That's the finding of Purdue University researcher Richard Hengst, who studies the physiology of dinosaurs to determine the efficiency of their breathing systems.
Hengst, a biologist at Purdue's North Central campus, found that while all early dinosaurs -- those dating back about 220 million years -- breathed inefficiently, dinosaurs that lived only 70 million to 65 million years ago had a much-improved system, resembling that of modern mammals.
However, the rate of improvement in respiratory capabilities differed significantly between the South American and North American dinosaurs, with the southern dinosaurs lagging behind their northern relatives in aerobic fitness for tens of millions of years.
"Whereas 140 million year ago, the Argentine dinosaurs breathed only slightly more efficiently than their more primitive ancestors, the North American dinosaurs of the same age had vastly improved their technique," he says. "The South Americans caught up with their northern relatives only in the later Cretaceous, or about 70 million years ago."
He will present his findings Saturday (10/11) during the annual meeting of the Society of Vertebrate Paleontologists in Chicago.
In his study, Hengst looked at how the ribs were attached to the vertebral column to determine the efficiency of the respiratory systems in a number of carnivorous theropods, a class of flesh-eating dinosaurs that walked mainly on their hind legs and included Tyrannosaurus rex.
By analyzing how the ribs pivoted around two prominent points, and studying how the ribs that make up the chest were aligned on the vertebrae, Hengst was able to determine how the dinosaurs' ribs moved during breathing.
"The more efficiently the chest expanded, the more air the animal could take in and therefore the more aerobic it could be," he says.
Though all of the of the early dinosaurs had a similar lineup of ribs to vertebrae, Hengst found that by the middle of the Jurassic period, or about 140 million years ago, many of the northern dinosaurs had evolved a system so that each rib moved in a specialized direction, allowing the dinosaur to consume up to 40 percent more air per breath for the same effort.
"This suggests that there was some kind of pressure on these dinosaurs to become more aerobically active over time," he says.
Last fall, during a trip to Argentina, Hengst compared the anatomy of the northern dinosaurs to their southern cousins. To his surprise, the southern dinosaurs living during the same period had retained much of their primitive respiratory structures, indicating that they weren't as active as their northern cousins, he says.
Though the reason for the difference in respiratory rates is not clear, Hengst says different hunting techniques may have played a role.
"The North American theropods may have been pursuit hunters, like wolves, which would require the ability to run over a long period of time, while the South American dinosaurs perhaps employed a 'dash and dine,' approach, an activity that required only a short burst of activity," he says.
Other possible explanations, says Hengst, are that the southern dinosaurs may have had more abundant food sources or a landscape that favored ambushing, factors that could have reduced the pressure to become more active.
"Whatever the reason, the fact that they eventually did catch up with the northern dinosaurs suggests that there was an optimal level of respiratory efficiency for the dinosaurs, and that all the dinosaurs eventually got there," he says.
Hengst also found similar differences in the respiratory systems of non-meat-eating dinosaurs in North America and South America.
Materials provided by Purdue University. Note: Content may be edited for style and length.
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