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ancestry, the ‘stem group’. As for osteich thyans,
although it is agreed that fossils from the
earliest Devonian
2,7
belong within the crown,
osteichthyan fragments of less-certain affin-
ity are also known from the Late Silurian3,
423 million to 416 million years ago.
But there’s more to this story, because the
question of gnathostome origins also involves a
pair of extinct groups of gnathostomes known
to appear earlier in the geological record, the
placoderms and acanthodians
1
. Importantly,
recent analys es
8
have begun to reveal new rela-
tionships between early vertebrates, in which
acanthodians and placoderms are scattered
among the early divisions of gnathostome evo-
lution; acanthodians, in particular, are crop-
ping up on chondrichthyan and osteichthyan
stem groups. The straightforward message
is that the origin of modern gnathostomes is
not a Devonian phenomenon, after all. The
basal divergence between osteichthyans and
chondrichthyans occurred somewhat earlier.
This, then, is the context within which to
place Guiyu oneiros, the new species of early
osteichthyan named and described by Zhu
et al.5. Preserved in 418-million-year-old
limestone in what is now southern China, the
fossils of Guiyu show the skeletal anatomy of a
small sarcopterygian, around 33 centimetres
long. The very fact that Guiyu can be identi-
fied as a sarcopterygian provides further and
arguably clinching evidence that a whole series
of major branching events within the gnatho-
stome crown group must have taken place well
before the end of the Silurian.
Like any other fossil, Guiyu has a mixture of
PALAEONTOLOGY
Beyond the Age of Fishes
Michael I. Coates
Discovery of an unusually intact and ancient fossil fish provides further evidence that the search for
modern vertebrate origins requires breaking out of the Devonian and into the preceding period.
As a rule, the earliest fossils of living groups
tend to be scrappy, and such fragments lend
themselves to contentious interpretations.
For ‘bony fishes’, Osteichthyes — the division
of vertebrates that includes everything from
humans to halibut — the record of articulated
fossils peters out within the Lower Devonian
1
,
some 400 million years ago. Earlier stretches
of osteichthyan history are littered with
fossil detritus, such as isolated teeth and scales.
In certain instances, bits and pieces have been
reassembled into conjectural species
2–4
, some
of which have surprising combinations of
anatomical features2. On page 469 of this issue,
Zhu et al.5 introduce a fresh — albeit long-dead
— fish into this poorly resolved
patch of vertebrate evolution.
Crucially, this piscine off-
shoot of our own distant past
is both unusually intact and
exceptionally old.
So what kind of fish is it? A
summary of vertebrate diver-
sity helps to make sense of the
answer. Of the 51,000 or more
living species of vertebrates,
99.9% have jaws: these are the gnathostomes.
Gnathostomes include the bony Osteich-
thyes and the cartilaginous Chondrichthyes.
Chondrichthyes (sharks, rays and chimae-
ras) account for only 2% of gnathostome
species, the Osteichthyes accounting for the
other 98%. Around half of the Osteichthyes
are Actino pterygii, or ‘ray-finned fishes’, and
half are Sarco pterygii, or ‘lobe-finned fishes’.
Actinopterygians include some 28,000 species,
from zebrafish to bichirs, and living sarcop-
terygian fishes are limited to three genera of
lungfishes and one coelacanth. Land-dwelling
tetrapods constitute the remaining majority of
sarcopterygians.
Thus far, the origins of these major divisions
of today’s gnathostomes can be traced back
to the Devonian, between 416 million and
359 million years ago, the Age of Fishes. Fossils
that are clearly chondrichthyan are known from
around 405 million to 400 million years ago
6
,
but we have little idea as to whether these belong
within the living radiation, the ‘crown group’,
or represent side branches of their common
Figure 1 | Newcomer to the Silurian seascape. This classic view of Silurian marine
life, published in the 1940s, is rich in invertebrates (corals, molluscs, arthropods,
echinoderms, and more besides). But it lacks fish. Armoured jawless fishes
existed throughout the Silurian (443 million to 416 million years ago), alongside
early jawed fishes (placoderms and acanthodians, extinct groups whose affinities
are the subject of debate8,10). A representative of modern fishes, Guiyu oneiros5 (inset), can now be added
to the picture. Guiyu is a Silurian-aged member of the sarcopterygians (extant representatives of which
include lungfishes, the coelacanth and all tetrapods). What else might be absent? Evidence of early
actinopterygians (ray-finned fishes) and chondrichthyans (sharks and chimaeras) must be lurking out
there, somewhere in the Silurian sediments. (Silurian scene by Z. Burian. Fish reconstruction by B. Choo.)
BRIDGEMAN ART LIBRARY/J. HOCHMAN, WWW.ZDENEKBURIAN.COM
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© 2009 Macmillan Publishers Limited. All rights reserved
primitive and advanced features. With regard
to its anatomical completeness, Guiyu provides
exceptional corroboration for the decidedly
odd reconstruction of the early osteichthyan
genus, Psarolepis2. Cobbled together from a
disparate set of fossils, the incongruent suite
of features9 displayed by Psarolepis has been
viewed with caution. Now, it turns out to be
thoroughly plausible. L ike Psarolepis and other
sarcopterygian fishes (including Latimeria,
the living coelacanth), the braincase of Guiyu
is divided into separate front and rear units.
Like Psarolepis, the cheek bones resemble those
of early actinopterygians. Like Psarolepis and
many other early gnathostomes
1
, including at
least one chondrichthyan
6
, the shoulder girdle
bears a spine in front of the pectoral fin. Simi-
larly, the dorsal-fin spine and anterior spine-
bearing plate of Guiyu are probably primitive.
These are all widespread features of early gna-
thostomes, and seeing such characteristics in
Guiyu provides a first glimpse of the sequential
order of anatomica l changes that resulted in the
standard set of sarcopterygian traits.
The evolutionary tree proposed by Zhu et al.5
(see Fig. 5 on page 473) adds to a growing set of
analyses of early osteichthyan and gnathostome
interrelationships8,10. Uncertainties still sur-
round the branching pattern of non-osteich-
thyans, but the addition of Guiyu t o th e ca st o f
early fishes does not change the basic pattern of
interrelationships among early osteichthyans.
Instead, it adds support to notable consisten-
cies in the emerging pattern of sarcopterygian
evolution, including the clustering of some of
the earliest-known examples to form an as-yet
unnamed group.
Finally, what does the conclusion that Guiyu
is unequivocally sarcopterygian imply? On
the whole, early fossils are thought to be unreli-
able as minimum-date markers of evolution-
ary branching events11, because they are less
complete and/or lack the full anatomical sig-
nature of the group to which they are assigned.
Guiyu might be an exception that proves the
rule, for it provides a new and exceptionally
reliable earliest fossil marker for a major split in
vertebrate evolution. By pushing a whole series
of branching points in gnathostome evolution
out of the Devonian and into the Silurian,
the discovery of Guiyu also signals that a sig-
nificant part of early vertebrate evolution is
unknown (Fig. 1).
The new shape of the gnathostome tree
shows that early sarcopterygians, as well as
actino pterygians and chondrichthyans, ought
to be turning up in Silurian sediments. But
where are they? Modern fish groups have
Silurian roots, but these fishes are consistently
absent from existing scenarios of Silurian life.
The discovery of Guiyu should provoke a rash
of new fieldwork and a fresh look at existing
collections of pre-Devonian fossils. ■
Michael I. Coates is in the Department of
Organismal Biology and Anatomy, University of
Chicago, Chicago, Illinois 60637, USA.
e-mail: mcoates@uchicago.edu
1. Janvier, P. Early Vertebrates (Oxford Univ. Press, 1996).
2. Zhu, M. et al. Nature 397, 607–610 (1999).
3. Botella, H. et al. Nature 448, 583–586 (2007).
4. Basden, A. M. & Young, G. C. J. Vert. Paleontol. 21,
754–766 (2001).
5. Zhu, M. et al. Nature 458, 469–474 (2009).
6. Miller, R. F. et al. Nature 425, 501–504 (2003).
7. Zhu, M. et al. Nature 441, 77–80 (2006).
8. Brazeau, M. D. Nature 457, 305–308 (2009).
9. Ahlberg, P. E. Nature 397, 564–565 (1999).
10. Friedman, M. J. Syst. Palaeontol. 5, 289–343
(2007).
11. Donoghue, P. C. J. & Benton, M. J. Trends Ecol. Evol. 22,
424–431 (2007).
ASTROPHYSICS
Quiet is the new loud
Daniel Proga
Understanding the mechanisms by which matter flows into black-hole
systems is pivotal to elucidating how such systems work. It seems that a
‘quiet’ mass outflow can play a hitherto-unknown part in the process.
All black-hole systems that accrete matter,
regardless of their size, are believed to have
very similar components and to operate in
a very similar way. Quasars — galaxies with
extremely bright nuclei powered by the accre-
tion of matter onto a supermassive black hole
— are an example of accreting black-hole sys-
tems at one end of the size range. At the other
end are their much smaller cousins, black-hole
binaries (a black hole with a star companion).
Black-hole binaries have fascinated astrono-
mers for years because they go through a cycle
of many different activity states. For example,
they can be in a state of high accretion and high
luminosity, in which they strongly emit both
‘soft’ (low energy) and ‘hard’ (high energy)
X-rays — the bright/soft state. Another state
is one of low accretion and low luminosity, in
which the hard-X-ray emission exceeds that of
soft X-rays — the faint/hard state.
On page 481 of this issue, Neilsen and Lee
1
report observations of a microquasar — a
black-hole binary that has a radio-emitting jet
of gas — known as GRS 1915 + 105. This black
hole has a mass 14 times that of the Sun, and
is accreting gas from its star. The authors have
discovered that, as the system changes from
the faint/hard accretion state to the bright/soft
state, the high-speed jet is rep laced by a much
slower, X-ray-absorbing wind (Fig. 1). The
authors conclude that the wind being launched
from the outer regions of the black hole’s accre-
tion disk competes with the jet for matter and
wins, taking matter away from the disk and
halting its flow into the jet.
Many quasars are found only in the
bright/soft accretion state. This is not because
they do not vary, but rather because they evolve
more slowly than their smaller counterparts.
Therefore, to understand quasars, astrono-
mers tend to study microquasars, which evolve
on timescales that are six to eight orders of
magnitude shorter than quasars.
Extensive obser vations of microquasars have
revealed that they can emit radiation over a
wide range of energies, from radio to X-rays
and γ-rays. Modelling of the observed spectra
shows that this broad emission is possible
because microquasars have different sources
of energy: ultraviolet and soft X-rays are emit-
ted by the accretion disk; hard X-rays are pro-
duced in the disk’s corona of very hot plasma;
and radio emission is generated by the narrow
jet of magnetized plasma
2,3
. Observe d changes
in emission can thus be linked to changes in the
sources of radiation.
Most studies of microquasars have focused
on their main, ‘loud’ components — that is, the
disk, corona and jet. These studies have identi-
fied a variety of accretion states4 (up to 14 states
in GRS 1915 + 105), and several correlations
and anti-correlations among the emissions in
different energy bands. For example, the radio
emission correlates with X-ray emission in
the faint/hard state
5
. In addition, such studies
have yielded clues to one of the main myster-
ies: how matter that plunges onto the black
hole can avoid crossing its event horizon (the
boundary below which nothing can escape),
and instead can escape as a jet with velocities
close to the speed of light. One clue is that jets
occur when matter accretion is in the form of
a thick flow.
But despite many successes, the most crucial
questions, such as how accretion disks work and
how jets are produced, remain unanswered. We
do not know exactly how the rotational energy
of the disk is dissipated, converted to heat and
finally radiated away. However, we do have a
good physical model for the outward transport
of angular momentum in the disk if magnetic
fields are present
6
. This is extremely important,
because such transport is crucial for accretion
to occur in the first place. As for jet production,
we still do not understand whether the jets are
powered by the rotational energy of the accre-
tion flow or by the energy of the fast-spinning
black hole
7,8
. In both cases, magnetic fields are
involved in transferring energy to the jet.
Microquasars thus continue to be the sub-
ject of intensive observational and theoretical
research. Neilsen and Lee’s work1 shows that
much can be learned from investigating the
‘quiet’ components of the systems, such as disk
414
NATURE|Vol 458|26 March 2009
NEWS & VIEWS
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© 2009 Macmillan Publishers Limited. All rights reserved
