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Dinosaurs' hollow bones may have given them the puff to lead active lifestyles. A fossil find shows that the group of dinosaurs that included Velociraptor and Tyrannosaurus rex probably used the same super-efficient respiratory system that birds have today.

The fossil, which is of a carnivorous dinosaur called Majungatholus atopus, shows that its bones included spaces for storing air. This would have allowed the species to have the quick metabolism necessary for an active predatory lifestyle.

Birds have fast metabolic rates thanks to their efficient way of extracting oxygen from the air. They have two lungs, as mammals do, but the airflow through them is controlled by a complex system of air sacs throughout the body. Most birds have nine such sacs, which also extend through their hollow bones.

Patrick O'Connor, of Ohio University in Athens, and Leon Claessens, of Harvard University in Cambridge, Massachusetts, compared the structure of air sacs in M. atopus 's vertebrae to those in more than 200 living birds. The structures were very similar, they report in this week's Nature¹.

"This study paints a clearer picture of how these organisms would have existed in their environment," says O'Connor. "It indicates that these animals had the potential for a high metabolic rate."

A perch in the family tree

Birds are thought to be direct descendants of theropod dinosaurs, the group to which M. atopus belongs. Palaeontologists already have evidence that the extinct creatures were similar to their descendants, with high growth rates, bird-like sleeping postures and even feathers.

"This study forms part of an increasingly robust story that says birds are essentially dinosaurs, but smaller," says Paul Barrett, a palaeontologist at the Natural History Museum in London. "Using functional work in live animals is a nice addition, and perhaps now you could go as far as saying dinosaurs had a bird-like metabolism."

The study shows that the efficient breathing system of birds is older than previously thought, but Barrett thinks there is more to come: "To me it seems that a breathing system like this is of more ancient origin, from nearer the base of the dinosaur family tree." He says that finding older dinosaur fossils would support this, and perhaps show that other bird-like characteristics are older than suspected.

Some palaeontologists still dispute that dinosaurs were closely related to birds, and have suggested that their breathing systems were more like those of crocodiles. "This work is another nail in the coffin for that competing theory," says Barrett. 

Ohio University, Athens

• References

  1. O'Connor P. M. O. & Claessens P. A. M et al. Nature, 436. 253 - 256 (2005). | Article |