A Nearly Modern Amphibious Bird from the Early Cretaceous of Northwestern China

Abstract

Three-dimensional specimens of the volant fossil bird Gansus yumenensis from the Early Cretaceous Xiagou Formation of northwestern China demonstrate that this taxon possesses advanced anatomical features previously known only in Late Cretaceous and Cenozoic ornithuran birds. Phylogenetic analysis recovers Gansus within the Ornithurae, making it the oldest known member of the clade. The Xiagou Formation preserves the oldest known ornithuromorph-dominated avian assemblage. The anatomy of Gansus, like that of other non-neornithean (nonmodern) ornithuran birds, indicates specialization for an amphibious life-style, supporting the hypothesis that modern birds originated in aquatic or littoral niches.

Full text

to such indirect dissociations, which is indi-
cated as Scheme C in Fig. 4.
The prediction (1) that N(O)–H bond fis-
sion after photoexcitation to the
1
ps* state
represents an efficient nonradiative decay path-
way for heteroaromatics like imidazole, pyrrole,
and phenol in the gas phase is thus con-
firmed, irrespective of whether the
1
ps* state
is populated by direct photoexcitation as in
imidazole or pyrrole, or indirectly, by radia-
tionless transfer from a photoprepared
1
pp*
state, as in phenol. The quantum yield for H
atom production after UV excitation of pyrrole
in the wavelength ranges discussed in this work
is estimated to be near unity. Due to the
similarity in the PESs for pyrrole and imid-
azole, we can also assume a high quantum yield
for the H atom production in this case. The
picture is less clear for phenol, but the fast H
atoms formed by the S
1
/S
2
coupling at l G
240 nm show anisotropic recoil distributions,
implying fast and efficient fragmentation.
Further, the recent PTS study of phenol photol-
ysis at 248 nm (25) reports no fragmentation
channels other than H þ C
6
H
5
O at that wave-
length. The available evidence thus suggests
that H atom loss is a major process after UV
excitation of these gas phase molecules. Fu-
ture challenges include the following: (i) ex-
tending such high-resolution PTS studies to
larger, less volatile biomolecules like adenine,
histidine, tyrosine, and tryptophan, and (ii)
exploring whether such photoinduced prompt
N(O)–H bond fission processes also operate in
the condensed phase.
References and Notes
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C. Jouvet, Phys. Chem. Chem. Phys. 4, 1093 (2002).
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3. Imidazole, pyrrole, and phenol were obtained commer-
cially (Aldrich, Gillingham, UK, 98%). Pyrrole, at its room
temperature vapor pressure (È11 torr) and diluted in
700 torr of Ar, was introduced into the photolysis region
in the form of a pulsed, skimmed molecular beam. For
imidazole and phenol, solid samples were packed in the
tube leading to the same pulsed molecular beam source
and heated to È100-C (imidazole) or 60-C (phenol), and
the resulting vapor entrained in 700 torr of Ar before
expansion into the interaction region.
4. L. Schnieder, W. Meier, K. H. Welge, M. N. R. Ashfold,
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5. B. Cronin, M. G. D. Nix, R. H. Qadiri, M. N. R. Ashfold,
Phys. Chem. Chem. Phys. 6, 5031 (2004).
6. m
H
and m
R
in Eq. 1 are the masses of the H atom (m
H
0
1.0079 dalton) and of the partner fragments (imid-
azole, pyrrolyl, and phenoxyl, with respective masses
m
R
0 67.07 dalton, 66.08 da lton, and 93. 11 dalton),
d 0 0.368 m is the length of the flight p ath, and t is the
measured H atom TOF.
7. The angular variation of the product yield is characterized
by the beta (b) parameter. Photoexcitation preferentially
selects molecules that are aligned so that their transition
moment (m) is parallel to e
phot
. Direct dissociation occurs
on a time scale that is much shorter than a classical
rotational period. The resulting fragments will recoil along
the axis of the breaking bond in the photoexcited
molecule, i.e., they will display a spatial anisotropy that
reflects the original mIe interaction. The photofragment
angular distribution is given by I(q) 0 [1 þ bP
2
(cosq)]/4p,
where q is the angle between the fragment recoil velocity
vector v and the TOF axis, and P
2
(x) 0 (3x
2
j 1)/2 is the
second-order Legendre polynomial. b takes limiting values
of þ2 in the case of prompt dissociation after excitation
via a parallel transition (i.e., m lies along the breaking
bond) and –1 in the case of a perpendicular transition.
Less anisotropic fragment recoil distributions (i.e., with b
closer to 0) are observed in the case of predissociations
(i.e., where the excited state lifetime is comparable to, or
longer than, the rotational period of the parent molecule).
8. R. N. Zare, Angular Momentum. Understanding Spatial
Aspects in Physics and Chemistry (Wiley, New York,
1988).
9. Equilibrium structures and vibrational frequencies
discussed in this work were calculated using Gaussian
03 (Revision B.04, M. J. Frisch et al., Gaussian, Inc.,
Pittsburgh, PA, 2003), B3LYP, with a 6-311G(d,p)
basis set.
10. Convention identifies the N atom involved in the NjH
bond in imidazole as atom 1, and the other heavy atoms in
the ring labeled by counting in a clockwise direction in the
case of the structure depicted in Fig. 1, i.e., C2, N3, etc.
11. A. J. Gianola et al., J. Phys. Chem. A 109, 11504 (2005).
12. Vibronic coupling is an example of a breakdown of the
Born-Oppenheimer separation, which applies when the
vibrational and electronic degrees are coupled sufficiently
strongly that they cannot be factored. In the case of an
electric dipole forbidden transition, inclusion of a
quantum of a nonsymmetric vibration changes the overall
(vibronic) symmetry and permits the transition (with an
intensity that depends on the degree of mixing of the
vibrational and electronic wave functions).
13. B. O. Roos, P.-A. Malmqvist, V. Molina, L. Serrano-Andres,
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Faraday Disc 127, 267 (2004).
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Spectrosc. 24, 402, 413 (1967).
18. C. Ratzer, J. Kupper, D. Spangenberg, M. Schmitt, Chem.
Phys. 283, 153 (2002).
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21. J. Lorentzon, P.-A. Malmqvist, M. Fu
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lscher, B. O. Roos,
Theor. Chim. Acta 91, 91 (1995).
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(2001).
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(2004).
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27. K. Daigoku, S. Ishiuchi, M. Sakai, M. Fujii, K. Hashimoto,
J. Chem. Phys. 119, 5149 (2003).
28. Two alternative numbering schemes for the normal mode
vibrations of benzene and benzene derivatives are
commonly found in the scientific literature (the so-called
Wilson and Herzberg assignments). We consistently use
the former. To assist readers familiar with the latter, the
Herzberg labels for the modes of phenol discussed in this
work are: n
16a
Y n
16
, n
16b
Y n
21
, n
18b
Y n
33
.
29. We are grateful to the Engineering and Physical Sciences
Research Council UK for financial support via the
Portfolio Partnership LASER and to K. N. Rosser for his
outstanding practical support of this work.
26 January 2006; accepted 24 April 2006
10.1126/science.1125436
A Nearly Modern Amphibious Bird
from the Early Cretaceous of
Northwestern China
Hai-lu You,
1
Matthew C. Lamanna,
2
*
Jerald D. Harris,
3
*
Luis M. Chiappe,
4
Jingmai O’Connor,
4
Shu-an Ji,
1
Jun-chang Lu¨,
1
Chong-xi Yuan,
1
Da-qing Li,
5
Xing Zhang,
6
Kenneth J. Lacovara,
7
Peter Dodson,
8
Qiang Ji
1
Three-dimensional specimens of the volant fossil bird Gansus yumenensis from the Early
Cretaceous Xiagou Formation of northwestern China demonstrate that this taxon possesses
advanced anatomical features previously known only in Late Cretaceous and Cenozoic ornithuran
birds. Phylogenetic analysis recovers Gansus within the Ornithurae, making it the oldest known
member of the clade. The Xiagou Formation preserves the oldest known ornithuromorph-dominated
avian assemblage. The anatomy of Gansus, like that of other non-neornithean (nonmodern)
ornithuran birds, indicates specialization for an amphibious life-style, supporting the hypothesis
that modern birds originated in aquatic or littoral niches.
N
eornithean (modern) birds are the most
diverse extant tetrapods, comprising
È10,000 species (1). Neornitheans, plus
the predominantly Late Cretaceous Hesperornithes
and Ichthyornis dispar, constitute the Ornithurae, a
clade that, along with a few other Cretaceous taxa,
comprises the Ornithuromorpha (2). Previously
reported, alleged Early Cretaceous ornithurans
are either fragmentary (3, 4), of debatable age
(5), or have received only limited examination
(6–8). Furthermore, they are rare compared to
members of the extinct clade Enantiornithes
(9, 10). Consequently, the early evolution of the
Ornithuromorpha and the phylogenetic, tempo-
ral, and paleoecological contexts of ornithuran
(and ultimately neornithean) origins remain ob-
scure. One of the first Early Cretaceous birds
discovered was Gansus yumenensis, based on
an isolated partial left pelvic limb from the
Lower Cretaceous (?Aptian-Albian, È115to105
million years ago; see supporting online material)
Xiagou Formation near Changma, Gansu Prov-
16 JUNE 2006 VOL 312 SCIENCE www.sciencemag.org
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REPORTS

ince, northwestern China (11) (Fig. 1). Gansus
was initially recognized as more closely allied
to neornithean birds than is Archaeopteryx; sub-
sequent discoveries have reinforced this hypoth-
esis (2, 12).
Here we describe five new, tern-sized
specimens of G. yumenensis, also from the
Xiagou Formation near Changma, that col-
lectively represent the entire skeleton except
the skull, mandibles, and cranial-middle
cervical vertebrae (Fig. 2 and figs. S1 to 6).
Unlike many compressed or split avian fossils
from the Jehol Group of northeastern China,
most of the new Gansus specimens consist of
three-dimensional, largely uncrushed and
undistorted bones, and many include soft
tissues. Autapomorphies are difficult to
pinpoint in the fragmentary Gansus holotype,
but the most frequently cited feature is pro-
nounced, distally projecting flexor tubercula
separate from, and distal to, the proxi-
moplantar ends of the pedal unguals (10, 11).
One of the new specimens has identical pedal
morphology (Fig. 2J and fig. S5); skeletal
elements of this specimen are indistinguishable
from their counterparts in the remaining four
skeletons, justifying the referral of all to G.
yumenensis. Unless otherwise noted, the phylo-
genetically important characters of Gansus
discussed below represent apomorphies relative
to more basal birds (see also supporting online
material).
The caudal cervical and 10 thoracic vertebrae
of Gansus are plesiomorphically not hetero-
coelous. The thoracics are excavated by deep,
emarginate, craniocaudally elongate, lateral pneu-
matic fossae. The cranialmost three to four tho-
racics possess well-developed ventral processes
(Bhypapophyses[) (Fig. 2G and fig. S4). The
cranialmost three of the 10 to 11 synsacral verte-
brae exhibit dorsally directed costal processes.
The tail comprises six to seven free vertebrae,
all with poorly developed cranial and caudal
zygapophyses, and a pygostyle the length of
three caudals. All specimens lack ossified gastra-
lia and uncinate processes.
A tall carina extends the length of the ster-
num (Fig. 2G and fig. S4). Closely spaced
coracoidal articular sulci embay the cranial
edge of the element. The thin U-shaped
furcula has an È38- intraclavicular angle.
The dorsally convex scapula tapers distally.
The strutlike, plesiomorphically apneumatic
coracoid possesses well-developed procora-
coid and lateral processes (Fig. 2E and fig.
S2), a deep circular scapular cotyle, and a
humeral articular facet situated well ventral to
the acrocoracoid process.
The proximal end of the plesiomorphi-
cally apneumatic humerus exhibits a ventral
tubercle, capital incisure, and domed artic-
ular condyle. The cranially projecting bi-
cipital crest bears a transverse sulcus on its
proximoventral surface and a tiny fovea
caudodistally. A shallow brachial fossa lies
proximal to approximately subequal, crani-
ally developed distal humeral condyles. The
ulna is slightly longer than the humerus and
displays a prominent bicipital tubercle and a
deep narrow brachial impression. The dorsal
trochlear surface of the dorsal condyle is
developed as a semilunate ridge. The carpal
trochlea (semilunate carpal) is completely
co-ossified with, and positioned proximal
to, major and minor metacarpals that may
also be fused distally. The weakly devel-
oped extensor process of the alular meta-
carpal extends cranially just beyond the
shelflike phalangeal articular facet. The
alular metacarpal does not reach the proxi-
mal terminus of the intermetacarpal space.
The craniocaudal diameter of the major
metacarpal is more than twice that of the
minor. The proximal phalanx of the major
digit is strongly dorsoventrally compressed,
flat caudally, and longer than the second
phalanx.
The iliac preacetabular alae extend crani-
ally beyond the synsacrum, overlapping the
caudalmost pair of thoracic ribs. A weak,
subtriangular preacetabular tuberculum (Bpec-
tineal process[) is situated cranioventral to the
acetabulum. A preacetabular (Bcuppedicus[)
fossa is absent. A well-developed antitro-
chanter occupies the caudodorsal corner of
the acetabulum. The pubis and ischium are
seamlessly fused to the ilium at the acetabu-
lum. The pubes are strongly retroverted,
nearly parallel to the ischia; their distalmost
È5 to 6% are in contact but not fused. The
iliac (Bdorsal[) process of the ischium,
situated approximately at midshaft, broadly
contacts, and may be co-ossified with, the
caudoventral surface of the ilium, enclosing a
craniocaudally elongate ilioischiadic fora-
men (Fig. 2B and fig. S1).
The femur exhibits a trochanteric crest
proximolaterally and a patellar sulcus craniodis-
tally. The lateral gastrocnemial (Bectocondylar[)
tubercle and condyle form a single distolateral
trochlear surface. The conjoined cranial and
lateral cnemial crests of the tibiotarsus make
up a single, well-developed, proximocranially
projected rounded crest (Fig. 2C and fig. S1).
A tibiotarsal extensor canal is present as an
emarginate groove that plesiomorphically
lacks a supratendinal pons. The tarso-
metatarsal articular surface of the approxi-
mately subequal distal condyles extends onto
the tibiotarsal caudal surface. The fibula
terminates proximal to the ankle. The hypo-
tarsus lacks crests and sulci (Fig. 2H and fig.
S4). Plantar displacement of the proximal end
of metatarsal III creates a deep dorsal
extensor sulcus that contains a pronounced
tibialis cranialis tuberosity and one vascular
foramen that penetrates to the plantar surface.
Metatarsal I is twisted and distally deflected
so that its plantaromedial surface is concave
proximal to its trochlea. The trochlea of
metatarsal II lies proximal to the proximal-
most extent of those of metatarsals III and IV.
It also exhibits marked plantar offset and
lateral rotation and is compressed mediolat-
erally as compared to those of metatarsals III
and IV. Metatarsals III and IV completely
1
Institute of Geology, Chinese Academy of Geological Sciences,
26 Baiwanzhuang Road, Beijing 100037, People’s Republic of
China.
2
Section of Vertebrate Paleontology , Carnegie Museum
of Natural History , 4400 Forbes Avenue, Pittsburgh, PA
15213–4080, USA.
3
Science Department, Dixie State College,
225 South 700 East, St. George, UT 84770, USA.
4
The
Dinosaur Institute, Natural History Museum of Los Angeles
County , 900 Exposition Boulevard, Los Angeles, CA 90007,
USA.
5
Fossil Research and Development Center, Third Geology
and Mineral Resources Exploration Academy of Gansu
Province, 1 Langongping Road, Lanzhou, Gansu 730050,
People’s Republic of China.
6
Provincial Museum of Gansu
Province, Nature Department, 3 West Xijin Road, Lanzhou,
Gansu 730050, People’s Republic of China.
7
Geology and
Paleontology Program, Department of Bioscience and
Biotechnology, Drexel University, 409 Stratton Hall, 32nd
and Chestnut, Philadelphia, P A 19104, USA.
8
Department of
Animal Biology, School of V eterinary Medicine, University of
Pennsylvania, 3800 Spruce Street, Philadelphia, P A 19104,
USA.
*To whom correspondence should be addressed. E-mail:
lamannam@carnegiemnh.org (M.C.L); jharris@dixie.edu
(J.D.H.)
Fig. 1. Map of China
(white), with Changma lo-
cality in Gansu Province and
fossil bird–producing de-
posits of the Jehol Group in
Liaoning Province (È20 00
km away) marked by bird
silhouettes.
www.sciencemag.org SCIENCE VOL 312 16 JUNE 2006
1641
REPORTS

enclose a large, proximodistally elongate,
distal vascular foramen. The first phalanges
of all pedal digits are longer than any of their
respective distal phalanges; all unguals are
small, short, and unrecurved.
Wing feathers preserved with one speci-
men (Fig. 2D and fig. S2) are asymmetrical
and virtually identical to those of volant mod-
ern birds. Semiplumes or down also appear to
be present.
Most Early Cretaceous bird species have been
recovered from northeastern China and Spain
and are primarily non-ornithuromorphs (enan-
tiornitheans plus more basal taxa) (8, 10, 13).
However, most of nearly 50 bird specimens
recovered from Changma appear to pertain
to Gansus. Thus, the XiagouFormationpre-
serves the oldest known avian assemblage
dominated by ornithuromorphs, possibly
indicating the first stages of the rise of
ornithuromorphs to diversity-based domi-
nance over enantiornitheans (supporting on-
line material).
Despite pectoral and alar features that
indicate powered flight, Gansus pelvic limb
elements exhibit specializations that are
characteristic of amphibious (14) birds. Oste-
ologically, these include the prominent, prox-
imally projecting cnemial crests on the
tibiotarsus; proximal position of the trochlea
of metatarsal II; elongate pedal digits EIII and
IV each longer than the tarsometatarsus
(tables S1 and S2)^ with elongate proximal
phalanges (all longer than any penultimate
phalanges); and unrecurved unguals with
large flexor tubercula. Except the last, these
features predominantly occur elsewhere in
undoubted foot-propelled divers such as
Hesperornis, loons (Gaviidae), and grebes
(Podicipedidae) (15); the cnemial crests are
more elongate in Gansus than in the slightly
younger hesperornithean Enaliornis (5). Sim-
ilar flexor tubercula characterize some extant
shorebirds (Charadriiformes) (3), herons (Ar-
deidae), and diving ducks (Anatidae) (16).
Moreover, one new Gansus specimen pre-
serves tubercular skin surrounding the pedal
digits (Fig. 2J and fig. S5) that indicate
interdigital telae (webbing) extending at least
to the proximal ends of the unguals. The
presence of web-footed birds in the Early
Cretaceous is supported by footprints else-
where in Asia (17). Gansus has typically been
considered a sandpiper (Scolopacidae) analog
(3, 11), implying that it was not amphibious
(that is, it lacked fully webbed feet and did
not dive) but instead waded and probed near-
shore sediments for food (1). Its anatomy,
however, demonstrates that it was more
similar to, but not as adept as, foot-propelled
diving birds such as grebes, loons, and diving
ducks.
In all consensus trees from a phylogenetic
analysis (see supplementary online material
for methods and details), Gansus occurs with-
Fig. 2. New specimens of G. yumenensis (Chinese Academy of Geological Sciences, Institute of
Geology, prefix CAGS-IG-04-). (A) CM-002, articulated caudal cervical, thoracic, synsacral, and
caudal vertebrae, pelvic girdle, and partial pelvic limbs in right dorsolateral view. (B) CM-002,
pelvis and synsacrum. (C) CM-002, proximal right tibiotarsus [indicated by arrowhead in (A)]. (D)
CM-004, nearly complete skeleton in ventral view with feathers (dark brown) on thoracic limbs,
lacking cranium, cranial-midcervical vertebrae, and both pelvic limbs. (E) CM-004, left coracoid in
ventral view [indicated by arrowhead in (D)]. (F) CM-001, partial right and left pelvic limbs. (G)
CM-003, nearly complete skeleton in ventral view, lacking cranium, cervical vertebrae, distal left
thoracic limb, and right and distal left pelvic limbs. (H) CM-003, proximal tarsometatarsus
[indicated by arrowhead in (G)]. (I) CM-008, partial pelvic limbs with soft-tissue preservation. (J)
CM-008, tubercular soft tissue preserved around toes [indicated by arrowhead in (I)]. (K)
Reconstruction of G. yumenensis based on new specimens. Elements shaded gray remain unknown.
Abbreviations in figure are as follows: cc, cnemial crests; ft, flexor tubercula; dp, dorsal process of
ischium; fu, furcula; ht, hypotarsus; pp, procoracoid process; sc, sternal carina; sy, synsacrum; vp,
ventral processes of cranial thoracic vertebrae.
16 JUNE 2006 VOL 312 SCIENCE www.sciencemag.org
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in the Ornithurae (Fig. 3). This position is con-
siderably more derived than that of all known
birds from the Jehol Group of northeastern
China. Gansus predates other, later Cretaceous
ornithurans that are considered to exhibit am-
phibious features Esuch as hesperornitheans^
or are thought to have occupied water-based
niches Esuch as Ichthyornis^ (5, 12, 18–23).
Some younger yet more basal ornithuromorphs,
including Apsaravis ukhaana (24)andPata-
gopteryx deferrariisi (25), occupied fully ter-
restrial niches, as did some later purported
neornitheans (12, 20, 24). Ichthyornis and Gansus
are recovered as proximal outgroups to the Ne-
ornithes, consistent with the hypothesis that ne-
ornitheans originated in an aquatic habitat (26).
Thus (Fig. 3), the most basal ornithuromorphs
appear to have evolved in a terrestrial/arboreal
context (25) but rapidly shifted to an aquatic
ecology (8)(Fig.3).
Before the end of the Cretaceous, some non-
ornithuran ornithuromorphs must have reverted
to a terrestrial life-style (24). The Cretaceous
existence of members of basal neornithean
clades EAnseriformes and possibly Gaviiformes
(19, 20, 22)^ implies that representatives of the
neornithean clades Galliformes and more basal
Palaeognathae, all known fossil and extant
members of which are terrestrial, must also have
existed during the Cretaceous (27). Thus, al-
though neornitheans may have originated in
aquatic niches, some basal neornitheans appar-
ently re-radiated into terrestrial niches before
the Cretaceous/Paleogene extinction event.
Consequently, contrary to recent hypotheses,
adaptation to an aquatic ecology appears to
have played little part in the survival of birds
across the K/P boundary (27).
References and Notes
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29. We thank Z.-c. Bai. and C. Peng and their field crew for
excavation and collection of the specimens; G.-h. Cui and
Y.-q. Zhang for specimen preparation; B. Livezey,
Z.-x. Luo, and R. Mulvihill for discussion; S. Yu for
translating (6); M. Klingler for Fig. 2K; and anonymous
manuscript reviewers. Funding was provided by the
Discovery Quest program for The Science Channel to H.-l.Y.,
M.C.L., and J.D.H.; the Carnegie Museum of Natural History
to M.C.L.; Dixie State College to J.D.H.; the Chinese
Geological Survey of the Ministry of Land and Resources of
China and the Ministry of Science and Tec hnology of China
(973 Project) to H-l.Y. and Q.J.; and the Gansu Bureau of
Geology and Mineral Resources to D.-q.L.
Supporting Online Material
www.sciencemag.org/cgi/content/full/312/5780/1640/DC1
Materials and Methods
SOM Text
Figs. S1 to S6
Tables S1 and S2
References
17 February 2006; accepted 19 April 2006
10.1126/science.1126377
Fig. 3. Phylogenetic position of G. yumenensis (50% majority-rule consens us tree) based on
holotype and new referred specimens; time scale is per (28). Thin line segments represent ghost
lineages; thick line segments represent known ranges of terminal taxa. Clades are denoted by
black circles (see supporting online material for clade definitions). The star represents the
temporal position of Enaliornis spp. Colors indicate known or inferred ecologies as follows: brown,
terrestrial/arboreal; b lue, aquatic/amphi bious; black, equivocal. Note the sequence of amphibious
taxa basal to the Neorni thes. Depicted divergence time s are intended as approxi mations only,
base d o n the oldest occurrence of an included species and sub sequent divergences. Mya, millio n
years ago.
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