Several recent papers on dinosaurs and birds have, for many of us paleontologists, more or less completely cinched the hypothesis (as much as can happen in science) that birds are living dinosaurs. Two such papers are:
- Birds have paedomorphic dinosaur skulls
But it is still very common for their to be doubt about birds as dinosaurs, not only from the general public, but from some of our colleagues, including ornithologists. Perhaps because of when I was born and the dinosaur books I was exposed to as a child and teenager, the discoveries over the past three decades have been magical but also very conclusive for me. I still wonder why anyone serious about evolution can still question the link between dinosaurs and birds, and I wanted to explore that in a series of posts.
I am a child of the late 70s and early 80s, which means that in addition to nostalgia about the Atari 2600, all that “cool” ’80s music and “classic rock,” and the Rubik’s Cube, my first and favorite books on dinosaurs were The Hot-Blooded Dinosaurs, Archosauria: A New Look at the Old Dinosaur, The Riddle of the Dinosaur, and, of course, The Dinosaur Heresies. These books and many other popular works of that time brashly proclaimed that previous generations of paleontologists had got it all wrong: dinosaurs were hot-blooded, active, and intelligent animals, and their descendants were none other than our feathered friends, the birds. Archaeopteryx, the “first bird,” was in fact the genetic lifeboat upon which dinosaurs would ride out their mistaken extinction at the end of the Cretaceous Period some 65 million years ago.
So it was a surprise to learn as an undergraduate that the hypothesis that birds are dinosaurs is not as new as I was led to think. “Darwin’s Bulldog,” the evolutionary biologist Thomas Henry Huxley, had noticed the striking similarities between the skeleton of Archaeopteryx and other small dinosaurs known at the time, including the bird-like Compsognathus, a little predator the size of a chicken.
But what happened? Why was the hypothesis of a dinosaur-bird relationship essentially discarded for most of the 20th century? The history of the people and politics behind the removal of birds from dinosaurs and their eventual reunification have been covered and detailed numerous times in books, blogs, and articles too numerous to mention. Instead, I wanted to focus on why birds were scientifically estranged from dinosaurs for so long.
First, let me briefly introduce a concept called parsimony. Parsimony is the default setting under which modern evolutionary biologists and paleontologists reconstruct the evolutionary tree of life. Boiled down to its basics, parsimony means that, all things being equal, the simplest explanation is preferred. In the context of vertebrate relationships, it means that we assume the presence of shared, specialized traits is due to common ancestry.
To demonstrate how parsimony works, let’s start with a (hopefully) non-controversial statement: you are a mammal. You share unique traits with dogs, naked mole rats, and the duck-billed platypus such as hair (yes, naked mole rats have some hair), the production of milk (in females), and a single lower jaw bone rather than multiple jaw elements. Now, we could say that you, your dog, the naked mole rats, and the duck-billed platypus each evolved these traits independently — in other words, humans, dogs, and all other mammals each re-invented hair, milk, and the single lower jaw bone. However, this would not be a very simple explanation (it would not be parsimonious). But we wouldn’t simply reject this hypothesis because it wasn’t parsimonious — we would also reject it because it was not supported by data from the fossil record and mammal embryology.
The issue with dinosaurs and birds has revolved around the interpretation of various traits and argumentation over something called convergent evolution. Granted, not all similar-looking traits are related to common ancestry. For example, a shark and a dolphin both have a stream-lined body form with fins.
At face value, we might conclude that these traits were evidence that sharks and dolphins shared a recent common ancestor. However, on closer inspection, we would begin to notice some large discrepancies. The skeletal structure of the shark is cartilaginous whereas that of the dolphin is bone. A shark’s skin is rough and covered in tooth-like scales, yet that of a dolphin is smooth and overlies a layer of blubber. Sharks breathe using gills, but dolphins have lungs and must surface occasionally to take in fresh air. Dolphins nurse their young on milk from mammary glands while shark pups must fend for themselves.
Eventually, it would occur to us that, more likely, the similar shapes of the shark and dolphin were not due to common ancestry but instead to a common environment: water. Water is denser than air, and there are only so many “solutions” to swimming fast in it. The shark and dolphin have converged onto a similar functional solution, the streamlining of their bodies and the possession of fins, to move fast in a dense medium.
So it has been argued for the striking similarities between birds and dinosaurs. For example, both birds and predatory dinosaurs are bipeds, so perhaps their bone structure and posture evolved independently because of a shared functional “need.” Both dinosaurs and birds are part of the broader Reptilia, and so perhaps the skeletal similarities in predatory dinosaurs and birds were independently evolved from a much early, reptilian skeletal framework.
Then there is the issue of what is missing. Birds have a furcula, a bone made by fusing the two collar bones together into a strut that resists the large forces generated by their flapping wings. Archaeopteryx has a furcula, but it was long supposed that dinosaurs did not have this structure. Embryonically, bird hands develop in such a way that it is the three middle digits that that remain (index, middle, and ring finger), whereas predatory dinosaurs have a thumb, index finger, and middle finger. And, the key feature held out for a long time as evidence that birds were not at all dinosaur relatives was their feathers. Where were the dinosaurs with feathers?
Stay tuned …