The Macro Library(beta)
The Origin of Birds
The origin of birds has long been considered a hallmark of the success of evolutionary theory. Just a two years after Darwin's work was published, the first Archaeopteryx was found in the Solnhofen limestone, having clear feather impressions but at the same time, many dinosaurian characteristics, seemingly an irrefutable confirmation of Darwin's theory1. Things became even more exciting when John Ostrom found Deinonychus antirrhopus in 19622, and his 1969 anlysis prompted the 1973 landmark paper suggesting that, in fact, birds are living
dinosaurs3. However, this very significant claim needs much good evidence and careful examination, for it is easy to make several fallacious inferences and, in fact, a few paleontologists have rejected the theropod ancestry of dinosaurs in favor of as-yet undiscovered "thecodont" ancestor, which was particularly reinvigorated in the 1990's with a text by Alan Feduccia4. In this article we will not address this alternative theory, which to date has largely lacked a
basis and is mostly an opposition argument, and relies on several problematic (and in some cases, outdated) claims5. Indeed, the theropod descent theory is so well supported that the issue is often not just the question of how less derived dinosaurs are to birds6 but in some cases the question instead becomes if some other dinosaur groups were, in fact, more derived than birds, such as the Alvarezsaurs, and finds such as those in Figure 1 only work to blur the lines further.
What we would most closely like to analyze, then, is how similar are many of these dinosaurs to birds? It would appear in the fossil record that theropods converged on various birdlike characteristics several times. In fact, Tyrannosaurus rex had a
rib segments and ossified sternum in
and oviraptorids suggest that many of the definitive skeletal features we traditionally associate with birds in fact originated seperately from flight, and was only later co-opted in the process known as
8. However, soft tissues would appear to be increasingly unsatisfactory in use as an evaluation of "bird-ness". Some analyses even find reason to believe that feathers can be found on all but the most basal
with even a light coat of feathers potentially covering T. rex, at least when it was young. Feathers, then, were not an adaptation for flight, with evidence for them arising very early, as shown in Figure 2.
This is even more strongly supported by finds such as that of a brooding Oviraptor9, in which case feathers merely for self-insulation may have been less of a requirement than the ability to thermoregulate eggs, much as modern birds today. Even some one of the most difficult features of birds to find in the fossil record, its unique air-sac system, may be a relatively basal development for saurischians in general10.
With so many avian character scattered throughout the fossil record of saurischians, it is evident that the primary arguments against the theropod hypothesis can't be well-established in fossil characters, and must instead lie elsewhere. One common argument is that the timing of the fossil record is at odd with the earliest appearance of birds. In particular, many of the most well-known relatives of birds do not appear until the Cretaceous — for example, Velociraptor is perhaps 70 MY younger than Archaeopteryx. However, this is not the problem it would first appear to be. There are other well-known dromaeosaurids such as Utahraptor that are only 25 MY younger than Archaeopteryx, less well-known ones such as Sinovenator that are 17 MY younger, and fragmentary speciemens that may be as much as 20 MY older than Archaeopteryx — and thus this argument is rendered ineffectual with a larger sampling size11. Furthermore, these gaps actually compare favorably to other gaps that are present and considered of little importance in the fossil record. Other contrary arguments focus on perhaps poor homologies of parts of the skeleton. However, most of these homologies have been well-established, such as the semilunate carpal — what was called the "half-moon shaped bone in the wrist" in the movie Jurassic Park — as shown in Figure 3
. The other contrary arguments often levelled focus on digit homlogy (in what amounts to semantics between diciplines) and in the methods of lung ventilation, but the treatment of both can be extensive, so we refer to the cited literature for a more in-depth discussion.
The final aspect of bird evolution to consider is, of course, the matter of flight. The advantages of flight are readily apparent, but the evolution of an evidently terrestrial animal such as more basal theropods into an animal capable of flight is a question that has been actively debated within paleontology. While clear and persuasive arguments have been made against using Archaeopteryx too much and relying on it as the model for the transition to flight and treating it as the de-facto direct ancestor of birds, it is nevertheless a useful model organism so long as one is aware that it is indeed just a model that came from a yet more primitive proto-bird, and almost certainly was not the direct ancestor of modern birds12. Furthermore, it is important to note that the "origin of flight" and the "origin of birds" may be linked, but they did not necessarily evolve concurrently and treating it as such could lead to many potentially fallacious inferences. Thus, for the two competing theories of
the fact that Archaeopteryx was apparently a terrestrial forager and escape climber (as shown in Figure 4)
does not necessarily impose either/both conditions on the direct ancestor of birds or on the origin of flight itself13. There are relative merits to both positions. The cursorial approach is perhaps easier to reconcile with what we currently have of the fossil record, which indicates that as a trend, dromaeosaurids shrank as they approached the K-Pg boundary. However, as the fossil record is very incomplete, and only fragmentary Jurassic dromaeosaurid fossils have been found, using fossils to justify this approach is somewhat fallacious, and the theory has to stand on its merit in a purely theoretical regime. The cursorial approach hypotheses that the origin of flight originated with the flight-stroke of birds, but highly modified. The arm structures of dromaeosaurids and related dinosaurs are very good for jackknifing forward and grasping. Since it is known that dromaeosaurids had flight feathers14, this action could have provided some degree of lift during predatory strikes, enabling long leaps; been used to capture insects; been used for display purposes, etc.
In contrast, while arboreality may be a bit more difficult to reconcile with the fossil record at first glance, there are reasons to support this view. Some studies suggest, for example, that dromaeosaurid "killer claws" were in fact best suited as climbing instruments, and that their shape and curvature were nonideal for cutting and tearing flesh. This is further supported by the short but sharp claws that many early birds still retained, which would also assist in climbing. However, selecting either arboreality or cursoriality as the "true" and final answer may be a false dichotomy. There is no reason to suppose that flight did not originate in some hybrid of these two approaches, much like the proposed model of Archaeopteryx in Figure 4. Also, while it goes against the generally-accepted axiom of
there is no reason why flight could not have independently evolved in the bird line more than once. While the origin of birds is fairly uncontroversial, the associated but independent question of the origin of bird flight has yet to be resolved, and may never be definitively answered.