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| [花之故都] |
[恐龙之乡] |
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| [鸟类的祖籍] |
[昆虫王国]] |
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| [鱼类乐园 |
[哺乳动物摇篮] |
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Dinosaurs take
to the air
Richard O. Prum
Flying birds evolved from a group of bipedal dinosaurs. The latest fossil
discoveries from China indicate that the dinosaurian ancestors of birds
had four wings — and that these animals glided rather than flapped.
| T |
hree questions lie at the heart of the
debate about the evolution of birds: the |
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origin of the group itself,
the origin of feathers and the origin of flight. Progress in
reconstructing the evolutionary history of dinosaurs has established
a well-corroboratedanswer to the first question1. Birds are a lineage
of dinosaurs, and are most closely related to dromaeosaurs and troodontids,both
of which belong to a group of bipedal,
carnivorous dinosaurs called theropods. Since 1997, new fossils and
insights fromdevelopmental biology have also supported a coherent
solution to the second question2.Feathers evolved in theropod dinosaurs
before the origin of birds or flight through a series of developmental
novelties. Now (page 335 of this issue), Xu et al.3 report spectacular
124–128-million-year-old fossils from Liaoning, China, that promise
to revolutionize discussion of the last question — how did avian flight
evolve? |
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origin of birds within the theropod dinosaurs1. Xu et al.3 describe
a four-winged
dromaeosaur, Microraptor gui, which they hypothesize glided from tree
to tree. They further
propose that gliding evolved initially in the four-winged ancestor
of the birds, dromaeosaurs,
and troodontids. Subsequently, the hindwings were lost with the origin
of powered, flapping
flight in the ancestor of Archaeopteryx and other birds. |
For more than a century, debate
on the
origin of bird flight has centred on two
different hypotheses4. According to arboreal
theories, flight arose in tree-dwelling creatures
through an intermediate gliding stage,
an idea that has been supported by the
observation that flight is energetically more
efficient at higher speeds (when more lift
is generated)5. Further, the flight stroke in
continuous, level flight is simpler than in
take-off from the ground. According to the
competing, cursorial theories, flight evolved
in ground-living animals via a powered
running stage. This view is supported by the
evidence that birds evolved from a lineage of
terrestrial, bipedal theropods, and that many
components of the avian flight apparatus
evolved originally in a terrestrial context4.
Moreover, aerodynamic models6 of the flight
stroke of Archaeopteryx, the earliest bird
accepted as such, indicate that its wings
could have provided thrust as well as lift, and
aided the legs in achieving enough ground
speed for a running take-off.
In a colourful and prescient paper of 1915,
however, William Beebe7 proposed that avian
flight evolved through a gliding, four-winged
— tetrapteryx — stage with wing feathers
on both the arms and the legs. Now Xu and
colleagues3 describe a small dromaeosaur,
Microraptor gui, that sports four wings of
fully modern, asymmetrical feathers on its
forelimbs and legs, and looks as if it could have
glided straight out of the pages of Beebe’s notebooks. |
Although the specimens of Microraptor
are younger than those of Archaeopteryx,
Microraptor is a basal member — an early
evolutionary branch — of the closest relatives
of Archaeopteryx and other birds (Fig. 1). In
support of the arboreal theory, Xu et al. propose
that the most recent common ancestor of
birds and dromaeosaurs was a four-winged
creature that lived in and glided among trees.
The evidence of an aerodynamic function
for Microraptor’s forelimb and leg feathers
is excellent. Asymmetrical feather vanes
have long been recognized as indicating
aerodynamic function in flight or gliding.
Furthermore, the feathers of both the
forewings and the hindwings increase in
asymmetry towards the end of the limb in a
striking match with the primary and secondary
feathers of a modern bird wing. The
discovery of dromaeosaur hindwings was
presaged last year by the description of 13-
cm-long modern, vaned feathers on the tibia
of an even larger, unnamed dromaeosaur
from Liaoning8, but that specimen lacked
enough detail to describe the entire structure.
The long tail of Microraptor also features a
terminal tuft of feathers like that found on
other basal dromaeosaurs8 and on another
theropod, the oviraptoran Caudipteryx9.
The discovery of a logical functional
intermediate provides striking support for |
the arboreal–gliding hypothesis
of the origin
of bird flight. However, substantial questions
remain. In particular, how did Microraptor
actually use its four wings? Perhaps because
flapping hindwings are so unlikely, Xu et al.
conclude that Microraptor merely glided,
and did not have a powered flight stroke.
Palaeontologists and functional morphologists
will be eager to study the shoulder and
wing anatomy to judge whether Microraptor
could sustain powered flight. More information
is also required on how the animal could
have rotated its legs to deploy its hindwings.
Xu et al. maintain that all four wings were
present in the earliest ancestors of birds and
dromaeosaurs, and that with the evolution
of powered flight the hindwings were lost
in the avian lineage before the advent of
Archaeopteryx. It is also possible, however,
that the hindwings are a unique feature of
dromaeosaurs. Palaeontologists will want to
re-examine specimens of Archaeopteryx for
any evidence of a vestigial hindwing (Beebe
found none). Regardless of the upshot of
those enquiries, there is no doubt that the
forewings of Microraptor, Archaeopteryx
and other birds are homologous and had an
aerodynamic function.
Xu et al. also argue that the extensively
feathered legs of Microraptorwould have been
incompatible with life on the ground. The
and troodontids. Subsequently, the hindwings were lost with the origin
of powered, flapping
flight in the ancestor of Archaeopteryx and other birds. |
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