(From NYT, 31/10/06)
Fossils of the largest known bird, an extinct flightless predator with a
skull the size of a horse?s and a menacing beak like an eagle?s, have been
discovered in Argentina, paleontologists reported last week.
The big bird, which stood about 10 feet tall and probably weighed 400 pounds,
was fleet of foot and able to chase down and devour rodents, reptiles and small
mammals 15 million years ago on the plains of Patagonia. Not for nothing are its
closely related species, a group known as phorusrhacids, more commonly called
the ?terror birds.?
Such avian giants evolved and prospered in the time of South America?s total
isolation from other continents. All of these birds were apparently flightless,
and most of them ate only plants. Until now, the only known species of
carnivorous terror birds averaged five to nine feet tall and had relatively
small heads.
Paleontologists said the new fossil discovery might force them to reconsider
previous ideas that the terror birds that evolved the biggest bodies were
significantly slower runners.
?This is not only the largest bird ever found,? said Luis M. Chiappe, curator
of vertebrate paleontology at the Natural History Museum of Los Angeles County.
?It also tells us the idea we have heard and repeated over the years may not be
entirely valid.?
Dr. Chiappe and Sara Bertelli, also at the Los Angeles museum, described the find in the current
issue of the journal Nature. They said the skull, more than 28 inches long, was
virtually complete and remarkably different from and at least 10 percent bigger
than skulls of related species. The researchers inferred the bird?s running
ability from its leg and foot bones.
?We conclude that reconstructions of the skull of gigantic phorusrhacids on
the basis of smaller relatives are unwarranted,? the two paleontologists wrote,
?and that the long-established correlation between their corpulence and reduced
cursorial agility needs to be re-evaluated.?
The skull and limb bones were found two years ago by a high school student,
Guillermo Aguirre-Zabala. They were embedded in 15-million-year-old rock
outcrops near the railroad station in his home village, Comallo, Argentina,
which is east of Bariloche, where the fossils, as yet unnamed, are kept at a
paleontological museum.
Before the discovery, knowledge of the skulls of large-bodied terror birds
was limited to fragmentary specimens collected more than two decades ago. So
scientists tended to interpret and illustrate the heads of the bigger birds as
scaled-up versions of the better-known smaller species. These smaller cousins
were about two to two and a half feet tall, big for birds but hardly the stature
of the predatory giants.
In their examination of the new specimen, Dr. Chiappe and Dr. Bertelli noted
many distinctive skull traits, particularly the shape and proportions of the
face, a lower and longer beak and a flat cranial roof.
Even more revealing, they said, were the long, slender leg bone and the foot
bones fused together. The foot bones bore a superficial resemblance to the
three-toed rhea, a living flightless bird. The newfound terror bird was indeed
big, they surmised, but ?substantially swifter? than had been assumed for other
enormous species of its group.
?It may not have been as fast as an ostrich,? Dr. Chiappe said, referring to
the speediest of living flightless birds, able to reach speeds around 45 m.p.h.
?But it clearly was a land bird that could run fast.?
And with this ability, these big birds appear to have become the top predator
of the continent for millions of years. Other fossil evidence shows that the
phorusrhacids lived the fast life between 60 million and 2 million years ago,
passing from the scene after the emergence of the Panamanian land bridge ended
South America?s isolation.
