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ORIGINAL PAPER
A Middle Jurassic heterodontosaurid dinosaur
from Patagonia and the evolution of heterodontosaurids
Diego Pol &Oliver W. M. Rauhut &Marcos Becerra
Received: 9 October 2010 / Revised: 28 February 2011 / Accepted: 3 March 2011
#Springer-Verlag 2011
Abstract Heterodontosauridae is a morphologically diver-
gent group of dinosaurs that has recently been interpreted as
one of the most basal clades of Ornithischia. Heterodonto-
saurid remains were previously known from the Early Jurassic
of southern Africa, but recent discoveries and studies have
significantly increased the geographical and temporal range for
this clade. Here, we report a new ornithischian dinosaur from
the Middle Jurassic Cañadón Asfalto Formation in central
Patagonia, Argentina. This new taxon, Manidens condorensis
gen. et sp. nov., includes well-preserved craniomandibular
and postcranial remains and represents the only diagnostic
ornithischian specimen yet discovered in the Jurassic of
South America so far. Derived features of its anatomy
indicate that Manidens belongs to Heterodontosauridae, as
the sister taxon of Heterodontosaurus and other South
African heterodontosaurids. The presence of posterior dentary
teeth with high crowns but lacking extensive wear facets in
Manidens suggests that this form represents an intermediate
stage in the development of the remarkable adaptations to
herbivory described for Heterodontosaurus. The dentition of
Manidens condorensis also has autapomorphies, such as
asymmetrically arranged denticles in posterior teeth and a
mesially projected denticle in the posteriormost teeth. At an
estimated total length of 60–75 cm, Manidens furthermore
confirms the small size of basal heterodontosaurids.
Keywords Ornithschia .Gondwana .Jurassic .Cañadón
Asfalto Formation .Heterodontosauridae
Introduction
The fossil record of ornithischian dinosaurs starts in the
Late Triassic (Casamiquela 1967) and extends up to the end
of the Cretaceous. However, the first 70 million years of
ornithischian evolution are still poorly known (Butler et al.
2006,2008a; Irmis et al. 2007), and our knowledge of early
ornithschians is so far based almost entirely on fossils from
a small number of geological units, such as the Elliot
Formation of southern Africa (Knoll 2005; Butler et al.
2007; Irmis and Knoll 2008; Rauhut and Lopez-Arbarello
2008). One of the most conspicuous groups in Early
Jurassic ornithischian assemblages is the Heterodontosauridae.
Whilst it was long thought to represent an early and highly
specialized lineage of derived clades of ornithischian
dinosaurs, such as ornithopods (e.g. Sereno 1986,1999;
Weishampel and Witmer 1990;Normanetal.2004a)or
marginocephalians (Xu et al. 2006), recent research has
reinterpreted this group as one of the most basal and
Communicated by Robert Reisz
Electronic supplementary material The online version of this article
(doi:10.1007/s00114-011-0780-5) contains supplementary material,
which is available to authorized users.
D. Pol (*)
CONICET, Museo Paleontológico Egidio Feruglio,
Fontana 140,
9100 Trelew, Argentina
e-mail: dpol@mef.org.ar
O. W. M. Rauhut
Bayerische Staatssammlung für Paläontologie und
Geologie and Department of Earth and Environmental Sciences,
LMU München, Richard-Wagner-Str. 10,
80333 Munich, Germany
e-mail: o.rauhut@lrz.uni-muenchen.de
M. Becerra
Departamento de Ciencias Geológicas,
Universidad de Buenos Aires,
Ciudad Universitaria Pab. II,
Ciudad Autónoma de Buenos Aires C1428EHA, Argentina
e-mail: tutunomaco@hotmail.com
Naturwissenschaften
DOI 10.1007/s00114-011-0780-5
successful radiation of ornithischians (Butler et al. 2007,
2008a). Furthermore, recent studies have demonstrated that
this group was more widely distributed both geographically
and stratigraphically than previously thought (Norman and
Barrett 2002, Zheng et al. 2009, Butler et al. 2010). These
studies have suddenly placed heterodontosaurids in a key
position for understanding the origin and early evolution of
ornithischian dinosaurs.
The Jurassic record of ornithischian dinosaurs from
Gondwana is of special interest for our understanding of the
early evolution of the group because basal members of the
major lineages are found in the Late Triassic–Early Jurassic of
southern Gondwana, indicating that the group as a whole
might have originated here (Rauhut and Lopez-Arbarello
2008). However, the Jurassic ornithischian fossil record from
Gondwana is currently restricted to the Early Jurassic of the
Upper Elliot Formation of southern Africa (Knoll 2005)and
the Late Jurassic Tendaguru Formation of Tanzania (Aberhan
et al. 2002;Maier2003; Rauhut and Lopez-Arbarello 2008).
Apart from these units, Gondwanan ornithischians are only
known from isolated remains (Barrett et al. 2008; Rauhut
and Lopez-Arbarello 2008) and footprints (e.g. Moreno et al.
2004) that do not provide information for our understanding
of the evolution of the group during the Jurassic.
The Cañadón Asfalto Formation of Chubut Province,
Argentina, has yielded the most diverse and important
Middle Jurassic terrestrial biota of Gondwana (Escapa et al.
2008). Vertebrate groups reported so far include basal
members of every major lineage of terrestrial vertebrates to
be expected in the Jurassic (Escapa et al. 2008), including a
wealth of dinosaur remains (Bonaparte 1979,1986; Rauhut
2003a,2005; Escapa et al. 2008), pterosaurs (Rauhut et al.
2001; Cordoniú et al. 2010), turtles (Sterli 2008), mammals
(Martin and Rauhut, 2005; Rauhut et al. 2002; Rougier et
al. 2007a,b) and anurans (Báez and Nicoli 2008). Until
recently, the dinosaur fauna was exclusively composed of
sauropod and theropod saurischians, with only a single
reported non-diagnostic specimen of an ornithischian
(Rauhut and Lopez-Arbarello 2008). Here, we describe
diagnostic ornithischian remains from a partially articulated
skeleton found in this unit and evaluate its phylogenetic
relationships and its significance for understanding the
evolution of heterodontosaurid dinosaurs.
Systematic palaeontology
Dinosauria Owen, 1842
Ornithischia Seeley, 1887
Heterodontosauridae Kuhn, 1966
Manidens condorensis gen. et sp. nov.
Etymology The genus name Manidens, from manus (Latin,
hand) and dens (Latin, tooth), refers to the similarity of the
posteriormost tooth to the human hand. The specific epithet
condorensis refers to the nearby village of Cerro Cóndor,
Chubut Province, Argentina.
Holotype MPEF-PV 3211 (Museo Paleontológico Egidio
Feruglio, Trelew, Argentina), partial associated skeleton,
including most elements of the skull and lower jaws,
cervical, dorsal and sacral vertebrae, left scapula and
coracoids, and almost complete pelvic girdles.
Referred material MPEF-PV 1719, 1786, 1718, 3810, 3811
isolated posterior teeth, from the same locality and horizon
as the holotype.
Locality and horizon Queso Rallado locality within the
Cañadón Asfalto Formation, 2.3 km west of the village of
Cerro Cóndor, Chubut Province, Argentina. The bone-
bearing layer is a silicified mudstone within a series of
lacustrine mudstones and limestones (Rougier et al. 2007b).
Further locality information is given in the Electronic
supplementary material (ESM) and stratigraphic informa-
tion can be found in Rougier et al. (2007b). GPS
coordinates of this locality are deposited in the MPEF
collections and are available upon request. The age of the
Cañadón Asfalto Formation is usually given as Callovian–
Oxfordian (e.g. Silva Nieto et al. 2002), but recent U–Pb
radioisotopic age determinations (Cabaleri et al. 2010) and
palynological research (Volkheimer et al. 2008) indicates
that it is probably considerably older, with dates ranging
from 171 ± 5 to 167 ± 4 Ma (Aalenian–Early Bathonian;
Salani 2007; Cabaleri et al. 2010).
Diagnosis Small heterodontosaurid (estimated body length
of approximately 50–60 cm) with the following autapo-
morphies: jugal with strongly developed, dorsally placed
lateral boss; dorsal part of the postorbital process of the
jugal very slender and flexes abruptly posteriorly at the
beginning of the articular facet for the postorbital; forebrain
facet on the ventral surface of the frontal enlarged and
with significantly raised margins; posterior teeth with
asymmetric arrangement of denticles and with a mesial
concavity in which the distal margin of the preceding
tooth is lodged; posteriormost dentary tooth “hand-
shaped”, with only one or two mesial denticles, the most
anterior of which diverges mesially from the mesial
margin of the crown; presence of small crenulations
along the cutting margin of each denticle.
Description
The holotype has preserved most elements of the skull and
lower jaws, most of the precaudal axial skeleton, including
cervical, dorsal and sacral vertebrae, the left scapula and
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coracoid, and almost complete pelvic girdles (Fig. 1). Most
of the elements of the skull roof are preserved, with the
exception of the premaxillae, lacrimals, prefrontals and
parietals, which could not be identified with confidence.
Furthermore, the braincase is almost completely preserved,
some fragments of the palate are present, and the mandible
is only missing the predentary (cand din Fig. 2), making
the skull of Manidens one of the most complete hetero-
dontosaurid skulls known. Skull elements were found
mainly disarticulated, although some were found in
articulation (most notably the almost complete right lower
jaw and the left temporal region) or retained a close
association (Figs. 1and 2).
Despite the disarticulation of the skull, a few aspects of
general skull shape and morphology can be evaluated. The
length of the right mandible is 63 mm without the
predentary, which might account for another 5–10 mm
(based on Heterodontosaurus, in which the predentary
accounts for approx. 10% of the length of the mandible).
Because mandible length is similar to skull length in basal
ornithischians, the skull of Manidens was most probably no
more than 75 mm long and thus closely comparable in size
to that of Fruitadens, one of the smallest known ornithis-
chian dinosaurs, for which Butler et al. (2010: Fig. 3e)
reconstructed a skull length of approximately 70 mm. The
preserved jugal and postorbital demonstrate that the orbit
was round and very large (Fig. 2), as in most basal
ornithischians, and, together with the rather small maxilla,
that the preorbital region of the skull was probably rather
short. The lower temporal fenestra was much narrower than
the orbit ventrally, but rapidly increased in anteroposterior
length dorsally, as indicated by the very long posterior
process of the postorbital and anterior branch of the
squamosal (Fig. 2).
The maxilla has a posterodorsally sloping anterior
margin, without a large anterior embayment for the
reception of the enlarged dentary tooth, as it is present in
all other known heterodontosaurids for which this element
is known (Tianyulong: Zheng et al. 2009;Fruitadens:
Butler et al. 2010;Abrictosaurus: Thulborn 1974;Lyco-
rhinus:Gow1975;Heterodontosaurus:Normanetal.
2004a), although the preservation of this area does not rule
out the possibility of a considerably smaller embayment.
The presence of such an embayment in Echinodon is
uncertain (Norman and Barrett 2002). A well-developed
buccal emargination is present and is separated from the
large antorbital fossa by a horizontal ridge, which becomes
more conspicuous posteriorly. This emargination seems to
be more strongly developed than in Echinodon (Norman
and Barrett 2002), Tianyulong (Zheng et al. 2009) and
Fig. 1 Preserved elements of M. condorensis gen. et sp. nov. Outline
reconstruction of the skeleton, indicating preserved elements, with
photographs of selected skeletal elements of the type specimen (MPEF
PV 3211). aPelvic girdle, lateral. bSchematic drawing of pelvic
girdle. cDorsal vertebrae, lateral. dCervical vertebrae, lateral. e
Quadrate, anterior. fTemporal (lateral) and occipital (posterior) skull
elements. fSchematic drawing of temporal and occipital region. il
illium, is ischia, jjugal, or occipital region of the braincase, pb pubis,
po postorbital, pod postorbital depression, pp preacetabular process,
sq lateral shelf of the squamosal. Dashed areas represent broken
surfaces and dotted areas represent sediment. Scale bars indicate
10 mm (a,b,f,g), 2 mm (c,d) and 1 mm (e)
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Fruitadens (Butler et al. 2010), but similar to the condition
in Lycorhinus (Gow 1975)andHeterodontosaurus (SAM
K337). The antorbital fenestra was very small and placed in
the posterodorsal margin of the antorbial fossa of the
maxilla, as in Lesothosaurus (Sereno 1991), Heterodonto-
saurus (Norman et al. 2004a) and other basal ornithischians
(Norman et al. 2004a,b; Barrett et al. 2005). The anterior
ramus of the jugal is slender and formed most of the ventral
margin of the orbit (Fig. 2). At the junction with the
postorbital ramus, a pronounced lateral boss is present, as in
Heterodontosaurus (Norman et al. 2004a) and, probably,
Abrictosaurus (Thulborn 1974). However, in contrast to the
former taxon, in which the boss is ventrally placed, it is
found at the dorsal margin of the bone in Manidens (aand
bin Fig. 2). A jugal boss seems to be absent in Tianyulong,
although the posterior part of the jugal is missing in this
Fig. 2 Anatomical details of the craniomandibular and dental remains
of M. condorensis gen. et sp. nov. aPhotographs of preserved cranial
and mandibular elements of the holotype (MPEF-PV 3211) super-
imposed on a skull reconstruction in lateral view. bReconstruction of
the skull and mandible in lateral view. cPhotograph of right lower jaw
and associated elements of the holotype (MPEF-PV 3211). d
Interpretative drawing of right lower jaw in lateral view (right maxilla,
left lower jaw and other bones shaded in grey); dashed areas represent
broken surfaces and dotted areas represent sediment. e–hSEM images
of posterior mandibular tooth in mesial (e) and bucal (f–i) views
corresponding to MPEF-PV 3810 (e), MPEF-PV 3811 (f)and
holotype MPEF-PV 3211 (g–i). ad anteriormost denticle mesially
offset, afo antorbital fossa, an angular, at anterior teeth of left dentary,
ca caniniform, cr crenulations, ddentary, dc denticles, dv dorsal
vertebra, ffrontal, gglenoid fossa, jb jugal boss, mmaxilla, mc mesial
cavity, nnasal, po postorbital qquadrate, qj quadratojugal, sq
squamosal, sa surangular, saf anterior surangular foramen. Scale bars
indicate 10 mm (a–d), 1 mm (e–g), 0.5 mm (h) and 0.1 mm (i)
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taxon (Zheng et al. 2009). The postorbital process of the
jugal flexes abruptly posteriorly at the beginning of the
articular facet for the postorbital (Fig. 1). The posterior
process of the jugal is short, much higher than the anterior
process, and bifurcates posteriorly, forming a wide angle
between the two processes (aand bin Fig. 2), also
resembling the condition in Heterodontosaurus (Norman
et al. 2004a). Although a bifurcated posterior end of the
jugal is also present in other basal ornithischians (Barrett
and Han 2009) and saurischians (e.g. Sereno and Novas
1993), the two processes usually form a narrower angle and
both contact the quadratojugal, whereas only the dorsal
process contacts the quadratojugal in Heterodontosaurus
and Manidens. The postorbital is triradiate and has a very
long posterior and very short anterior process. A well-
developed lateral boss is present at the posterodorsal margin
of the orbit just below the junction between the anterior and
ventral rami of the postorbital, as in Lesothosaurus (Sereno
1991) and many saurischians (e.g. Sereno and Novas
1993). Posterior to this boss, the postorbital of Manidens
has a depression, resembling the condition of Heterodonto-
saurus tucki (SAM K337). The posterior end of a stout,
rod-like palpebral is preserved above the anterior process of
the left postorbital, indicating that this bone was very long
and probably transversed the whole length of the orbit, in
contrast to the situation in Heterodontosaurus, but as in
some other ornithischians, such as Agilisaurus (Peng 1992;
Barrett et al. 2005). The squamosal has a long, straight
lateral shelf dorsally above its ventral process, considerably
more developed than a similar structure in Heterodonto-
saurus (Norman et al. 2004a). Anterior to this shelf, the
long anterior process of the squamosal flexes notably
ventrally to meet the posterior process of the postorbital,
indicating that the main body of the squamosal was
considerably tilted, as in Heterodontosaurus (Norman et
al. 2004a), Lesothosaurus (Sereno 1991) and Hypsilopho-
don (Galton 1974). The left and right nasals and frontals are
fused at their midline sutures without any visible suture, but
apparently not with each other. The nasals are flat and
gradually widen posteriorly. Their lateral margins are
slightly thickened, as is the case in Heterodontosaurus
(Butler et al. 2008b). The frontal has a considerably
widened, strongly rimmed ventral facet for the forebrain.
The quadrate is slender and only slightly flexed posteriorly.
The quadrate foramen is developed as a large embayment in
the lateral flange of the bone, as in Hypsilophodon (Galton
1974) and iguanodontians. The lateral condyle for the
articulation with the lower jaw is much larger than the medial
condyle, as in other heterodontosaurids and ceratopsians
(Butler et al. 2010).
In the braincase, the foramen magnum has been
dorsoventrally flattened by deformation, but it was
originally larger than the stout, semicircular occipital
condyle (Fig. 1). The paroccipital processes are dorso-
ventrally high, relatively short and have rounded lateral
margins. The post-temporal fenestra is developed as a
large foramen within the paroccipital process, well
separated from the exoccipital–squamosal suture, as in
Heterodontosaurus (Norman et al. 2004a)andGaspar-
inisaura (Coria and Salgado 1996). The basal tubera are
separated by a wide notch below the condyle. The
basisphenoid is wide and short, with a well-developed
ventral basisphenoid recess, resembling the condition in
some basal theropods (Raath 1985; Rauhut 2003b).
No external mandibular fenestra is present in the lower
jaw (Fig. 2), unlike the situation in Heterodontosaurus and
Tianyulong (Norman et al. 2004a; Zheng et al. 2009) and
most other basal ornithischians (e.g. Haubold 1990; Sereno
1991; Butler et al. 2007), but similar to Abrictosaurus
(Thulborn 1974), Scelidosaurus (Owen 1863) and more
derived cerapodans. The dentary is robust and dorsoven-
trally high, in contrast to the more slender dentaries in other
heterodontosaurids (Thulborn 1974; Norman and Barrett
2002; Norman et al. 2004a; Zheng et al. 2009; Butler et al.
2010). It has a strongly enlarged, caniniform first dentary
tooth typical of many heterodontosaurids (Norman et al.
2004a; Zheng et al. 2009; Butler et al. 2010). The dorsal
part of its posterior end extends posterodorsally to form the
anterior margin of the coronoid process, as in all ornithi-
schians. The latter is a well-developed, triangular eminence
that rises to about 160% of the height of the dentary at the
tooth row, which is less than in Tianyulong (Zheng et al.
2009) and Abrictosaurus (Thulborn 1974), although this is
mainly due to the unusual robustness of the dentary. The
surangular forms the entire posterior part of the process in
lateral view, and there is an enlarged anterior surangular
foramen near its dorsal margin, just posterior to the apex of
the coronoid process (Fig. 2). Posteriorly, the surangular
becomes rapidly lower towards the jaw articulation, which
is located considerably ventral to the level of the tooth row
as in Heterodontosaurus and derived ornithopods (Weish-
ampel 1984). The glenoid facet is developed as an elongate
concavity, which is considerably longer anteroposteriorly
than the distal quadrate head and is delimited anteriorly by
a well-developed, triangular dorsal projection of the
surangular. The retroarticular process is a stout, poster-
oventrally directed process with a median ridge along its
posterodorsal surface resembling the condition of some
ornithopods, such as Thescelosaurus (Galton 1997). As in
most ornithischians, this process is shorter than the
mandibular glenoid articulation.
The dentition is strongly heterodont. The exact tooth
count cannot be established due to overlap and incomplete-
ness of some of the elements. However, the lower dentition
is composed of at least 11 teeth, including the hypertro-
phied anterior caniniform of the dentary (cand din Fig. 2)
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This number is slightly lower than in Heterodontosaurus
and Abrictosaurus, which have 13 dentary teeth (Norman et
al. 2004a), but higher than in Tianyulong (nine dentary
teeth; Zheng et al. 2009) and comparable to Fruitadens
(9–11 dentary teeth; Butler et al. 2010). The caniniform is a
stout, recurved pointed tooth that seems to lack marginal
denticles, as in Fruitadens (Butler et al. 2010), but in
contrast to the situation in Heterodontosaurus,Lycorhinus
and Abrictosaurus (Charig and Crompton 1974; Hopson
1975). The first three post-caniniform teeth are very small
and well spaced from each other (but poorly preserved in
MPEF-PV 3211; din Fig. 2). The subsequent dentary teeth
rapidly increase in size posteriorly and become mesiodis-
tally expanded, but the distal-most two teeth slightly
decrease in size so that the highest tooth crowns are
situated at the mid-length of the posterior region of the
tooth row (cand din Fig. 2), similar to the situation in
Fruitadens (Butler et al. 2010) and Abrictosaurus (Thul-
born 1974). In contrast, although there is also an increase in
tooth size from the first to the third post-canine tooth in
Tianyulong and Heterodontosaurus, the decrease in at least
the mesiodistal size of the posteriormost teeth in these taxa
is much less marked (Hopson 1980; Norman et al. 2004a;
Zheng et al. 2009). The crowns of posterior teeth are
asymmetrical, unlike the much more symmetrical dentary
teeth of other heterodontosaurids (Thulborn 1974; Charig
and Crompton 1974; Hopson 1975; Norman and Barrett
2002; Zheng et al. 2009; Butler et al. 2010). They are leaf-
shaped, lateromedially compressed and have a weakly
developed central ridge on the lingual and labial surfaces
(fand gin Fig. 2), more marked than in Fruitadens (Butler
et al. 2010), but apparently less conspicuous than in
Heterodontosaurus (Hopson 1980). Unlike the situation in
Echinodon (Norman and Barrett 2002), Tianyulong (Zheng
et al. 2009) and Fruitadens (Butler et al. 2010), the tooth
crowns are considerably higher than wide, comparable to
the situation in Lycorhinus (Hopson 1975) and Hetero-
dontosaurus (Hopson 1980; Norman et al. 2004a). The
margins of the crowns bear well-developed denticles,
usually one or two mesial to the apex and four or five
distal to the apex, creating the aforementioned asymmetry
in the tooth crown (fand gin Fig. 2). The margin of each
denticle bears small crenulations, which are likely formed
exclusively by ridges on the enamel (g–iin Fig. 2), and
have not been reported in any other heterodontosaurid so
far, although subdivided denticles are present in some other
ornithischians (Bakker et al. 1990; Rauhut 2002). All
crowns have a gentle distal curvature that is more
conspicuous in the apical region (f–hin Fig. 2). Both the
number of denticles and the apical curvature of the crown
vary along the tooth row. The posteriormost dentary tooth
has a distinct “hand-like”appearance, with the anteriormost
denticle being offset mesially from the mesial margin (fin
Fig. 2). The posteriormost six asymmetric teeth are higher
and tightly appressed against each other, unlike the more
widely spaced teeth in Tianyulong (Zheng et al. 2009) and
Fruitadens (Butler et al. 2010), but similar to the situation
in Lycorhinus (Hopson 1975)andHeterodontosaurus
(Hopson 1980; Norman et al. 2004a,b). Each crown bears
a mesial groove, delimited by two ridges (ein Fig. 2),
which houses the distal margin of the preceding tooth, as in
the closely packed dental batteries of more derived
ornithischians.
The cervical vertebrae are shorter than the dorsal
vertebrae and have short and stout diapophyses and para-
pophyses. Strongly elongate hypertrophied epipophyses are
present in the preserved anterior cervicals, as in Hetero-
dontosaurus (Santa Luca 1980). Cervical and dorsal neural
spines are anteroposteriorly elongate and low. There are six
sacral vertebrae, the neural spines of which form a
continuous sheet of bone over the ilium. An anterior caudal
vertebra has a low elongate centrum with well-developed
chevron facets.
Only the left coracoid and proximal portion of the
scapula are preserved. The coracoid has a well-developed,
hook-like posteroventral process that is separated from the
glenoid cavity by a wide notch. The pelvic girdle is
complete, with the exception of the distal ends of the
pubes and ischia (Fig. 1). The ilium is low and elongate,
with a preacetabular process that accounts for approximate-
ly 50% of the total length of the bone. A longitudinal ridge
extends along the lateral surface of the preacetabular
process, but is less well-developed than in Heterodonto-
saurus (Santa Luca 1980). Unlike the latter taxon and other
basal ornithischians, in which the pubic peduncle is longer
than the ischial peduncle, the former is subequal in length
or even slightly shorter than the latter in Manidens.Asin
all ornithischians, the pubis is opisthopubic with a very
slender posteroventral shaft (Fig. 1). The prepubic process
is short, only little extending beyond the pubic peduncle of
the ilium anteriorly, and robust. A small obturator foramen
is present below the acetabulum. The ischium is more
robust than the pubis and has an extensive medial suture
along its shafts, which are rectangular in cross-section. Not
enough of the shaft is preserved to establish the presence or
absence of an obturator process.
Discussion
To test the phylogenetic relationships of the new taxon, we
included it in a slightly modified version of the data matrix
of Butler et al. (2010), which focuses on the interrelation-
ships of basal ornithischian clades and includes the most
extensive taxon sampling of heterodontosaurids published
to date. The final data matrix included 51 taxa and 230
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characters (see ESM) and was analysed using equally
weighted parsimony in TNT, version 1.1 (Goloboff et al.
2008a,b). We performed a traditional heuristic search with
1,000 replicates with random addition sequences followed
by tree bissection–reconnection (TBR) branch swapping,
followed by a second round of TBR.
The analysis resulted in 216 MPTs with a length of 551
steps, the strict consensus of which (Fig. 3) is well resolved
for basal ornithischians. In particular, the results show
phylogenetic relationships for heterodontosaurids that have
both evolutionary and biogeographic implications for the
group. M. condorensis was found to be a heterodontosaurid
more closely related to Early Jurassic southern African
forms than to the later northern representatives of the clade.
The position of Manidens within Heterodontosauridae is
supported by the presence of three of the six unambiguous
synapomorphies of this clade [lateral condyle of quadrate
larger than medial (character 63.2), coronoid process well-
developed (character 101.1), enlarged caniniform anterior
dentary tooth (character 124.1)]. The clade of Manidens
and the Early Jurassic taxa from South Africa clusters all
heterodontosaurids from Gondwana (node 2 in Fig. 3) and
is supported by two unambiguous synapomorphies [lack of
alveolar foramina (character 126.1) and apicobasally high
mid-dentary and maxillary tooth crowns (character 228.0)]
and one ambiguous synapomorphy [angular occupying
more than half the depth of the mandible at the level of
the coronoid process (character 230.1)]. However, Man-
idens is placed as the sister taxon of the South African clade
(node 1 in Fig. 3) since it lacks the unambiguous
synapomorphy of this group [systematic development of
wear facets along the entire tooth row (character 222.1)].
See ESM for a full list of synapomorphic characters of each
clade. The discovery of Manidens in the Middle Jurassic of
Patagonia and the phylogenetic hypothesis presented here
have important implications for understanding several
evolutionary aspects of heterodontosaurids, including the
pace and mode of their radiation and biogeographic history,
the evolution of body size and the evolution of their
adaptations to herbivory.
Radiation and distribution of Heterodontosauridae
Given the wide geographic range of other Early–Middle
Jurassic dinosaurs, such as basal theropods and sauropodo-
morphs (Weishampel et al. 2004; Smith et al. 2007), the
Early
Cretaceous
Early
Jurassic
Middle
Jurassic
Late
Jurassic
Aptian
Barremian
Hauterivian
Valanginian
Berriasian
Tithonian
Kimmeridgian
Oxfordian
Callovian
Bathonian
Bajocian
Aalenian
Toarcian
Pliensbachian
Sinemurian
Hettangian
Rhaetian
Norian
Carnian
Late
Triassic
Herrerasaurus
Pisanosaurus mertii
Echinodon becklessi
Tianyulong confuciusi
Fruitadens haagarorum
Manidens condoriensis
Abrictosaurus consors
Heterodontosauridae
Heterodontosaurus tucki
Lycorhinus angustidens
Eocursor parvus
Lesothosaurus diagnosticus
Stormbergia dangershoeki
Agilisaurus louderbacki
Hexinlusaurus multidens
Orodromeus makelai
Cerapoda
Scutellosaurus lawleri
Emausaurus ernstii
Scelidosaurus harrisonii
Stegosauria
Ankylosauria
Thyreophora
Genasauria
BMNH A100
Ornithischia
1
2
i
i
Fig. 3 Results of the phylogenetic analysis showing the proposed
interrelationships of M. condorensis within ornithischians. The
phylogenetic tree is plotted against geological time to calibrate the
phylogeny and highlight the extensive ghost lineages in basal
heterodontosaurids (grey lines). Derived clades of Ornithischia have
been collapsed into single taxonomic units for clarity. For details of
phylogenetic analysis, see text and ESM
Naturwissenschaften
apparent restriction of derived heterodontosaurids to southern
Gondwana is somewhat surprising. Furthermore, the fact that
the two most basal heterodontosaurids, Fruitadens and
Tianyulong, are currently only known from the Northern
Hemisphere (though in much higher stratigraphic levels)
seems to contradict the hypothesis that the early ornithischi-
an radiation took place in southern Gondwana (Rauhut and
Lopez-Arbarello 2008). However, the phylogenetic uncer-
tainty on the position of some forms, such as Echinodon
from the Middle Purbeck Beds (Berriasian) of southern
England (see Sereno 1997; Norman and Barrett 2002),
fragmentary remains from the Late Triassic of southern
Patagonia (Báez and Marsicano 2001) and the undescribed
remains from the Kayenta Formation (Attridge et al. 1985;
Sues et al. 1994;Sereno1997), precludes the assessment of a
robust biogeographic scenario for heterodontosaurids. Fur-
thermore, the minute size of some heterodontosaurids (see
below) and the generally poor Early and Middle Jurassic
fossil record may explain their disparate distribution. Thus,
future finds may change our view of both heterodontosaurid
and basal ornithischian evolution and biogeography.
However, based on the currently available information, the
phylogenetic analysis indicates that there was an early
radiation of heterodontosaurids at least in the Early Jurassic
and probably in the Late Triassic (as suggested by the remains
from this time found in southern Patagonia; Báez and
Marsicano 2001). The phylogenetic position of Early
Jurassic forms indicates that several lineages must have been
generated in this initial radiation of Heterodontosauridae,
including the lineages leading towards Tianyulong and
Fruitadens and the derived and probably endemic clade
from southern Gondwana (node 2 in Fig. 3). The occurrence
of Manidens in the Middle Jurassic of central Patagonia
demonstrates that this clade still persisted at least into the
Middle Jurassic. The initial radiation of heterodontosaurids
(and the phylogenetic positions of Tianyulong and Fruita-
dens, and probably Echinodon) implies at least two other
long-lived lineages of heterodontosaurids (Fig. 3). These
mark a major mismatch between the phylogenetic topology
obtained for heterodontosaurids and the order of appearance
of taxa in the fossil record (see ESM), which further
underlines the extremely poor fossil record of this clade.
The undescribed form from the Kayenta Formation indicates
that the group might have achieved a Pangean distribution by
the Early Jurassic. Thus, it is expected that future discoveries
will show an even higher diversity and wider geographic
distribution of heterodontosaurids in the Early Jurassic (and
possibly also in the Late Triassic).
Ontogenetic stage and body size
One of the aspects of the evolution of Heterodontosauridae that
was recently emphasized is that post-Early Jurassic forms
represent some of the smallest ornithischian dinosaurs,
including the smallest known adult ornithischian to date (i.e.
Fruitadens;Butleretal.2010). This is confirmed by the find
of Manidens: Based on the closely comparable size of the
cranial remains, this taxon was probably in the same size
range as Fruitadens, for which Butler et al. (2010) estimated a
total body length of 65–75 cm and a mass of <1 kg. One
important question, of course, is whether the type of
Manidens represents a juvenile or an adult or subadult
individual. Butler et al. (2008b) demonstrated that ontogenetic
changes in Heterodontosaurus included a relative lengthening
of the snout, an increase of the number of teeth and the fusion
of several sutures in the skull, such as the nasal–nasal suture.
Although the relatively large orbits, short snout and low
number of teeth in Manidens (in comparison with adult
Heterodontosaurus) might thus be taken as indications that
the type specimen represents a juvenile, there are several
aspects that argue against this interpretation. First, the number
of teeth in the dentary of Manidens (11) is lower than that of
large individuals of Heterodontosaurus (13; Norman et al.
2004a), but it is comparable to or even higher than that in the
basal heterodontosaurids Tianyulong (9) and Fruitadens
(9–11), which are of comparable size (Zheng et al. 2009;
Butler et al. 2010). Second, the interfrontal and internasal
contacts in the type specimen of Manidens are fused without
any visible suture, which, according to Butler et al. (2008b),
would indicate that it is not a juvenile. Finally, the neural
arches of the presacral vertebrae in the type specimen are also
fused to their respective centra without any visible suture.
Butler (2010: 676) discussed the timing of neurocentral fusion
in ornithischians and came to the conclusion that “neuro-
central fusion may only have occurred in the very oldest
individuals of basal ornithischian species.”Thus, the type
specimen might not only represent a sexually mature, but
even an old individual (see also Brochu 1996; Irmis 2007).
Given that other basal ornithischians, such as Pisanosaurus
(Bonaparte 1976), Eocursor (Butler 2010), Lesothosaurus
(Sereno 1991;Knoll2002)andScutellosaurus (Colbert
1981) are slightly larger animals, there seems to have been
a tendency towards miniaturization in basal heterodontosaur-
ids. The “juvenile”characters of small heterodontosaurids,
such as the low number of teeth and the large orbit and short
snout, might thus be explained as a result of heterochrony,
which seems to have played a role in miniaturization in other
reptiles as well, such as lepidosaurs (Rieppel 1984). The
same process might have played a role in the evolution of
maniraptoran theropods, as well, in which small forms also
display large orbits (e.g. Turner et al. 2007).
Evolution of herbivory in Heterodontosauridae
The discovery of Manidens and the phylogenetic hypothesis
presented here indicate a trend towards greater sophistication
Naturwissenschaften
in the adaptations to herbivory within the phylogeny of
Heterodontosauridae. As Butler et al. (2010)noted,three
adaptations to herbivory are present in Heterodontosaurus,
including closely packed teeth, high tooth crowns, and
extensive wear facets on the maxillary and dentary teeth.
Basal heterodontosaurids, such as Tianyulong and Fruita-
dens, lack these adaptations and bear lower crowns that are
more widely spaced and lack extensive wear facets. The new
taxon fills the gap between the basal forms and the derived
South African clade of Heterodontosaurus in having high
and closely appressed crowns but lacking extensive wear
facets. Thus, the pattern of character acquisition in hetero-
dontosaurids indicates that an increase in tooth crown height
preceded the development of extensive wear facets in this
clade (Fig. 3). This is in marked contrast to the evolutionary
pattern observed in other groups of ornithischian dinosaurs in
which the evolution of sophisticated chewing mechanisms
(involving intensive wear on the teeth) predated the increase
in relative crown height and the appearance of high tooth
batteries in iguanodontians (Weishampel 1984) and ceratop-
sians (You and Dodson 2004;Xuetal.2006). The evolution
of herbivory in these other groups of dinosaurs resembles the
pattern of character acquisition present in other groups of
vertebrates (e.g. Cenozoic herbivorous mammals; Pascual
1996), placing the evolution of herbivory in heterodonto-
saurids as a distinct case among amniotes. It is noteworthy
that this increased sophistication of the dietary adaptations in
the southern African heterodontosaurids, which might
account for their relative diversity in the Elliot Formation,
does not contradict the idea that the more generalized
dentition of basal heterodontosaurids favoured their longev-
ity as evolutionary lineages (Butler et al. 2010).
The discovery of a heterodontosaurid in the Middle
Jurassic of Patagonia underlines the recently recognized
diversity of the clade, increases the recently extended
geographical and temporal range of this clade, and shows
that heterodontosaurids remained a component of herbivo-
rous dinosaurian faunas of Gondwana at least until the
Middle Jurassic. Given the general rarity of small terrestrial
vertebrates in many Jurassic localities, the absence or rarity
of heterodontosaurids might represent an artefact of the
fossil record. The relative diversity and abundance of these
animals in comparison to that of large-bodied dinosaurs
was probably higher than previously suspected and will
likely be further appreciated in light of future discoveries.
Acknowledgements We thank Guillermo Rougier for collecting the
specimen described here. Fieldwork was only possible with the
friendly support of the Farias family, the Subsecretaría de Cultura of
the Chubut province and the School number 31 at Cerro Cóndor.
Leandro Canessa carried out the very delicate preparation of the type
specimen and Pablo Puerta and Leandro Canessa prepared the isolated
teeth. Fieldwork was supported by the project “Paleontological
Exploration of Patagonia”(Fundación Antorchas and University of
Louisville) and NSF DEB-0946430 for the study of Mesozoic
mammals from South America (to Guillermo Rougier). This research
was conducted with the support of ANPCYT (grant PICT 1756) and
international collaboration grant CONICET-Chinese Academy of
Sciences to DP, and DFG (grant RA 1012/9-1) to OR. Access to
SEM lab was possible thanks to ALUAR Aluminio Argentino, and the
technical help of Mr. Jaime Groizard is deeply appreciated. R. Butler
is deeply thanked for providing constructive comments and discussion
about this specimen. Line drawings of 2 have been conducted by Mr.
Jorge Gonzalez. Ms. Sheena Kaal is thanked for access to comparative
specimens of Heterodontosaurus at the Iziko-South African Museum
to DP. Mr. Zheng Xiao-Ting is thanked for access to comparative
specimens of Tianyulong at the Shandong Tianyu Museum of Nature
to DP. Ms. Zhang Xiaomei is also thanked for her help, and Dr. Xu
Xing is thanked for providing access to other relevant comparative
material. Critical comments by Richard Butler and four anonymous
reviewers have considerably helped to improve the manuscript.
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