SMALL holes discovered in the skeleton of the oldest known bird may clarify a
long-standing evolutionary puzzle.
Many palaeontologists believe that birds evolved from two-legged predatory
dinosaurs called theropods. If true, then Archaeopteryx, the earliest
known flying bird, should have shared key characteristics of both theropods and
modern birds.
Hollow bones are a common feature of dinosaurs, modern birds and
Archaeopteryx. In today鈥檚 birds, air sacs linked to their respiratory
system fill these cavities, including those in the backbone, pelvis and the long
bones of the leg. Theropods have similar air sacs in their vertebrae. But
scientists had found no sign of such air-sac holes in Archaeopteryx.
How, then, asked the sceptics, could its ancestors be theropods?
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So Peter Makovicky and three of his colleagues at the American Museum of
Natural History in New York took a fresh look at two Archaeopteryx
fossils. They found evidence of air sacs, in the form of telltale connecting
holes called foramen, in several neck bones (Nature, vol 395, p
374).
Earlier researchers missed them, says Mokovicky, because they concentrated on
long bones such as the leg and pelvis and neglected the vertebrae. 鈥淣ow that
we鈥檝e identified them in Archaeopteryx, we have a connection through
the phylogenetic tree,鈥� he says.
Not everyone is convinced, however. John Ruben of Oregon State University in
Corvallis believes that the air sacs don鈥檛 tell us anything about which
dinosaurs were birds鈥� ancestors, because dinosaurs breathed in a fundamentally
different way. 鈥淭hese aren鈥檛 the critical bones for the function of the avian
lung,鈥� he says.
Ruben adds that modern birds also have air sacs in their abdomens. Dinosaurs,
in contrast, did not have these abdominal air sacs, according to an unpublished
study of one of the best-preserved fossils, an Italian dinosaur called Scipionyx.