 |
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| |
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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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feathers extend all
the way down the leg, much
further than they do in Beebe’s mythical
tetrapteryx. Dragging your wing feathers in
the dirt would doubtless be aerodynamically
disadvantageous, but it will require detailed
reconstructions of Microraptor’s hindlimbs
with feathers attached to rule out the possibility
that it could have walked and run.
Finally, although Xu and colleagues
report that Microraptor has the anatomical
features of a dromaeosaur, firm conclusions
about the evolution of bird flight will require
new systematic analyses incorporating this
and other newly discovered theropod species
from Liaoning to confirm their phylogenetic
position. Sceptics will argue in any case that
Microraptor and dromaeosaurs are more
closely related to modern birds than is
Archaeopteryx — but then they will also have
to address the problem of why a bird that
could flap its wings perfectly well would
evolve a second pair of wings.
Birds are traditionally considered to be
animals with a difference: that is, to be a
distinct vertebrate class despite their origins
within the reptiles. But advances in palaeontology,
phylogenetics and evolutionary biology
have erased the anatomical gap between
birds and their dinosaur ancestors10. Now
that dromaeosaurs have taken to the air, in
the form of Microraptor, there remain no
major traits that are unique to birds — with |
the possible exception
of powered flight.
Although some may be irked at this lost distinction,
the benefits will be a fuller, more
integrated understanding of avian biology.
This new evidence of an arboreal, gliding
stage in the evolution of bird flight complements
the evidence of terrestrial evolution
in the theropod dinosaurs. Terrestrial theropod
dinosaurs had evolved for millions of
years before the ancestors of Microraptor
and the birds took to the trees or to the air.
Moving beyond the arboreal versus cursorial
debate over the origins of bird flight4, the task
ahead is to understand which components of
the avian flight apparatus evolved in a terrestrial
and which in an arboreal context.
Richard O. Prum is in the Department of Ecology
and Evolutionary Biology, and the Natural History
Museum and Biodiversity Research Center,
University of Kansas, Lawrence, Kansas 66045, USA.
e-mail: prum@ku.edu
1. Sereno, P. Science 284, 2137–2147 (1999).
2. Prum, R. O. & Brush, A. H. Q. Rev. Biol. 77, 261–295 (2002).
3. Xu, X. et al. Nature 421, 335–340 (2003).
4. Padian, K. & Chiappe, L. M. Biol. Rev. 73, 1–42 (1998).
5. Rayner, J. M. V. in The Beginnings of Birds (eds Hecht, M. K.,
Ostrom, J. H., Viohl, G. & Wellnhofer, P.) 279–287 (Freunde
des Jura-Museum, Eichst?tt, 1985).
6. Burgers, P. & Chiappe, L. M. Nature 399, 60–62 (1999).
7. Beebe, W. H. Zoologica 2, 38–52 (1915).
8. Norell, M. et al. Nature 416, 36–37 (2002).
9. Ji, Q., Currie, P. J., Norell, M. A. & Ji, S.-A. Nature 393,
753–761
(1998).
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10.Padian, K. Nature
393, 729–730 (1998). foraminiferal abundance indicate that the
Holocene was punctuated by a quasi-periodic recurrence of increased
monsoonactivity on
timescales of 1,000 years.
This picture is supported by oxygen isotope
analyses2 — a part indicator of water
temperature — of the same species in a
sediment core from the Somali continental
margin to the south. The isotope record
documents rapid changes in the summer
temperature of surface water during the
early Holocene, 11,000–6,500 years ago. In
turn, both records fit in nicely with isotope
analysis3 of a stalagmite, from Hoti Cave in
northern Oman, that likewise indicates
much variability in monsoon intensity over
that interval.
The work of Gupta et al.1 also takes the
record of monsoon variability further into
the mid- and late Holocene. They detect
continued swings in monsoon intensity,
including a notable shift from strong to
weak activity during the transition from
the Medieval Warm Period (AD800–1300) to
the Little Ice Age (AD 1300–1870). This is
confirmed by faunal and organic molecular
data from the region4,5: monsoon variation,
including an increase in strength during
the past century or so, is evidently a robust
feature of its climatic history.
To add to the excitement, Gupta and colleagues’
record shows some resemblance to |
Global
change
..........................
Monsoon
linkages
Rainer Zahn
..........................
An excellent sediment record from the Arabian Sea traces recent patterns
in the activity of the Asian monsoon. It reveals both variability
in monsoon strength and links with climatic events elsewhere.
The monsoon is
the main determinant of environmental
conditions over much of Asia, and so affects the most densely
populated region on Earth. Differential
heating of the north Indian Ocean and the
northwest Pacific, and of the Asian landmass,
cause the seasonal reversal of monsoon
winds. In summer, these winds blow northwards
over the northern Indian Ocean,
carrying huge amounts of moisture over the
neighbouring land. The ensuing heavy rainfall
can have devastating consequences for
human life and livelihood. Conversely, agriculture
in Asia depends on monsoon rains;
and the seasonal upwelling of nutrient-laden
subsurface waters, driven by monsoon winds,
is essential to the success of coastal fisheries.
Understanding monsoon history and
past dynamics is necessary for improving
our knowledge of |
the monsoon
system
and how it may respond to changing global
conditions. On page 354 of this issue1, Gupta
et al. take us a step further down that road.They present a
fine-scale palaeoceanographic
record, from a marine sediment core
from the Arabian Sea, that traces the operation
of the Asian monsoon 11,000 years back
in time — that is, over the Holocene, the
epoch that spans the interval from the end of
the last glacial period until the present day.
The record is derived from fluctuations
in the abundance of a planktonic organism,
a foraminifer, that is known to thrive particularly
well in waters that provide an ample
supply of food. The link with monsoon intensity
is through coastal upwelling, induced by
monsoon winds, that stimulates marine biological
productivity off Oman, including the
growth of this foraminiferal species. Gupta
and colleagues’ record is particularly valuable
because the core comes from a depositional
environment in which sediments have
accumulated swiftly, so limiting the extent
to which burrowing organisms have mixed
the sediment column. This record, then, is
a very precise one, and the variations in |
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that seen in the distribution of haematite in
Holocene sediments from the North Atlantic.
Haematite is believed to be a key indicator of
sand debris that was frozen into icebergs and
carried across the North Atlantic, and therefore
of the rhythmic recurrence of cold spells
in the region. A link between cold episodes in
the Atlantic and a weakened Asian monsoon
has already been documented for the last
glacial period, when cold spells — the socalled
Dansgaard/Oeschger and Heinrich
events — periodically produced arctic conditions
in the North Atlantic region6–8. The
Dansgaard/Oeschger and Heinrich events
caused far more dramatic environmental
changes than any of the climatic cycles in the
Holocene, so their influence on climatic
regimes well beyond the Atlantic region is not
surprising. Remarkably, however, the findings
of Gupta et al. indicate that this linkage
continued into and throughout the current
warm period of the past 11,000 years, even
though the climatic anomalies have been
far smaller.
The monsoon system does not operate in
isolation, of course. It is only one of many
participants in the global dance of climate
oscillators and dipoles (see Box 1), and many
of these climatic regimes have evidently
undergone rapid swings during the
Holocene9–11. Following the initial appearance
of ice-core palaeoclimate records from
Greenland, a belief that climate has been
highly stable during the Holocene briefly
fluttered through the scientific community. |
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