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SYNOPSIS Ornithischia is a familiar and diverse clade of dinosaurs whose global phylogeny has remained largely unaltered since early cladistic analyses in the mid 1980s. Current understanding of ornithischian evolution is hampered by a paucity of explicitly numerical phylogenetic analyses that consider the entire clade. As a result, it is difficult to assess the robustness of current phylogenetic hypotheses for Ornithischia and the effect that the addition of new taxa or characters is likely to have on the overall topology of the clade. The new phylogenetic analysis presented here incorporates a range of new basal taxa and charac-ters in an attempt to rigorously test global ornithischian phylogeny. Parsimony analysis is carried out with 46 taxa and 221 characters. Although the strict component consensus tree shows poor resolution in a number of areas, application of reduced consensus methods provides a well-resolved picture of ornithischian interrelationships. Surprisingly, Heterodontosauridae is placed as the most basal group of all well-known ornithischians, phylogenetically distant from a stem-defined Ornithopoda, creating a topology that is more congruent with the known ornithischian stratigraphical record. There is no evidence for a monophyletic 'Fabrosauridae', and Lesothosaurus (the best-known 'fabrosaur') occupies an unusual position as the most basal member of Thyreophora. Other relationships within Thyreophora remain largely stable. The primitive thyreophoran Scelidosaurus is the sister taxon of Eurypoda (stegosaurs and ankylosaurs), rather than a basal ankylosaur as implied by some previous studies. The taxonomic content of Ornithopoda differs significantly from previous analyses and basal relationships within the clade are weakly supported, requiring further investigation. 'Hypsilopho-dontidae' is paraphyletic, with some taxa (Agilisaurus, Hexinlusaurus, Othnielia) placed outside of Ornithopoda as non-cerapodans. Ceratopsia and Pachycephalosauria are monophyletic and are united as Marginocephalia; however, the stability of these clades is reduced by a number of poorly preserved basal taxa. This analysis reaffirms much of the currently accepted ornithischian topology. Nevertheless, in-stability in the position and content of several clades (notably Heterodontosauridae and Ornithopoda) indicates that considerable future work on ornithischian phylogeny is required and causes problems for several current phylogenetic definitions.
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Journal of Systematic Palaeontology 6(1): 1–40 Issued 22 February 2008
doi:10.1017/S1477201907002271 Printed in the United Kingdom C
The Natural History Museum
The phylogeny of the ornithischian
dinosaurs
RichardJ.Butler
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK &
Department of Palaeontology, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK
Paul Upchurch
Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK
David B. Norman
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK
SYNOPSIS Ornithischia is a familiar and diverse clade of dinosaurs whose global phylogeny has
remained largely unaltered since early cladistic analyses in the mid 1980s. Current understanding of
ornithischian evolution is hampered by a paucity of explicitly numerical phylogenetic analyses that
consider the entire clade. As a result, it is difficult to assess the robustness of current phylogenetic
hypotheses for Ornithischia and the effect that the addition of new taxa or characters is likely to have
on the overall topology of the clade.
The new phylogenetic analysis presented here incorporates a range of new basal taxa and charac-
ters in an attempt to rigorously test global ornithischian phylogeny. Parsimony analysis is carried out
with 46 taxa and 221 characters. Although the strict component consensus tree shows poor resolution
in a number of areas, application of reduced consensus methods provides a well-resolved picture
of ornithischian interrelationships. Surprisingly, Heterodontosauridae is placed as the most basal
group of all well-known ornithischians, phylogenetically distant from a stem-defined Ornithopoda,
creating a topology that is more congruent with the known ornithischian stratigraphical record. There
is no evidence for a monophyletic ‘Fabrosauridae’, and Lesothosaurus (the best-known ‘fabrosaur’)
occupies an unusual position as the most basal member of Thyreophora. Other relationships within
Thyreophora remain largely stable. The primitive thyreophoran Scelidosaurus is the sister taxon of
Eurypoda (stegosaurs and ankylosaurs), rather than a basal ankylosaur as implied by some previous
studies.
The taxonomic content of Ornithopoda differs significantly from previous analyses and basal
relationships within the clade are weakly supported, requiring further investigation. ‘Hypsilopho-
dontidae’ is paraphyletic, with some taxa (Agilisaurus,Hexinlusaurus,Othnielia) placed outside
of Ornithopoda as non-cerapodans. Ceratopsia and Pachycephalosauria are monophyletic and are
united as Marginocephalia; however, the stability of these clades is reduced by a number of poorly
preserved basal taxa.
This analysis reaffirms much of the currently accepted ornithischian topology. Nevertheless, in-
stability in the position and content of several clades (notably Heterodontosauridae and Ornithopoda)
indicates that considerable future work on ornithischian phylogeny is required and causes problems
for several current phylogenetic definitions.
KEY WORDS vertebrate palaeontology, Ornithischia, systematics, cladistics, Dinosauria
E-mail: R.Butler@nhm.ac.uk; Email: P.Upchurch@ucl.ac.uk; Email: dn102@esc.cam.ac.uk
Contents
Introduction 1
Institutional abbreviations 2
Previous analyses of ornithischian phylogeny 2
Traditional classifications 2
Cladistic studies 3
Material and Methods 5
The aim of the analysis 5
Phylogenetic framework 5
Selection of ingroup taxa 5
2R. J. Butler et al.
Supraspecific taxa 5
Included species level taxa 7
Excluded species level taxa 11
Selection of outgroup taxa 11
Analyses 12
Search methods 12
Testing the support for relationships 12
Randomisation tests 12
Bremer support 12
Bootstrapping 14
Results 16
Ornithischian monophyly 16
Pisanosaurus mertii 17
Heterodontosauridae 17
‘Fabrosaurids’ 18
Thyreophora 19
Basal neornithischians 19
Ornithopoda 20
Hypsilophodontidae 20
Marginocephalia 21
Ceratopsia 21
Pachycephalosauria 22
Conclusions 22
Stability, instability, and future directions in ornithischian phylogeny 22
Implications for phylogenetic taxonomy of Ornithischia 22
Acknowledgements 23
References 23
Appendix 1 – Specimens and references used for coding operational taxonomic units 29
Appendix 2 – Character list 30
Appendix 3 – Data matrix 35
Appendix 4 – Tree descriptions 40
Introduction
The clade Ornithischia represents a major grouping within
Dinosauria, the most familiar and widely popularised of
all extinct organisms. An ornithischian, Iguanodon Mantell,
1825, was the second dinosaur genus to be named, while
of the three taxa explicitly included within Dinosauria by
Owen (1842), two (Iguanodon and Hylaeosaurus) were later
recognised as ornithischians. Since these early discoveries, a
large number of genera and species of ornithischians have
been named: a recent review (Weishampel et al. 2004a)
recognised over 180 valid genera. The earliest ornithischi-
ans are known from the Carnian stage of the Late Trias-
sic (Casamiquela 1967) and the clade disappeared in the
mass extinctions at the end of the Cretaceous. During this
time ornithischians achieved a global distribution and are
now known from every continent, including Antarctica (e.g.
Hooker et al. 1991; Weishampel et al. 2004b).
Ornithischians appear to have been extremely scarce
during the Late Triassic (Sereno 1997) and remained un-
common (although apparently more diverse) during the Early
and Middle Jurassic; during this time interval terrestrial ver-
tebrate faunas are dominated by saurischians (Weishampel
et al. 2004b). Ornithischians became much more abundant
during the Late Jurassic and Early Cretaceous and ornith-
ischian diversity peaked during the Campanian stage of the
Late Cretaceous. Late Triassic–Early Jurassic ornithischians
were generally small-bodied (1–2 m in length) bipedal curs-
ors (e.g. Lesothosaurus diagnosticus: Thulborn 1972; Sereno
1991a), but during the rest of the Mesozoic they diversified
into a considerable range of morphologies and sizes, with
many groups reverting to quadrupedality. The vast majority
of ornithischians are believed to have been herbivorous, al-
though some basal forms have been interpreted as potentially
omnivorous (Barrett 2000).
One major problem in understanding ornithischian
evolution is that, to date, there are few published numer-
ical phylogenetic analyses dedicated solely to Ornithischia
(Weishampel 2004). It is difficult, therefore, to assess the re-
lative phylogenetic support for ornithischian clades, to look
at the effects upon phylogenetic results of adding or deleting
taxa or characters, or to test alternative hypotheses of taxon
or character evolution. Here, we present a new phylogen-
etic analysis of Ornithischia, as part of an on-going study of
ornithischian phylogeny.
Phylogeny of ornithischian dinosaurs 3
Institutional abbreviations
BMNH =Natural History Museum, London, UK
BRSMG =Bristol City Museum and Art Gallery, Bris-
tol, UK
BP =Bernard Price Institute for Palaeontological
Research, Johannesburg, South Africa
BYU =Earth Science Museum, Brigham Young
University, Provo, Utah, USA
CAMSM =Sedgwick Museum, University of Cam-
bridge, Cambridge, UK
CEUM =Prehistoric Museum, College of Eastern
Utah, Price, Utah, USA
CV =Chongqing Natural History Museum,
Chongqing, People’s Republic of China
GCC =Museum of the Chendgu University of
Technology (formerly Chendgu College of
Geology), Chengdu, People’s Republic of
China
GI =Geological Institute, Ulaanbaatar, Mongo-
lia
GSM =Geological Survey Museum, Keyworth,
UK
GZG =Geowissenschaftliches Zentrum der Uni-
versit ¨
at G¨
ottingen, G¨
ottingen, Germany
IGCAGS =Institute of Geology, Chinese Academy
of Sciences, Beijing, People’s Republic of
China
IVPP =Institute of Vertebrate Paleontology and
Paleoanthropology, Bejing
MCZ =Museum of Comparative Zoology, Harvard,
USA
MB =Museum f¨
ur Naturkunde, Berlin, Germany
MCF-PVPH =Museo Carmen Funes, Paleontolog´
ıa de
Vertebrados Plaza Huincul, Argentina
MNA =Museum of Northern Arizona, Flagstaff,
USA
MOR =Museum of the Rockies, Bozeman,
Montana, USA
MPM =Museo Padre Molina, Rio Gallegos, Santa
Cruz, Argentina
MWC =Museum of Western Colorado, Grand Junc-
tion, Colorado, USA
OUM =University Museum of Natural History, Ox-
ford, UK
PVL =Fundaci´
on Miguel Lillo, Universidad
Nacional de Tucum´
an, San Miguel de Tu-
cum´
an, Argentina
ROM =Royal Ontario Museum, Toronto, Ontario,
Canada
SAM-PK =Iziko South African Museum, Cape Town,
South Africa
SDSM =South Dakota School of Mines, Rapid City,
South Dakota, USA
SGWG =Sektion Geologische Wissenschaften Gre-
ifswald, Ernst-Moritz Universit¨
at, Greif-
swald, Germany
UC =University of Chicago, Chicago, USA
UCMP =University of California Museum of Pale-
ontology, Berkeley, USA
YPM =Peabody Museum of Natural History,
Yale University, New Haven, Connecticut,
USA
ZDM =Zigong Dinosaur Museum, Dashanpu,
People’s Republic of China
ZPAL =Institute of Paleobiology of the Polish
Academy of Sciences, Warsaw, Poland.
Previous analyses of
ornithischian phylogeny
Traditional classifications
Although material from the clade began to be described and
named in the early 19th century (Mantell 1822, 1825, 1833),
the recognition that ornithischians formed a grouping dis-
tinct from other reptiles (including other dinosaurs) was not
reached until the work of Seeley (1887). Seeley was the first
to identify and articulate a fundamental morphological dicho-
tomy within the then described dinosaurian taxa. He recog-
nised two orders, distinguished mainly on the basis of pelvic
anatomy, that he named Ornithischia and Saurischia. Major
subdivisions of Seeley’s Ornithischia (Stegosauria, Ornitho-
poda, Ceratopsia) were identified by Marsh (1877a, 1881,
1890) and Marsh (1894) later grouped these subdivisions
together as the order Predentata, generally considered syn-
onymous with Ornithischia.
Nopcsa (1915) proposed a subdivision within Ornithis-
chia, between the bipedal, unarmoured forms (ornithopods)
and a new suborder that he named Thyreophora (compris-
ing ankylosaurs, stegosaurs and ceratopsians). The group-
ing Thyreophora was often ignored by later workers, but
the name was revived by Norman (1984) and Sereno (1984,
1986), although its current usage (for a clade consisting of
all armoured ornithischians) differs somewhat from that pro-
posed by Nopcsa (i.e. in current usage ceratopsians are not
members of Thyreophora).
Romer (1956) divided Ornithischia into four suborders:
Ornithopoda (including all bipedal forms), Stegosauria, An-
kylosauria and Ceratopsia. Most bipedal and relatively ‘un-
specialised’ taxa were included within Ornithopoda; other
suborders were believed to be derived from within the or-
nithopods, although only the ceratopsians were explicitly
linked with a particular group of ornithopods (the psit-
tacosaurids).
Thulborn (1971) suggested that most of the major
groups of ornithischians (e.g. iguanodontids, pachycephalo-
saurs, ceratopsians) were descended from a Late Triassic–
Late Cretaceous ‘hypsilophodont plexus’, an implicitly para-
phyletic grade of small, primitive, bipedal ornithopods. An-
kylosaurs and stegosaurs were considered to occupy a prim-
itive position outside of this plexus, sharing ancestors with
the earliest hypsilophodontids. The details of this classifica-
tion were questioned by Galton (1972), who removed Ech-
inodon becklesii and Fabrosaurus australis from the family
Hypsilophodontidae to form the family Fabrosauridae (see
also Galton 1978), which included those taxa he considered
to represent the most basal known ornithischians. Marya´
nska
&Osm
´
olska (1974) emphasised the morphological distinct-
ness of the pachycephalosaurs, previously included as the
family Pachycephalosauridae within Ornithopoda, and cre-
ated a new suborder of Ornithischia, Pachycephalosauria.
In summary, pre-cladistic ornithischian classifications
tended to recognise either four (Romer 1956) or five
4R. J. Butler et al.
(Marya´
nska & Osm´
olska 1974) suborders within Ornithis-
chia. One of these suborders (Ornithopoda) was an implicitly
paraphyletic grouping of taxa, defined on the basis of ple-
siomorphic characters (e.g. bipedality). Only a few workers
(e.g. Thulborn 1971; Galton 1972) attempted to identify the
pattern of interrelationships between or within these clades.
Cladistic studies
The first cladistic studies of ornithischian phylogeny were
published simultaneously by Norman (1984), Milner &
Norman (1984) and Sereno (1984); the results of Norman
(1984) and Sereno (1984) are shown in Fig. 1A, B. Norman
(1984) proposed that Ornithischia could be divided into two
major groupings: Thyreophora, comprising the ankylosaurs
and stegosaurs, and Ornithopoda, which Norman expanded to
include ceratopsians. Norman positioned fabrosaurs as basal
ornithopods, suggested that ceratopsians and iguanodontians
(referred to as ‘dryosauroideans’ in his cladogram) shared
a common ancestor to the exclusion of hypsilophodontids,
but considered the position of heterodontosaurids and pachy-
cephalosaurs to be problematic and unresolved. The phylo-
geny of Milner & Norman (1984) concentrated on relation-
ships within Ornithopoda and effectively represented a subset
of the analysis of Norman (1984).
The phylogeny presented by Sereno (1984) differed
significantly from that of Norman (1984). Sereno proposed
that Fabrosauridae was polyphyletic and positioned Lesotho-
saurus diagnosticus (previously included within Fabrosaur-
idae) as the most basal known ornithischian. His conception
of Ornithopoda was much more restrictive than that of previ-
ous workers and excluded ceratopsians, pachycephalosaurs
and ‘fabrosaurs’, while including heterodontosaurids. His
conception of Thyreophora also differed substantially from
that of Norman (1984), comprising a clade consisting of an-
kylosaurs, stegosaurs, pachycephalosaurs and ceratopsians.
The phylogeny of Cooper (1985) was similar in many
aspects (Fig. 1C) to that of Sereno (1984). However, Cooper
positioned Heterodontosauridae as the sister group to the
Pachycephalosauria–Ceratopsia clade and considered Fab-
rosauridae to represent the most basal clade within Ornitho-
poda.
Marya´
nska & Osm´
olska (1985) outlined a phylogeny
(Fig. 1D) that differed in several key points from that of
Norman (1984) and Sereno (1984). Marya´
nska & Osm´
olska
proposed that ankylosaurs and stegosaurs did not share a
common ancestor but, instead, represented serial outgroups
to more derived ornithischians, and that heterodontosaurids
formed the sister group to a clade consisting of Ornitho-
poda (including Lesothosaurus diagnosticus), Pachycephalo-
sauria and Ceratopsia. Following Sereno (1984), Marya´
nska
&Osm
´
olska united Pachycephalosauria and Ceratopsia to
form a clade to the exclusion of ornithopods.
The most influential published work on ornithischian
phylogeny was produced by Sereno (1986) and his res-
ults (Fig. 1E) have dominated subsequent understanding
of ornithischian phylogeny (see, e.g. Weishampel et al.
1990, 1992, 2004a; Fastovsky & Weishampel 1996, 2005;
Currie & Padian 1997; Sereno 1997, 1999a). Sereno (1986)
modified his earlier (Sereno 1984) hypothesis by uniting or-
nithopods, pachycephalosaurs and ceratopsians (following
Norman (1984) and Marya´
nska & Osm´
olska (1985)) in a
clade that he termed Cerapoda. Within Cerapoda, hetero-
dontosaurids were positioned as basal ornithopods, while
the clade containing Pachycephalosauria and Ceratopsia was
named Marginocephalia. Sereno followed Norman (1984) in
restricting Thyreophora to ankylosaurs, stegosaurs and two
basal armoured forms (Scutellosaurus lawleri,Scelidosaurus
harrisonii), while thyreophorans and cerapodans were united
as the clade Genasauria. Sereno continued to consider Fab-
rosauridae polyphyletic and positioned Lesothosaurus dia-
gnosticus as the sister taxon of Genasauria.
Following the work of Sereno (1986), ornithischian
workers tended to focus on relationships within the major
ornithischian clades; e.g. the phylogeny of basal Ornitho-
poda has been analysed by Weishampel & Heinrich (1992),
Winkler et al. (1997), Scheetz (1998, 1999), Weishampel
et al. (2003) and Norman et al. (2004c), amongst others.
However, there have been only a few attempts to test the
global phylogeny of Ornithischia.
Sereno (1997: figs 1, 2) presented an ornithischian
cladogram, but this was not supported by a data matrix or in-
formation about analyses. In a review paper, Sereno (1999a)
considered ornithischians within a larger scale analysis of di-
nosaurian phylogeny. This included nine separate data sets,
each of which analysed separate portions of the dinosaur-
ian tree. Four of these data sets (data sets 1–4) dealt with
ornithischians. Data set 1 analysed basal dinosaurian phylo-
geny and, within this framework, tested both ornithischian
monophyly and interrelationships. Data sets 2–4 analysed the
within-clade phylogeny of Thyreophora, Ornithopoda and
Marginocephalia. Results generally supported the findings of
Sereno (1986) and differed only in the inclusion of additional
taxa (e.g. the basal thyreophoran Emausaurus ernsti) and the
identification of the Late Triassic taxon Pisanosaurus mertii
as the most basal ornithischian. Monophyly of Thyreophora,
Ornithopoda and Marginocephalia (and a taxonomic content
for these clades consistent with the phylogenetic results of
Sereno (1986)), was assumed prior to analysis.
Buchholz (2002) carried out an ornithischian analysis
with 27 taxa and 97 characters. Taxonomic sampling was re-
stricted mostly to taxa generally considered as ornithopods,
with Marginocephalia coded as a composite taxon and basal
thyreophorans excluded from the analysis. Buchholz found
support for a sister-grouping of heterodontosaurids and mar-
ginocephalians and for paraphyly of hypsilophodontids. Un-
fortunately, although a character-list was published, a matrix
was not provided and these results cannot be reassessed.
Liu (2004) tested the global phylogeny of Ornithischia
with a large-scale analysis including 44 taxa and 326 char-
acters. Interesting results were reported: Fabrosauridae and
Hypsilophodontidae were found to be paraphyletic, Leso-
thosaurus was positioned as a basal member of Thyreo-
phora, Agilisaurus louderbacki (often considered to repres-
ent a basal ornithopod, e.g. Norman et al. 2004c) grouped
outside of Cerapoda and Marginocephalia and Iguanodontia
were united as sister groups. Unfortunately, this analysis has,
to date, been published in abstract form only. It is, therefore,
impossible, at this stage, to reanalyse the data. However, the
reported results of Liu (2004) clearly highlight the import-
ance of new phylogenetic analyses.
One of us (Butler 2005) included a cladistic analysis
(23 taxa, 73 characters) of Ornithischia within a review of the
‘fabrosaurid’ ornithischians of the upper Elliot Formation of
southern Africa. That analysis recovered interesting results,
including positioning heterodontosaurids and the Middle
Phylogeny of ornithischian dinosaurs 5
Figure 1 Previous numerical ornithischian phylogenies, simplified and redrawn from the original publications. A, Norman (1984); B, Sereno
(1984); C, Cooper (1985); D, Marya´nska & Osm´olska (1985); E, Sereno (1986 ). H1, H2, alternative positions proposed by Norman (1984) for the
clade Heterodontosauridae; P1, P2, alternative positions proposed by Norman (1984) for the clade Pachycephalosauria.
6R. J. Butler et al.
Jurassic taxa Agilisaurus louderbacki and Hexinlusaurus
multidens outside of Cerapoda; that analysis represented an
early iteration of the analysis presented here and is super-
seded by the present study.
Although a number of major phylogenetic studies of Or-
nithischia have been published (Norman 1984; Sereno 1984,
1986, 1999a; Cooper 1985; Marya´
nska & Osm´
olska 1985),
the majority have failed to include crucial information, in-
cluding: (1) a character–taxon data matrix; (2) details of the
specimens and references used in coding operational taxo-
nomic units; (3) tree searching methods; (4) the number and
scores of the most parsimonious trees recovered by search
methods; (5) tests of data robustness and support for particu-
lar clades (e.g. bootstrapping, decay analysis). Most previous
studies have simply presented a fully-resolved tree and lists
of apomorphies for particular clades. It is neither possible to
independently rerun these analyses and recover their results,
nor is it possible to assess data robustness, the relative sup-
port for clades, or the support for alternative phylogenetic
hypotheses.
With the exception of the preliminary study of Butler
(2005), Sereno (1999a) is the only published ornithischian
analysis that includes a data matrix that can be rerun and
reanalysed. However, there are problems with this analysis.
Only a limited number of ornithischian taxa were included
and, in addition, monophyly of major clades (such as Mar-
ginocephalia) was assumed prior to analysis; unfortunately,
monophyly of clades such as Marginocephalia are questions
that ornithischian analyses still need to resolve.
Material and Methods
The aim of the analysis
The aim of this analysis is to test the global phylogeny of Or-
nithischia, concentrating on the phylogenetic relationships
of basal forms. Questions concerning the monophyly of the
Dinosauria, the phylogenetic relationships of basal ‘dino-
sauriformes’ and basal saurischians, and the relationships of
derived taxa within the major ornithischian clades, are bey-
ond the scope of this analysis. The intention is to test the
phylogenetic framework upon which the current understand-
ing of ornithischian evolution is based.
Phylogenetic framework
All published cladistic analyses have shown Dinosauria to
form a monophyletic clade (e.g. Gauthier 1986; Benton
& Clark 1988; Novas 1996; Sereno 1999a; Benton 2004;
Langer 2004; Langer & Benton 2006). Dinosauria includes
two subclades: Saurischia and Ornithischia (Gauthier 1986;
Sereno 1986). A number of outgroups to Dinosauria have
been identified; the most proximate of which appear to be sev-
eral ‘dinosauriform’ taxa, exemplified by Lagerpeton (Ser-
eno & Arcucci 1993), Marasuchus (Sereno & Arcucci 1994)
and Silesaurus (Dzik 2003; Langer & Benton 2006). Suc-
cessively more distant outgroups to Dinosauria within Archo-
sauria include Pterosauria (although see Bennett 1996; Peters
2000), Scleromochlus, Crurotarsi, Proterochampsidae, Eu-
parkeria,Erythrosuchus and Proterosuchus (Sereno 1991b;
Benton 2004).
Table 1 provides phylogenetic definitions for ornithis-
chian clade names discussed in the text (modified from Ser-
eno 1998, 1999b; Wagner 2004). There is a conflict in the lit-
erature between the names Neornithischia (Cooper 1985) and
Cerapoda (Sereno 1986), which have both been applied to
the clade consisting of ornithopods, pachycephalosaurs and
ceratopsians (e.g. Sereno 1999a; Weishampel 2004). Here,
we follow Buchholz (2002) and Barrett et al. (2005) by us-
ing both names: Neornithischia is applied to a stem-based
clade, while Cerapoda is used for a node-based clade. In ad-
dition, our use of Ornithopoda differs from that of those au-
thors who have defined this taxon as a node-based clade util-
ising Heterodontosaurus as an internal specifier (e.g. Sereno
1998); we instead use Ornithopoda for a stem-based clade
(Buchholz 2002; Wagner 2004; Norman et al. 2004c).
Selection of ingroup taxa
A taxonomic review of Ornithischia was carried out and a
number of supraspecific, species level and outgroup oper-
ational taxonomic units (OTUs) were selected. Weishampel
et al. (1990, 1992, 2004a) served as the source for this review.
Coding of taxa for cladistic analysis was based, where pos-
sible, on first-hand examination of specimens, supplemented
with information from the literature. Appendix 1 provides de-
tails of the references and specimens used for coding OTUs.
The choice of ingroup taxa is discussed in greater detail be-
low and outgroup taxa are discussed in the following section.
Supraspecific taxa
A number of authors (e.g. Wiens 1998; Prendini 2001) have
suggested that supraspecific taxa should be avoided in phylo-
genetic analysis when possible, as the coding of such taxa
is problematic, and simulations tend to suggest that splitting
such taxa into species level terminals provides better results.
Ideally, therefore, any analysis of Ornithischia should util-
ise only species level terminal taxa. Nevertheless, the use of
species level taxa as exemplars for major clades was con-
sidered impractical for this analysis for a number of reasons.
First, the choice of exemplar taxa is not always obvious. For
instance, the clade Ankylosauria is nearly universally accep-
ted as monophyletic and is well-supported by anatomical
evidence. However, there is little consensus as to phylogen-
etic relationships within Ankylosauria (e.g. Kirkland 1998;
Carpenter 2001; Vickaryous et al. 2001, 2004; Parish 2003)
and justifying the use of one taxon, or several taxa, as exem-
plars is difficult. In addition, many of the apparently basal an-
kylosaur taxa that might be used as exemplars are fragment-
ary (e.g. Cedarpelta bilbeyhallorum, Mymoorapelta maysi),
incompletely described in the literature (Gastonia burgei), or
based upon juvenile material (Liaoningosaurus paradoxus).
In such a situation it can be advantageous to code a supra-
specific taxon to represent the clade. The use of supraspecific
taxa has the additional advantage of reducing the number of
OTUs that must be included in the analysis. This is important,
because it allows heuristic searches to be carried out in an
acceptable timeframe and allows much more detailed exam-
ination of the data. For these reasons, selected supraspecific
taxa were included in this analysis.
As outlined by Bininda-Emonds et al. (1998), the cor-
rect use of supraspecific taxa in phylogenetic analyses has
two requirements. Firstly, the taxa must be monophyletic.
Only supraspecific taxa that are generally accepted as
Phylogeny of ornithischian dinosaurs 7
Table 1 Phylogenetic definitions for the major ornithischian clades discussed in this analysis (modified from: Sereno 1998, 1999b; Buchholz
2002; Wagner 2004).
Clade name Phylogenetic definition
Dinosauria Owen, 1842 Triceratops horridus Marsh, 1889, Passer domesticus (Linnaeus, 1758), their most
recent common ancestor and all descendents.
Saurischia Seeley, 1887 All dinosaurs more closely related to Passer domesticus (Linnaeus, 1758) than to
Triceratops horridus Marsh, 1889.
Ornithischia Seeley, 1887 All dinosaurs more closely related to Triceratops horridus Marsh, 1889 than to either
Passer domesticus (Linnaeus, 1758), or Saltasaurus loricatus Bonaparte & Powell,
1980.
Genasauria Sereno, 1986 Ankylosaurus magniventris Brown 1908, Stegosaurus stenops Marsh, 1877a,
Parasaurolophus walkeri Parks, 1922, Triceratops horridus Marsh, 1889,
Pachycephalosaurus wyomingensis (Gilmore, 1931), their most recent common
ancestor and all descendents.
Thyreophora Nopcsa, 1915 All genasaurians more closely related to Ankylosaurus magniventris Brown, 1908
than to Parasaurolophus walkeri Parks, 1922, Triceratops horridus Marsh, 1889, or
Pachycephalosaurus wyomingensis (Gilmore, 1931).
Eurypoda Sereno, 1986 Ankylosaurus magniventris Brown, 1908, Stegosaurus stenops Marsh, 1877a,their
most recent common ancestor and all descendents.
Ankylosauria Osborn, 1923 All ornithischians more closely related to Ankylosaurus magniventris Brown, 1908
than to Stegosaurus stenops Marsh, 1877a.
Stegosauria Marsh, 1877aAll ornithischians more closely related to Stegosaurus stenops Marsh, 1877athan to
Ankylosaurus magniventris Brown, 1908.
Neornithischia Cooper, 1985 All genasaurians more closely related to Parasaurolophus walkeri Parks, 1922, than
to Ankylosaurus magniventris Brown, 1908 or Stegosaurus stenops Marsh, 1877a.
Cerapoda Sereno, 1986 Parasaurolophus walkeri Parks, 1922, Triceratops horridus Marsh, 1889, their most
recent common ancestor and all descendents.
Ornithopoda Marsh, 1881 All genasaurians more closely related to Parasaurolophus walkeri Parks, 1922, than
to Triceratops horridus Marsh, 1889
Marginocephalia Sereno, 1986 Triceratops horridus Marsh, 1889, Pachycephalosaurus wyomingensis (Gilmore,
1931), their most recent common ancestor and all descendents.
Ceratopsia Marsh, 1890 All marginocephalians more closely related to Triceratops horridus Marsh, 1889 than
to Pachycephalosaurus wyomingensis (Gilmore, 1931).
Pachycephalosauria Marya´nska & Osm´olska, 1974 All marginocephalians more closely related to Pachycephalosaurus wyomingensis
(Gilmore, 1931) than to Triceratops horridus Marsh, 1889.
monophyletic were utilised here. Secondly, it must be pos-
sible to code them as a single OTU in a manner that maintains
their position on a cladogram with respect to a solution in-
cluding all species. Several authors (e.g. Bininda-Emonds
et al. 1998; Wiens 1998) suggest that the ‘ancestral’ method,
whereby the character states of a hypothetical ancestor (the
‘groundplan’) are reconstructed on the basis of prior phylo-
genetic analyses, is the most successful method of coding
supraspecific taxa. The ‘ancestral’ method (the methodology
is outlined by Langer & Benton 2006) was used in this ana-
lysis. The eight supraspecific OTUs used are discussed in
greater detail below:
1. Ankylosauria. Ankylosauria is defined as all taxa more
closely related to Ankylosaurus magniventris than to
Stegosaurus stenops (Sereno 1998) and includes the
subclades Ankylosauridae and Nodosauridae. Included
taxa and diagnostic features are listed by Vickaryous
et al. (2004). The known temporal range of the clade is
Callovian to Maastrichtian (Middle Jurassic–Late Creta-
ceous).
The most recent review of Ankylosauria (Vickaryous
et al. 2004) recognised over 40 valid species. The phylo-
geny assumed here for character coding represents a con-
sensus of the following published phylogenies: Kirkland
(1998), Vickaryous et al. (2001), Hill et al. (2003) and
Vickaryous et al. (2004). The phylogenetic analysis of
Carpenter (2001) is not used, because it utilised a com-
partmentalisation technique and does not, therefore, rep-
resent a global phylogeny.
2. Stegosauria. Stegosauria is defined as all taxa more
closely related to Stegosaurus stenops than to Ankylo-
saurus magniventris (Sereno 1998). A full listing of in-
cluded taxa and synapomorphies supporting monophyly
of the clade is given in Galton & Upchurch (2004b). Ste-
gosaurs form a relatively small but well-known and well-
supported clade of ornithischians, known mostly from the
Middle to Late Jurassic, with fragmentary forms known
from the Early Cretaceous.
Sereno & Dong (1992) provided the first phylo-
genetic analysis of stegosaurs, but considered only a
few taxa. They proposed that Huayangosaurus taibaii
represents the most basal member of the clade, with
Dacentrurus armatus positioned as the sister-group
to all more derived stegosaurs. Galton & Upchurch
(2004b) have provided the most complete analysis to
date; the basal positions of Huayangosaurus and Da-
centrurus were confirmed by their analysis, but little
resolution was found amongst more derived stego-
saurs.
8R. J. Butler et al.
3. Rhabdodontidae. Rhabdodontidae is defined as Zalmoxes
robu stus,Rhabdodon priscus, their common ancestor and
all of its descendents (Weishampel et al. 2003) and the
temporal range of the clade extends from the Late San-
tonian to the Maastrichtian (Late Cretaceous). Synapo-
morphies supporting the clade are given by Weishampel
et al. (2003).
4. Dryosauridae. Dryosauridae includes the taxa Dry-
osaurus altus,Dryosaurus lettowvorbecki,Valdosaurus
canaliculatus and Valdosaurus nigeriensis (Norman
2004) and is defined as Dryosaurus altus and all taxa
more closely related to it than to Parasaurolophus walkeri
(Sereno 1998). The clade is known from the Late Jurassic
and Early Cretaceous. Potential synapomorphies of this
clade include: lacrimal inserts into notch in the maxilla;
very wide brevis shelf on the ilium; large, deep pit on the
femoral shaft, at the base of the fourth trochanter; digit I
of the pes is absent or vestigal.
5. Ankylopollexia. Ankylopollexia is defined as Campto-
saurus dispar,Parasaurolophus walkeri, their common
ancestor and all descendents (Sereno 1998). A listing of
included taxa and synapomorphies supporting monophyly
of the clade is given by Norman (2004). The clade ex-
tends from the Kimmeridgian to the Maastrichtian (Late
Jurassic–Late Cretaceous).
Ankylopollexia was erected by Sereno (1986) for or-
nithopods exhibiting derived features of the teeth and
manus, in particular the modification of manual digit I to
accommodate a spine-like pollex. Within this clade are
Camptosauridae and Styracosterna, both of which have
been given stem-based phylogenetic definitions by Sereno
(1998). The monophyly of Ankylopollexia is universally
supported by phylogenetic analysis and its interrelation-
ships are relatively well understood (e.g. Norman 2002,
2004).
6. Pachycephalosauridae. Pachycephalosauridae is defined
as all taxa more closely related to Pachycephalosaurus
wyomingensis than to either Homalocephale calathocer-
cos or Goyocephale lattimorei. Known taxa are restricted
to the Late Cretaceous (Campanian–Maastrichtian) and
synapomorphies are given in Sereno (2000).
A number of explicit, numerical phylogenetic analyses
of Pachycephalosauridae have been carried out in recent
years (Sereno 2000; Williamson & Carr 2002; Sullivan
2003; Marya´
nska et al. 2004).These studies indicate a
basal position for Stegoceras and the existence of a de-
rived clade containing Tylocephale,Prenocephale and
Pachycephalosaurus, amongst others.
7. Psittacosauridae. Psittacosauridae is defined as all taxa
more closely related to Psittacosaurus mongoliensis than
to Triceratops horridus and contains the genera Hong-
shanosaurus and Psittacosaurus. Synapomorphies of the
clade are given by Sereno (2000). The genus Psit-
tacosaurus is probably the most speciose and diverse of
all dinosaur genera, although the exact number of spe-
cies recognised is controversial (Sereno 1990b;You&
Dodson 2004). Relationships within the clade are poorly
understood. Members of Psittacosauridae are known from
the Early Cretaceous.
8. Unnamed taxon (Coronosauria +Leptoceratopsidae).
This unnamed clade is defined as Leptoceratops gracilis,
Protoceratops andrewsi and Triceratops horridus,their
common ancestor and all of its descendents and includes
taxa ranging from the Turonian to the Maastrichtian (Late
Cretaceous). Synapomorphies of this node are given by
Sereno (2000), Makovicky (2001) and Makovicky &
Norell (2006).
Neoceratopsian phylogeny has undergone a number of
rigorous studies in recent years (Sereno 2000; Makovicky
2001; Xu et al. 2002; You & Dodson 2003, 2004;
Chinnery 2004; Makovicky & Norell 2006) and a broad-
scale consensus has been reached that Liaoceratops and
Archaeoceratops represent basal taxa and that other neo-
ceratopsian taxa form a distinct clade. This clade of
more derived neoceratopsians remains unnamed and,
generally, comprises a clade known as Leptoceratop-
sidae and a clade known as Coronosauria, which in turn
comprises Protoceratopsidae and Ceratopsoidea (Sereno
2000; Makovicky 2001; Xu et al. 2002; Chinnery 2004).
Included species level taxa
Following the selection and definition of supraspecific taxa,
the status of all taxa not included in one of these derived
clades was assessed, using first-hand observations and the
literature. For each taxon a decision was made as to whether
it should be included in phylogenetic analysis or not. Included
species level taxa are discussed in this section; excluded spe-
cies level taxa (and the reasons for exclusion) are discussed
in the following section.
Abrictosaurus consors (Thulborn, 1974) is known from
a single partial skull and postcranial skeleton (BMNH
RUB54, holotype; formerly UCL B54) from the upper
Elliot Formation of Lesotho (Early Jurassic: Hettangian–
Sinemurian). Autapomorphies have not previously been
defined for Abrictosaurus and this taxon is provisionally dia-
gnosed by the following combination of characters: arched
diastema between premaxilla and maxilla present; enlarged
caniniform teeth absent from the premaxilla and dentary. The
single specimen of Abrictosaurus was initially described as a
new species of the heterodontosaurid Lycorhinus (Thulborn
1974); however, it can clearly be distinguished from Ly-
corhinus on the basis of dental characters (Hopson 1975).
Hopson (1975) tentatively referred the specimen BMNH
A100 (discussed below) to Abrictosaurus;however,thisre-
ferral has not been supported by subsequent work. Abricto-
saurus and BMNH A100 are included here as separate OTUs.
Agilisaurus louderbacki Peng, 1990 is known from a
complete, articulated skull and postcranial skeleton (ZDM
T6011, holotype) from the Lower Shaximiao Formation of
Sichuan Province, People’s Republic of China (Middle Jur-
assic: ?Bajocian, Chen et al. 1982; ?Bathonian–Callovian,
Dong & Tang 1984). Agilisaurus can be distinguished
by the following autapomorphies: presence of a palpeb-
ral/supraorbital bar that traverses the width of the orbit;
anteriormost dentary teeth conical, resembling premaxillary
teeth; presence of a series of low, anterolaterally directed
ridges on the orbital portion of the frontal; presence of an
excavated area immediately anterior to the antorbital fossa
(modified from Barrett et al. 2005).
Anasibetia saldiviai Coria & Calvo, 2002 is known
from a partial skeleton with skull fragments (MCF-PVPH-
74, holotype) as well as a number of referred speci-
mens (see Coria & Calvo 2002) from the Lisandro Form-
ation of Neuqu´
en Province, Argentina (Late Cretaceous:
Cenomanian). This taxon can be diagnosed on the basis of its
Phylogeny of ornithischian dinosaurs 9
anteroventrally orientated occipital condyle and the presence
of an ilium with preacetabular process longer than 50% of
the total ilium length (modified from Coria & Calvo 2002).
Archaeoceratops oshimai Dong & Azuma, 1997 is
known from a well-preserved skull and postcranial material
(IVPP V11114, holotype; IVPP V11115, paratype) from the
Xinminbao Group of Gansu Province, China (Early Creta-
ceous: Aptian–Albian) and is characterised by the presence of
an excavation on the lateral surface of the ischiadic peduncle
of the ilium, as well as by a unique character combination
(modified from You & Dodson 2003).
The specimen BMNH A100 (formerly UCL A100)
comprises a partial, disarticulated skull from the upper El-
liot Formation of South Africa. Assignment of this speci-
men to Lycorhinus (Thulborn 1970b, 1974; Gow 1990) was
not based on unique characters, but on general similarity. A
number of subsequent authors criticised the referral of this
specimen to Lycorhinus: Galton (1973a: caption to fig. 2)
referred BMNH A100 to Heterodontosaurus sp.; Charig &
Crompton (1974) considered BMNH A100 to be generic-
ally distinct from both Heterodontosaurus and Lycorhinus;
while Hopson (1975) provisionally referred BMNH A100 to
Abrictosaurus.
The taxonomy of the Elliot Formation heterodontosaur-
ids is poorly resolved and requires further work. At present
there is no consensus as to the taxonomic status of BMNH
A100; however, this specimen is known from relatively com-
plete and informative cranial remains and has received a de-
tailed description (Thulborn 1970b) and is thus included in
the phylogenetic analysis here as a separate OTU.
Bugenasaura infernalis Galton, 1995 was erected for a
partial skull and postcranial fragments (SDSM 7210, holo-
type) from the Hell Creek Formation of South Dakota, USA
(Late Cretaceous: Maastrichtian). This taxon is diagnosed by
the following features: no edentulous region at the anterior
end of the premaxilla; very deeply recessed cheek tooth row,
with a massive and deep dentary and a very prominent over-
hanging ridge (with a braided appearance) on the ventral
part of the maxilla; distal end of palpebral obliquely trun-
cated with ridges medially (modified from Galton 1999). A
new skeleton of Bugenasaura is known (MOR 979; R.J.B.
pers. obs. 2004); however, this specimen has not been fully
prepared and is currently undescribed.
Chaoyangsaurus youngi Zhao et al. 1999 is based upon
a partial skull and fragmentary postcranial elements (IG-
CAGS V371, holotype) from the Tuchengzi Formation of
Liaoning Province, China (Middle or Late Jurassic: Middle
Jurassic, Zhao et al. 1999; Tithonian, Weishampel et al.
2004b). Chaoyangsaurus is distinguished by the follow-
ing autapomorphic features: quadratojugal overlaps posterior
side of the quadrate shaft; quadrate slopes strongly anteriorly;
ridge present between the planar lateral and ventral surfaces
of the angular (modified from Zhao et al. 1999).
Echinodon becklesii Owen, 1861bis based upon frag-
mentary cranial material (see Norman & Barrett 2002) from
the Purbeck Formation of England (Early Cretaceous: Berri-
asian). Echinodon can be diagnosed by the presence of one,
or possibly two, caniniform teeth situated at the anterior end
of the maxilla (Norman & Barrett 2002).
Emausaurus ernsti Haubold, 1990 is known from a par-
tial skull and postcranial fragments (SGWG 85, holotype),
from an unnamed unit in Germany (Early Jurassic: Toarcian).
Emausaurus can be diagnosed by the possession of a large,
triangular plate-like palpebral, the robust lateral margin of
which bears a number of low ridges.
Gasparinisaura cincosaltensis Coria & Salgado, 1996
is known from numerous specimens (see Coria & Salgado
1996; Salgado et al. 1997) from the Rio Colorado Forma-
tion of Patagonia, Argentina (Late Cretaceous: Coniacian–
Santonian) and is diagnosed by the following characters:
anteroposteriorly wide ascending process of lacrimal con-
tacts ventral process of postorbital posteriorly; infratemporal
fenestra bordered entirely ventrally by quadratojugal; apex
of arched dorsal margin of infratemporal fenestra positioned
posterior to mandibular articulation; fully fused greater and
lesser trochanters; condylid of femur laterally positioned
(modified from Coria & Salgado 1996).
Goyocephale lattimorei Perle et al. 1982 is known from
a relatively complete skeleton with partial skull (GI SPS
100/1501, holotype), from an unnamed unit, Mongolia (Late
Cretaceous: ?late Santonian or early Campanian). One auta-
pomorphy has been identified: the lateral margin of the skull
is sinuous in dorsal view (Sereno 2000).
Heterodontosaurus tucki Crompton & Charig, 1962 is
known from a nearly complete skull (SAM-PK-K337, holo-
type) from the Clarens Formation (=Cave Sandstone) of
South Africa (Early Jurassic: Sinemurian) and a referred
skull and postcranial skeleton (SAM-PK-K1332, Santa Luca
et al. 1976; Santa Luca 1980) from the upper Elliot Form-
ation of South Africa. A number of features may be auta-
pomorphic for this taxon, although it is possible that some
may prove to be present in other, poorly known, heterodon-
tosaurids, or may eventually prove to be ornithischian ple-
siomorphies. These possible autapomorphies include: dorsal
process of premaxilla does not form contact with nasals; an-
terior, accessory opening present within the antorbital fossa;
squamosal–quadratojugal contact is anteroposteriorly broad;
paroccipital processes are very deep dorsoventrally; paired,
deep recesses on the ventral surface of the basisphenoid;
basisphenoid processes are extremely elongated; cingulum
is completely absent on cheek-teeth; ischium with elongate
flange on lateral margin.
Hexinlusaurus multidens (He & Cai, 1983) is known
from two partial skulls and postcranial skeletons (ZDM
T6001, holotype; ZDM T6002, paratype) from the Lower
Shaximiao Formation of China. Hexinlusaurus can be distin-
guished by the presence of a marked concavity that extends
over the lateral surface of the postorbital (Barrett et al. 2005).
Homalocephale calathocercos Marya´
nska &
Osm´
olska, 1974 is known only from a skull and par-
tial postcranial skeleton (GI SPS 100/1201, holotype) from
the Nemegt Formation of Mongolia (Late Cretaceous: ?late
Campanian or early Maastrichtian). Homalocephale is
diagnosed by the presence of a postacetabular process of the
ilium that is crescent-shaped and ventrally deflected (Sereno
2000).
Hypsilophodon foxii Huxley, 1869 is known from nu-
merous specimens (see Galton 1974a) from the Wessex
Formation of the Isle of Wight, UK (Early Cretaceous: Bar-
remian). Autapomorphies have not been previously defined
for Hypsilophodon, but include the presence of a large fora-
men in the ascending process of the maxilla that communic-
ates medially with the antorbital fossa (Galton 1974a: fig. 3).
Although material of Hypsilophodon has been reported from
continental Europe (Sanz et al. 1983) and North America
(Galton & Jensen 1979), none of this material can be
10 R. J. Butler et al.
confidently referred to this taxon and, at present, Hypsilo-
phodon is only known from the UK.
Jeholosaurus shangyuanensis Xu et al. 2000 is known
from two specimens (IVPP V12529, holotype; IVPP
V12530, referred) from the Yixian Formation of Liaoning
Province, China (Early Cretaceous) and is characterised by
the following combination of characters: six premaxillary
teeth; foramina present on dorsal surface of nasal; large fo-
ramen present in quadratojugal; predentary about 1.5 times
as long as the premaxilla; pedal phalanx 3–4 times longer
than other phalanges of pedal digit 3 (modified from Xu
et al. 2000).
Lesothosaurus diagnosticus Galton, 1978 is known
from a number of nearly complete skulls and disarticulated
postcranial skeletons, while Stormbergia dangershoeki But-
ler, 2005 is known from three partial skeletons. Both taxa are
from the upper Elliot Formation of South Africa and Leso-
tho. A full discussion of the hypodigm and diagnosis of each
taxon can be found in Butler (2005).
Liaoceratops yanzigouensis Xu et al. 2002 is known
from two complete skulls (IVPP V12738, holotype; IVPP
V12633, referred specimen) from the Lower Yixian Forma-
tion of Liaoning Province, China (Early Cretaceous). Liao-
ceratops is characterised by the following features: sutures
between premaxilla, maxilla, nasal and prefrontal intersect-
ing at a common point high on the side of the snout; posses-
sion of several tubercles on the ventral margin of the angular;
a foramen on the posterior face of the quadrate near the artic-
ulation with the quadratojugal; small tubercle on the dorsal
border of the foramen magnum; thick posterior border of the
parietal frill (Xu et al. 2002).
Lycorhinus angustidens Haughton, 1924 is known from
a left dentary (SAM-PK-K3606, holotype) and two provi-
sionally referred specimens (BP/1/4244, left maxilla, holo-
type of Lanasaurus scalpridens Gow, 1975, referred to Ly-
corhinus angustidens by Gow 1990; BP/1/5253, left max-
illa, referred to Lycorhinus angustidens by Gow 1990) from
the upper Elliot Formation of South Africa. There has been
considerable controversy over the validity of Lycorhinus an-
gustidens. Haughton (1924) named Lycorhinus for a partial
left dentary that he believed represented a cynodontid syn-
apsid. Crompton & Charig (1962) reidentified Lycorhinus as
a heterodontosaurid and, later Charig & Crompton (1974)
considered it a nomen dubium. Thulborn (1970b) assigned
the specimen BMNH A100 to Lycorhinus;however,thisas-
signment was not supported by most subsequent authors
(Galton 1973a; Charig & Crompton 1974; Hopson 1975,
1980). Hopson (1975) demonstrated that Lycorhinus could be
distinguished from other heterodontosaurids (Abrictosaurus
and Heterodontosaurus). Finally, Gow (1990) referred the
maxillae BP/1/4244 (holotype of Lanasaurus Gow, 1975)
and BP/1/5253 to Lycorhinus.
The validity of Lycorhinus requires reassessment and
only a preliminary diagnosis is suggested here, based upon
the marked medial curvature of the dentary and maxillary
tooth rows (Gow 1990). As discussed by Hopson (1975), a
unique combination of plesiomorphic and derived characters
is probably also diagnostic for Lycorhinus.
Micropachycephalosaurus hongtuyanensis Dong, 1978
is known from a partial skull and skeleton (IVPP V5542,
holotype) from the Wangshi Formation of Shandong
Province, People’s Republic of China (Late Cretaceous:
Campanian). The holotype of Micropachycephalosaurus is
extremely fragmentary and many elements were erroneously
identified in the original description. A full review and rede-
scription is being prepared (R. J. B. & Q. Zhao, unpublished
results). Although Sereno (2000) has suggested that autapo-
morphic features are absent, the presence of prominent vent-
ral grooves on the most posterior dorsal vertebrae appears
to be autapomorphic for Micropachycephalosaurus and this
taxon is here included in the phylogenetic analysis.
Orodromeus makelai Horner & Weishampel, 1988 is
known from abundant and well-preserved material (see
Scheetz 1999) from the Upper Two Medicine and Judith
River formations of Montana, USA (Late Cretaceous: ?Late
Campanian). Orodromeus is characterised by the following
combination of characters: prominent boss on anterolateral
maxilla; subnarial depression on premaxilla–maxilla bound-
ary; midline depression on nasals; boss on jugal; postorbital
with distinct projection into orbit; dentition plesiomorphic
with ridges absent lingually and labially.
Othnielia rex (Marsh, 1877b) is known from a num-
ber of specimens (YPM 1915, holotype, left femur; referred
specimens (see Galton 1983) include: BYU ESM-163R, ar-
ticulated, near-complete postcranial skeleton described by
Galton & Jensen 1973) from the Morrison Formation of
the USA (Late Jurasssic: Kimmeridgian–Tithonian). YPM
1915 is the holotype of Nanosaurus rex Marsh, 1877b,which
was made the type species of the genus Othnielia by Galton
(1977). This specimen (an isolated femur) lacks obvious auta-
pomorphies, although it may be diagnosable on the basis of a
unique character combination. Referral of specimens to Oth-
nielia follows Galton (1983), pending a review of the validity
of this taxon.
Parksosaurus warreni (Parks, 1926) is known from a
single, relatively complete, skull and skeleton (ROM 804,
holotype) from the Horseshoe Canyon Formation of Alberta,
Canada (Late Cretaceous: Maastrichtian). Parksosaurus is
distinguished by a dorsoventrally broad posterolateral pro-
cess of the premaxilla and a postorbital process of the jugal
that expands posterodorsally.
Pisanosaurus mertii Casamiquela, 1967 is known from
a single partial skeleton (PVL 2577, holotype) from the Is-
chigualasto Formation (Late Triassic: Carnian) of Argentina.
As discussed by Sereno (1991a), this taxon can be diagnosed
by the following characters: anteroposterior depth of distal
end of the tibia is greater than maximum transverse width;
calcaneum is transversely narrow.
Scelidosaurus harrisonii Owen, 1861ais known from
several partial skeletons from the Lower Lias of Dorset, Eng-
land (Early Jurassic: late Sinemurian). Owen (1861a)de-
scribed material which he referred to Scelidosaurus, includ-
ing a femur (GSM 109560), articulated knee-joint (BMNH
39496), ungual phalanx (GSM 10956), a partial juvenile
skeleton (Philpott Museum, Lyme Regis, unnumbered, casts
are catalogued as BMNH R5909) and a near-complete skull
(BMNH R1111). Owen (1863) described the near-complete
postcranial skeleton associated with the skull BMNH R1111,
while Lydekker (1888) later designated the articulated knee-
joint as the lectotype. Newman (1968) recognised that the
material described by Owen (1861a) represents a composite
of theropod (GSM 109560; BMNH 39496; GSM 10956) and
ornithischian (Philpott Museum, juvenile skeleton; BMNH
R1111) material, and Charig & Newman (1992) formally
designated the skull and postcranial skeleton (BMNH R1111)
as a replacement lectotype. The juvenile described by Owen
Phylogeny of ornithischian dinosaurs 11
(1861a) probably represents a second individual of Scelido-
saurus (Galton 1975). Further material has come to light in
recent years, including BMNH R6704 (Rixon 1968; Charig
1972: fig. 6A; considered as a possible new taxon of basal
thyreophoran by Coombs et al. 1990), BRSMG Ce12785
(Barrett 2001) and CAMSM X 39256. Reports of the genus
Scelidosaurus in the Kayenta Formation of Arizona (Padian
1989) and the Lower Lufeng of China (Lucas 1996: see Tat -
isaurus, below) cannot be substantiated at present. No dia-
gnosis based upon synapomorphies has ever been published
for Scelidosaurus and a full diagnosis must await redescrip-
tion of this taxon (D. B. Norman, unpublished results).
Scutellosaurus lawleri Colbert, 1981 is known from
several partial skeletons (MNA P1.175, holotype; MNA
P1.1752, paratype; for referred material see Rosenbaum &
Padian 2000) described by Colbert (1981) and Rosenbaum &
Padian (2000) from the Kayenta Formation of Arizona, USA
(Early Jurassic: Sinemurian–Pliensbachian). Unique features
include: dorsal and ventral margins of the preacetabular pro-
cess of the ilium are drawn out medially into distinct flanges
which converge upon one another anteriorly; elongate tail of
at least 58 caudal vertebrae (R. J. B. & S. C. R. Maidment,
unpublished results).
Stenopelix valdensis Meyer, 1857 is known from a
single partially articulated postcranial skeleton, preserved
as impressions in sandstone blocks, from the Obernkirchen
Sandstein of Germany (Early Cretaceous: Berriasian); latex
casts prepared by Sues & Galton (1982) make detailed exam-
ination of the material possible. The ischium of Stenopelix
has the following autapomorphies: distinct bend at mid-shaft;
broadest at mid-shaft and tapers anteriorly and posteriorly;
blade is transversely arched distally, being ventrally convex
and dorsally concave (modified from Sereno 1987).
Talenkauen santacrucensis Novas et al. 2004 is based
upon a partial skull and postcranial skeleton (MPM-10001,
holotype) from the Pari Aike Formation, Santa Cruz
Province, Argentina (Late Cretaceous: Maastrichtian). This
taxon can be diagnosed by the presence of well-developed
epipophyses on cervical 3 and plate-like uncinate processes
on the rib-cage (Novas et al. 2004).
The genus Tenontosaurus Ostrom, 1970 is character-
ised by the following features: dorsoventrally tall maxilla,
nearly full height of the rostrum; orbit square; 12 cervical
vertebrae; elongate tail (59+ caudals). Two species are re-
cognised and included here: Tenontosaurus tilletti Ostrom,
1970 is known from abundant material (see Forster 1990)
from the Cloverly Formation of Montana, USA (Early Creta-
ceous: Aptian–Albian) and Tenontosaurus dossi Winkler
et al. 1997 is known from two specimens from the Twin
Mountains Formation of Texas, USA (Early Cretaceous).
The species are distinguished by the retention in Tenonto-
saurus dossi of premaxillary teeth and a postpubic process
equal in length to the ischium (Winkler et al. 1997).
Thescelosaurus neglectus Gilmore, 1913 is known
from numerous specimens (see Galton 1997) from the
Lance Formation of Wyoming, the Hell Creek Formation of
Montana, the Scollard Formation of Alberta and the French-
man Formation of Saskatchewan (Late Cretaceous: ?late
Campanian–Maastrichtian). The taxonomic history of Thes-
celosaurus has been summarised by Galton (1995, 1997)
and his taxonomic assignments are followed provisionally
here, with Thescelosaurus edmontonensis being considered
a junior synonym of T. neglectus. It should be noted that
synonymy is based on general similarity, rather than on auta-
pomorphic features (see Galton 1995); future revision of the
Thescelosaurus material may indicate the presence of two,
or more, distinct taxa. One possible autapomorphy is recog-
nised here: a large notch or foramen within the supraoccipital,
dorsal to the foramen magnum (see Galton 1997).
Wannanosaurus yansiensis Hou, 1977 is known from
a partial skull and postcranial skeleton (IVPP V4447, holo-
type) and some referred postcranial elements (IVPP V4447.1,
paratype), from the Xiaoyan Formation of Anhui Province,
China (Late Cretaceous: Campanian). Wannanosaurus is dis-
tinguished by the extreme flexure of the humerus, with prox-
imal and distal ends set at approximately 30to one another
(modified from Sereno 2000).
Yandusaurus hongheensis He, 1979 is known from a
partial skull and postcranial material (GCC V20501, holo-
type) from the Upper Shaximiao Formation of Sichuan,
People’s Republic of China (Late Jurassic: ?Oxfordian,
Weishampel et al. 2004b). All of the anatomical features pre-
viously used to diagnosis Yandusaurus (He & Cai 1984) have
wider distributions amongst basal ornithopods. However, one
autapomorphic feature is apparent in the holotype (R. J. B.,
pers. obs. 2004; Barrett et al. 2005); the mid–posterior cer-
vicals have circular, pit-like depressions developed at the
base of their postzygapophyses.
Zephyrosaurus schaffi Sues, 1980 is known from an in-
complete skull and postcranial fragments (MCZ 4392, holo-
type) from the Cloverly Formation of Montana, USA (Early
Cretaceous: Aptian–Albian) and is characterised by the fol-
lowing unique combination of characters: prominent boss
on anterolateral maxilla; short, massive, triangular palpeb-
ral; boss on jugal; postorbital with distinct projection into or-
bit; dentary/maxillary teeth with numerous subparallel ridges
connecting to marginal denticles.
Excluded species level taxa
A number of taxa have been considered non-diagnosable
nomina dubia by most recent reviews and are here excluded
from analysis. These taxa include ‘Camptosaurusleedsi,
Fabrosaurus australis,Geranosaurus atavus,‘Hypsilopho-
donwielandi,Laosaurus celer,Laosaurus minimus,Lus-
itanosaurus liasicus, Nanosaurus agilis and Sanpasaurus
yaoi.
Many ornithischian or putative ornithischian taxa have
been erected solely or largely on the basis of dental re-
mains, including: Alocodon kuehnei,Crosbysaurus harri-
sae,Drinker nisti,Ferganocephale adenticulatum,Galto-
nia gibbidens,Gongbusaurus shiyii, Krzyzanowskisaurus
hunti, Lucianosaurus wildi, Pekinosaurus olseni, Phyllodon
henkeli,Protecovasaurus lucasi,Revueltosaurus callenderi,
Siluosaurus zhangqiani,‘Stegosaurusmadagascariensis,
Taveirosaurus costai, Tecovasaurus murryi and Trimucrodon
cuneatus. Recent work has demonstrated that at least some
of these taxa pertain to non-ornithischian clades (Parker et al.
2005a; Irmis et al. 2007) and the taxonomic validity of many
of these tooth taxa is additionally questionable (Weishampel
et al. 2004a). Furthermore, tooth taxa add little new anatom-
ical information to the analysis and suffer from extremely
high (more than 95%) levels of missing data. For these reas-
ons these taxa have been excluded.
Technosaurus smalli, from the Cooper Canyon Form-
ation (Late Triassic: Norian) of Texas, was described as a
‘fabrosaurid’ ornithischian by Chatterjee (1984) on the basis
12 R. J. Butler et al.
of a single fragmentary skeleton. Sereno (1991a) sugges-
ted that the holotype of Technosaurus contains elements of
both an ornithischian and a sauropomorph. Irmis et al. (2005,
2007) agreed that the specimen is a composite of at least two
taxa, but suggested that the posterior portion of the lower jaw
is referable to the pseudosuchian Shuvosaurus, whereas other
parts of the holotype may represent a Silesaurus-like taxon.
They do not consider any of the material to be ornithischian.
In view of the considerable confusion as to the association
of the holotype of Technosaurus, it is here excluded from
phylogenetic analysis.
Norman et al. (2004b) considered Tatisaurus oehleri
and Bienosaurus lufengensis from the Lower Lufeng Form-
ation (Early Jurassic: Sinemurian) of China as valid taxa
and possible basal thyreophorans. However, reassessment of
Tatisaurus (Norman et al. 2007) suggests that it is a no-
men dubium. Autapomorphic characters are not evident in
the original description of Bienosaurus (Dong 2001) and the
validity of this taxon is uncertain. As a result, both taxa are
excluded from the current phylogenetic analysis.
Xiaosaurus dashanpensis from the Lower Shaximiao
Formation of Sichuan Province, China (Middle Jurassic:
?Bajocian, Chen et al. 1982; ?Bathonian–Callovian, Dong
& Tang 1984) is based upon very fragmentary remains, but
appears to be diagnosable on the basis of the possession of a
proximally straight humerus that lacks the medial curvature
seen in all other basal ornithischians (Barrett et al. 2005).
However, the whereabouts of the holotype and referred ma-
terial are currently unknown (Barrett et al. 2005) and un-
available for further study and the original description (Dong
& Tang 1983) provides few anatomical details. As a result,
we do not include Xiaosaurus in the present study.
The location of the holotype of ‘Gongbusauruswu-
caiwanensis is currently unknown (X. Xu, pers. comm.,
2004) and the original description (Dong 1989) is brief and
poorly figured and does not allow the recognition of auta-
pomorphies, or a unique character combination. Therefore,
despite the fact that recent reviews (e.g. Norman et al. 2004c)
retain ‘Gongbusauruswucaiwanensis as a valid taxon, we
here exclude it from the phylogenetic analysis.
Notohypsilophodon comodorensis was described by
Mart´
ınez (1998) on the basis of a partial skeleton from the
Bajo Barreal Formation, Chubut Province, Argentina (Late
Cretaceous: ?Cenomanian). Mart´
ınez (1998) listed a number
of potential autapomorphies of Notohysilophodon;however,
most of these appear to have wider distributions within Or-
nithischia (e.g. the reduction of the deltopectoral crest of the
humerus is seen in other South American ornithopods, Novas
et al. 2004) or represent plesiomorphies (distal end of fibula
reduced, astragalus with ‘stepped’ proximal surface). Noto-
hypsilophodon may represent a valid taxon, but this cannot
be ascertained from the published description and this taxon
is provisionally excluded from the phylogenetic analysis.
Four taxa (Atlascopcosaurus loadsi,Fulgurotherium
australe,Leaellynasaura amicagraphica,Quantassaurus in-
trepidus) have been named on the basis of cranial and post-
cranial material from the Early Cretaceous of Australia (Rich
& Vickers-Rich 1989, 1999). Although all of these taxa have
been considered valid by a recent review (e.g. Norman et al.
2004c), the type specimens of all four taxa are fragmentary
and unambiguous autapomorphies have not yet been defined.
Referral of additional material to any one of these taxa is
problematic. Although we accept that some or all of these
taxa may prove to be diagnostic with further study, we here
provisionally exclude them from phylogenetic analysis as
we have been unable to examine the majority of the material
first-hand.
A number of taxa (Auroraceratops rugosus,
Changchunsaurus parvus,Xuanhuaceratops niei,Yamacer-
atops dorngobiensis,Yinlong downsi) were described after
the current analysis was carried out and thus have not been
included. We plan to include these taxa in future iterations
of this analysis.
Selection of outgroup taxa
Three outgroup taxa were chosen, based upon the phylo-
genetic framework outlined above. Recent, comprehens-
ive, phylogenetic analyses of basal dinosaurs have demon-
strated that Herrerasaurus ischigualastensis Reig, 1963 is
the most basal known member of Saurischia (Langer 2004;
Langer & Benton 2006). In addition, this taxon is known
from well-preserved complete material and has been extens-
ively described (Novas 1993; Sereno 1993; Sereno & Novas
1993). Marasuchus talampayensis (Romer, 1972) represents
a well-known dinosaurian outgroup and Euparkeria capensis
Broom, 1913 represents a basal archosaur, phylogenetically
distant from Dinosauria and lacking the numerous derived
specialisations seen in many other more proximate dinosaur-
ian outgroups, such as pterosaurs and crurotarsans.
Analyses
Search methods
The full matrix (Appendix 3) consists of 46 taxa (43 in-
group taxa and 3 outgroup taxa), coded for 221 characters
(Appendix 2). The data matrix was constructed using the
NEXUS Data Editor (http://taxonomy.zoology.gla.ac.uk/rod/
NDE/nde.html). Prior to analysis, safe taxonomic reduc-
tion (Wilkinson 1995c) was carried out using the pro-
gram TAXEQ3 (Wilkinson 2001a). Safe taxonomic reduc-
tion identifies taxa that can be excluded without affecting
the inferred relationships of the remaining taxa. The matrix
does not contain any taxonomic equivalents and all taxa were
included in subsequent analyses.
Analyses were carried out in PAUP4.0b10 (Swofford
2002); all characters are treated as unordered and equally
weighted and polymorphisms are treated as uncertainty.
Branches with a minimum length of zero were collapsed dur-
ing searches (the ‘-amb’ option); this setting recovers only
‘strictly supported’ trees (Nixon & Carpenter 1996; Kearney
& Clark 2003), but can result in trees that are not of min-
imum length and cannot be considered as most parsimonious
trees (MPTs); (Wilkinson 1995a). As a result we filtered
the resultant set of trees to ensure that only minimum length
trees were retained. Analysis was conducted using a heuristic
search with 10,000 replicates and TBR branch-swapping,
each starting tree being produced by random stepwise addi-
tion.
The analysis recovered 3787 trees; filtering these trees
so that only minimum length trees were retained resulted
in 756 MPTs of 477 steps (Consistency Index (CI) =0.505,
Retention Index (RI) =0.732, Rescaled Consistency Index
(RC) =0.370). Strict and 50% majority-rule component
Phylogeny of ornithischian dinosaurs 13
Figure 2 Strict component consensus (left) and 50% majority-rule consensus (right) of 756 most parsimonious trees (MPTs) produced by
analysing a data matrix of 46 taxa and 221 characters. Values above nodes on the Strict component consensus represent bootstrap proportions.
Values beneath nodes on the Strict component consensus indicate Bremer support. Bremer support values of +1 or less are not shown. Numbers
beneath nodes on the 50% majority-rule consensus indicate the percentage of MPTs in which that node appears (nodes with no values beneath
them appear in all MPTs).
consensus trees (Fig. 2) and an Adams consensus tree, were
calculated using PAUP. The strict component consensus
(SCC) tree contains two major polytomies and contains a
much lower degree of resolution than the majority-rule or
Adams consensus trees; the latter observation suggests that
the low degree of resolution in the SCC tree results from
a number of taxa acting as ‘wildcards’, as a result of high
amounts of missing data, or character conflict, or both. A
maximum agreement subtree was also calculated that ex-
cludes 8 taxa (Echinodon,Lycorhinus,Bugenasaura,Jeholo-
saurus,Talekauen,Thescelosaurus,Yandusaurus,Zephyro-
saurus; see Fig. 3).
An additional search was carried out using a demonstra-
tion version of TNT (Tree Analysis Using New Technology)
14 R. J. Butler et al.
Figure 3 Maximum agreement subtree of 756 most parsimonious trees (MPTs) produced by analysing a data matrix of 46 taxa and 221
characters. Eight (Echinodon,Lycorhinus,Bugenasaura,Jeholosaurus,Talekauen,Thescelosaurus,Yandusaurus and Zephyrosaurus)ofthe
original taxa are excluded.
v1.0, downloaded from www.zmuc.dk/public/phylogeny. A
‘New Technology’ search was carried out, using a random
addition-sequence, 1000 replicates and default settings for
the ‘Sect. Search’, ‘Ratchet’, ‘Drift’ and ‘Tree Fusing’ op-
tions. The search recovered 119 trees of 477 steps; the con-
sensus of these trees matched the consensus of the 756 MPTs
recovered by PAUP. That TNT failed to find trees shorter
than 477 steps suggests that this is the minimum tree length.
The set of 756 MPTs recovered by PAUPforms the basis
for subsequent discussion.
In an attempt to resolve further relationships com-
mon to all 756 MPTs and to identify the most unstable
taxa, reduced consensus techniques (Wilkinson 1994, 1995b,
2003) were applied to the data. The most commonly used
consensus methods are strict component consensus (SCC)
trees, which include all terminal taxa and all the clades
Phylogeny of ornithischian dinosaurs 15
(components) common to all MPTs. However, the strict com-
ponent method has problems of insensitivity and may fail to
represent relationships that are common to the set of MPTs,
but cannot be expressed as shared clades (Wilkinson 2003).
Reduced consensus methods identify ‘n-taxon statements’;
n-taxon statements express cladistic relationships (e.g. A and
C are more closely related to each other than either is to E),
but need not include all terminal taxa. Reduced consensus
methods delete unstable taxa to produce more informative
consensus trees, which represent n-taxon statements.
Reduced consensus was applied using the ‘strict’ pro-
gram of REDCON 3.0 (Wilkinson, 2001b) and the results
corroborated using RADCON (Thorley & Page 2000), identi-
fying a profile of eight strict reduced consensus (SRC) trees,
the first of which includes all taxa and corresponds in to-
pology to the SCC tree of the 756 MPTs. The remaining
SRC trees exclude one or more unstable ‘wildcard’ taxa, res-
ulting in an increase in resolution. Seven taxa (Echinodon,
Lycorhinus,Zephyrosaurus,Talenkauen,Yandusaurus,Gas-
parinisaura,Parksosaurus) are identified as unstable by these
analyses. We combined six of the SRC trees (those exclud-
ing Echinodon,Lycorhinus,Zephyrosaurus,Talenkauen and
Yandusaurus) to produce an informative derivative SRC tree
(Fig. 4). We use this derivative SRC tree as the basis for
optimisation of synapomorphies (Appendix 4) and for much
of the subsequent discussion, and it represents our preferred
hypothesis of interrelationships.
Testing the support for relationships
Randomisation tests
PAU Pwas used to run a Permutation Tail-Probability (PTP)
test using 1000 randomised replicates of the reduced data
set (Faith & Cranston 1991; Kitching et al. 1998). The ran-
domised replicates are created by randomly permuting the
character states assigned to taxa, decreasing character con-
gruence to a level that would be expected by chance alone.
The MPT length is then calculated for each replicate and the
distribution of MPT lengths for the replicates is compared
to the length of the original MPT. The PTP test has been
criticised (Bryant 1992; Carpenter 1992) and Kitching et al.
(1998) suggested that it could best be used as an independent
evaluation of the explanatory power of the data set, rather
than as a criterion for acceptance or rejection of any partic-
ular cladogram. In this case, the results of this test indicate
that the most parsimonious tree length (477 steps) lies out-
side the range of minimum tree lengths obtained from the
randomised data (P=0.001). This indicates that a signific-
ant phylogenetic signal is present in the data set and is not
completely obscured by character conflict and missing data.
Bremer support
‘Traditional’ decay analysis, or Bremer support, measures
the number of additional steps required before the clade is
lost from the strict consensus of near-minimum length clado-
grams (Bremer 1988; Kitching et al. 1998). Bremer support
was calculated for nodes present in the SCC tree by search-
ing in PAUPfor the shortest trees not compatible with a
particular node, using the CONVERSE option.
Bremer support values are shown in Fig. 2. Most nodes
have a decay index of +1, i.e. they are absent from the strict
consensus of all trees of 478 steps or less. Stronger support is
found only within the clades Thyreophora and Iguanodontia,
but even here support is relatively low. However, it is possible
that a few unstable ‘wildcard’ taxa, such as those identified
by reduced consensus techniques (see above), can obscure
support for relationships, resulting in lower decay indices
than might be otherwise expected.
Wilkinson et al. (2000) proposed a new technique,
double decay analysis (DDA), which provides Bremer sup-
port for all strictly supported n-taxa relationships. An attempt
was made to apply DDA using RADCON (Thorley & Page
2000); however, the large size of the data set meant that
this approach was not feasible due to time and memory con-
straints. For particular areas of interest PAUPwas used to
write backbone constraints that could then be used to test
the decay indices of n-taxa statements (see individual ex-
amples below). This allowed an assessment of the effect that
wildcard taxa have upon the Bremer support for clades.
Bootstrapping
Bootstrap analysis generates ‘pseudoreplicate’ data sets by
randomly sampling with replacement a proportion of the
characters, deleting some characters randomly and reweight-
ing other characters randomly. The MPTs are generated for
each pseudoreplicate and the degree of conflict between res-
ulting MPTs is assessed using a 50% majority rule consensus
tree. Clades that are supported by a large number of charac-
ters, with low levels of homoplasy, would be expected to have
high bootstrap values, whereas bootstrap values should be
lower for clades supported by only a few, or by homoplastic,
characters. The bootstrap should, therefore, be considered as
a one-sided test of a cladogram (Kitching et al. 1998): groups
that are recovered are supported by the data, but groups that
are not recovered (or that have low bootstrap values) cannot
be rejected.
A bootstrap analysis was carried out using PAUPwith
1000 replications. To allow the analysis to be carried out
within a reasonable length of time the MAXTREES option
of PAUP was set to 1000. This means that for each pseu-
doreplicate data set the search for MPTs was truncated once
1000 trees had been found. Figure 2 shows the results of
the bootstrap analysis. Bootstrap support is weak throughout
much of the tree.
Unstable ‘wildcard’ taxa can obscure levels of bootstrap
support for relationships (Wilkinson 2003). One potential
solution is to use the majority-rule bootstrap reduced con-
sensus (MBRC) technique developed by Wilkinson (1996).
The MBRC method provides bootstrap proportions for all
‘n-taxon statements’ and is implemented in the REDBOOT
program of REDCON 3.0. Unfortunately, REDBOOT could
not be used for this analysis due to the large size of the
data matrix. An alternative method for assessing the im-
pact of ‘wildcards’ on bootstrap support is used here. All
trees recovered by the bootstrap analysis were saved to a
NEXUS treefile. The five unstable taxa removed in the de-
rivative SRC tree (Echinodon,Lycorhinus,Zephyrosaurus,
Talenkauen and Yandusaurus) were pruned a posteriori from
this set of bootstrap trees, with duplicate topologies cre-
ated by this deletion being collapsed. Following pruning of
these five taxa a new majority-rule bootstrap reduced con-
sensus tree was generated containing recalculated bootstrap
values (Fig. 4). For further areas of particular interest addi-
tional potentially unstable taxa were identified and pruned
16 R. J. Butler et al.
Figure 4 Derivative strict reduced consensus tree derived by a posteriori pruning of five unstable taxa (Echinodon,Lycorhinus,
Zephyrosaurus,Talenkauen and Yandusaurus) from the set of 756 most parsimonious trees (MPTs) generated by the full analysis. The number
above each node is a unique identifier used in the tree description (see Appendix 4). The number beneath a node represents the bootstrap
proportion for that node (taken from the reduced bootstrap analysis). Note the increased levels of bootstrap support for a number of nodes
when compared to Fig. 2. Abbreviations: ORN., Ornithischia; HETERODONT., Heterodontosauridae; GENA., Genasauria; NEORN., Neornithischia;
ORNITH., Ornithopoda; MARG., Marginocephalia; CERAT., Ceratopsia; PACHY., Pachycephalosauria.
Phylogeny of ornithischian dinosaurs 17
and bootstrap values recalculated. The effect of ‘wildcards’
on bootstrap values is discussed further below.
Results
Ornithischian monophyly
Most of the characters identified as synapomorphic for Or-
nithischia (Appendix 4) have been identified by previous
authors (Norman 1984; Sereno 1984, 1986, 1991a, 1999a;
Cooper 1985; Marya´
nska & Osm´
olska 1985; Weishampel
2004; Norman et al. 2004a; Butler 2005). Some new potential
ornithischian synapomorphies are proposed by this analysis.
The presence of a buccal emargination has previously been
considered (Sereno 1986, 1999a) to be synapomorphic for a
less inclusive clade of ornithischians, Genasauria, and absent
in the basal ornithischian Lesothosaurus diagnosticus.This
analysis alternatively suggests that the presence of a weak or
incipient buccal emargination (generally correlated with the
presence of ‘cheeks’, see Galton 1973a) is a synapomorphy
of Ornithischia. As noted by Butler (2005: 204), in Lesotho-
saurus there is a weak anteroposteriorly extending ridge (re-
ferred to, below, as the ‘maxillary ridge’) above the row of ex-
ternal maxillary foramina, forming the ventral margin of the
external antorbital fenestra. Below this eminence the lateral
surface of the maxilla is gently bevelled such that the maxil-
lary tooth row is slightly inset along at least the posterior two-
thirds of its length. A similar weak medial offset of the tooth
row is seen in other basal ornithischians such as Abricto-
saurus consors (BMNH RUB54) and Scutellosaurus lawl-
eri (Colbert 1981) and is proposed here to be homologous
with the well-developed buccal emargination seen in many
other ornithischians. Irmis et al. (2007) criticised Butler
(2005) for using the same coding for the weak buccal
emargination of Lesothosaurus and the well-defined buccal
emargination of taxa such as Heterodontosaurus tucki.How-
ever, the coding of Butler (2005) referred only to the presence
of an emargination (not how well-developed it was), which
we propose is homologous in Lesothosaurus and Hetero-
dontosaurus. Irmis et al. (2007) additionally stated that in
ornithischians the maxillary ridge was separated from the
ventral margin of the external antorbital fenestra. This is true
for some ornithischians (e.g. Hypsilophodon foxii, Galton
1974a) but in many basal ornithischians such as Lesotho-
saurus (BMNH R8501) and Heterodontosaurus (SAM-PK-
K1332; Norman et al. 2004c: fig. 18.1) the maxillary ridge
does form the ventral margin of the external antorbital fen-
estra.
Another potential ornithischian synapomorphy is the
size and position of the posttemporal foramen. In ornith-
ischian outgroups the posttemporal foramen is relatively
large and positioned on the boundary between the parietal
and the paroccipital process (e.g. Euparkeria capensis, Ewer
1965: fig. 2B; basal sauropodomorphs, Galton & Upchurch
2004a;Herrerasaurus ischigualastensis, Sereno & Novas
1993: fig. 8C), whereas in basal ornithischians (e.g. Lesotho-
saurus diagnosticus, Sereno 1991a, Sereno 1991a: fig. 11C;
Heterodontosaurus tucki, Weishampel & Witmer 1990b:fig.
23.1), the posttemporal foramen is reduced in size and en-
tirely enclosed by the paroccipital process. This character was
independently identified as an ornithischian synapomorphy
by Langer & Benton (2006).
Some previously suggested ornithischian synapo-
morphies are not supported by this analysis. For example
Sereno (1999a) suggested that an elongate posterolateral
process of the premaxilla diagnoses the clade; however,
an elongate posterolateral process is present in the basal
archosaur Euparkeria (Ewer 1965), the basal saurischian
Herrerasaurus (Sereno & Novas 1993) and the problem-
atic non-ornithischian ‘dinosauriform’ Silesaurus opolen-
sis (Dzik 2003), and may be plesiomorphic for Dinosauria
(Marya´
nska & Osm´
olska 1985).
Many of the characters identified by this and previ-
ous analyses as synapomorphic for Ornithischia describe the
anatomy of the dentary and maxillary teeth. These synapo-
morphies have been used to refer taxa named on the basis
of isolated teeth to Ornithischia (e.g. Hunt & Lucas 1994).
However, recent discoveries suggest that ornithischian-like
teeth have evolved a number of times within Archosauria.
For instance, recently discovered cranial and postcranial ma-
terial of the putative ornithischian Revueltosaurus callenderi
suggests that this taxon is actually a non-ornithischian archo-
saur that appears to be phylogenetically closer to crocodylo-
morphs than to dinosaurs and that its ornithischian-like den-
tition evolved independently (Parker et al. 2005a,b; Irmis
et al. 2007). In addition, ornithischian-like teeth have been
described in Silesaurus opolensis (Dzik 2003), while many
of the characteristic features of the ornithischian dentition
also occur in taxa as diverse as therizinosaurs, basal sauro-
podomorphs and aetosaurs, suggesting that dental characters
(perhaps not surprisingly) may be subject to particularly high
levels of homoplasy.
The identification of ornithischian-like dental character
states in non-ornithischian taxa highlights the problems in-
herent in referring fragmentary material to specific clades.
As noted by Butler et al. (2006) and Irmis et al. (2007), most
Triassic taxa named upon the basis of isolated teeth cannot,
in general, be referred to Ornithischia with certainty unless
they demonstrate, or are associated with material demon-
strating, unique synapomorphies of an ornithischian clade
(i.e. features that are not independently synapomorphic for
other clades).
Pisanosaurus mertii
Pisanosaurus mertii is generally believed to be the oldest
known ornithischian (Casamiquela 1967), but both the as-
sociation of the material included within the holotype speci-
men and its phylogenetic position, have proved controversial.
Sereno (1991a) proposed that the holotype specimen con-
tained material from more than one individual; he suggested
that the skull fragments, partial impression of the pelvis and
distal hind limb might belong together, but that the fragment-
ary scapula and other postcranial bones were too small to be
referred to the same individual. However, Bonaparte (1976)
noted that the vertebrae were recovered in articulation with
the skull and that the skeleton may have been complete prior
to weathering, and a recent review supported the idea that
the type specimen represents a single individual (Irmis et al.
2007).
Bonaparte (1976) referred Pisanosaurus to Heterodon-
tosauridae on the basis that both share subcylindrical, closely
packed cheek teeth, with wear facets forming a more or
less continuous surface extending along the tooth row. How-
ever, Weishampel & Witmer (1990a) and Sereno (1991a)
18 R. J. Butler et al.
considered Pisanosaurus to be the most basal known or-
nithischian. Weishampel & Witmer (1990a) suggested that
the similarities between Pisanosaurus and heterodontosaur-
ids are plesiomorphic, while Sereno (1991a: 174) noted that:
‘The [wear] facets ...do not form a continuous occlusal sur-
face as occurs in Heterodontosaurus [tucki].’ More recently,
Norman et al. (2004a) have once again emphasised the mor-
phological similarities between the cranial material of Pis-
anosaurus and that of heterodontosaurids.
The SCC tree (Fig. 2) recovered by this analysis posi-
tions Pisanosaurus in an unresolved polytomy at the base of
Ornithischia. However, the 50% majority-rule consensus tree
(Fig. 2), maximum agreement subtree (Fig. 3) and the derivat-
ive SRC tree (Fig. 4) all support the position of Pisanosaurus
as the most basal known ornithischian. These consensus trees
additionally position heterodontosaurids as a monophyletic
clade of non-genasaurians, close to the base of Ornithischia
(discussed below). Pisanosaurus and heterodontosaurids are
not separated in the SCC (this is the result of the instability of
the fragmentary wildcard taxon Echinodon becklesii); only
one node separates Pisanosaurus from heterodontosaurids in
the derivative SRC tree. This node is weakly supported by
bootstrap proportions: it does not appear in the total-evidence
bootstrap analysis (support of only 37%) and has support of
only 54% in the reduced bootstrap analysis (Fig. 4). Only one
character (Character 206, Appendix 2) unambiguously sup-
ports this node. Constraining Pisanosaurus and heterodon-
tosaurids to form a monophyletic clade requires only one ad-
ditional step. This suggests that the two opposing phylogen-
etic positions for Pisanosaurus, as either a non-genasaurian
basal ornithischian (Sereno 1991a) or a heterodontosaurid
(Bonaparte 1976), are not necessarily mutually exclusive.
Pisanosaurus may indeed represent a heterodontosaurid and
the evidence for this should be reconsidered by future work.
Heterodontosauridae
The position of heterodontosaurids within Ornithischia is one
of the most problematic areas in ornithischian phylogeny and
there is no current consensus on this topic. Four alternative
phylogenetic positions have been proposed: (1) as basal or-
nithopods (e.g. Crompton & Charig 1962; Thulborn 1971;
Galton 1972; Santa Luca et al. 1976; Sereno 1984, 1986,
1999a; Gauthier 1986; Weishampel 1990; Weishampel &
Witmer 1990b; Smith 1997; Norman et al. 2004c); (2) as
the sister group to Marginocephalia (Marya´
nska & Osm´
olska
1984; Cooper 1985; Olshevsky 1991; Zhao et al. 1999; Buch-
holz 2002; You et al. 2003; Norman et al. 2004c; Xu et al.
2006); (3) as the sister group to Ornithopoda + Margino-
cephalia (Cerapoda) (Norman 1984; Marya´
nska & Osm´
olska
1985; Butler 2005); (4) as the most basal well-known ornith-
ischians (Bakker & Galton 1974; Olsen & Baird 1986). This
analysis supports the fourth of these positions and the phylo-
genetic support for this is discussed below, although a full
review of the anatomical evidence will be presented else-
where (R. J. B., unpublished results).
Four taxa (Heterodontosaurus tucki,Abrictosaurus con-
sors,Echinodon becklesii,Lycorhinus angustidens) and one
specimen (BMNH A100) previously referred to Heterodon-
tosauridae were included in this analysis. These taxa do not
form a clade in the SCC tree, but are included in a large
polytomy at the base of Ornithischia (Fig. 2). However, re-
duced consensus trees indicate that this basal polytomy is
the result of the unstable and problematic taxon Echinodon
(which is highly fragmentary, with 83% missing data); ex-
clusion of Echinodon results in the remaining four OTUs
forming a heterodontosaurid clade, while further exclusion
of Lycorhinus (86% missing data) in the derivative SRC tree
resolves relationships within Heterodontosauridae (Fig. 4).
The heterodontosaurid node is weakly supported by
bootstrap proportions in the total-evidence bootstrap ana-
lysis (support of only 19%); however, bootstrap support is
considerably higher (68%) in the reduced bootstrap analysis
(Fig. 4), suggesting that low bootstrap support for Heterodon-
tosauridae is probably the result of fragmentary and unstable
wildcard taxa. To test whether similar ‘hidden’ Bremer sup-
port exists for Heterodontosauridae we wrote a backbone
constraint that specified that Heterodontosaurus,Abricto-
saurus and BMNH A100 form a clade to the exclusion of
other ornithischians, but did not specify the position of the
five unstable taxa (Echinodon,Lycorhinus,Zephyrosaurus,
Talenkauen and Yandusaurus) discussed above and removed
in the derivative SRC tree. The Bremer support for this back-
bone constraint was +2, again suggesting that low support
for Heterodontosauridae is the result of unstable wildcard
taxa.
The MPTs recovered in this analysis do not support
a link between heterodontosaurids and ornithopods (phylo-
genetic hypothesis 1, above). In order to test this further we
ran the following constrained analyses: firstly, Ornithopoda
(sensu Weishampel 1990; Sereno 1999a), containing hetero-
dontosaurids, ‘hypsilophodontids’ and iguanodontids, was
constrained as monophyletic, although Hypsilophodontidae
(sensu Weishampel & Heinrich 1992) was not constrained as
a monophyletic clade; secondly, a backbone constraint was
specified that required that Heterodontosaurus and Ankylo-
pollexia be more closely related to each other than either
is to marginocephalians, thyreophorans or ornithischian out-
groups. This constraint does not specify the complete content
of Ornithopoda.
Templeton non-parametric tests were carried out using
PAU Pthat compared trees within the profile of 756 MPTs
generated by the unconstrained analysis with all trees re-
covered by the constrained analyses. Ideally all MPTs should
be compared with all trees recovered by the constrained ana-
lyses; however, as this is time intensive (this would involve
756 separate Templeton tests) a subset of the MPTs (every
25th MPT) was used.
Under the first constraint, 1812 trees of 500 steps were
found, 23 steps longer than the most parsimonious tree
length. These trees were a significantly worse (p=0.0004–
0.01) explanation of the data than the most parsimonious
topology. The second constraint recovered 2034 trees of 490
steps (13 extra steps). Some, but not all, of these trees were
significantly (p=0.03–0.07) worse explanations of the data
than the most parsimonious topology.
A sister group relationship between heterodontosaurids
and marginocephalians (phylogenetic hypothesis 2, above)
is not found by this analysis; instead heterodontosaurids are
positioned more basally within Ornithischia (Fig. 4). To test if
any support is present in the data for a Heterodontosauridae–
Marginocephalia sister grouping we constrained such a clade
to be monophyletic and 420 trees of 487 steps (10 extra steps)
were recovered; Templeton tests indicate that these trees are
not a significantly worse explanation of the data (p=0.128–
0.1803).
Phylogeny of ornithischian dinosaurs 19
A number of derived character states are absent in het-
erodontosaurids, but occur in all cerapodan ornithischians.
These include: loss of the squamosal–quadratojugal con-
tact; closure of the external mandibular fenestra (reversed
in some psittacosaurids); reduction of the pubic peduncle
of the ilium; development of an elongated rod-like prepubic
process; modification of the fossa trochanteris into a dis-
tinct constriction separating the femoral head and greater
trochanter; and the anteroposterior expansion of the greater
trochanter of the femur relative to the anterior trochanter
(Characters 52, 104, 178, 193, 194, 198, 199: Appendix 2). If
heterodontosaurids are positioned within Cerapoda, as either
basal marginocephalians or basal ornithopods, then a sub-
stantial amount of homoplasy must be invoked to explain the
distribution of these derived character states.
This analysis finds support for an alternative position,
not suggested by previous cladistic phylogenetic analyses
(although suggested by the non-cladistic work of Bakker &
Galton 1974; Olsen & Baird 1986), that heterodontosaurids
are not members of Genasauria and are actually situated close
to the base of Ornithischia. This basal position is supported by
a number of characters (Appendix 4), including: absence of
a spout-shaped mandibular symphysis; premaxillary crowns
not expanded mesiodistally or apicobasally above the root; al-
veolar foramina absent medial to maxillary and dentary tooth
rows; retention of epipophyses on cervical vertebrae; manus
length more than 40% of the combined length of the humerus
and radius; penultimate phalanges of the second and third
manual digits more elongate than the proximal phalanges;
extensor pits present on the dorsal surface of the distal end of
metacarpals and manual phalanges; manual unguals strongly
recurved with prominent flexor tubercles (Characters 97, 113,
126, 133, 156, 159, 162, 163: Appendix 2).
A basal position for heterodontosaurids is more con-
sistent with the stratigraphic record than previous hypotheses
and has important implications for our understanding of early
ornithischian anatomy, palaeobiology and evolution that will
be explored by future work. However, substantial further
work is required on heterodontosaurid anatomy and char-
acter homology before this hypothesis can be considered
well-supported.
‘Fabrosaurids’
Sereno (1984, 1986, 1991a, 1999a) proposed that the ple-
siomorphic ornithischian Lesothosaurus diagnosticus is the
sister group of Genasauria, a position similar to that pro-
posed by Galton (1972) and this interpretation has been fol-
lowed by later authors (e.g. Weishampel & Witmer 1990a;
Norman et al. 2004a). This contrasts with previous views
of ornithischian phylogeny (Thulborn 1971; Norman 1984;
Cooper 1985; Marya´
nska & Osm´
olska 1985) that usually
considered Lesothosaurus to have affinities with Ornitho-
poda and, sometimes, a larger group that additionally in-
cluded pachycephalosaurs and ceratopsians (Norman 1984;
Butler 2005). The interpretation of the position of Lesotho-
saurus suggested by Sereno (1984, 1986, 1991a, 1999a)
is based upon a number of putative synapomorphies sup-
posedly shared by Genasauria and absent in Lesothosaurus;
Butler (2005) recently demonstrated that most of these char-
acters are either present in Lesothosaurus or absent in some
basal thyreophorans/neornithischians. Butler (2005) sugges-
ted three characters that might group Lesothosaurus with
basal neornithischians. Of these characters, reduction of the
forelimb (character 153, Appendix 2) is here optimised as
independently gained in Lesothosaurus,Agilisaurus louder-
backi and pachycephalosaurs; the presence of a dorsal groove
on the ischium (character 183, Appendix 2) may be a gena-
saurian plesiomorphy as it appears to be present in the thyreo-
phoran Scutellosaurus lawleri (UCMP 130580, R. J. B., pers.
obs., 2005); while the reduction of metatarsal one (char-
acter 211, Appendix 2) optimises as an ornithischian ple-
siomorphy, present in heterodontosaurids.
This analysis resolves Lesothosaurus as the most basal
thyreophoran (Figs 2–4; see also Liu 2004); however, only
one unambiguous character (character 106, Appendix 2) sup-
ports this position. In Lesothosaurus an anteroposteriorly ex-
tending ridge is present on the lateral surface of the surangu-
lar, immediately anterodorsal to the glenoid and dorsal to the
surangular foramen (BMNH RUB17; a previous reconstruc-
tion (Sereno 1991a) shows this feature as less prominent than
actually occurs in the syntype and referred specimens). This
feature is also present in the basal thyreophorans Scelido-
saurus (BMNH R1111), Emausaurus (Haubold 1990) and
Scutellosaurus (UCMP 130580). In all four taxa the ridge is
prominent and transversely thickened, overhangs the lateral
surface of the angular and laterally delimits a narrow, dorsally
facing shelf. Statistical support for this position of Leso-
thosaurus is weak (total-evidence bootstrap proportion =
40%; Bremer support =+1), with the low bootstrap support
reflecting the fact that this node is supported by only a single
character.
All previous and current interpretations of the exact
phylogenetic position of Lesothosaurus are problematic and
poorly supported. However, it seems clear that Lesothosaurus
is positioned close to the base of Genasauria, as either the
sister-taxon to this clade (Sereno 1986, 1999a), the most
basal known neornithischian (Butler 2005), or the most basal
thyreophoran (this analysis). In addition, autapomorphies ap-
pear to be difficult to delimit for Lesothosaurus (Butler 2005).
Taken in combination, these observations suggest that the
anatomy of Lesothosaurus may be close to that of the ances-
tral genasaurian and that this taxon remains a good ‘ancestral’
taxon for ornithischian functional studies.
A monophyletic Fabrosauridae was not supported by
this analysis. Peng (1997) identified a number of characters
that he suggested linked Lesothosaurus and Agilisaurus to
the exclusion of other ornithischians. While some of these
characters have more widespread distributions within Or-
nithischia (e.g. Characters 173, 176, 183, 211, Appendix 2),
one character (humerus strongly reduced in length, Char-
acter 153, Appendix 2) is only known in Lesothosaurus
and Agilisaurus amongst basal ornithischians. This character,
therefore, provides some evidence for a monophyletic Fab-
rosauridae. However, Agilisaurus is more derived than Leso-
thosaurus in a large number of other character states (e.g.
Characters 101, 104, 178, 184, 194, Appendix 2), suggesting
that Fabrosauridae sensu Peng (1997) is defined largely on
the basis of symplesiomorphies and is probably paraphyl-
etic. Five additional steps are required for Lesothosaurus
and Agilisaurus to form a clade to the exclusion of other
taxa.
Neither Butler (2005) nor this analysis have been able to
find anatomical evidence for a South African ‘fabrosaurid’
clade (sensu Knoll 2002a,b), consisting of Lesothosaurus
and Stormbergia dangershoeki, although only one additional
20 R. J. Butler et al.
step is required for such a clade to be recovered. One un-
ambiguous synapomorphy positions Stormbergia as a basal
neornithischian, separate from Lesothosaurus:presenceofa
tab-shaped obturator process on the ischium (Character 184,
Appendix 2).
Thyreophora
The monophyly of Thyreophora (ignoring, at present, the
possible most basal thyreophoran Lesothosaurus diagnos-
ticus, discussed above) is well-established and uncontro-
versial and the clade including Scutellosaurus lawleri,
Emausaurus ernsti,Scelidosaurus harrisonii and Eurypoda
(Ankylosauria + Stegosauria) is comparatively well-
supported by total-evidence bootstrap proportions and
Bremer support (Fig. 2). The characters that unambiguously
support the monophyly of this clade (Appendix 4) have
already been identified by Sereno (1986, 1999a), with the
exception of the presence of cortical remodelling of cra-
nial elements (Character 89, Appendix 2), which has been
previously considered to be limited to ankylosaurs, but is
also known in Scutellosaurus (UCMP 130580, jugal; Rosen-
baum & Padian 2000: fig. 5C), Emausaurus (SGWG 85) and
Scelidosaurus (BMNH R1111; Carpenter 2001).
The phylogenetic position of Scelidosaurus harrisonii
has proved controversial, despite being known from a num-
ber of well-preserved specimens. This is probably because
no comprehensive description of the taxon has been car-
ried out since the work of Owen (1861a, 1863), although
a redescription is in progress (D.B.N. unpublished results).
Sereno (1986, 1999a) suggested 16 characters that link an-
kylosaurs and stegosaurs (as the clade Eurypoda) to the ex-
clusion of Scelidosaurus. Carpenter (2001) made the most
extensive case to date in favour of positioning Scelidosaurus
as the most basal ankylosaur; however, Carpenter did not
include the eurypodan characters of Sereno (1999a)inhis
analysis. At least four of the five synapomorphies identified
by Carpenter as linking Scelidosaurus with ankylosaurs ap-
pear to have a more widespread distribution within Thyreo-
phora (Maidment et al., 2006; R. J. B. & S. C. R. Maidment,
unpublished results).
This analysis does not find a sister group relationship
between Scelidosaurus and ankylosaurs. A large number of
anatomical features are shared by stegosaurs and ankylo-
saurs, but are absent in Scelidosaurus (Appendix 4; see also
Sereno 1986, 1999a). It is possible that a number of these
features could have evolved in parallel in both clades as ad-
aptations to large size (e.g. development of the preacetabular
process of the ilium, Character 166, Appendix 2; reduction
of the fourth trochanter, Character 201, Appendix 2), but
that this homoplasy has not yet been recognised as a result
of the scarcity of well-preserved Early and Middle Jurassic
eurypodans. It is also possible that the distribution of charac-
ter states has been complicated by coding both Stegosauria
and Ankylosauria as supraspecific clades: a full considera-
tion of the phylogenetic position of Scelidosaurus requires
an analysis that includes a large number of stegosaur and
ankylosaur taxa. Unfortunately such an analysis has not yet
been carried out and is beyond the scope of this work. How-
ever, the evidence presented by this analysis casts doubts
upon interpretation of Scelidosaurus as a basal ankylosaur.
Constraining Scelidosaurus to be the sister group of Ankylo-
sauria results in 2270 trees of 485 steps (8 extra steps), but is
not a significantly worse explanation of the data (Templeton
test, p=0.04–0.13).
Basal neornithischians
Most previous analyses have included all neornithischian
taxa within Cerapoda, as members of either Ornithopoda or
Marginocephalia (e.g. Sereno 1999a). Only Butler (2005) has
previously identified non-cerapodan neornithischians. This
analysis finds four OTUs (Stormbergia dangershoeki,Agil-
isaurus louderbacki,Hexinlusaurus multidens,Othnielia rex;
the lattermost is identified as a non-cerapodan neornithis-
chian in the derivative SRC tree: Fig. 4) that form a pectinate
series of successively closer sister taxa to Cerapoda.
Referral of Stormbergia to Neornithischia is supported
by only a single unambiguous character (possession of an
obturator process on the ischium, Character 184, Appendix
2), and measures of support for this node are weak (Bremer
support =+1, bootstrap =33%). Referral of this taxon to
Neornithischia is, therefore, preliminary (although we have
not yet been able to identify evidence that might support
its referral to any other ornithischian clade) and a better
supported phylogenetic position must await the discovery of
specimens of this taxon with cranial material.
Apparent support is also weak for the position of Agil-
isaurus and Hexinlusaurus as basal neornithischians. How-
ever, bootstrap support for the node supporting the clade Oth-
nielia + Cerapoda, to the exclusion of Agilisaurus and Hexin-
lusaurus, is relatively high (79%) in the reduced bootstrap
analysis (Fig. 4). Five unambiguous characters (Appendix 4)
suggest that Hexinlusaurus is more closely related to cera-
podans than Agilisaurus. This is of interest as many authors
have synonymised the two, although recent work (Barrett
et al. 2005) supports generic-level distinction. Four addi-
tional steps are required for Agilisaurus and Hexinlusaurus
to group as sister taxa, although this does not represent a
significantly worse explanation of the data (Templeton test,
p=0.10–0.35). Constraining these taxa as ornithopods (as
suggested by previous authors, e.g. Weishampel & Heinrich
1992; Norman et al. 2004c) requires 13 additional steps and
is a significantly worse explanation of the data than the most
parsimonious topology (Templeton test, p=0.01–0.02).
The phylogenetic position of Othnielia is problematic.
Othnielia has been invariably referred to Ornithopoda, usu-
ally as a member of Hypsilophodontidae (e.g. Weishampel &
Heinrich 1992); by contrast, the derivative SRC tree (Fig. 4)
recovered by this analysis excludes Othnielia from Cerapoda,
as a result of the retention of a relatively short postacetabular
process (Character 174, Appendix 2) and a relatively reduced
and splint-like first metatarsal (Character 211, Appendix 2).
However, the position of Othnielia appears to be unstable
and weakly supported and this may reflect the near-absence
of cranial data for this taxon.
Ornithopoda
A monophyletic Ornithopoda is not recovered in the SCC tree
(Fig. 2), but is present in the derivative SRC tree (Fig. 4) fol-
lowing the a posteriori pruning of five unstable taxa, includ-
ing the putative ornithopods Yandusaurus,Zephyrosaurus
and Talenkauen. However, the content of the ornithopod clade
recovered in the derivative SRC tree (Fig. 4) differs signific-
antly from that suggested by previous analyses. A number of
Phylogeny of ornithischian dinosaurs 21
taxa generally referred to Ornithopoda (heterodontosaurids,
Agilisaurus,Hexinlusaurus,Othnielia) are here positioned
elsewhere within Ornithischia; this suggests that Ornitho-
poda, as conceived by some previous authors (e.g. Sereno
1986, 1999a; Norman et al. 2004c), may be polyphyletic; al-
ternatively, although we do not promote this view (we prefer
the stem-based definition for Ornithopoda given in Table 1),
it is conceivable that the definition of Ornithopoda could be
expanded to include some, or all, of these taxa, as well as
ceratopsians and pachycephalosaurs.
Hypsilophodontidae, as generally conceived
(Weishampel & Heinrich 1992), does not form a mono-
phyletic clade; instead ‘hypsilophodontids’ appear to
represent a paraphyletic grade of basal neornithischian and
basal ornithopod taxa (see below). More derived ornithopod
relationships generally follow that suggested by previous
authors (e.g. Weishampel et al. 2003).
The derivative SRC tree (Fig. 4) suggests that Ornitho-
poda includes a paraphyletic assemblage of ‘hypsilophodon-
tids’ (Orodromeus makelai,Jeholosaurus shangyuanensis,
Hypsilophodon foxii,Parksosaurus warreni,Gasparinisaura
cincosaltensis,Bugenasaura infernalis,Thescelosaurus neg-
lectus) and Iguanodontia (comprising rhabdodontids, tenon-
tosaurs, dryosaurids and ankylopollexians). However, sup-
port is weak at the base of Ornithopoda, with most basal
nodes having very low (less than 50%) total-evidence boot-
strap values. The characters that support basal nodes in the
derivative SRC tree tend to have limited or poorly understood
distributions. For example, Ornithopoda is supported by the
presence of a fossa-like depression on the premaxilla–maxilla
boundary (Character 13, Appendix 2), the possession of nar-
row and elongate (more than twice as long as wide) frontals
that contact the nasals anterior to the orbits (Character 64,
Appendix 2), and the possession of a foramen positioned
on the dorsal part of the surangular–dentary joint (Charac-
ter 105, Appendix 2). The first character is only known in
Orodromeus,Jeholosaurus and Hypsilophodon. It appears
to be absent in more derived ornithopods and a similar dis-
tribution occurs for the second character (also present in
Zephyrosaurus, Sues 1980).
This analysis suggests that Ornithopoda is a poorly-
supported clade and the weak support at the base of Or-
nithopoda may be a result of the instability of fragment-
ary (e.g. Zephyrosaurus,Yandusaurus) or incompletely de-
scribed (e.g. Talenkauen) taxa. A further cause of the weak
support for Ornithopoda may be the repositioning of het-
erodontosaurids as basal ornithischians. Heterodontosaurids
have effectively acted as an outgroup for analyses of ornitho-
pod phylogeny. For instance, Sereno (1999a) suggested that
the following characters were derived features of Euornitho-
poda: reduction of the external antorbital fenestra; closure
of the external mandibular fenestra; reduction of the delto-
pectoral crest of the humerus; enlargement of the prepubic
process of the pubis; presence of an obturator process on the
ischium. The plesiomorphic state for each of these characters
is present in Heterodontosauridae. However, if heterodonto-
saurids are repositioned basally within Ornithischia it be-
comes apparent that each of the above characters suggested
by Sereno (1999a) as diagnostic for Euornithopoda actually
has a more widespread distribution within Cerapoda. For in-
stance, the external mandibular fenestra is closed in basal
neornithischians (e.g. Agilisaurus, Peng 1992), ceratopsians
(e.g. Zhao et al. 1999; You & Dodson 2004) and pachyceph-
alosaurs (Marya´
nska et al. 2004), the prepubic process of the
pubis is elongated in some basal neornithischians (Hexin-
lusaurus, He & Cai 1984), pachycephalosaurs (Marya´
nska
et al. 2004: fig. 21.4C) and basal ceratopsians (e.g. Psit-
tacosauridae, Sereno 1987; Archaeoceratops, You & Dodson
2003) and the obturator process is present on the ischium
in basal neornithischians (Stormbergia, SAM-PK-K1105,
BMNH R11000, Butler 2005; Agilisaurus, ZDM T6011;
Hexinlusaurus, He & Cai 1984, ZDM T6001). Thus the ap-
parently strong support for Ornithopoda found by previous
analyses (e.g. Sereno 1999a) may be in part an artefact of an
incorrect phylogenetic placement of heterodontosaurids.
Hypsilophodontidae
Early conceptions of Hypsilophodontidae were of an impli-
citly paraphyletic ‘plexus’ of plesiomorphic, small, bipedal
ornithischians (e.g. Thulborn 1971). Such a grouping was
incompatible with cladistic study and a number of authors
attempted to develop a more restrictive, monophyletic, defin-
ition of the group (e.g. Norman 1984; Sereno 1984, 1986;
Cooper 1985; Sues & Norman 1990; Weishampel & Heinrich
1992). Sereno (1986) and Weishampel & Heinrich (1992)
considered the following taxa as members of Hypsilopho-
dontidae: Thescelosaurus neglectus,Othnielia rex,Hypsilo-
phodon foxii,Zephyrosaurus schaf,Orodromeus makelai
and Yandusaurus (the taxa Yandusaurus hongheensis and
Hexinlusaurus multidens were included as a single OTU;
however, these taxa are distinguishable at generic level: Bar-
rett et al. 2005). Sues & Norman (1990) referred a num-
ber of other small ornithopods (e.g. Agilisaurus louderbacki,
Gongbusaurus shiyii,‘Gongbusaurus’ wucaiwanensis)to
Hypsilophodontidae, while Bugenasaura infernalis has been
referred to the clade by Galton (1999). However, recent ana-
lyses (Scheetz 1998, 1999; Winkler et al. 1998; Weishampel
et al. 2003; Norman et al. 2004c) have found little evidence
for hypsilophodontid monophyly.
A monophyletic Hypsilophodontidae was not recovered
by this analysis (Figs 2–4). Although ‘hypsilophodontids’ are
generally thought of as rather anatomically conservative, it is
clear that they are actually relatively diverse, with some taxa
(e.g. Thescelosaurus) showing greater similarities to more
derived, iguanodontian ornithopods, while other taxa appear
to have much more plesiomorphic morphologies (e.g. Oro-
dromeus) and some may not even be referable to Ornithopoda
(e.g. Agilisaurus,Hexinlusaurus,Othnielia).
‘Hypsilophodontids’ appear to represent a grade of
basal neornithischian and basal ornithopod taxa. Further
work should be carried out on the relationships of these basal
taxa. This analysis finds weak support for some groupings,
such as Gasparinisaura and Parksosaurus and more in depth
work in this part of the tree might clarify such possibilities.
Marginocephalia
Only limited character evidence has been discovered pre-
viously for Marginocephalia (although see Xu et al. 2006),
leading some authors to doubt its validity (Dodson 1990; Sul-
livan 2006). This analysis supports marginocephalian mono-
phyly; however, total-evidence bootstrap values (less than
50%) and Bremer support (+1) are very low for this clade.
This might suggest that the clade is supported by a relatively
low number of characters. However, character optimisation
22 R. J. Butler et al.
(Appendix 4) indicates that Marginocephalia is supported by
8 unambiguous synapomorphies and as many as 17 char-
acters under accelerated transformation (ACCTRAN). This
suggests that there is considerable character evidence for
Marginocephalia, but that support for the clade is obscured
by one or more wildcard taxa. The putative basal pachyceph-
alosaur Stenopelix valdensis and the problematic cerapodan
Micropachycephalosaurus hongtuyanensis are identified be-
low as wildcard taxa and it is possible that the apparent low
support for Marginocephalia may result, in part, from the
instability of these taxa. To test this possibility a backbone
constraint was written that did not specify the position of
Stenopelix,Micropachycephalosaurus, or the five unstable
taxa identified in the derivative SRC tree (Fig. 4), but spe-
cified that all other pachycephalosaurs and ceratopsians were
more closely related to one another than any of them were
to ornithopods, thyreophorans, or non-cerapodan neornithis-
chians. This constraint had a Bremer support of +4. This in-
creased Bremer support suggests that there is relatively strong
‘hidden’ support for Marginocephalia. To confirm this, we
pruned the five unstable taxa identified in the derivative SRC
tree, Micropachycephalosaurus and Stenopelix from the set
of trees recovered by the bootstrap analysis and generated
a new 50% majority-rule bootstrap reduced consensus tree,
which demonstrates increased (70%) bootstrap support for
Marginocephalia.
Three key characters have been identified by previous
authors (Sereno 1984, 1986, 1987, 1999a, 2000; Cooper
1985; Marya´
nska & Osm´
olska 1985) as supporting mar-
ginocephalian monophyly: posterior expansion of the pari-
etosquamosal obscures the occiput in dorsal view, premaxil-
lae excluded from the border of the internal nares, postpubis
reduced or absent (Characters 68, 84, 188/189, Appendix
2). Other suggested marginocephalian synapomorphies (see,
e.g., Cooper 1985; Sereno 1987) have not been supported by
subsequent work. This analysis supports the validity of these
three marginocephalian characters.
A number of other potential marginocephalian synapo-
morphies are identified here. Unambiguous synapomorphies
include: absence of elliptic fossa on the nasals (although a
fossa is present in at least one basal ceratopsian, Liaoceratops
yanzigouensis), three premaxillary teeth, elongate and strap-
like scapula blade and the absence of an obturator process
on the ischium (Characters 19, 112, 150, 184, Appendix 2).
ACCTRAN indicates additional potential synapomorphies,
most of which have rather uncertain distributions and suffer
from missing data. For example, node-like ornamentation
is present on the jugal–postorbital bar and angular (Charac-
ters 41, 108, Appendix 2) of pachycephalosaurs and the basal
ceratopsians Chaoyangsaurus youngi and Archaeoceratops
oshimai. However, the distribution of this character remains
uncertain as ornamentation is absent in other basal ceratop-
sians such as Psittacosauridae.
Ceratopsia
Ceratopsia is recovered as a monophyletic clade in this ana-
lysis, although decay indices and bootstrap values suggest
that support for the clade is weak. However, this may result
from the inclusion within the clade of the fragmentary and
problematic taxon Micropachycephalosaurus hongtuyanen-
sis, which appears to act as a ‘wildcard’, or the unstable
position of the putative basal pachycephalosaur Stenopelix
valdensis (see below), or both. The Bremer support for the
ceratopsian node (including Micropachycephalosaurus)is
+1. To test the effect of Micropachycephalosaurus and Sten-
opelix upon the apparent support for Ceratopsia an n-taxon
backbone constraint was defined which specified that Chaoy-
angsaurus, Psittacosauridae and neoceratopsians were more
closely related to each other than to ornithopods, pachyceph-
alosaurs, or non-cerapodans; however, it did not specify the
position of Micropachycephalosaurus,Stenopelix,orthefive
wildcard taxa identified by the derivative SRC tree (Fig. 4).
PAU Pwas then used to search for the shortest trees in-
compatible with the backbone constraint. The shortest tree
recovered was of 481 steps (Bremer support of +4), suggest-
ing that strong support exists for the ceratopsian clade, but is
obscured by the unstable nature of several taxa. To confirm
this, we pruned seven taxa (see above) including Micropachy-
cephalosaurus and Stenopelix from the set of trees recovered
by the bootstrap analysis and generated a new majority-rule
bootstrap reduced consensus tree, which demonstrates very
high (89%) bootstrap support for Ceratopsia.
Why does Micropachycephalosaurus act as a wildcard
and why does this taxon group with ceratopsians when it
has been traditionally referred to Pachycephalosauria (Dong
1978; Marya´
nska et al. 2004)? The holotype of Micropachy-
cephalosaurus (IVPP V 5542), originally described by Dong
(1978), is extremely fragmentary and comprises a quadrate
and partial tooth row, vertebral elements (including an in-
complete sacrum), a partial ilium and the partial left femur
and tibia. Dong (1978) referred Micropachycephalosaurus to
Pachycephalosauria on the basis of a supposedly thickened
skull roof; however, no material of the skull roof is cur-
rently present in the holotype specimen (R. J. B. pers. obs.,
2004; R. J. B. & Q. Zhao, unpublished results). In addition,
other pachycephalosaurian synapomorphies (e.g. preacetab-
ular process of ilium expands mediolaterally anteriorly in
dorsal view; elongate caudal ribs) are absent. Cerapodan fea-
tures (particularly of the femur) are present, but few charac-
ters can be identified that unambiguously link Micropachy-
cephalosaurus with pachycephalosaurs, ornithopods or cer-
atopsians. The only character that can be identified as po-
tentially linking Micropachycephalosaurus with ceratopsi-
ans is the enlargement of the lateral condyle of the quadrate
(Character 63, Appendix 2); however, this character is ho-
moplastic, with the derived state also known in heterodonto-
saurids (Heterodontosaurus, SAM-PK-K337) and some or-
nithopods (e.g. Weishampel et al. 2003). Only one additional
step is required for Micropachycephalosaurus to group with
either pachycephalosaurs or ornithopods, instead of ceratop-
sians. A ceratopsian identity appears to be poorly supported
and the position of Micropachycephalosaurus within Cera-
poda cannot be clarified with any certainty (R. J. B. & Q.
Zhao, unpublished results).
Pachycephalosauria
Pachycephalosauria is a well-established clade and all pub-
lished ornithischian phylogenetic analyses have found it to
be monophyletic. This analysis finds Pachycephalosauria
to be a monophyletic clade including Stenopelix valdensis,
Wannanosaurus yansiensis,Goyocephale lattimorei,Homa-
locephale calathocercos and Pachycephalosauridae; how-
ever, Bremer support (+1) and total-evidence bootstrap val-
ues (55%) show surprisingly low support for Pachyceph-
alosauria (Fig. 2), although bootstrap support is stronger
for more derived groupings within the clade. However, as
Phylogeny of ornithischian dinosaurs 23
with Ceratopsia, the real support for the clade may be ob-
scured by the presence of one or more ‘wildcard’ taxa. The
most obvious candidate is the problematic basal taxon Sten-
opelix. To test this possibility, a backbone constraint was
defined that specified that Wannanosaurus,Goyocephale,
Homalocephale and Pachycephalosauridae are more closely
related to one another than to ceratopsians, ornithopods, or
non-cerapodans (the phylogenetic positions of the five wild-
card taxa identified by reduced consensus, Stenopelix and
the problematic taxon Micropachycephalosaurus were not
specified by this constraint). The shortest tree incompatible
with this constraint was 480 steps long (3 steps longer than
the MPTs), suggesting that unstable taxa are decreasing the
apparent support for Pachycephalosauria. A new majority-
rule bootstrap reduced consensus tree, following the prun-
ing of seven taxa (see above) demonstrates exceptionally
high (95%) bootstrap support for Pachycephalosauria (Wan-
nanosaurus,Goyocephale,Homalocephale, Pachycephalo-
sauridae). Thus, although support for pachycephalosaurian
monophyly may be strong, it is obscured by the presence
in the data set of a number of wildcard taxa of uncertain
position.
Conclusions
Stability, instability, and future directions
in ornithischian phylogeny
The results of this phylogenetic analysis differ in many re-
spects from those of previous analyses and reinforce the
importance of constant reassessment of phylogenetic hypo-
theses. Furthermore, the results of this analysis highlight
areas of stability within ornithischian phylogeny and also
problematic areas in which future efforts should be concen-
trated.
At a broad-scale, the widely accepted framework of or-
nithischian phylogeny, developed by Norman (1984), Sereno
(1984, 1986, 1991a, 1999a), Cooper (1985) and Marya´
nska
&Osm
´
olska (1985), amongst others, has proved relatively
stable (Fig. 4). There appears to be a basal split within Ornith-
ischia between thyreophorans and neornithischians. Within
Neornithischia two monophyletic clades occur, Ornithopoda
and Marginocephalia; the latter is subdivided into Ceratop-
sia and Pachycephalosauria. In many cases this analysis has
strengthened this basic framework; for example, by identify-
ing additional marginocephalian synapomorphies. However,
a number of unstable and problematic areas within this frame-
work have also been identified.
Heterodontosaurids have long been recognised as the
most unstable of ornithischian taxa and the clade has been
positioned in four very different positions within Ornith-
ischia. This analysis positions heterodontosaurids as non-
genasaurian basal ornithischians, but we additionally note
that recent authors have found anatomical evidence and
phylogenetic support for the idea that heterodontosaurids
may form the sister clade to Marginocephalia (e.g. Xu et al.
2006). Resolving the position of heterodontosaurids within
Ornithischia is one of the most important tasks facing ornith-
ischian phylogeneticists and future work should aim to com-
bine the data set provided here with that of Xu et al. (2006).
The removal of heterodontosaurids from the base of Or-
nithopoda highlights another major area of instability within
Ornithischia: basal ornithopod phylogeny. Removing hetero-
dontosaurids here results in a number of taxa long considered
as ornithopods (e.g. Agilisaurus louderbacki,Hexinlusaurus
multidens,Othnielia rex) grouping outside of Cerapoda, as
non-ornithopods. This analysis suggests that the number of
taxa that can be referred to Ornithopoda is more restrictive
than hypothesised by previous workers. In addition, recog-
nising robust ornithopod synapomorphies is extremely diffi-
cult and basal relationships (amongst the ‘hypsilophodontid’
taxa) are poorly resolved and weakly supported. Consider-
able future work is required to determine exactly which taxa
can be referred with confidence to this clade, what charac-
ters diagnose the clade and how basal taxa are related to one
another.
Implications for phylogenetic taxonomy of
Ornithischia
Sereno (1998, 1999b) provided node-based and stem-based
phylogenetic definitions for a large number of ornithischian
clades; alternative definitions for a number of major ornithis-
chian clades have since been provided by Buchholz (2002),
Wagner (2004), various authors in Weishampel et al. (2004a)
and this study (Table 1). The results of this analysis highlight
some of the problems that can occur when clade names are
given phylogenetic definitions on the basis of assumed stabil-
ity and later phylogenetic analyses generate alternate topolo-
gies. For instance, this analysis positions heterodontosaurids
near to the base of Ornithischia, rather than as ornithopods as
suggested by many previous authors. Sereno (1998) provided
a node-based definition for the clade Ornithopoda: ‘Hetero-
dontosaurus,Parasaurolophus, their most recent common
ancestor and all descendents’. When this definition is ap-
plied to the phylogeny produced by this analysis (Fig. 4),
virtually all ornithischians (with the exception of Pisano-
saurus mertii), including ankylosaurs, stegosaurs, ceratop-
sians and pachycephalosaurs, are included within Ornitho-
poda. This is a radically different, and rather unsatisfactory,
taxonomic content to that currently understood. The prob-
lem is avoided if a conservative stem-based definition is used
for Ornithopoda (see Table 1; Buchholz 2002; Wagner 2004;
Norman et al. 2004c). Likewise, Sereno (1998) provided a
stem-based definition for the clade Euornithopoda: ‘All or-
nithopods closer to Parasaurolophus than to Heterodonto-
saurus.’ Applying this definition to the phylogeny produced
by this analysis results in Euornithopoda effectively refer-
ring to the same clade as Genasauria. Again, this is an un-
satisfactory taxonomic content for Euornithopoda, radically
different from its currently understood content. In this case it
would probably have been better not to provide a phylogen-
etic definition for Euornithopoda, given the known unstable
phylogenetic position of the external specifier, Heterodonto-
saurus tucki.
Another taxonomic problem is highlighted by this ana-
lysis. Iguanodontia has generally been used for those taxa
more derived within Ornithopoda than Hypsilophodontidae.
However, such a definition is problematic if, as sugges-
ted by this analysis, Hypsilophodontidae is paraphyletic.
Sereno (1998) proposed the stem-based definition, ‘All eu-
ornithopods closer to Parasaurolophus than to Hypsilopho-
don’. Application of this definition to the phylogeny pro-
duced by this analysis suggests that taxa such as Thescelo-
saurus neglectus and Parksosaurus warreni are iguanodon-
tians. However, such taxa have generally been considered as
24 R. J. Butler et al.
‘hypsilophodontid’ basal ornithopods, outside of Iguanodon-
tia. In response to this problem, Norman (2004) used the
name Iguanodontia for: ‘all euornithopods closer to Edmon-
tosaurus than to Thescelosaurus’. Such a definition may be
satisfactory, provided that the relatively derived position for
Thescelosaurus suggested by Norman et al. (2004c) is ac-
curate. If future analyses find that Thescelosaurus groups
more basally within Ornithischia than other ‘hypsilophodon-
tids’ (see, for instance, Buchholz 2002) then the definition
of Iguanodontia will again be problematic and a node-based
definition for the clade (e.g. Weishampel et al. 2003) may
be preferable. At present we refrain from proposing an al-
ternative phylogenetic definition for Iguanodontia, due to the
instability of basal ornithopod phylogeny.
Acknowledgements
This work was carried out as part of a R.J.B.’s PhD thesis
at the University of Cambridge and the Natural History Mu-
seum, London, and was supervised by P.U., D.B.N., and
Angela Milner. Many thanks are due to Paul Barrett, Jenny
Clack, Catherine Forster and Susannah Maidment, for en-
couragement, critical comments and discussion. Adam Yates
and an anonymous reviewer provided helpful comments that
greatly improved the final version of this manuscript. For
discussion and/or access to unpublished material we addi-
tionally thank Peter Buchholz, Stefan Gabrial, Peter Galton,
Randy Irmis, Peter Makovicky, Joe Parish, Laura Porro, Al
Santa Luca, Paul Sereno, Rod Scheetz, Robert Sullivan, Ron
Tykoski and Adam Yates. PhD funding was provided by
a NERC studentship (NER/S/A/2002/10338) to the Depart-
ment of Earth Sciences at the University of Cambridge, linked
to a CASE award from the Natural History Museum, London.
Additional funding was provided by the Society of Verteb-
rate Paleontology Predoctoral Grant, the Department of Earth
Sciences and Emmanuel College, Cambridge, the Cambridge
Philosophical Society, the Cambridge European Trust, the
Worts Travelling Scholars Fund, the Jurassic Foundation and
the Samuel P. and Doris Welles Research Fund. For access
to specimens in their care, thanks are due to: S. Chapman
(BMNH), R. Clark (BRSMG), D. Pemberton (CAMSM), P.
Jeffries (OUM), S. Kaal (SAM-PK), E. Butler (NM), M.
Raath (BP), M. Holland and J. Horner (MOR), C. Herbel
(SDSM), W. Simpson and P. Makovicky (FMNH), C. Schaff
(MCZ), K. Stadtman (BYU), D. Burge and J. Bird (CEUM),
J. Foster (MWC), B. Albright and J. Gillette (MNA), P. Hol-
royd and K. Padian (UCMP), Z. Zhou and X. Xing (IVPP), G.
Peng (ZDM), L. Kui and X-L. He (GCC), S. Zhou and G. Wei
(CV), M. Reich (GZG), J. Koppka and I. Hinz-Schallreuter
(SGWG), D. Unwin (MB) and H. Osm´
olska and J. Dzik
(ZPAL).
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