Fossils of a newly discovered species of dinosaur -- a 10-meter-long, elephant-weight predator -- were discovered in 1996 along the banks of Argentina's Rio Colorado, and are now being reported after a long period of careful study. This dinosaur dates to about 85 million years (which falls within the Cretaceous period).
Perhaps the most interesting feature of Aerosteon riocoloradensis is that it demonstrates the evolution of a bird-like respiratory system in an animal that is definitely not bird-like in most other ways. Indeed, the authors of this paper imply that this dinosaur's respiratory system represents an early phase in the evolution of the bird's respiratory system. This is a case of an adaptation arising in one context and later being used in an entirely different context.
This is how evolution often works: An adaptation arises for one reason, and then that adaptation is employed in a similar way but for a very different purpose (it is "exapted" as Stephen Gould would say) in a later organism. In this case, we see a lung adapted for a certain kind of efficiency in a terrestrial organism. Later, if the authors are correct, this same efficiency-enhancing adaptation is used in birds as an adaptation related to flight, an entirely different reason.
In order to understand the significance of this find, it may be helpful to first discuss some interesting things about tetrapod lungs, and bird lungs in particular.
Let's start with a useful digression about breathing. Obviously, the rate of energy demand by an air breathing organism (which might relate to the organism's movement pattern) determines the required breathing rate. It is interesting to note, then, that the breathing rate of many organisms is determined to some degree ... sometimes completely ... by the movement pattern of the organism. The classic example of this is the medium to large size cursorial (ground-walking) mammalian quadruped, such as a horse or a cheetah (perhaps the two most studied such animals in relation to running). When a horse or cheetah is running, the animal automatically exhales when it hits the ground, and thus inhales when it is in the air-borne part of its stride. Thus, the rate of breathing is determined by the rate of running. That may sound useful because one would want to increase breathing rate when running faster, but in truth, this is NOT even close to the best engineered system. The fact is that as a cheetah or a horse runs father, the rate of breathing should increase much more than it is allowed to by the movement pattern. This is the limiting factor on distance and/or speed of these mammals.
This limiting factor (breathing rate) affects two different systems, and which one conks out first will depend on the situation: 1) This limits the rate of oxygen intake; and 2) this limits the cooling effects of breathing, which for many quadrupedal mammals would be the main effect. Cheetahs do not stop running after a quick burst because they are tired out. They stop because they are overheated.
Humans do not have this limitation. Being bipedal, humans can increase their breathing rate as they run. For this reason a fit human with proper experience can run for hours without stopping, while a typical quadruped (like a deer, antelope, horse, or cheetah) must stop much more frequently. Some human groups today and in recent times are known to have hunted this way: Running the animal down. Don't try this with a cheetah, they can bite and pant at the same time!
If this was a constraint on birds ... say, if every flap of the wing required a breathing cycle ... then you would see birds standing around resting all of the time, and there would be no long-range migratory species of birds. Also, many birds fly (typically while migrating) at very high altitudes, where there is less oxygen per breath. And, of course, generally speaking flight is very demanding in terms of energy (this affects a wide range of adaptations. See this for example.)
Birds manage this problem by having a very different respiratory system than other tetrapods. Bird lungs are fairly rigid. They do not expand and contract, letting air in and pushing air out in a tidal fashion, as mammalian lungs do. Rather, they are set up to have air pass through them in one direction (from front to back) continuously. The movement of air in mammals is facilitated by the diaphragm contracting and expanding, and thus inflating and deflating the lung. In birds, the movement of air is facilitated by a system of air sacs. Many of the air sacs act as bellows moving the air at a continuous and controlled rate through the lungs. This rate can be increased or decreased as needed irrespective of the movement of the animal's body.
One of the interesting features of bird air sacs is that some part of some of the air sacs grow into some of the bird's bones.
This is the best scenario for a paleontologist. Soft tissue adaptations are nice, but typically do not preserve as fossils. Therefore, we don't know what early bird lungs look like. But since the air sacs involve the bones, which do preserve as fossils, there is hope to understand the evolution of this trait in birds.
It turns out that Aerosteon riocoloradensis has numerous pneumatic adaptations in its skeleton (see the diagram at the top of this post). The PLoS paper clearly documents these; There is no question whatsoever that these are present, and there is no question as to what they are for. It is almost certainly the case that Aerosteon riocoloradensis had a somewhat bird-like respiratory system.
But, if you look at Aerosteon riocoloradensis as reconstructed, it clearly did not fly. Also, as a biped, it may not have been as constrained in its breathing cycle as a mammalian horse or cheetah would be (I'm oversimplifying that argument a bit). But there are at least two reasons that this dinosaur would benefit from this adaptation: First, long distance high energy cursorial hunting would be facilitated by an extremely efficient respiratory system. In other words, this large allosaurus like dinosaur would have been able to prance along after its prey for longer than the less aerobically efficient, possibly quadrupedal tidal-lung equipped prey would be able to prance away. What a great hunting adaptation: To not have to stop and rest as often as your prey. Second, this adaptation would have been very effective for cooling down.
The authors of this paper have added a third benefit of the rigid lung facilitated by air sacs: Reduction of upper body mass to allow the bipedal animal to be better balanced. This idea relates to one of the odd features of Aerosteon riocoloradensis's air sacs: Some of the air sacs are located in an unusual place, involving the "belly ribs" on the lower part of the body. The authors see this as evidence that the front of the animal had a system of air tubes in it's skin, which they view as an adaptation for cooling.
The authors have combined the results of this analysis with other information to provide a model for the evolution of avian air sacs and one way bellows-driven lung ventilation. Here (paraphrased from the original paper for easier reading) are the four phases they outline, which you can link to the diagram provided below.
- Phase I
- Elaboration of paraxial cervical air sacs in basal theropods (bipedal dinosaurs) no later than the earliest Late Triassic.
- Phase II
- Differentiation of so-called avian ventilatory air sacs, including both cranial (clavicular air sac) and caudal (abdominal air sac) divisions, in basal "tetanurans" (theropod dinosaurs) during the Jurassic. A heterogeneous respiratory tract with flexible air sacs that could be used as bellows, in turn, suggests the presence of rigid, dorsally attached lungs with flow-through ventilation, as found today in birds.
- Phase III
- Evolution of a primitive air pump involving the sternum and ribs in maniraptoriform theropods, the group that includes today's birds, before the close of the Jurassic.
- Phase IV
- Evolution of an advanced air pump involving the sternum and rib in maniraptoran theropods before the close of the Jurassic.
- The advent of avian unidirectional lung ventilation is not possible to pinpoint, as osteological correlates have yet to be identified for uni- or bidirectional lung ventilation.
- The origin and evolution of avian air sacs may have been driven by one or more of the following three factors: flow-through lung ventilation, locomotory balance, and/or thermal regulation.
In addition, the authors conclude:
In addition to the importance of this find in relation to the evolution of respiratory systems in dinosaurs and birds, Aerosteon riocoloradensis is also important for another reason. From the paper:
The new theropod is particularly interesting for another reason; it represents a previously unrecorded lineage of large-bodied predator in the early Late Cretaceous (Santonian, ca. 84 Ma) of South America. Most large-bodied Cretaceous theropods on southern continents (South America, Africa, Madagascar, India) pertain to one of three distinctive and contemporary clades: abelisaurids..., spinosaurids..., or carcharodontosaurids .... This predatory triumvirate persisted for millions of years on both South America and Africa.... Although initially thought to be a late-surviving carcharodontosaurid..., Aerosteon preserves cranial bones that bear none of the distinguishing features of carcharodontosaurids.... Rather, Aerosteon represents a distinctive basal tetanuran lineage that has survived into the Late Cretaceous on South America and is possibly linked to the allosauroid radiation of the Jurassic.
Sereno PC, Martinez RN, Wilson JA, Varricchio DJ, Alcober OA, et al. (2008) Evidence for Avian Intrathoracic Air Sacs in a New Predatory Dinosaur from Argentina. PLoS ONE 3(9): e3303. doi:10.1371/journal.pone.0003303.
I haven't spent enough time digging into this subject to be sure, but I kind of like the idea that the earliest feathered flight arose in the late Triassic. Chatterjee describes a feathered flyer with a deep sternum from about this time.
I'm not so enamored of the notion of the transition to flight via climbing and gliding off trees. Instead, IMO everybody's ignored the "wing in ground effect" (WIGE) that can allow an airfoil close to the ground to "fly" with much less energy (less than 1/10th?) than free flight out of ground effect. Also, an animal using WIGE can get forward impulse from its feet, rather than having to use its wings as propellers. This means they don't need to evolve specialized adaptations for flight, just hold their feathered arms out like sails, and run. Sort of like what ostriches do. (I wonder whether ostriches don't have much more endurance running than similar-sized mammals. IIRC there weren't any studies of the subject when I looked a while back.)
The relevance here is that if WIGE had evolved early, many (perhaps all) theropods may have evolved from an ancestor using it. We would see multiple independent evolution of the deep sternum structure (which, IIRC, is actually needed to anchor the muscles that pull the wings up, although I don't remember the exact logic). We would also see multiple evolution of large size, abandonment of flight (even WIGE), and growth into massive predators. It makes perfect sense to me that the Tyrannosaurs might be evolved from a flying animal. (Allosaurus is somewhat less plausible.)
In this scenario, the Jurassic "archeopteryx" might be descended from a long line of small WIGE fliers that had retained a fully workable ancestral form from the late Triassic.
This is also relevant to the pass-through lungs. It seems likely that they evolved independently (from the mammalian form) from the simple sack-lungs possessed by the small ancestors of the diapsids. Both lung forms probably (IMO) evolved during (or before) the Triassic as the need for extended exercise drove improved lungs. As with the inner ears, we shouldn't (IMO) regard one form as "primitive" from which the other evolved, but see them as independent developments from a truly primitive form: the simple sack-lung.
There's no point in reading that without also reading this:
and points linked therefrom.
I overstated the case, above; Aerosteon is interesting despite Sereno's lapses outlined as noted. But this is also worth consulting:
The species is a nice design, but that does not mean that birds came from dinosaurs.
Nothing can possibly mean that birds came from dinosaurs, just as nothing can show that hominids came from apes. That is because birds are dinosaurs, just as hominids are apes. (One can determine the latter from behavior alone.)