Monday, 29 July 2013

Birds- An Origin Simplified

When I talk to people about dinosaurs, more often than not, two things will eventually come up. The first, as always, is Jurassic Park. Do I like it (yes), how realistic is it (not terribly), and could we ever do something like that (probably not in the foreseeable future). And, aside from Tyrannosaurus, the one animal that always gets brought up in the wake of this movie is Velociraptor.

Everyone thinks it was awesome, even if we can now look back at it's depiction in the film and acknowledge the flaws present- despite close relatives coming from the region, Velociraptor was not found in the western United States (it's a Mongolian genus). It was not as big as depicted in the film. As Dr. Grant puts it, Velociraptor was between 5 and 6 feet high and 9 feet long. In reality, Velociraptor was roughly the size of a small turkey and likely not much more dangerous to something the size of a human (it's cousin Deinonychus, however, does bear a similar design to what is shown in the film and is also found in the USA). Another thing that science has proven and we've been getting good at warming up to is the fact that Velociraptor, and most likely all of its relatives, had feathers. This is a trait I see more and more in popular depictions of these animals. However, I've also found that the average person without a degree in palaeontology often struggles to put the pieces together and fully grasp the evolutionary implications we draw from the fact that the 'raptors' had feathers. Does that mean they could fly? Did birds really come from dinosaurs? Modern birds aren't dinosaurs, though... Right? These are questions I hear often. Let's clear up this messy subject.

What you must first understand is that the historical perception of scientists and the public as a whole really affects on how we think about birds and dinosaurs today. So I hope you'll be able to suffer me a jaunt down history lane. The group Dinosauria was first coined by Sir Richard Owen, the legendary English anatomist in 1842. Back in those days, only three types of what would later be called dinosaurs were known: Megalosaurus, Iguanadon and Hylaeosaurus. Not too much was known about these creatures aside from the fact that they had similar hip and limb structures, and so Owen united them based on that. It was obvious that these creatures were 'reptiles' (I use the term loosely), and since the general conception of reptiles back then was of creatures that were dull, ugly and sluggish, early palaeontologists depicted these first dinosaurs as such. Soon the world became dinosaur crazy and many different species were being discovered throughout Europe, the opening North American frontier, and other such places. This led us to gain a more three-dimensional view of dinosaurs as perhaps not as lazy and lizard-like as we'd assumed. While the Bone Wars raged across the American West, a very special fossil was unearthed in Germany.

 It was the skeleton of a small, carnivorous Theropod from the late Jurassic. If that had been all that was preserved, we would have called it a Theropod dinosaur and left it at that. However, surrounding the body, limbs, and tail was a coat of fossilized feathers. Feathers identical in structure to that of a modern bird. Now, in those days, the origin of birds was a pretty big mystery. The early supporters of Darwin's evolutionary theory were busy putting together how organisms were related, but no one had yet found anything to really put the birds alongside. Scientists like Edward D. Cope noticed similarities between the skeletons of Theropod dinosaurs and modern birds (the shape of the bones and structures, especially in the limbs, the upright posture of the body, the way the head was supported, the presence of hollow bones... the list goes on), but this wasn't enough to convince everyone. Not until this little feathered dinosaur emerged from Germany, which was named Archaeopteryx. Thomas H. Huxley, the other legendary English anatomist and die-hard supporter of Darwin, looked at Archaeopteryx and saw that it was certainly a dinosaur- it had a long bony tail (as opposed to the short stump present in birds), a mouth full of teeth (all modern birds have toothless beaks), and three dinosaurian fingers (in birds, one finger has been lost and the other two have fused into one). However, it's birdlike features balanced out its dinosaurian features- it's feathers were well suited for some degree of flight and its fingers were much more slender and elongated than what was seen in other dinosaurs at that time. Huxley saw this as evidence that birds had evolved from dinosaurs, and that Archaeopteryx was and early along the continuum between the two. We might even call it the first bird.

Huxley's idea, however, was largely ignored by the scientific community, who still saw dinosaurs, for the most part, as slow, scaly behemoths who lived and died, leaving no descendants. Then, in the 1970's, discoveries were made, and perceptions began to change. John Ostrom discovered our old friend Deinonychus, and noted the slender, gracile, upright (one might say bird-like) structure of the skeleton, which implied an active and fast lifestyle for the animal. Around the same time, Robert Bakker published The Dinosaur Heresies, in which he argued that many, if not most, dinosaurs were not the stupid, cold-blooded, lizard-like creatures we thought they were. Instead they were active, fast, and highly successful in their time (some may have even been warm-blooded, just like mammals and -gasp- birds). Based on his analysis of Deinonychus, Ostrom reawakened Huxley's theory that birds had evolved from dinosaurs. He argued that the two groups were so closely related that we really should start thinking of birds as living dinosaurs in their own right. His evidence was convincing, and scientists started to listen.

The argument that birds are dinosaurs received even more merit when, in the following decades, small Theropod fossils began turning up from China that bore what were obviously feathers. Scientists also noted the presence of a furcula (aka wishbone) in Theropod dinosaurs, something also seen only in birds. The evidence was now pretty well undeniable. Palaeontologists now assert that we now must think of birds as highly adapted dinosaurs that evolved the ability to fly and survived the Cretaceous extinction. As Dr. Grant says in Jurassic Park, bet you'll never look at birds the same way again.

But hang on a minute! Let's go back to the raptors. How do we know that they had feathers? We haven't found Velociraptors or Deinonychus with preserved feathers! True, but, look at it this way- let's say your grandfather has brown skin. So does your father. Let's also say you have a child, and they too  have brown skin. It would make a lot of sense if you yourself had brown skin too. Now apply that logic into a more broad evolutionary view. If the older relatives of a species have a certain trait (such as feathers) and the younger relatives of said species also have that trait, it would make sense for the species in question to have that trait too. Let's look at Velociraptor. One of the most significant feathered dinosaurs we've found so far is a creature called Microraptor. Microraptor was an older relative of Velociraptor and Deinonychus and all the other 'raptors'. It also just so happens to have a good coat feathers. Logic dictates then, that if more primitive raptors such as Microraptor had feathers, and their more evolved cousins the birds had them too, then dinosaurs like Velociraptor almost certainly had a good coat of feathers. From this we've been able to infer that other types of dinosaurs that were closely related to the raptors (the technical term is Dromaeosaurids, and they're considered to be the 'sister-group' to the birds), such as the Ornithomimids, Oviraptorids, and even the fearsome Tyrannosaurs had feathers too.

Alright, you're likely asking yourself, dinosaurs had feathers. So could they all fly? I mean, why else have feathers if not to fly with them? Do we consider all feathered dinosaurs to be birds? Let me answer these in order: we figure that at least some feathered non-avian dinosaurs could glide, such as Microraptor. Not sustained powered flight, mind you. Just glide. Dinosaurs likely evolved feathers at first not for flying but for another purpose- insulation.

The difference between cold-blooded and warm-blooded animals is that cold-blooded ones get their heat from the outside environment (their blood isn't actually cold). They get all the heat they need from the sun, and are able to tolerate wider ranges of temperatures. As a result, they generally have slower metabolisms. That's why creatures like lizards are so sluggish. Warm-blooded animals, such as mammals and birds, generate their own heat from inside their bodies. In order to do this, they have to have much faster metabolisms, and are generally more active than cold-blooded animals. Now, in order to keep their body temperature at a constant level, most warm-blooded creatures living in environments that aren't always very warm need some way to keep the heat from escaping their bodies. They need insulation. Mammals got around that by evolving hair which effectively traps heat inside and keeps the cold out. Theropod dinosaurs, being warm-blooded, found a similar solution by evolving modified scales that became more and more filamentous and hair-like. These modified scales became effective as an insulatory device. After a long time, some of these dinosaurs began to use these modified scale for not just keeping warm but also getting airborne. This is how and why feathers first appeared. This principle is still seen in modern emus, who instead use their feathers for keeping cool air in and warm air out in the hot Australian outback.

Now, where do we draw the line between things that are dinosaurs but not birds and things that are both dinosaurs and birds? Well, most scientists use Archaeopteryx as a sort of benchmark. Archaeopteryx is more bird-like than the next closest things to birds, the Dromaeosaurids. Therefore, if we consider Archaeopteryx to be the earliest bird, everything more bird-like than it gets labelled as a bird, while everything less bird-like is just a dinosaur. This, however, doesn't mean that all modern birds evolved from Archaeopteryx. It simply means that Archaeopteryx is an early offshoot from the common ancestor that gave rise to it and all modern birds, and who probably looked a lot like Archaeopteryx, who would also share an earlier common ancestor with the Dromaeosaurids. The more fossils we find and the more ideas we set forth, the clearer a picture we will get.

It's often hard to look at chickadees, robins, and sparrows, feeding at a backyard feeder while chirping to each other and flitting around the yard, and see them as in with a group that also contains the ferocious Tyrannosaurus rex. This peaceful, domestic scene is far removed from savage Mesozoic many millions of years ago. But science is filled with surprising revelations that often boggle the mind. You never know what you'll dig up.

3 comments:

  1. Very cool! I found you from Reddit. I've done a similar blog for a while now and I've got to say it's been very rewarding personally and very useful professionally (I've been using it as an informal portfolio).

    You said that you hoped someone out there would like it. I like it. Keep it up!

    ReplyDelete
    Replies
    1. Oh good! Glad to see someone at least gave it a read. Thanks for the encouragement and I hope youll come back to read more.

      Delete
  2. Loved it! Thanks for the awesome article!

    ReplyDelete