Population and genetics

Population and genetics

a 1-3 page summary. summary should be focused on the work performed by the authors and their interpretations of the outcome (methods, results, and discussion sections) DO NOT PARAPHRASE THE ABSTARCT. be sure to include complete citation {Author’s name. Year of publication. Title of Article. Journal Name. Volume:page numbers}

File #1
removal, which has been shown to be the ratedetermining
step on similar surfaces (12–14, 17).
Relative to their core-shell precursors, the nanocage
models showed substantially enhanced activity,
which is attributed to the shortening of Pt-Pt
interatomic distances (table S2).
We evaluated the long-term stability of the
catalysts through an accelerated durability test
(Fig. 4, C and D). The Pt octahedral nanocages
showed the best performance, with the ORR mass
activity only reduced by 36% after 10,000 cycles,
still showing 3.4-fold enhancement relative to the
pristine Pt/C. The ECSAs of the cubic and octahedral
nanocages only dropped by 13 and 6% after
5000 cycles and by 32 and 23% after 10,000 cycles,
respectively. During the durability test, the holes
in the walls of the nanocages were slightly enlarged
(fig. S7). These results demonstrate that the
excellent durability associated with the core-shell
catalysts was not affected by the selective removal
of Pd cores.
REFERENCES AND NOTES
1. J. Chen, B. Lim, E. P. Lee, Y. Xia, Nano Today 4, 81–95
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2. F. A. de Bruijn, V. A. T. Dam, G. J. M. Janssen, Fuel Cells
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3. X. Lu et al., Nano Lett. 7, 1764–1769 (2007).
4. S. Xie et al., Angew. Chem. Int. Ed. Engl. 51, 10266–10270
(2012).
5. C. Chen et al., Science 343, 1339–1343 (2014).
6. A. Funatsu et al., Chem. Commun. (Camb.) 50, 8503–8506
(2014).
7. H. Li et al., Angew. Chem. Int. Ed. Engl. 52, 8368–8372
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8. H. Duan et al., Nat. Commun. 5, 3093 (2014).
9. R. R. Adzic et al., Top. Catal. 46, 249–262 (2007).
10. K. Sasaki et al., Nat. Commun. 3, 1115 (2012).
11. M. Shao et al., Chem. Commun. (Camb.) 49, 9030–9032
(2013).
12. S. Xie et al., Nano Lett. 14, 3570–3576 (2014).
13. J. Park et al., ACS Nano 9, 2635–2647 (2015).
14. X. Wang et al., Nat. Commun. 6, 7594 (2015).
15. Y. Yin et al., Science 304, 711–714 (2004).
16. M. Jin et al., Nano Res. 4, 83–91 (2011).
17. Materials and methods are available as supplementary
materials on Science Online.
18. X. Xia et al., Proc. Natl. Acad. Sci. U.S.A. 110, 6669–6673
(2013).
19. M. Heggen, M. Oezaslan, L. Houben, P. Strasser, J. Phys. Chem.
C 116, 19073–19083 (2012).
20. J. Erlebacher, M. J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki,
Nature 410, 450–453 (2001).
21. V. A. Baheti, R. Ravi, A. Paul, J. Mater. Sci. Mater. Electron. 24,
2833–2838 (2013).
22. N. M. Markovic, P. N. Ross, Surf. Sci. Rep. 45, 117–229
(2002).
23. J. Zhang, H. Yang, J. Fang, S. Zou, Nano Lett. 10, 638–644
(2010).
24. J. Wu, A. Gross, H. Yang, Nano Lett. 11, 798–802
(2011).
25. S.-I. Choi et al., Nano Lett. 13, 3420–3425 (2013).
26. C. Cui, L. Gan, M. Heggen, S. Rudi, P. Strasser, Nat. Mater. 12,
765–771 (2013).
27. L. Gan et al., Science 346, 1502–1506 (2014).
28. B. Han et al., Energy Environ. Sci. 8, 258–266 (2015).
ACKNOWLEDGMENTS
The syntheses were supported by start-up funds from the
Georgia Institute of Technology (to Y.X.). As jointly supervised
PhD students from Xiamen University, L.Z. and X.W. were also
partially supported by fellowships from the China Scholarship
Council. The theoretical modeling work at University of
Wisconsin–Madison was supported by the U.S. Department of
Energy (DOE)–Basic Energy Sciences (BES), Office of Chemical
Sciences, grant DE-FG02-05ER15731. Calculations were
performed at supercomputing centers located at the
Environmental Molecular Sciences Laboratory, which is
sponsored by the DOE Office of Biological and Environmental
Research at the Pacific Northwest National Laboratory; Center
for Nanoscale Materials at Argonne National Laboratory,
supported by DOE contract DE-AC02-06CH11357; and National
Energy Research Scientific Computing Center, supported by
DOE contract DE-AC02-05CH11231. Part of the electron
microscopy work was performed through a user project
supported by the Oak Ridge National Laboratory’s Center for
Nanophase Materials Sciences, which is a DOE Office of Science
User Facility. J.L. gratefully acknowledges the support by Arizona
State University and the use of facilities in the John M. Cowley
Center for High Resolution Electron Microscopy at Arizona State
University. Data described can be found in the main figures and
supplementary materials. The authors declare no conflict of
interests.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/349/6246/412/suppl/DC1
Materials and Methods
Figs. S1 to S8
Tables S1 to S3
References (29–37)
8 March 2015; accepted 15 June 2015
10.1126/science.aab0801
EVOLUTION
A four-legged snake from the Early
Cretaceous of Gondwana
David M. Martill,1 Helmut Tischlinger,2 Nicholas R. Longrich3
Snakes are a remarkably diverse and successful group today, but their evolutionary origins are
obscure. The discovery of snakes with two legs has shed light on the transition from lizards to
snakes, but no snake has been described with four limbs, and the ecology of early snakes is
poorly known. We describe a four-limbed snake from the Early Cretaceous (Aptian) Crato
Formation of Brazil. The snake has a serpentiform body plan with an elongate trunk, short tail,
and large ventral scales suggesting characteristic serpentine locomotion, yet retains small
prehensile limbs. Skull and body proportions as well as reduced neural spines indicate fossorial
adaptation, suggesting that snakes evolved from burrowing rather than marine ancestors.
Hooked teeth, an intramandibular joint, a flexible spine capable of constricting prey, and the
presence of vertebrate remains in the guts indicate that this species preyed on vertebrates and
that snakes made the transition to carnivory early in their history. The structure of the limbs
suggests that they were adapted for grasping, either to seize prey or as claspers during mating.
Together with a diverse fauna of basal snakes from the Cretaceous of South America, Africa, and
India, this snake suggests that crown Serpentes originated in Gondwana.
S
nakes are among the most diverse groups
of tetrapods, with >3000 extant species exploiting
a remarkable range of niches (1).
Snakes inhabit deserts and rainforests, mountains
and oceans; and despite lacking limbs,
employ an extraordinary range of locomotor
styles, including crawling, burrowing, climbing,
swimming, and even gliding (1). All snakes are
predators, but they consume a wide range of prey,
from insects to large mammals (1). This diversity
is made possible by a specialized body plan, including
an elongate body with reduced limbs, a
flexible skull and ribs to swallow large prey (2),
and a specialized forked tongue and vomeronasal
organ to detect chemical gradients (1). The
origins of this body plan remain unclear, however
(1). One scenario holds that it originated in
a marine environment, whereas others argue that
it results from adaptation for a fossorial lifestyle.
New fossils (2–4), including snakes with hindlimbs
(5, 6), have shed light on the lizard-to-snake
transition, but no snake has been reported with
four limbs. The ecology of early snakes is also uncertain.
Although alethinophidians are primarily
carnivorous (1), Typhlopidae and Anomalepididae,
which are basal with respect to Alethinophidia
(7–9), are insectivorous (1). This suggests that
early snakes were insectivores, although adaptations
for carnivory in stem snakes (2) suggest
that carnivory may be primitive (2, 5).
Here we report a fossil snake from the Early
Cretaceous of Gondwana, which sheds light on
these issues. Tetrapodophis amplectus gen. et sp.
nov. (holotype BMMS BK 2-2) is distinguished
from all other snakes by the following combination
of characters: 160 precaudal and 112 caudal
vertebrae, short neural spines, four limbs, metapodials
short, penultimate phalanges hyperelongate
and curved, phalangeal formula 2?-3-3-3-3?
(manus) 2-3-3-3-3 (pes).
The fossil (Fig. 1) comes from the Nova Olinda
Member of the Early Cretaceous (Aptian) Crato
Formation, Ceará, Brazil (10). The specimen is preserved
on laminated limestone as part and counterpart.
The matrix is typical of the Nova Olinda
Member in being composed of fine-grained laminated
micrite with elongated pellets on the surface
of the slab representing coprolites of the fishDastilbe.
As is typical of Crato vertebrates, the skeleton is
articulated and the bones are a translucent orangebrown
color; soft tissues are also preserved.
The snake affinities of Tetrapodophis are demonstrated
by derived features of the skull, axial
416 24 JULY 2015 • VOL 349 ISSUE 6246 sciencemag.org SCIENCE
1
School of Earth and Environmental Sciences, University of
Portsmouth, Portsmouth PO1 3QL, UK. 2
Tannenweg 16,
85134 Stammham, Germany. 3
Department of Biology and
Biochemistry and Milner Centre for Evolution, University of
Bath, Claverton Down, Bath BA2 7AY, UK.
RESEARCH | REPORTS
on February 16, 2016 Downloaded from
skeleton, limbs, integument, and even behavior
(Figs. 2 to 4) (* = snake autapomorphy). Snakelike
features of the skull include a short rostrum,
a long braincase, and a nasal descending lamina.
The mandible is bowed, with a deep subdental
ridge and an intramandibular joint formed by a
concave splenial cotyle contacting the angular, as
in Dinilysia (11). Teeth exhibit the ophidian condition,
being unicuspid and hooked, with expanded
bases. Implantation is subacrodont, with teeth
separated by interdental ridges; replacement
teeth are oriented subhorizontally.* Snake-like
features of the axial skeleton include an elongate
trunk with over 150 vertebrae,* zygosphenezygantrum
articulations, a vaulted neural arch
with posterolateral tuberosities,* short neural
spines, haemal keels, large subcentral fossae/
foramina, tubercular processes of the ribs, and
lymphapophyses. The ilium is long and slender
as in other snakes; the fibula is bowed as in Najash
(5) and Simoliophiidae (12). Transverse belly scales*
are preserved, and the presence of a vertebrate
in the gut suggests a snake-like feeding strategy
in which proportionately large prey are ingested
whole. Although many of these features occur
in other long-bodied squamates, only snakes
exhibit all of them, and many of these characters
are uniquely ophidian.
Tetrapodophis exhibits a number of primitive
characters, however. The nasal is L-shaped, as in
Dinilysia (11) and Simoliophiidae. The maxilla’s
facial process is reduced as compared to those of
lizards but tall relative to those of crown snakes,
as in Coniophis (2). The subdental ridge is shallow
posteriorly, a primitive feature shared with
Najash (5) and Coniophis (2). Unlike crown snakes,
in which a convex splenial condyle articulates
with the angular, the splenial exhibits a concave
cotyle, as in Dinilysia (11). Prezygapophyseal
processes are absent as in other stem snakes;
synapophyses are kidney-shaped, lacking the distinct
condyle and planar cotyle of alethinophidians.
Strikingly, Tetrapodophis retains reduced but apparently
functional forelimbs and hindlimbs.
To test Tetrapodophis’ ophidian affinities, we
used a morphological matrix (13, 14) to conduct
four phylogenetic analyses: with and without molecular
backbone constraint (8) and with equal
and implied weighting (15). In each analysis,
Tetrapodophis emerges as a basal snake (Fig. 5)
but is closer to modern snakes than the putative
Jurassic-Cretaceous stem ophidians Parviraptor,
Diablophis, Portugalophis, and Eophis. When a
molecular backbone is used (Fig. 5), Tetrapodophis
emerges as sister to Coniophis, and snakes emerge
as sister to the Mosasauria; i.e., Pythonomorpha,
as in a recent combined analysis (7).
As the only known four-legged snake, Tetrapodophis
sheds light on the evolution of snakes
from lizards. Tetrapodophis lacks aquatic adaptations
(such as pachyostosis or a long, laterally
compressed tail) and instead exhibits features of
fossorial snakes and lizards: a short rostrum and
elongation of the postorbital skull, a long trunk
and short tail (16, 17), short neural spines (18),
and highly reduced limbs (16, 17). Tetrapodophis
therefore supports the hypothesis that snakes
evolved from burrowing (2, 5, 6) rather than marine
(19) ancestors. Although the current analysis
suggests a sister-group relationship between
Mosasauria and Serpentes, Cretaceous aquatic
snakes (Simoliophiidae) are recovered nested
within crown Serpentes, and aquatic habits are
therefore derived (2, 7).
Tetrapodophis also sheds light on the evolution
of snake feeding. Tetrapodophis exhibits adaptations
for carnivory, including recurved claw-like
teeth to seize large prey and an intramandibular
joint allowing the gape to expand to swallow
large prey. Along with the presence of a vertebrate
in the gut, these feature show that Tetrapodophis
preyed on vertebrates. Similar adaptations occur
in other early snakes (2, 11), suggesting that
snakes made the transition to carnivory early in
their history and that the insectivorous lifestyle
of typhlopids and anomalolepidids is derived.
The structure of the spine may represent another
such adaptation for carnivory. Elongate
bodies and reduced limbs evolved many times
among squamates (13, 17), occurring in burrowing
SCIENCE sciencemag.org 24 JULY 2015 • VOL 349 ISSUE 6246 417
imj sp
sdr ld rd
rm
lm
pm
nas fp par
fr
1 mm sp 1 mm
sdr
ld
rd mec
dt
rep
idr
mt
Fig. 2. T. amplectus, skull and jaws. (A) Skull. (B) Left mandible in medial view. Abbreviations: dt, dentary tooth; fp, facial process of maxilla, fr, frontal; idr,
interdental ridges; imj, intramandibular joint lm, left maxilla, ld, left dentary; mt, maxillary teeth; nas, nasal, par, parietal; pm, premaxilla; rd, right dentary; rd,
right dentary; rep, replacement teeth; sdr, subdental ridge; sp, splenial.
10 mm
Fig. 1. T. amplectus, holotype part and counterpart. (A) Counterpart, showing skull and skeleton
impression. (B) Main slab, showing skeleton and skull impression.
RESEARCH | REPORTS
and terrestrial forms (17) as well as in aquatic
mosasaurs. Yet snakes are unique among longbodied
squamates in having over 150 precaudal
vertebrae. This permits extreme flexibility of the
spinal column, so that the entire body can coil
into tight loops. The fact that other long-bodied
squamates lack this feature suggests that it is not
related to locomotion. We propose that the increased
number of trunk vertebrae may be an
adaptation allowing the body to be used to constrict
prey. Tetrapodophis exhibits both an increased
number of precaudal vertebrae and a high degree
of flexibility, with the body forming a tight coil
anteriorly and a series of sinuous curves posteriorly,
suggesting that constriction was developed
even in the earliest snakes.
The structure of the limbs may represent another
predatory adaptation. The snake-like spine
and reduced limbs of Tetrapodophis suggest that
the animal engaged in characteristic serpentine
locomotion, with the limbs playing little or no
role in locomotion. However, the specialized structure
of the limbs suggests that they were functional.
Given Tetrapodophis’ presumed fossorial
or semifossorial habits, digging is a plausible
function, but the limbs lack fossorial specializations.
Instead, the manus and pes exhibit slender
isodactyl digits with hyperelongate penultimate
phalanges and abbreviated proximal phalanges.
This suite of characters recalls the prehensile feet
of scansorial birds, sloths (20), and bats, suggesting
a grasping or hooking function. The limbs
may have functioned for grasping prey, or perhaps
mates. Climbing is another possibility, although
the low neural spines seem inconsistent
with this function. Regardless, Tetrapodophis
shows that after the initial evolution of serpentine
locomotion, the limbs were repurposed for
another function.
Finally, Tetrapodophis sheds light on the geographic
origin of snakes. The Serpentes, Iguania,
and Anguimorpha form the Toxicofera (7–9), with
the oldest iguanian and anguimorph fossils coming
from Laurasia (13). These patterns suggest
that the center of toxicoferan diversification is
Laurasia and that the ancestors of snakes probably
originated there. The identification of possible
stem ophidians from the Jurassic and Early Cretaceous
of Laurasia (4) would support this hypothesis.
However, the most basal divergences within
crown Serpentes, including Anomalolepididae
and Typhlopidae, Aniliidae and Tropidophiidae,
418 24 JULY 2015 • VOL 349 ISSUE 6246 sciencemag.org SCIENCE
mt
as
cal
ph
ph
un
I
II
III
IV V
II
III
il
il
tib tibb
pes
pes
fem
fem hu
ra
mann
ul
fib fibf b
tib tib
pes
pes
fem
fem
fib fib
sr
lym
il sr
i 1 mm 1 mm 1 mm
1 mm
1 mm
mc
ph
ph
un
I
II III
IV
V
hu
ra
man
ul
mc
ph
ph
un
I
II III
IV
V
Fig. 4. T. amplectus appendicular morphology. (A) Forelimb. (B) Manus. (C) Hindlimbs and pelvis. (D) Pes. (E) Pelvis. Abbreviations: fem, femur; fib, fibula;
hu, humerus; il, ilium; lym, lymphapophysis; man, manus; mc, metacarpal; mt, metatarsals; ph, phalanges; ra, radius; sr, sacral rib; tib, tibia; ul, ulna; un, ungual.
Fig. 3. T. amplectus
axial column. (A)
Cervicals and anterior
presacrals. (B) Midthorax,
showing ventral
scales. (C) Posterior
thorax, showing gut
contents. Abbreviations:
gc, gut contents;
nsp, neural spines;
poz, postzygapophysis;
prz, prezygapophysis;
syn, synapophyses vb,
vertebrate bone; vs,
ventral scales; zga,
zygantrum; zgs,
zygosphene.
nsp poz
zga zgs
syn
gc
prz
vb
1 mm
1mm
nsp poz
zga zgs
syn
prz
gc
vb
1 mm
vs 1mm
RESEARCH | REPORTS
are endemic to or originate in South America and
Africa, hinting at Gondwanan origins (1). Furthermore,
during the middle Cretaceous, Gondwana was
home to a diverse fauna of basal snakes, including
Coniophiidae, Russellophiidae, Madtsoiidae (21),
and Simoliophiidae (22) in the Cenomanian of
Africa, the Cenomanian-Turonian Najash (18, 23) in
South America, and now Tetrapodophis from the
Aptian of South America. Snakes are far less diverse
in the Cretaceous of Laurasia, with a single lineage
appearing in the Cenomanian (24) of North America;
alethinophidians do not appear until the
Maastrichtian in North America (14) and Europe (25).
These patterns suggest that the Serpentes represent
an endemic Gondwanan radiation that saw limited
dispersal to Laurasia during the Cretaceous.
Snakes appear to have been part of a highly
endemic herpetofauna that evolved in the Cretaceous
in Gondwana. In this fauna, notosuchian
crocodiles (26) and rhynchocephalians (27) played
a major role, whereas squamates appear to have
been less diverse and disparate than in Laurasia.
The exception is the snakes, which radiated to
produce small burrowers, large constrictors,
and aquatic forms (21, 22). Much of this herpetofauna
appears to have become extinct during
the Cretaceous-Paleogene extinction (Notosuchia)
or was greatly reduced in diversity in the Cenozoic
(Rhynchocephalia). Snakes, meanwhile, not
only survived but became diverse and widespread
in the Paleogene (14), perhaps in response to ecological
release provided by the end-Cretaceous
mass extinction (14).
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of Brazil, D. M. Martill, G. Bechly, R. F. Loveridge, Eds.
(Cambridge Univ. Press, Cambridge, 2007), pp. 44–62.
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ACKNOWLEDGMENTS
Thanks to B. A. S. Bhullar, J. A. Gauthier, and J. C. Rage for
discussions and to the anonymous reviewers whose comments
improved this paper. The holotype (BMMS BK 2-2) is housed at
the Bürgermeister-Müller-Museum, Solnhofen, Germany.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/349/6246/416/suppl/DC1
Supplementary Text
Figs. S1 to S7
Table S1
References (27–44)
Character-Taxon Matrix
Constraint Tree
23 February 2015; accepted 16 June 2015
10.1126/science.aaa9208
SCIENCE sciencemag.org 24 JULY 2015 • VOL 349 ISSUE 6246 419
SCOLECOPHIDIA
ANGUIMORPHA
MOSASAURIA
IGUANIA
Coniophis precedens
ANILIIDAE
TROPIDOPHIIDAE
CALABARIIDAE
UNGALIOPHIINAE
ERYCINAE
BOINAE
CYLINDROPHIIDAE
ANOMOCHILIDAE
UROPELTIDAE
XENOPELTIDAE
LOXOCEMIDAE
PYTHONIDAE
XENODERMATIDAE
BOLYERIIDAE
ACROCHORDIDAE
PAREATIDAE
VIPERIDAE
HOMALOPSIDAE
LAMPROPHIIDAE
ELAPIDAE
COLUBRIDAE
ALETHINOPHIDIA
SERPENTES
PYTHONO-
MORPHA
OPHIDIA Tetrapodophis amplectus
Parviraptor estesi
Portugalophis lignites
Diablophis gilmorei
Eophis underwoodi
Dinilysia patagonica
MADTSOIIDAE
Pachyrhachis problematicus
Euopodophis descouensi
Haasiophis terrasanctus
intramandibular joint
constriction
limbs reduced
elongate body
Najash rionegrina
forelimbs lost
80
70
60
50
40
30
20
10
JURASSIC CRETACEOUS PALAEOGENE NEOGENE Oligocene Eocene Paleocene Maastrichtian Campanian Santonian Coniacian Turonian Cenomanian Aptian Albian Barremian Hauterivian Tithonian Berriasian Valanginian Oxfordian
Bajocian
Bathonian
Callovian
Aalenian
Kimmeridgian
Miocene
Pleistocene
Pliocene
170
160
150
140
130
120
110
100
90
K-Pg
Fig. 5. Phylogenetic position of T. amplectus.
A strict consensus of 85 most parsimonious
trees found using implied weights and molecular
constraint is shown (see the supplementary
materials) for a matrix of 632 characters
and 205 taxa.
RESEARCH | REPORTS
DOI: 10.1126/science.aaa9208
Science 349, 416 (2015);
David M. Martill et al.
A four-legged snake from the Early Cretaceous of Gondwana
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374 24 JULY 2015 • VOL 349 ISSUE 6246 sciencemag.org SCIENCE
Silence or destruction by
microRNA p. 380
Conducting fibers that don’t INSIGHTS fade in the stretch p. 382 ?
Four legs
too many?
PALEONTOLOGY
PERSPECTIVES
A long-bodied fossil snake
retains fore- and hindlimbs
A classic Gary Larson cartoon shows a
robed and bearded figure rolling out
clay strips, with the caption: “God
makes the snake.” Body elongation
was certainly fundamental in the evolution
of snakes from lizards, as was
the shrinking and ultimately the loss of limb
pairs (limb reduction). However, informative
early fossils are rare, and many details of the
transition remain unresolved. A remarkable
fossil described on page 416 of this issue by
Martill et al. ( 1) brings fresh perspective to
the debate. The aptly named Tetrapodophis
combines a snakelike body with fore- and
hindlimbs bearing five well-developed digits
(see the illustration).
Snakelike bodies evolved several times
through geological history. Among amniotes
(reptiles, birds, and mammals), they occur
only in Squamata, the group comprising lizards
and snakes. Within Squamata, however,
this body form has arisen independently
at least 26 times ( 2) (see the figure). Body
elongation is always correlated with limb
reduction ( 2), and the forelimbs are usually
lost first (Bipes and Bachia are rare exceptions).
One explanation is that as the body
lengthens, coordination of limb movements
becomes increasingly difficult. Moreover, a
serpentine body moves most effectively by
lateral undulation, a movement in which
limbs can become a hindrance, especially
in narrow spaces. Researchers have identified
a threshold body length at which limb
A four-limbed snake from the Cretaceous. Artist’s impression of Tetrapodophis, which retains four limbs, each with
five digits, in an elongated body with 160 precaudal vertebrae.
By By Susan Evans
ILLUSTRATION: JULIUS CSOTONYI
Department of Cell and Developmental Biology, University
College London, London, UK. E-mail: s.e.evans@ucl.ac.uk
Published by AAAS
on February 16, 2016 Downloaded from
SCIENCE sciencemag.org 24 JULY 2015 • VOL 349 ISSUE 6246 375
reduction begins, and no
known squamate with more
than 70 precaudal (before-tail)
vertebrae retains four complete
limbs ( 2). Tetrapodophis
( 1), with around 160 precaudals,
is therefore exceptional.
Efforts to reconstruct the
evolutionary stages in the
snake body plan are hampered
by a lack of consensus
on snake relationships and ancestral
lifestyle. Analyses using
molecular data group snakes
with terrestrial lizards like
iguanas and Komodo dragons
(Iguania and Anguimorpha)
( 3) and generally hypothesize
a burrowing or semiburrowing
ancestry ( 1– 3). However,
some analyses that include
anatomical characters place
them with extinct Cretaceous
(~100 to 66 million years ago)
marine lizards, the mosasaurs
( 3, 4). This has prompted the
suggestion of a marine swimming
ancestry for snakes ( 4).
Molecular divergence estimates
date snake origins
to the Jurassic (~150 million
years ago) ( 5), but the earliest
uncontested fossils are isolated
vertebrae from the midCretaceous
(~113 million years ago) of North
America ( 5). These vertebrae come from
terrestrial deposits but are otherwise fairly
uninformative. More instructive are several
articulated skeletons or partial skeletons
from slightly younger (~100 million-year-old)
deposits. The largest set consists of several
related marine snakes from the Middle East,
North Africa, and southern Europe. These
fossil snakes have 140 to 155 precaudal vertebrae
and a short tail. They show no trace
of forelimbs or shoulder girdle but do have
small hindlimbs; only one [Haasiophis ( 6)]
preserves digits. The relationships of these
limbed marine snakes remain controversial,
but many analyses ( 1, 3, 6, 7) nest them
among modern snakes, rather than nearer
the base of the snake evolutionary tree. This
implies either that hindlimbs were reduced
more than once within snakes, or that the
limbs redeveloped in some lineages (6).
A second set of early fossil snakes comes
from terrestrial deposits in South America.
The most complete, 95-million-year-old Najash
( 7), resembles the fossil marine snakes
in having small hind legs without preserved
digits but is more primitive ( 1, 7). Tetrapodophis
is also from South America, and from a
deposit that yields a mix of freshwater and
terrestrial species, but it is older (~113 million
years old). Martill et al. ( 1) place it on
the stem of the snake evolutionary tree, below
Najash and close to another early terrestrial
snake, the North American Coniophis,
represented by vertebrae and attributed jaw
elements.
Whereas fossils can yield information on
the sequence of anatomical changes involved
in any major transition, developmental biology
helps to explain how these changes occurred.
Evolution of the snake body form
combined axial elongation, limb loss, and
reduced regionalization ( 8, 9). Whether and
how these components are linked developmentally
remains uncertain. In all vertebrate
embryos, individual vertebrae develop from
segments (somites) that form at regular
intervals. To increase vertebral numbers,
somite formation must either continue for
longer or occur at a faster rate. Snakes use
both strategies ( 8). Individual vertebrae then
acquire positional identity along the body
axis through the overlapping expression domains
of Hox genes. In a typical tetrapod, the
boundaries between major vertebral regions
(such as the neck and the trunk) coincide
with Hox gene expression boundaries.
In a pioneering study of Python development,
Cohn and Tickle ( 10) reported a
marked expansion of the typical Hox expression
domains, particularly
those normally associated
with the neck-trunk boundary.
They argued that the neck
had been lost in snakes and
that this loss disrupted the
molecular signals required
for forelimb positioning and
outgrowth. However, in another
snake, Pantherophis,
the Hox expression domains,
although expanded and without
sharp boundaries, retain
a regionalized pattern comparable
to that of lizards with a
distinct neck ( 9, 11). A parallel
study of vertebral anatomy
across a wide range of snakes
( 12) revealed a similar regionalized
pattern, implying that
snakes have a neck of 10 to 12
segments.
Like that of a lizard, the
vertebral column of Tetrapodophis
has distinct regions,
including 10 to 11 short-ribbed
neck vertebrae adjacent to
the tiny forelimbs. This neck
length is within the range of
some generalized terrestrial
lizards and matches that proposed
by the developmental
( 9, 11) and anatomical ( 12)
studies. Thus, as in long-bodied
lizards, elongation of the snake skeleton
occurred in the trunk region and not the
neck. Moreover, if Tetrapodophis is correctly
interpreted as a stem-snake, that elongation
preceded loss of the forelimbs.
Love them or loathe them, snakes have
long fascinated humans. The combined efforts
of paleontology and developmental
biology have gone some way toward unraveling
the early history of snakes, but many
questions remain as to their origins, relationships,
character evolution, and ancestral
lifestyle. Resolution of these questions
depends, ultimately, on the recovery of further
fossils and their thorough and objective
analysis. ¦
REFERENCES
1. D. M. Martill et al., Science 349, 416 (2015).
2. M. C. Brandley, J. P. Huelsenbeck, J. J. Wiens, Evolution 62,
2042 (2008).
3. T. W. Reeder et al., PLOS ONE 10, e0118199 (2015).
4. M. W. Caldwell, Zool. J. Linn. Soc.125, 115 (1999).
5. J. J. Head, Palaeontol. Electronica 18, 1 (2015).
6. E. Tchernov, O. Rieppel, H. Zaher, M. J. Polcyn, L. L. Jacobs,
Science 287, 2010 (2000).
7. S. Apesteguía, H. Zaher, Nature 440, 1037 (2006).
8. C. Gomez et al., Nature 454, 335 (2008).
9. J. M. Woltering et al., Dev. Biol. 332, 82 (2009).
10. M. J. Cohn, C. Tickle, Nature 399, 474 (1999).
11. N. Di-Poï et al., Nature 464, 99 (2010).
12. J. J. Head, P. D. Polly, Nature 520, 86 (2015).
PHOTO: (A) D. M. MARTILL/UNIVERSITY OF PORTSMOUTH; ILLUSTRATION: (B) P. HUEY/SCIENCE
Limbs or no limbs. (A) Martill et al. report the discovery of a four-limbed snake,
Tetrapodophis amplectis, from the Cretaceous. (B) Schematic showing independent
development of the long bodied, limb-reduced body plan among squamates (not to scale).
Lerista
Acontias
Lialis
Dibamus Pantherophis
Hassiophis
Ophisaurus
Bachia
10 mm
Bipes
Amphisbaena
Amphisbaenia
Scincidae
Gekkota
Dibamidae
Tetrapodophis
Serpentes
Anguidae
Gymnophthalmidae
B
A
10.1126/science.aac5672
Published byAAAS
DOI: 10.1126/science.aac5672
Science 349, 374 (2015);
Susan Evans
Four legs too many?
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