Conventional wisdom has it that the evolution of birds is connected with the evolution of flight. At first glance, this seems to make sense. The entire anatomy of birds seems to have been extensively modified to serve the stringent demands of the airborne life, and almost all modern birds can fly. The few that cannot almost certainly evolved from flying ancestors.
As is so often the case, conventional wisdom is wrong. It underestimates the capacity of the fossil record to surprise and shock. A report in the 19 March 1998 Nature describes a creature that looks superficially like a dinosaur, but detailed examination of its skull shows it to have been more closely related to modern birds than is the famous first bird, Archaeopteryx. But this new creature, and its close relatives, did not have wings and could not fly. If they had feathers, they are not preserved. Not only that, the forelimbs of at least one of these creatures were unusually short and stubby, a fossilized insult to those too closely wedded to the idea that the evolution of flight is a necessary accompaniment to the evolution of birds.
The animal concerned is called Shuvuuia deserti, from the spectacularly fossil-rich Cretaceous (about 75 Ma) deposits in the Gobi Desert of Mongolia. The fossil is described by Luis Chiappe of the AMNH and his colleagues: 'shuvuu' is a Mongolian word for 'bird'.
The story starts in 1987, when a joint Soviet-Mongolian expedition found remains of what looked like a turkey-sized, bipedal, running dinosaur similar to creatures such as Velociraptor. This is no surprise. Small, carnivorous dinosaurs like this are well known from the Gobi. It was not until 1992, when the Mongolian-American team discovered more remains of the same animal, that the creature was realized to be something truly odd.
In 1993, the Mongolian-American team described the creature in Nature. Superficially, it looked like a small, running dinosaur, with a long, snaky tail, long, spindly legs and a long neck. The arms were extremely strange: although very robustly built, they were very short and stubby, and the solid, blocky 'hands' each had just one finger, terminating with a huge spike. The animal was called Mononykus -- 'single spike'.
Although Mononykus looked like an aberrant dinosaur, closer examination revealed a host of bird-like features in the backbone, pelvis and hindlimbs. Controversially, the detailed anatomy of Mononykus looked even more birdlike than that of Archaeopteryx.
Archaeopteryx is famous as the earliest known bird. It has a long, reptilian tail and jaws with teeth, but its forelimbs are clearly modified into wings, and it had feathers remarkably like those of modern birds. For decades, Archaeopteryx has been seen as a kind of 'missing link' between reptiles and birds, a primitive staging post on the way towards fully modern birds, and fully modern flight. To claim, therefore, that Mononykus was even more closely related to modern birds than Archaeopteryx, even though Mononykus did not have wings and could not have flown, caused a stir among evolutionary biologists.
What was lacking from the skeletons of Mononykus found in Mongolia in the early 1990s were good-quality skulls. Although several Cretaceous animals, from South and North America as well as Mongolia, have been recognized as forming a distinctive group of flightless, bird-like dinosaurs, no really good skull material has been described. The skulls of birds show a number of extremely distinctive specializations. The presence or absence of these features in a skull of Mononykus or one of its relatives would help test the idea that they were more closely related to birds than Archaeopteryx. This is the gap that Chiappe and colleagues now fill with Shuvuuia, an animal similar to Mononykus but with two, complete, beautiful, but birdlike skulls.
Although Shuvuuia did not have a beak, its slender jaws bore tiny teeth, and its skull was able to move so that it could raise its upper jaw, with respect to the braincase, a facility that modern birds have.
This work illustrates that the path of evolution does not always run as smoothly as we like to think. No sooner had birds become airborne (as with Archaeopteryx) but some contrived ways to shed the habit of flight. Shuvuuia, Mononykus and its relatives represent a successful radiation of flightless birds that evolved before birds had achieved what many consider to be their modern form. That they look like dinosaurs is not entirely a coincidence, for they form part of the increasing body of evidence to show that birds and dinosaurs are close relatives.
© Macmillan Magazines Ltd 1998 - NATURE NEWS SERVICE
Note: This item from the Nature News Service is mounted on this Web page by special permission of Nature, while the Nature site is being revised..
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