurs were huge, powerful land animals with scaly skin while birds are relatively small, feathered creatures with beaks and wings and hollow bones! Unfortunately, if you had this thought, your image of dinosaurs has been thoroughly poisoned by Hollywood. While there were certainly some massive dinosaur creatures, there were also quite a number of dinosaur species which remained relatively small. In particular, we know of many species of Theropods– that is, the dinosaurs that walked on two legs– which were about the same size as modern chickens or small turkeys (for example, Oviraptor and Compsognathus). Furthermore, one of the defining characteristics of Theropods is that they have hollow bones. It might surprise you to learn that even the mighty Tyrannosaurus rex had far more in common with a rooster than with a crocodile– including (but certainly not limited to) hollow bones and even a wishbone! As for the scaly skin, even today, this feature persists in birds. Just look at the legs and feet of almost any bird, and you’ll notice the same scaly nobs of flesh that you might have seen on the velociraptors in Jurassic Park– and as you would certainly see on the fossilized dinosaur skin which has been found over the last century.
Ahh, but feathers! You might rightly proclaim that feathers are one of the defining features of birds. Dinosaurs didn’t have feathers, did they? Actually, it seems that a great many of them did. Since 1994, paleontologists have found fossils of more than 30 different species of dinosaur with preserved feathers. This even includes a few Tyrannosauroids, like the Dilong paradoxus, implying that even the ever-popular T. rex may have been feathered. Other fossil finds, particularly of the Oviraptor, have indicated that these feathers might have had some particularly beneficial uses to dinosaurs, even excluding the flight with which they are now commonly associated– like keeping a nest of eggs warm while brooding, in exactly the same manner as a chicken does!
What about their wings, then? Wings are very different from arms, and dinosaurs didn’t fly! So where did wings come from? This is a common question, but it’s built upon some very bad misconceptions. If you ever examine the skeletal structure of a bird’s wing, you’ll notice that they aren’t as distinct from arms as you might have thought. Wings have humerus, radius, ulna, carpus, metacarpus, and digit bones just like a T. rex’s forelimbs, or even your own arms. Furthermore, it is readily apparent that wings do not necessarily imply flight: kiwis, penguins, emus, ostriches, quails, cassowaries, grebes, rails, and even a number of species of duck are all examples of flightless birds, which yet have wings. Just take a look at the following image. On the left is Compsognathus, a perfect example of what one would immediately think of when someone mentions the word “dinosaur;” in the middle is Archaeopteryx, a fully feathered creature that could fly for short distances and which had a classically dinosaur-looking toothed skull; on the right is a common chicken. Notice, in particular, the bones in the arms of Compsognathus as compared to the bones in the wings of Archaeopteryx and the chicken.
Even the toothless beaks of modern birds can be traced back through the toothed beaks of extinct birds to the socketed teeth of older archosaurs. Enantiornithines are unmistakably birds, even capable of flight, but they still retained the socketed teeth of their ancestors. One such bird, the Sulcavis geeorum, was the subject of an exciting find from early 2013. As I mentioned earlier, the skull of the Archaeopteryx was distinctly saurian, with socketed teeth and no beak. It is very clear from the fossil record that the beaked skulls of modern birds have descended from their toothy, saurian ancestors.
If you are still having trouble in seeing that modern birds are dinosaurs, I’ll borrow this fabulous quote from Aron Ra‘s fantastic video entitled “Pterosaurs are Terrible Lizards,” a set of questions which he very commonly asks of Creationists and others who deny the science of evolutionary biology:
Are mallards related to pochards, wood ducks and muscovys? Are all ducks also related to geese and other Anseriformes? Are Anseriformes related to Galliformes and other Neognathae? Are Neognathae related to Palaeognathae? Are any extant (still alive) birds related to Hesperornis, Ichthyornis, Enantiornis or other Euronithes? Are Euronithes related to Confuciusornis or Archaeopteryx? Are all early Aves (birds) related to Microraptor, Velociraptor or other non-avian Dinosaurs? Are dinosaurs related to Pterosaurs, Phytosaurs and other Archosaurs?
Tracing backward, in this manner, can make it easier to see the connections and relationships involved. If you don’t think that birds are dinosaurs, then where do you draw the line? At what point did these creatures cease to be dinosaurs? The answer is that they didn’t, just like platypuses have not ceased to be mammals just because they display some traits which are amazingly unique. Navigating the branching clades of phylogeny can be a beautiful and wondrous exercise.
So, if you want to ask me what my favorite dinosaur is, I do not say that it is the T. rex or the Triceratops or the Brachiosaurus. I don’t point to the Velociraptors or the Dilophosaurus which were depicted in that classic movie, Jurassic Park. My favorite dinosaur is one that lives side-by-side with mankind, one which is incredibly beautiful and which continues to impress scientists with its intelligence and capabilities. My favorite dinosaur is an avialan theropod called Corvus corax– the common raven.