A Light, Quick, Killing Machine

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When you purchase some frozen chicken from the supermarket, you are really buying the frosted remains of a living dinosaur. Over the past decade an abundance of fossils from China has convincingly illustrated that birds evolved from small, predatory dinosaurs, and even the giant Tyrannosaurs might have sported some downy fuzz during part of their lives. With each passing year, new evidence further underlines that many of the characteristics we consider to be indicative of birds first appeared in dinosaurs. A new dinosaur from Argentina, however, illustrates that some of these traits appeared much earlier than previously thought.

Announced last week in the open-access journal PLoS One, Aerosteon riocoloradensis was a large, meat-eating dinosaur that lived about 84 million years ago in what is now South America. Earlier in geologic history, South America belonged to a larger landmass called Gondwana. Because of this connection, many of the dinosaurs that lived in South America also lived in other parts of the world that were once connected to it, like Africa.

In these southern continents there were three major groups of large, predatory dinosaurs. Some of the more familiar ones were the sail-backed hunter Spinosaurus, the large-skulled Giganotosaurus, and the horned carnivore Carnotaurus. They are representatives of the three main groups of theropods that were present across Gondwana. But strangely, Aerosteon did not fit in with any of these groups. At first is showed the most similarity to relatives of Giganotosaurus, but as researchers studied it more closely they were surprised to find that Aerosteon was more closely related to a terror of North America, Allosaurus.

The presence of an Allosaurus relative in South America during this time -- the Late Cretaceous -- was shocking by itself, but the skeletal architecture of Aerosteon was even more intriguing. As paleontologists studied the bones they saw curious features that are also seen in birds, like parts of the vertebrae that looked like they had been scooped out with a spoon. These were places in the bones that had been invaded by air sacs.

In living birds, air sacs are connected to the lungs and penetrate surrounding bone, particularly vertebrae. These bones are called "pneumatic." The structures are essential to birds' unique pattern of breathing, where inhaled air moves continuously throughout the series of air sacs, allowing birds to extract more oxygen. They also reduce weight by hollowing bone, which helps flying species to take to the air. Aerosteon showed similar features in its vertebrae as well as other parts of the skeleton like its hips, wishbone, and bones along the belly (called gastralia). Other dinosaurs, like the theropod Majungasaurus from Madagascar and many of the long-necked sauropod dinosaurs like Diplodocus were known to possess pneumatic bones previously, but Aerosteon exhibited the most extensive hollowing throughout its body seen so far.

As similar to birds as they may have been, Aerosteon and Majungasaurus did not belong to the group of theropods directly related to the earliest birds, and sauropods were even further removed from bird ancestry. Aerosteon, sauropods, and the bird ancestors all were saurischian dinosaurs, however, and shared a common ancestor over 230 million years ago. Indeed, the presence of pneumatic bones in so many different saruischians suggests that the presence of air sacs in the body was something already present in the common ancestor for saurischians.

While a definitive common ancestor for saurischians may never be found, the prediction is confirmed by the fact that one of the earliest saurischian dinosaurs, Eoraptor, also had bones shaped by air sacs. This means that one of the key adaptations of birds living today was already present (even if not used in the same way) in small, predatory dinosaurs over 200 million years ago.

Aerosteon was not a flying dinosaur, however, so why do its bones clearly indicate the presence of air sacs? Given that the first avian dinosaurs did not evolve until about 150 million years ago -- or 75 million years after the first evidence of air sacs in the skeleton -- pneumatic bones must have evolved for a different reason. In their study of Aerosteon, the researchers proposed that the extent of air sacs in individual dinosaurs varied.

Even between individual animals, the amount that bones were sculpted by air sacs differs. More extensive air sacs would have been favored in dinosaurs where it lightened the skeleton enough to provide them an advantage, primarily through requiring less energy to move around.

Once air sacs penetrated a greater amount of the skeleton they could have been co-opted by evolution for other uses. Since they would have arisen from the lungs to begin with, the air sacs may have aided the breathing of some dinosaurs. This is a possibility for Aerosteon, but the researchers cautioned that just because its air sacs may have been used in breathing does not mean that it breathed just like living birds do.

Given that birds are the only living animals with these extensive air sacs, there are no precise living equivalents to compare Aerosteon to. Still, once dinosaurs had air sacs that contributed to breathing, the air sacs could be further adapted by evolution to make breathing more efficient. Hollowed bones even more similar to those of living birds have been described from coelurosaurs, the predatory dinosaurs from which birds evolved.

The description of Aerosteon is remarkable not only because it is another impressive meat-eating dinosaur to add to the predatory pantheon, but also because it raises some important new questions about dinosaurs as living creatures. How did a relative of a North American dinosaur make it to South America and persist there for so long? Did Aerosteon have close relatives whose remains are yet to be found elsewhere in South America? What advantages did the air sacs in its skeleton provide? How did the air sacs evolve into organs that helped dinosaurs breathe? Could they have been used to help regulate the body temperature, too? Such questions will keep paleontologists working for many years to come.

Here is a two-part critical analysis of the Aerosteon that I found interesting.

Photo Credit: PlosOne

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