Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina

Abstract

Pisanosaurus mertii was originally described on the basis of an incomplete skeleton from the early Late Triassic (Carnian) of northern Argentina. It is consistently regarded by most authors as a very basal ornithischian, the sister group of remaining members of the clade. The referral to Ornithischia is based mainly on tooth-bearing bones and tooth morphology. On the other hand, the postcranium is recognized as strikingly plesiomorphic for ornithischians, and even for dinosaurs. The recent description of non-dinosaurian dinosauriforms of the clade Silesauridae having ornithischian-like dentition invites a review of the phylogenetic affinities of Pisanosaurus. In this regard, an overview of the holotype specimen allows a reanalysis of previous anatomical interpretations of this taxon. The phylogenetic analysis presented here suggests that Pisanosaurus may be better interpreted as a member of the non-dinosaurian Silesauridae. It shares with silesaurids reduced denticles on the teeth, teeth fused to maxilla and dentary bone, sacral ribs shared between two sacral vertebrae, lateral side of proximal tibia with a fibular flange, and dorsoventrally flattened pedal ungual phalanges. The present analysis indicates that Pisanosaurus should be removed from the base of the Ornithischia and should no longer be considered the oldest representative of this dinosaurian clade.

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Phylogenetic reassessment of Pisanosaurus mertii
Casamiquela, 1967, a basal dinosauriform from
the Late Triassic of Argentina
Federico L. Agnolín & Sebastián Rozadilla
To cite this article: Federico L. Agnolín & Sebastián Rozadilla (2017): Phylogenetic reassessment
of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of
Argentina, Journal of Systematic Palaeontology, DOI: 10.1080/14772019.2017.1352623
To link to this article: http://dx.doi.org/10.1080/14772019.2017.1352623
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Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967,
a basal dinosauriform from the Late Triassic of Argentina
Federico L. Agnol
ın
a,b
*and Sebasti
an Rozadilla
a
a
Laboratorio de Anatom
ıa Comparada y Evoluci
on de los Vertebrados, Museo Argentino de Ciencias Naturales ‘Bernardino
Rivadavia’, Av.

Angel Gallardo 470, C1405DJR Buenos Aires, Argentina;
b
Fundacion de Historia Natural ‘Felix de Azara’,
Universidad Maim
onides, Hidalgo 775, C1405BDB Buenos Aires, Argentina
(Received 24 November 2016; accepted 26 June 2017; published online 4 August 2017)
Pisanosaurus mertii was originally described on the basis of an incomplete skeleton from the early Late Triassic (Carnian)
of northern Argentina. It is consistently regarded by most authors as a very basal ornithischian, the sister group of
remaining members of the clade. The referral to Ornithischia is based mainly on tooth-bearing bones and tooth
morphology. On the other hand, the postcranium is recognized as strikingly plesiomorphic for ornithischians, and even for
dinosaurs. The recent description of non-dinosaurian dinosauriforms of the clade Silesauridae having ornithischian-like
dentition invites a review of the phylogenetic affinities of Pisanosaurus. In this regard, an overview of the holotype
specimen allows a reanalysis of previous anatomical interpretations of this taxon. The phylogenetic analysis presented here
suggests that Pisanosaurus may be better interpreted as a member of the non-dinosaurian Silesauridae. It shares with
silesaurids reduced denticles on the teeth, teeth fused to maxilla and dentary bone, sacral ribs shared between two sacral
vertebrae, lateral side of proximal tibia with a fibular flange, and dorsoventrally flattened pedal ungual phalanges. The
present analysis indicates that Pisanosaurus should be removed from the base of the Ornithischia and should no longer be
considered the oldest representative of this dinosaurian clade.
Keywords:Pisanosaurus mertii; Ornithischia; Silesauridae; Late Triassic; Argentina
Introduction
Pisanosaurus mertii was originally described by Casami-
quela (1967) on the basis of a poorly preserved but articu-
lated skeleton (Bonaparte 1976). The only known
specimen comes from the upper levels of the Ischigualasto
Formation, late Carnian–Early Norian (late Triassic), at
the Hoyada de Las Lajas fossiliferous locality, La Rioja
province, Argentina (see details in Novas 2009). As noted
by previous authors (Irmis et al. 2007; Sereno 2012), the
poor preservation of the specimen is the largest difficulty
to overcome when interpreting its morphology.
In the original description, Casamiquela (1967) consid-
ered Pisanosaurus to belong to the Ornithischia, and since
then, it has invariably been considered the oldest representa-
tive of that dinosaurian clade (Bonaparte 1976,1996;Novas
1989,1996,2009; Norman & Weishampel 1990; Sereno
1991,2012;Normanet al.2004; Brusatte et al.2010;
Langer et al.2010; Butler 2010). Its phylogenetic position
within ornithischians is problematic. Originally regarded as
an ornithopod, its aberrant features led it to be placed in its
own family: Pisanosauridae Casamiquela, 1967. Later, it
was allied to basal ornithischians such as Heterodontosauri-
dae, ‘Fabrosauridae’ or even ‘hypsilophodontid’ ornitho-
pods (Casamiquel 1967; Thulborn 1971,1972; Galton
1972,1986; Charig & Crompton 1974; Bonaparte 1976;
Cooper 1985; Weishampel 1984; Crompton & Attridge
1986; Sereno 2012), or as the most basal member of all
known ornithischians (Novas 1989,1996;Weishampel&
Witmer 1990; Sereno 1991,1999; Langer 2004; Butler
2005; Butler et al. 2007,2008; Langer & Benton 2006;
Boyd 2015). The combination of highly derived tooth mor-
phology and plesiomorphic postcranial anatomy resulted in
uncertainty about the phylogenetic position of Pisanosau-
rus, and based on this incongruence, some authors suggested
that the only known specimen may be a chimaera (Sereno
1991,2012;Normanet al. 2004), and even doubted its
ornithischian affinities (Thulborn 2006;Bonaparte2007;
Irmis et al. 2007;Olsenet al. 2011; Padian 2013).
Pisanosaurus is one of a few proposed late Triassic
ornithischians, and the only one that is unambiguously
Upper Triassic in age (Olsen et al.2011; Padian 2013).
The phylogenetic placement of Pisanosaurus is of prime
importance because it would be the only unambiguously
*Corresponding author. Email: fedeagnolin@yahoo.com.ar
ÓThe Trustees of the Natural History Museum, London 2017. All rights reserved.
Journal of Systematic Palaeontology, 2017
https://doi.org/10.1080/14772019.2017.1352623
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dated Upper Triassic ornithischian in the world (Olsen
et al.2011) and so would have the potential to shed much
light on ornithischian origins and early dinosaur evolution
in general.
As recognized by most authors, the ornithischian affili-
ation for Pisanosaurus is based mainly on its derived den-
tition and tooth-bearing bones. On this evidence, Norman
et al.(2004) indicated that cranial remains of Pisanosau-
rus (mainly the dentition) suggest its placement within
Genasauria (Thyreophora CNeornithischia) or even Cera-
poda (Marginocephalia COrnithopoda). However, it is
worth noting that archosaurs, other than ornithischians,
showing such a specialized dentition were unknown
before the early 2000s. This dramatically changed with
the description of Silesaurus opolensis, a non-dinosaurian
dinosauriform with strongly specialized ornithischian-like
dentition, coming from the Upper Triassic of Poland
(Dzik 2003). In the same line, Parker et al. (2005; see also
Irmis et al.2007) indicated that the discovery of this kind
of dentition in the pseudosuchian Revueltosaurus and the
basal silesaurid dinosauriforms hints that the diversity of
Triassic archosaurs with highly derived masticatory mor-
phology is still poorly understood. This finding resulted in
the recognition that herbivorous-like dentition was far
more widespread than previously thought, and that the
acquisition of herbivory occurred more than once within
archosaurs.
Silesaurus was the first recognized member of Silesaur-
idae, a clade that is currently recognized as the sister
group to Dinosauria (Dzik 2003; Irmis et al.2007; Bru-
satte et al.2010; Langer et al.2010; Nesbitt et al.2010;
Nesbitt 2011). Since their recognition, silesaurids have
been described from Poland (Dzik 2003; Piechowski &
Dzik 2010), North America (Ezcurra 2006), Brazil (Feri-
golo & Langer 2007), Africa (Nesbitt et al.2010; Kam-
merer et al.2012; Peecook et al.2013) and Argentina
(Irmis et al. 2007; Mart
ınez et al. 2013). In this context,
several authors (Irmis et al. 2007; Novas 2009; Padian
2013) expressed some doubts on the ornithischian and
even dinosaurian affinities of Pisanosaurus.
Addressing the same issue, Dzik (2003) and Ferigolo &
Langer (2007) proposed, without a numerical phyloge-
netic analysis, that silesaurids may be the ancestors of
Ornithischia. This hypothesis was criticized by some
authors, and most numerical analyses have found silesaur-
ids to lie outside Dinosauria (Nesbitt et al.2010; Nesbitt
2011; Bittencourt et al. 2014). More recently, Langer &
Ferigolo (2013) presented a more detailed analysis in
which silesaurids were found to be polyphyletic, and
some were basal ornithischians. However, as recognized
by Langer & Ferigolo (2013), this hypothesis was not
strongly supported, and most subsequent authors have not
followed this proposal.
Thus, the recognition of ornithischian-like dentition in
non-dinosaurian dinosauriforms of the clade Silesauridae
invites a reassessment of some anatomical features of
Pisanosaurus and a reconsideration of its phylogenetic
position on the basis of the discoveries made over the last
few decades. We also redescribe and analyse in detail
some anatomical details of this taxon, in order to help elu-
cidate its phylogenetic position and skeletal morphology.
Material and methods
Pisanosaurus mertii comes from the Upper Triassic (late
Carnian–Early Norian) Ischigualasto Formation of north-
western Argentina (Casamiquela 1967; Martinez et al.
2011,2013). The holotype specimen was found at the
Hoyada de Cerro Las Lajas fossil locality in La Rioja
province (Casamiquela 1967; Bonaparte 1976). As indi-
cated by Bonaparte (1976), it came from the uppermost
levels of the Ischigualasto Formation, and was discovered
in the same strata and locality as the holotype of Venatico-
suchus rusconii (Bonaparte 1970; Von Baczko et al.
2014) and a specimen referred to Trialestes romeri (Bona-
parte 1970; Leucona et al. 2016). The uppermost levels of
these outcrops have been dated at 225.9 §0.9 Ma
(Mart
ınez et al.2011).
Association of the elements in the specimen
The holotype and only known specimen (PVL 2577 in the
Colecci
on de Paleontolog
ıa de Vertebrados, Instituto
Miguel Lillo, Tucum
an, Argentina) consists of a fragmen-
tary skeleton including partial upper and lower jaws, a
series of seven articulated dorsal vertebrae, four fragmen-
tary vertebrae of uncertain position in the column (identi-
fied as caudal vertebrae by Casamiquela (1967) and
Thulborn (2006), and as cervical vertebrae by Bonaparte
(1976)), the impression of the central portion of the pelvis
and sacrum, an articulated partial hind limb including the
right tibia, fibula, proximal tarsals and pedal digits III and
IV, the distal ends of the right and left femora, a left scap-
ular blade (currently lost), a probable metacarpal III, and
the impressions of some metacarpals (currently lost).
Sereno (1991,2012) claimed that the skull fragments,
pelvis and distal hind limb may well belong to a single
individual, but the fragmentary scapula, metacarpus and
vertebral column belonged to another taxon. Sereno
(1991) indicated that the proportions of the scapula and
vertebrae do not resemble the shape or relative size of
comparable bones in heterodontosaurids. Furthermore, he
argued that there was weak justification for the association
of bones and impressions, now lost, which differ markedly
from those in other heterodontosaurids. Following this,
Norman et al.(2004) suggested that the crania and post-
crania might belong to separate taxa based on their con-
flicting phylogenetic affinities, with skull bones resem-
bling ornithischians, and postcrania that are congruent
2 F. L. Agnol
ın and S. Rozadilla
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with an animal of non-dinosaurian affinities. However, as
contested by Irmis et al. (2007), the specimen cannot be
regarded as a chimaera based simply on character incon-
gruence between different regions of the body. Further-
more, Novas (2009) accepted the association of the
elements in the holotype of Pisanosaurus, which was
found articulated, as indicated by Casamiquela (1967) and
Bonaparte (1976). The latter included a sketch of the spec-
imen as found by him in the field, and clearly demon-
strates that it belongs to a single, articulated individual.
This was pointed out by J. F. Bonaparte (pers. comm.),
who corroborated the finding and degree of articulation of
the specimen. Therefore, we here consider the holotype of
Pisanosaurus mertii to be a single individual, as originally
advocated by Casamiquela (1967).
Systematic palaeontology
Dinosauromorpha Benton, 1984
Dinosauriformes Novas, 1992
Family Silesauridae Langer, Ezcurra, Bittencourt
& Novas, 2010
Genus Pisanosaurus Casamiquela, 1967
Pisanosaurus mertii Casamiquela, 1967
Emended diagnosis. Pisanosaurus is a basal dinosauri-
form diagnosable by the following autapomorphies: cen-
tral teeth bilobate in occlusal view, showing well-
developed mesial and distal grooves; distal end of the tibia
anteroposteriorly longer than transversely wide; bilobate
astragalus in distal view; ascending process of the astraga-
lus being subquadrangular and robust in lateral view;
intense transversal compression of the calcaneum.
Descriptive notes. Because Pisanosaurus mertii was
described and commented on in detail by several authors
(e.g. Casamiquela 1967; Bonaparte 1976; Sereno 1991,
2012; Novas 1989,2009), we will not attempt a complete
description of the specimen. Thus, only novel observa-
tions and the reinterpretation of some anatomical features
are noted below.
Tooth-bearing bones. The poor preservation of the
specimen presents the largest difficulty in interpreting its
morphology, and the recognition of several features is ten-
tative. For example, as recognized by Casamiquela
(1967), lower jaw bony elements are very difficult to
delimit. In fact, post-dentary elements are badly weath-
ered and sutures between the bones are not recognizable;
thus, the extension and morphology of articular, surangu-
lar, coronoid and other bones are not certain (Fig. 1).
Casamiquela (1967) and Bonaparte (1976) indicated
that a mandibular fenestra was absent in Pisanosaurus,a
conclusion followed by Irmis et al. (2007). In contrast,
Sereno (1991,2012) proposed that a small opening may
represent a small mandibular fenestra. More specifically,
Sereno (2012, p. 18) stated that “The curved margin of a
small mandibular fenestra appears to be preserved in two
short sections in the wall of the adductor fossa.” However,
a detailed examination of the actual specimen indicates
that the fenestra is a broken surface of the mandible, as all
its margins comprise irregularly broken bone edges
(Fig. 2). We were unable to find any sign of the presence
of a mandibular fenestra in any portion of the specimen.
Given this, we concur with the original interpretation of
Casamiquela (1967) and consider the apparent mandibular
fenestra to be a preservational artifact.
Langer (2004) indicated that ornithischians, including
Pisanosaurus, share the presence of an expanded ventral
border of the mandibular symphysis. In this regard,
Langer & Benton (2006) proposed that the ventral margin
of the rostral tip of the dentary in Pisanosaurus is medi-
ally expanded, so that the symphysis is caudally extended.
However, the symphyseal region of the dentary in Pisano-
saurus is greatly damaged and a dentary symphysis or
articulation scar is not clearly recognizable in it. Although
a transverse thickening of the anterior portion of the den-
tary exists in the specimen, this is strongly distorted and
weathered, and its recognition as part of the dentary sym-
physis is far from certain.
It is worth mentioning that the maxilla and dentary in
Pisanosaurus have about 15 tooth positions, as occurs in
Silesaurus and Sacisaurus (Ferigolo & Langer 2007), as
well as heterodontosaurids (Sereno 2012; Butler et al.
2012).
Dentition. The dentition of Pisanosaurus has been
interpreted in different ways by authors who studied the
holotype. In fact, both maxillary and dentary teeth are
strongly fragmented, weathered and cracked, with the
whole snout surface being composed of tiny intermixed
fragments of bone and enamel. Because of this, a correct
interpretation of the number and morphology of the teeth
is difficult (Fig. 3).
At first sight, the teeth are not set in sockets or dis-
crete alveoli in the dentary or the maxilla. The better
preserved tooth bases are on the dentary teeth, and
available roots are continuous with the dentary bone,
indicating some kind of ankylothecodonty (sensu Chat-
terjee 1974). Casamiquela (1967) considered that the
first dentary tooth was absent and a subcircular impres-
sion of the bone, interpreted by Casamiquela (1967)as
a possible broken root, waspresent.However,this
structure is continuous with the dentary bone and is
better interpreted as a cross section of the base of the
root of an ankylosed tooth (Fig. 4). Computed tomog-
raphy (CT) scanning would confirm the hypothesis of
ankylothecodont dentition but, unfortunately, because
Pisanosaurus is a unique and very valuable specimen,
it is not currently possible to scan it.
Phylogenetic reassessment of a Late Triassic basal dinosauriform 3
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The teeth are mesiodistally compressed and labiolin-
gually expanded, the anterior teeth being much more
labiolingually compressed than the posterior ones. In
occlusal view, the main axis of the crowns is obliquely
oriented with respect to the main axis of the dentary.
Anterior and posterior teeth are smaller dorsoventrally,
Figure 1. Pisanosaurus mertii holotype. Right lower mandible in medial (A, C) and lateral (B, D) views. Abbreviations: d, dentary
bone; gf, glenoid fossa; pd, postdentary bones; f, broken area interpreted by some authors as the mandibular fenestra. Scale bar D5 cm.
4 F. L. Agnol
ın and S. Rozadilla
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lower than more centrally located teeth. Anterior dentary
teeth are relatively globular, and show gently convex
mesial, distal, labial and lingual surfaces when observed
in occlusal view (Fig. 5). In some teeth, there appears to
exist an oblique masticatory facet on the labial surface.
Central teeth are bilobate in occlusal view, and show
well-developed mesial and distal grooves, a condition
unknown in other herbivorous taxa and a trait that may be
an autapomorphy of Pisanosaurus. In other herbivorous
basal dinosauriforms, such as Silesaurus,Fruitadens,Mani-
dens and Panphagia, the teeth are leaf-shaped and ovoidal
in cross section, having a mesiodistally extended major axis
(Dzik 2003;Mart

ınez & Alcober 2009; Butler et al. 2010;
Norman et al.2011; Becerra et al.2013). This morphology
is very different from the condition in Pisanosaurus.
Central and posterior teeth show globous and mesiodis-
tally expanded crown bases, a condition more evident in
the posteriormost teeth that consequently show a slight
overlap of their bases. The base is extensive due to the
presence of a strongly bulbous, step-like labial cingulum
(contra Norman et al.2004)(Fig. 5). This large cingulum
is even more developed in posterior teeth. The crown-root
constriction is deep, and the base of the crown is mesio-
distally and labiolingually wider than the root. The root is
subcircular in section.
Maxillary teeth are relatively globular and homoge-
neous in size and shape. They show a slightly convex
labial margin and a convex lingual masticatory surface.
The lingual cingulum is globous and strongly convex. The
mesial and distal margins are smooth, devoid of carinae
or denticles (Fig. 5).
Both dentary and maxillary teeth lack any sign of den-
ticulated carinae (Fig. 5). In fact, carinae are not clearly
recognizable on any teeth. It is probable that previous
authors (e.g. Casamiquela 1967; Bonaparte 1976) mistook
broken and fragmented pieces of enamel present on the
mesial and distal margins of several teeth for carinae and
denticles. The enamel of the teeth is of similar thickness
in both labial and lingual margins. B
aez & Marsicano
(2001) indicated that ridges were absent in the maxillary
teeth of Pisanosaurus. Although the crowns of the maxil-
lary teeth are strongly damaged and their surface is nearly
unrecognizable, some teeth bear clear dorsoventral ridges
and a large primary ridge appears to be present in at least
a few teeth, resembling some heterodontosaurids (Sereno
2012).
As correctly interpreted by Sereno (1991), the teeth do
not form a palisade or continuous masticatory surface as
advocated by some authors (Bonaparte 1976; Norman
et al.2004). In fact, the angle of the masticatory surfaces
varies from tooth to tooth, and tooth margins are not
squared for close apposition. Furthermore, there are no
clear masticatory facets in most available posterior maxil-
lary and dentary teeth. Small, obliquely oriented wear fac-
ets appear to be present in the anteriormost dentary teeth.
We interpret supposed wear facets in posterior teeth as
possible deformed and weathered surfaces rather than
masticatory facets. Given this, it is worth mentioning that
the purported masticatory surface on the crown of several
dentary teeth is not flat but slightly convex, contrasting
with the condition of true masticatory surfaces present in
most basal ornithischians (Galton 1973; Sereno 2012).
Uncertain vertebrae. Bonaparte (1976) mentioned
the presence of cervical vertebrae (Fig. 3). This author
reconstructed the prezygapophyses as relatively long and
finger-like, whereas the postzygapophyses are short, stout
and located at mid-length of the centrum. The centrum is
elongate and parallelogram-shaped. The caudal articular
surface is sub-quadrangular in contour, with a concave
ventral margin. This element is very different from the
cervical vertebrae described for basal dinosauriforms and
ornithischians (as noted by Sereno 2012). In contrast, the
elongate prezygapophyses and the anterior position of the
postzygapophyses are very similar to caudal vertebrae
(Casamiquela 1967), being almost indistinguishable from
those described in basal dinosauriforms such as Sacisau-
rus (Langer & Ferigolo 2013). In this regard, Langer &
Benton (2006) followed Casamiquela (1967) in consider-
ing these elements to be caudal vertebrae. However, due
to the incomplete and distorted nature of these vertebrae,
we prefer to consider them of uncertain position within
the vertebral series.
Dorsal vertebrae. In Pisanosaurus these are repre-
sented by a poorly preserved and highly distorted series of
seven complete and one incomplete element. In spite of
strong deformation, most morphological features are still
recognizable (Fig. 6). Vertebral centra are very elongate
and strongly transversely compressed, with a rounded
Figure 2. Pisanosaurus mertii holotype. A, view of the com-
plete right mandible; red [grey] square marks the enlarged sec-
tion. B, broken surface interpreted by some authors as the
mandibular fenestra. Not to scale.
Phylogenetic reassessment of a Late Triassic basal dinosauriform 5
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ventral margin, lacking any sign of a longitudinal keel.
The lateral surfaces of the centra bear a poorly developed
and shallow anteroposteriorly extended ellipsoidal exca-
vation. The neural spines are subrectangular in contour,
proximodistally tall, and strongly anteroposteriorly elon-
gate. However, because of their poor preservation, the
total length of the spines cannot be ascertained. The verte-
brae show strong and complex systems of laminae on their
neural spine and centrum. Well-developed prezygo-para-
pophyseal laminae are present, with acute and subhorizon-
tally oriented prezygo-diapophyseal and postzygo-
diapophyseal laminae. Parapo-diapophyseal and centro-
diapophyseal laminae are acute, well developed and
obliquely oriented (Fig. 7). In ornithischians such laminae
are occasionally present, as in Lesothosaurus (Butler et al.
2012), but are notably feebly developed.
Figure 3. Pisanosaurus mertii holotype. A, B, incomplete right maxilla in lateral (A) and occlusal (B) views. C, D, possible metacarpal
III in lateral? (C) and medial? (D) views. E, two fragmentary ?cervical vertebrae in right lateral view purported cervical vertebrae. F–I,
third right metatarsal in medial (F), anterior (G), posterior (H) and lateral (I) views. J–M, fourth right metatarsal in anterior (J), lateral
(K), medial (L) and posterior (M) views. Abbreviations: ns, neural spine; prz, prezygapophysis; vc, vertebral centrum. Scale bars:
A–D D1 cm; E–M D2 cm.
6 F. L. Agnol
ın and S. Rozadilla
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Sacral count. The sacral count in Pisanosaurus has
been interpreted in different ways by several authors. This is
because the sacrum is preserved only as moulds in rock and
is therefore difficult to interpret. Bonaparte (1976) recog-
nized the presence of five sacral vertebrae, Langer & Benton
(2006)suggestedfiveorfoursacralelements,andSereno
(1991) interpreted two sacrals, while Irmis et al. (2007)con-
sidered that there is no trace of sacral elements on the pre-
served remains, an opinion refuted by Sereno (2012).
We concur with Irmis et al. (2007) that some features
previously considered to be impressions of sacral ribs are
actually cracks in the matrix, and there is insufficient
fidelity to determine whether any of the centra are fused
to each other. A detailed examination of the block and its
latex mould suggests the existence of four sacrals. In fact,
the centrum of the first caudal vertebra is preserved and is
more transversely compressed than centra here interpreted
as sacrals (Fig. 8). On the other side, the last dorsal is also
poorly preserved as a mould. This element is more trans-
versely compressed than sacral elements and lacks
expanded sacral ribs.
In ventral view, sacral centra are transversely wide. The
centrum here interpreted as belonging to the first sacral
shows a base for the articulation with the sacral rib, which
is shared with the second sacral vertebra. In the same
way, the second sacral shares with the third sacral a well-
developed base of the sacral rib on the anterior corner of
the centrum. Due to the poor preservation we were not
able to determine the presence of caudosacral or dorsosac-
ral vertebrae (noticed by Langer & Benton 2006).
Langer & Benton (2006) indicated that the sacral ribs of
Pisanosaurus are not fan-shaped, like those of typical
ornithischians. Thus, they suggested that those of the two
primordial elements do not cover the entire medial surface
of the iliac alae. However, although the base of possible
sacral ribs 1–3 is present, its poor state of preservation
prevents the recognition of its shape; thus, the morphol-
ogy of sacral ribs is here regarded as uncertain for
Pisanosaurus.
Pelvis. This is represented by fragments of bones and
bone impressions in rock. Because of the poor state of
preservation of the elements, its interpretation is dubious
in several respects, and possibly because of this Casami-
quela (1967) did not mention it in his original description.
In this regard, it is without doubt the most complex and
difficult element to interpret among the material of Pisa-
nosaurus. Bonaparte (1976) gave a detailed sketch and
reconstruction of this block, interpreting the medial sur-
face of the pelvis and femoral head to be preserved as
moulds, an interpretation adopted by later authors. How-
ever, it is more probable that the pelvis is preserved in lat-
eral rather than medial view. In fact, the sacrum is
articulated and preserved in life position with respect to
the pelvis. This indicates that pelvic elements (Fig. 9) are
preserved in lateral view (contra Bonaparte 1976; Sereno
1991). On the other hand, as indicated below, the pre-
served portion of the femoral head appears to be exposed
in lateral view. Thus, we consider that the pelvis in Pisa-
nosaurus is exposed in lateral aspect.
Bonaparte (1976) interpreted Pisanosaurus as having a
relatively derived ornithischian-like opisthopubic pelvis.
More recently, Sereno (1991,2012) reinterpreted the pelvis
as plesiomorphically propubic, having a dorsoventrally
Figure 4. Pisanosaurus mertii holotype. A, B, anterior end of right dentary showing the absence of alveoli on dentary teeth; shaded area
indicates the base of anteriormost dentary tooth. Scale bar D2 mm.
Phylogenetic reassessment of a Late Triassic basal dinosauriform 7
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wide puboischial plate and no postpubic process, a mor-
phology very different from that reported in ornithischians
(Sereno 1986)(Fig. 10). Our study finds that the pelvis of
Pisanosaurus is not ornithischian-like but similar to that of
basal dinosauriforms such as Marasuchus and Silesaurus
(Sereno & Arcucci 1994; Dzik 2003). A detailed inspection
of this specimen also allows reinterpretation of some char-
acters. At first the supposed articulation between the pubis
Figure 5. Pisanosaurus mertii holotype. Selected teeth. A, B, posterior teeth of right dentary in lingual view. C–F, central dentary teeth
in lingual (C, D) and occlusal (E, F) views. G, H, central maxillary teeth in occlusal view, showing the apomorphic bilobate occlusal
contour of the teeth. Abbreviations: cg, cingulum; g, median groove; pr, possible primary ridge. Scale bar D2 mm.
8 F. L. Agnol
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Figure 6. Pisanosaurus mertii holotype. Dorsal vertebrae in left lateral (A, C) and right lateral (B, D) views. Scale bar D5 cm.
Phylogenetic reassessment of a Late Triassic basal dinosauriform 9
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and the ischium is not recognizable. The assumed suture
between the two bones recognized by Bonaparte (1976)
and Sereno (1991,2012) is a dorsoventral natural fissure in
the rock, and we were not able to recognize a clear articula-
tory surface between these bones.
The portion of ilium above the impression of the femo-
ral head was interpreted by Bonaparte (1976) and Sereno
(1991) as part of the lateral surface of the ilium. However,
because it is at a more internal level than the pubic pedun-
cle of the ilium, it may in fact represent a portion of the
internal wall of the acetabulum. Furthermore, Bonaparte
(1976) interpreted part of the proximal end of the pubis as
the posterior process ( Dpostpubic process). However,
this impression of bone is medially positioned and, based
on the preserved portion, very probably surrounded medi-
ally the femoral head. This suggests that the acetabulum
of Pisanosaurus was probably closed or nearly so
(Fig. 10), in contrast with the interpretations of Bonaparte
(1976) and Sereno (1991,2012). It also indicates that the
femoral head is exposed in lateral rather than in medial
view.
The iliac peduncles from ischium and pubis are broad
and poorly defined, and the acetabulum is anteroposter-
iorly expanded and dorsoventrally low, as recognized in
previous reconstructions (Bonaparte 1976; Sereno 1991).
This kind of acetabulum is congruent with that of non-
dinosaurian dinosauriforms, such as Marasuchus and Sile-
saurus, rather than that of ornithischians such as Eocursor
(Butler 2010) and Heterodontosaurus (Santa Luca 1980;
Figure 7. Pisanosaurus mertii holotype. Reconstructed mid-
dorsal vertebra in right lateral view. Abbreviations: cdl, centro-
diapophyseal lamina; cve, lateral excavation; dp, diapophysis;
ns, neural spine; p, parapophysis; pdl, diapoparapophyseal lam-
ina; popl, postzygodiapophyseal lamina; poz, postzygapophysis;
prz, prezygapophysis; prpl, prezygodiapophyseal lamina; vc,
vertebral centrum. Not to scale.
Figure 8. Pisanosaurus mertii holotype. Mould of ventral surface of sacral vertebrae. A, preserved mould; B, latex mould; C, interpreta-
tion of sacral imprints on the latex mould. Abbreviations: c1, first caudal vertebra; s1–4, sacral vertebrae 1–4; sr, sacral ribs. Scale bar D
5 cm.
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Galton 2014). It is noteworthy that Lesothosaurus shows a
well-developed medial flange that almost closes the ace-
tabulum (Sereno 1991).
Femur. The femur is represented by two elements:
the distal half of the left femur and the distal end of the
right femur, both badly weathered (Fig. 11). The distal
end of the femur is strongly craniocaudally bowed and not
transversely expanded, being subequal in width to the pre-
served portion of the diaphysis. The anterior surface of
the shaft lacks a conspicuous cranial intermuscular line. It
also lacks any sign of a flexor groove, and the patellar
groove is deep and narrow, surrounded by two ridges of
bone. These acute ridges, extending dorsally from the lat-
eral and medial condyles, reach the proximal shaft. The
distal lateral condyle is located posteriorly on the lateral
surface of the bone, and the fibular condyle and fibular
groove appear to be reduced. The intercondylar groove is
deep and ‘U’-shaped in distal view. A clear concave
groove or surface separating the lateral condyle and the
tibiofibular crest is absent.
Tibia. This was accurately described by Bonaparte
(1976), Sereno (1991) and Irmis et al.(2007), and will not
be redescribed here. However, we add some traits not
described by those authors. In medial view the proximal
tibia shows an elongate impression for the origin of M.
tibialis anterior, which is bounded cranially by a proxi-
modistallly extended ridge, as occurs in Sacisaurus (Feri-
golo & Langer 2013).
In lateral view, the proximal margin of the tibia shows a
proximodistally extended, but low, fibular crest that cau-
dally bounds the distal end of the incisura tibialis (Fig. 12).
Calcaneum. Bonaparte (1976) interpreted the pres-
ence of a calcaneal tuber on the caudal margin of the
bone, followed by later authors (Novas 1989; Langer &
Benton 2006). In contrast, Sereno (1991) considered the
structure an overhanging posterodistal corner of the fibula.
A detailed inspection of the specimen corroborates
Bonaparte’s assertion. In fact, the posterior process in the
calcaneum resembles in position and gross morphology
the calcaneal tuber of other basal dinosauriforms (Arcucci
1987; Nesbitt et al.2010). Furthermore, there is no sign of
separation or suture between the main body of the calca-
neum and the purported posterodistal portion of the fibula
in either medial or lateral view (Fig. 13).
Irmis et al. (2007) concluded that the calcaneum had a
concave dorsal surface that articulated with the fibula.
However, as noticed by Novas (1989), the articulation of
the calcaneum and fibula of Pisanosaurus is complex. In
lateral view, the calcaneum articulates with the fibula by
the means of a concave dorsal surface, as pointed out by
Irmis et al.(2007). However, in medial view, the dorsal sur-
face of the calcaneum is not concave at all. It shows a
strongly convex dorsal surface that becomes slightly flat to
concave towards the caudal end of the bone. This complex
morphology is reminiscent of the condition shown by Sile-
saurus (Dzik 2003)andAsilisaurus (Nesbitt et al. 2010).
Distal tarsals. Sereno (1991) reported the total
absence of distal tarsals. However, a distal tarsal 4 still
articulated to metatarsal IV was illustrated and described
by Casamiquela (1967). We found this tarsal among the
elements of the holotype specimen of Pisanosaurus
(Fig. 14). In proximal view this bone is subtriangular in
shape, with a convex lateral edge and a short and rounded
caudomedial process. The medial margin shows gently
concave anteromedial and posteromedial edges that con-
verge at a central ridge. The proximal surface shows a shal-
low, obliquely oriented groove that runs anterolaterally to
posteromedially. The area between the anterior and medial
edges is flat and slightly taller than the rest of the bone. In
distal view, the element maintains its subtriangular contour.
Two excavations are present. A smaller subcircular pit is
medially located, probably as a surface for the attachment
of part of the proximal end of metatarsal III. A wider
groove-like excavation runs anterolaterally to posterome-
dially, as occurs in the proximal surface of the bone. This
could receive the proximal end of metatarsal IV. In medial
Figure 9. Pisanosaurus mertii holotype. Mould of right pelvis
in lateral view. Abbreviations: fh, femoral head; ili, ilium; izq,
ischium; pub, pubis. Scale bar D5 cm.
Phylogenetic reassessment of a Late Triassic basal dinosauriform 11
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view, the bone shows flattened proximal and distal surfa-
ces, with convex anterior and posterior margins.
Metatarsals. Proximally, metatarsals III and IV are
very narrow transversely (Figs 3,15). The proximal end
of metatarsal III is teardrop-shaped in proximal view,
with the anterior margin more transversely expanded than
the posterior margin. Metatarsal IV in proximal view is
strongly transversely compressed, and shows an acute
caudal margin. In contrast to Bonaparte’s (1976) recon-
struction, and following Sereno’s (1991) observations, the
metatarsus was very probably strongly appressed and
bunched (Fig. 16). In fact, the shaft of metatarsal IV
shows in medial view a distally extended wide surface for
articulation of metatarsal III, suggesting that both bones
were firmly attached.
Although metatarsals are poorly known in silesaurids,
the metatarsal IV in Sacisaurus and Silesaurus is also
strongly transversely compressed (Langer & Ferigolo
2013), a condition reminiscent of Pisanosaurus but also
present in basal saurischians (Novas 1996).
Available phalanges are elongate, with transversely
expanded proximal and distal articular surfaces (Fig. 17).
The claw of digit IV is elongate and dorsoventrally com-
pressed, and has an acute and pointed distal end. The ven-
tral surface is nearly flat, and shows deep collateral
grooves that delimitate large lateral and medial expansions.
Metacarpal. A small long bone is preserved that may
belong to the forelimb. Based on general proportions and
size it can be identified tentatively as a third metacarpal.
The element is relatively stout, and shows a concave
Figure 10. Figure showing different restorations of pelvis of Pisanosaurus mertii in right lateral view. A, modified from Sereno (1991,
2012); B, modified from Bonaparte (1976); C, present paper. Abbreviations: ac, acetabulum; fh, femoral head; ili, ilium; is, isquium;
pub, pubis. Not to scale.
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Figure 11. Pisanosaurus mertii holotype. Distal end of left femur in anterior (A), medial (B), lateral (C), posterior (D, E) and distal (F)
views. Abbreviations: fc, fibular condyle; ig, intercondylar groove; tc, tibial condyle; tfc, tibiofibular crest. Scale bar D2 cm.
Figure 12. Pisanosaurus mertii holotype. Right tibia and astragalus in medial (A), posterior (B), anterior (C), lateral (D, E) views.
Abbreviations: aap, ascending process of astragalus; cn, cnemial crest; fcd, fibular condyle; fcr, fibular crest; m, lateral crest. Scale
bar D2 cm.
Phylogenetic reassessment of a Late Triassic basal dinosauriform 13
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lateral margin (Fig. 3). The medial margin is nearly
straight and shows a flat proximal portion that suggests
appression with metacarpal II. The distal end of the bone
is badly damaged. It shows the proximal portion of the
distal condyles, which are separated by a relatively nar-
row, deep and proximodistally extended groove. The
proximal end of the bone is elliptical, with its major axis
mediolaterally oriented.
Discussion
Phylogenetic analysis
The phylogenetic analysis is based on the inclusive archo-
saur phylogeny published by Nesbitt (2011), with the
modifications and restrictions identified by Bittencourt
et al. (2014). The coding of several character states was
modified from that of previous authors (Supplemental
Appendix 1), and characters 292 and 293 were added
based on previous studies (Supplemental Appendix 2).
The matrix is composed of 293 characters scored for 37
taxa. Although Nesbitt (2011) proposed synonymy
between Lewisuchus and Pseudolagosuchus, we codify
these genera as separate taxa following Bittencourt et al.
(2014). We added Echinodon,Fruitadens and Tianyulong
to this data matrix with the aim of improving the sampling
of basal ornithischians.
The phylogenetic analysis was performed using TNT
1.1 (Goloboff et al.2008). All characters were equally
weighted and treated as unordered. Heuristic searches
were performed after 1000 pseudoreplicates of the
WAGCTBR search strategy, with 10 random addition
sequences after each search, saving 100 trees at each
replicate.
The analysis resulted in 20 most parsimonious trees
(MPTs) of 795 steps in length, a consistency index of
0.43, and a retention index of 0.61. The strict consensus
tree yielded an unresolved polytomy at the base of Dino-
sauriformes, including Pisanosaurus,Marasuchus, the
silesaurids Asilisaurus,Diodorus,Sacisaurus,Eucoeloph-
ysis and Pseudolagosuchus, and Lewisuchus (Figure 18A;
Supplemental Appendices 3 and 4).
Following computation of an Adam Consensus Tree,
we excluded the wild-card taxa Eucoelophysis,Pseudola-
gosuchus and Lewisuchus. This new analysis resulted in
82 most parsimonious trees of 775 steps. The basal poly-
tomy at Dinosauriformes resulted in a monophyletic Sile-
sauridae, with Pisanosaurus included within this clade.
The nesting of Pisanosaurus within Dinosauria needs four
extra steps, and within Ornithischia seven additional steps.
The inclusion of Pisanosaurus within Heterodontosauri-
dae results in a tree of 784 steps.
Figure 13. Pisanosaurus mertii holotype. Right fibula and cal-
caneum in lateral (A), medial (B), anterior (C) and posterior (D)
views. Abbreviations: CC, calcaneum; CT, calcaneal tuber.
Scale bar D2 cm.
Figure 14. Pisanosaurus mertii holotype. Right distal tarsal 4 in
distal (A, D), proximal (B, E), and lateral (C, F) views. Abbrevi-
ation: cmp, caudomedial process. Scale bar D1 cm.
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Bremer support is generally low (Fig. 18C), indicating
weak support for most clades, as previously discussed by
Nesbitt (2011) and Bittencourt et al. (2014).
Comparisons of Pisanosaurus with ornithischian
dinosaurs and silesaurids
In the original description of Pisanosaurus, Casamiquela
(1967) considered that it was a very distinct ornithischian,
and thus merited a family: Pisanosauridae. This author
noted a large number of plesiomorphic features in the
skeleton of Pisanosaurus, and detailed similarities with
Poposaurus (then considered an ornithischian, but now a
well-known pseudosuchian; Long & Murry 1995; Gauth-
ier et al. 2011). Subsequent authors considered Pisano-
saurus to be related to Heterodontosauridae (Bonaparte
1976; Sereno 1986,1999,2012) or to be the basalmost
ornithischian (Novas 1989; Weishampel & Witmer 1990;
Butler et al.2007,2008; Boyd 2015). Because of strong
differences between the available elements of
Pisanosaurus and other taxa, some authors proposed that
it represented a chimaeric individual (Sereno 1991,2012),
a hypothesis that was dismissed by the original discoverer
of the specimen (Bonaparte 1976), as also noted by Novas
(2009) and discussed above.
In the present phylogenetic analysis, only seven extra
steps are needed to place Pisanosaurus as the most basal
ornithischian. In spite of this, several features of the post-
cranial skeleton, mainly those of vertebrae and hind limb,
are congruent with the interpretation that Pisanosaurus
may not even be a dinosaur. Because of the strong similari-
ties between Pisanosaurus and ornithischians on one hand,
and with non-dinosaurian silesaurids on the other, summary
comparisons are here carried out with these taxa.
As explained above, the dentition and tooth-bearing
bones of Pisanosaurus have been regarded since its
Figure 16. Reconstruction of metatarsus of Pisanosaurus mertii
in anterior view. A, reconstruction from the current paper. B,
reconstruction from Bonaparte (1976). Abbreviation: Mt, meta-
tarsal. Shaded area represents metatarsal II, and obscure grey
indicates overlapping between metatarsals II and III. Not to
scale.
Figure 15. Pisanosaurus mertii holotype. Right distal metatar-
sals III and IV in proximal (top) and anterior (bottom) views.
The arrow indicates anterior direction. Abbreviation: mt, meta-
tarsal. Scale bar D2 cm.
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original description as possessing a large number of
ornithischian traits (Casamiquela 1967; Bonaparte 1976).
The preserved dentary of Pisanosaurus shows a combina-
tion of characters that was considered typical or diagnos-
tic of Ornithischia: relatively short with a very tall
coronoid process, reduced external mandibular fenestra
(absent in the case of Pisanosaurus), and ventrally dis-
placed glenoid (Weishampel & Witmer 1990; Norman
et al. 2004; Butler 2005; Langer & Benton 2006;Fig. 1).
These features are not easy to assess in silesaurids because
of the incompletely preserved published specimens. How-
ever, the posterior portion of the mandible in Silesaurus
shows a relatively small, eye-shaped external mandibular
Figure 17. Pisanosaurus mertii holotype. A–D, phalanges III-1
to III-3 of right foot in medial (A, C) and ventral (B, D) views.
E, F, phalanges IV-1 to IV-5 of right foot in medial view. Scale
bars: A–D D2 cm; E, F D1 cm.
Figure 18. A, strict consensus cladogram showing the nesting of
Pisanosaurus within Silesauridae; Lewisuchus and Pseudolago-
suchus were pruned from the analysis. B, strict consensus clado-
gram showing Pisanosaurus in a polytomy with basal
dinosauriforms; Pseudolagosuchus and Lewisuchus were
retained as valid terminal taxa. C, cladogram showing Bremer
support values.
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fenestra and a very tall coronoid process (Dzik & Sulej
2007). In other silesaurids, such as Sacisaurus and Tech-
nosaurus, the concave dorsal margin of the dentary sug-
gests that its posterior end was probably elevated, as
occurs in ornithischians. Furthermore, the extent of devel-
opment of the dentary in the coronoid region in Pisano-
saurus cannot be established with confidence because this
area of the lower jaw is broken (Norman et al.2004).
A ventrally displaced mandibular glenoid is more wide-
spread than previously thought, as can be seen in Eorap-
tor,Panphagia and sauropodomorphs more derived than
Thecodontosaurus and Anchisaurus, in addition to basal
ornithischians (Gauthier 1986; Langer & Benton 2006;
Martinez & Alcober 2009; Sereno et al.2013). In other
basal dinosaurs, however, the condyle is in line with the
tooth row, as seen in Herrerasaurus,Staurikosaurus and
basal theropods (Gilmore 1920; Welles 1984; Colbert
1989; Charig & Milner 1997; Sampson et al. 1998). Sile-
saurus is similar to ornithischians and Pisanosaurus in
that the glenoid is ventrally displaced with respect to the
horizontal axis of the dorsal margin of the dentary (Dzik
& Sulej 2007).
Sereno (1991; see also Norman et al.2004) recognized
that the cheek emargination of maxilla and dentary of
Pisanosaurus was a feature present in ornithischians more
derived than ‘fabrosaurids’ (i.e. Genasauria). This condi-
tion is often related to the presence of cheeks (Galton
1973; Paul 1984). However, the presence of this feature is
dubious in the basal ornithischian Lesothosaurus (Norman
et al.2011).Moreover, this is not unique to derived orni-
thischians, also occurring in some silesaurids, including
Technosaurus and Sacisaurus at least (Chatterjee 1984;
Langer & Ferigolo 2013), and the pseudosuchian Revuel-
tosaurus (Parker et al.2005). Basal sauropodomorphs and
therizinosaur theropods (Paul 1984; Bonaparte & Pumares
1995; Yates 2003; Barrett & Upchurch 2007) also show
emarginated teeth, contrasting with remaining sauri-
schians (Langer & Benton 2006).
Preserved teeth of Pisanosaurus also show several traits
that were considered diagnostic of ornithischians by previ-
ous authors (Figs 1,5). Probably the main feature that pro-
moted referral of Pisanosaurus to Ornithischia was its
barricade-like dentition (Bonaparte 1976; Norman et al.
2004). However, as interpreted by Sereno (1991), the
present revision indicates that the teeth in Pisanosaurus
do not form a ‘barricade’ or a continuous masticatory sur-
face (contra Bonaparte 1976; Norman et al. 2004), as
described in some Heterodontosauridae (e.g. Butler et al.
2009; Butler 2010; Norman et al.2011; Sereno 2012;
Becerra et al. 2013).
Pisanosaurus shares with ornithischians gross dental
features including: (1) largest teeth on the caudocentral
part of the series; (2) basal cingulum more expanded lin-
gually; (3) non-recurved and low-crowned teeth; (4) well-
developed constriction between root and crown; and (5)
teeth asymmetrical in mesial and distal views (Sereno
1986; Hunt & Lucas 1994; Norman et al.2004; Ferigolo
& Langer 2007). However, such a combination of charac-
ters is present in several Triassic herbivorous archosaur
lineages, including the pseudosuchian Revueltosaurus
(Irmis et al.2007) and silesaurids such as Sacisaurus and
Silesaurus (Dzik 2003; Ferigolo & Langer 2007). Further-
more, an asymmetrical basal cingulum is also reported in
several silesaurids (Ferigolo & Langer 2007; Langer &
Ferigolo 2013).
Presence of sub-triangular teeth without caudal curva-
ture in maxillary and dentary teeth are features considered
by Sereno (1986,1991) to be apomorphic for ornithi-
schians, including Pisanosaurus. Langer & Benton (2006)
differentiated this condition from that of sauropodo-
morphs and Silesaurus in which tooth crowns are often
asymmetrical, with the rostral border longer than the cau-
dal, giving a slight caudal inclination to the crown. How-
ever, no caudal tooth inclination is present in Silesaurus,
Asilisaurus and Sacisaurus (Dzik & Sulej 2007; Nesbitt
et al. 2010; Langer & Ferigolo 2013), indicating that these
characters are more widespread than previously thought.
In spite of gross similarities, Pisanosaurus, silesaurids
and ornithischians differ from sauropodomorphs in the
combination of bulbous teeth, smaller (or absent) den-
ticles and a well-developed constriction between root and
crown (Irmis et al.2007).
The presence of epipophyses in cervical vertebrae was
regarded as a diagnostic trait of dinosaurs by Novas
(1996), being present in most basal ornithischians such as
Heterodontosaurus and Lesothosaurus (Santa Luca 1980;
Sereno 1991; Butler 2010; Galton 2014). If we follow the
interpretation of Bonaparte (1976) regarding the identity
of some isolated vertebrae of Pisanosaurus as cervicals,
none of these elements bear epipophyses, and are thus ple-
siomorphic with respect to dinosaurs (Fig. 3). Nonethe-
less, within Ornithischia epipophyses are present in most
anterior cervical vertebrae. However, as discussed above,
the exact position of these vertebrae in the backbone of
Pisanosaurus is still uncertain; therefore, the presence of
epipophyses on cervical vertebrae is equivocal.
As noted by previous authors, Pisanosaurus shows some
features that strongly differ from those of ornithischians.
This genus shows a very plesiomorphic vertebral column
(Figs 6,7,19). The vertebral centra are very elongate and
transversely compressed, differing from the short and stout
dorsal vertebrae of known ornithischians, including hetero-
dontosaurids (Sereno 2012; Butler et al. 2012; Galton
2014). Furthermore, there are strong prezygodiapophyseal,
postzygodiapophyseal and infradiapophyseal laminae in
available vertebrae, which are absent in most ornithischians
(e.g. Laquintasaura,Eocursor,Heterodontosaurus,Scutel-
losaurus: Colbert 1981;Britt1993; Butler et al. 2007,
2008; Butler 2010; Galton 2014; Barrett et al. 2014), with
the exception of some derived ornithopods (e.g. Cumnoria,
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Mantellisaurus,Valdosaurus: Galton & Powell 1980; Bar-
rett et al.2011; Butler et al. 2012), but are a common fea-
ture in basal dinosauriforms (e.g. Silesaurus: Piechowski &
Dzik 2010) and saurischians (Bonaparte 1999; Wilson
1999). In addition, a lateral constriction and excavation of
the dorsal centra is also absent in basal ornithischians,
whereas it is present in basal dinosauriforms (e.g. Marasu-
chus,Silesaurus:Bonaparte1976; Piechowski & Dzik
2010) and saurischians (O’Connor 2006).
In Pisanosaurus the bases of the sacral ribs are shared
by two sacral vertebrae (Dzik 2003; Nesbitt 2011), as apo-
morphically occurs in ornithischians and silesaurids such
as Silesaurus,Asilisaurus and Lesothosaurus,andthistrait
was considered by Langer & Ferigolo (2013)tobediag-
nostic of an Ornithischia CSilesauridae clade. It differs
from the morphology of other dinosaurs, in which each
sacral rib articulates with one vertebra (Nesbitt 2011).
Another portion of the skeleton of Pisanosaurus that is
strongly different from that of ornithischians is the pelvis.
In spite of its poor preservation we can make a tentative
reconstruction of acetabular morphology in Pisanosaurus.
As noted by previous authors (Sereno 1991,2012), the
pelvis is not opisthopubic and the ischiac-pubis contact is
dorsoventrally extended, features absent in known ornithi-
schians. Furthermore, the acetabulum is anteroposteriorly
long, dorsoventrally low, and medially closed, a plesio-
morphic combination of characters absent in ornithi-
schians (although a nearly closed acetabulum is present in
Lesothosaurus: Sereno 1991; Baron et al.2017), but pres-
ent in basal dinosauriforms such as Marasuchus (Novas
1996) and Silesaurus (Dzik 2003). On the other hand, our
interpretation indicates that the acetabulum was closed in
Pisanosaurus, resembling the plesiomorphic condition
found in silesaurids (Peecook et al. 2013) and other non-
dinosaurian archosaurs (Parker 2008; Nesbitt 2011), and
basal dinosaurs such as Saturnalia and Guaibasaurus
(Bonaparte et al. 1999; Langer et al.1999), but contrast-
ing with the open acetabulum of remaining dinosaurs
(Novas 1996; Nesbitt 2011).
The femur of Pisanosaurus shows additional similari-
ties with silesaurids, including a popliteal fossa between
the posterior condyles of the femur that is strongly
expanded proximally, a character present in Asilisaurus,
Silesaurus and Sacisaurus (Nesbitt et al.2010;Fig. 11).
Furthermore, unlike basal dinosaurs such as Lesothosau-
rus,Scutellosaurus,Saturnalia,Guaibasaurus and Her-
rerasaurus (Colbert 1981; Novas 1994; Bonaparte et al.
1999; Langer 2003; P. M. Barrett, pers. comm.), a promi-
nent cranial intermuscular line along the anterior surface
of the femur is totally absent. In this regard, the morphol-
ogy of Pisanosaurus recalls that of Sacisaurus (Langer &
Ferigolo 2013). Pisanosaurus differs in these features
from the condition of basal ornithischians such as Eocur-
sor,Stormbergia,Scutellosaurus and Heterodontosaurus
(Butler et al. 2007; Butler 2010; Galton 2014).
As pointed out by Irmis et al.(2007), beyond the autapo-
morphic features of the tibia and astragalus, these elements
in Pisanosaurus are plesiomorphic for Dinosauriformes. In
this respect, Ferigolo & Langer (2007) indicated that in
Sacisaurus the cnemial crest is not well developed and
proximally projected as in most dinosaurs (Novas 1996),
but is instead similar to the condition in Pisanosaurus
(Bonaparte 1976). In fact, in Pisanosaurus as in Sacisaurus
and Silesaurus, the cnemial area of the tibia shows its cra-
nial apex distally displaced from the proximal margin of
the bone in lateral and medial views (Langer & Ferigolo
2006). This condition contrasts with that of dinosaurs,
including basal ornithischians such as Eocursor,Lesotho-
saurus (Thulborn 1972)andHeterodontosaurus,inwhich
the cnemial crest is proximally projected (Santa Luca
1980;Novas1994,1996; Butler 2005;2010;Mart

ınez &
Alcober 2009; Bittencourt & Kellner 2009; Galton 2014).
Furthermore, as detailed by Irmis et al. (2007), Pisanosau-
rus is plesiomorphic in lacking a concave posterolateral
margin in distal view as in Lesothosaurus, other ornithi-
schians (Sereno 1991), basal sauropodomorphs and thero-
pods (Langer 2004), resembling silesaurids in this regard
(Dzik 2003).
As indicated by Novas (1989), Sereno (1991) and
Weishampel & Witmer (1990), the anteroposterior length
of the distal end of the tibia is greater than its transverse
width, a condition unknown in dinosauriforms and sile-
saurids, and this trait may be autapomorphic for Pisano-
saurus (Fig. 20). In addition, the distal end of the tibia
shows a descending process, as typically occurs in dino-
saurs and silesaurids (Ferigolo & Langer 2007). This
expands laterally and abuts but does not overlap the fibula,
contrasting with the extensive overlap seen in theropods
and in ornithischians, in which it forms an outer malleolus
(Colbert 1981; Butler 2005; Ferigolo & Langer 2007;
Irmis et al. 2007)(Fig. 20). In this aspect Pisanosaurus
resembles Silesaurus and Sacisaurus (Dzik 2003; Langer
& Ferigolo 2013). As indicated by Nesbitt (2011), the
Figure 19. Comparative figure of right lateral view of mid-
dorsal vertebrae.: A, reconstruction of Pisanosaurus mertii;B,
Silesaurus opolensis (based on interpretation of Piechowski &
Dzik 2010 and personal observations of specimens); and C, Het-
erodontosaurus tucki (based on interpretation of Santa Luca
1980, and personal observations of specimens). Not to scale.
18 F. L. Agnol
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outer malleolus of Silesaurus and also Pisanosaurus not
only reaches the fibula but is much more laterally pro-
jected than the flange present in basal dinosaurs (Langer
& Ferigolo 2013).
In lateral view, the proximal margin of the tibia shows a
proximodistally extended, but low, fibular crest (Fig. 12).
This structure caudally bounds the distal end of the inci-
sura tibialis, as occurs in Sacisaurus (Langer & Ferigolo
2013). A fibular flange is unknown in basal ornithischians
but is present in theropods (Rauhut 2003) and silesaurids
(e.g. Silesaurus,Sacisaurus: Dzik 2003; Langer & Feri-
golo 2013). The proximal tibia of Heterodontosaurus,
Tianyulong,Fruitadens and possibly Scutellosaurus
shows a crest located along the lateral surface that superfi-
cially resembles the fibular crest of theropods and sile-
saurids (Santa Luca 1980; Colbert 1981; Butler et al.
2007,2012; Zhen et al. 2009; Galton 2014). However, in
contrast to saurischians, silesaurids and Pisanosaurus,in
these ornithsichians it forms a prominent lateral flange
starting just distal to the fibular condyle, being extended
along the proximal third of the bone (Galton 2014).
In contrast to ornithischians (e.g. Heterodontosaurus
Lesothosaurus and Scelidosaurus: Colbert 1981; Sereno
1991; Galton 2014), the ascending process of the
astragalus is very well developed (Sereno 1991)and
anteroposteriorly extended. In fact, the low ascending
process is subquadrangular in contour in lateral view,
and its thickness is more than half the depth of the
astragalus (Novas 1989). The anteroposterior extension
of the ascending process is present in some basal sauro-
podomorphs (e.g. Lessemsaurus and Riojasaurus: Bona-
parte 1972; Pol & Powell 2007), and was regarded as
primitive for dinosaurs by Novas (1989). In addition,
the position of the ascending process of the astragalus,
located at the lateral surface of the astragalar body, is
similartothatinbasaldinosaurs(Normanet al. 2004),
basal dinosauriforms (e.g. Dzik 2003) and some ornithi-
schians (Novas 1989).
The astragalar body in Pisanosaurus exhibits a trans-
verse width that is shorter than its anteroposterior length,
and shows deep anterior and posterior grooves, which
result in a bilobate contour when the astragalus is viewed
distally (Fig. 21). This morphology is unknown in most
other taxa, with the exception of the basal herrerasaurian
Chindesaurus and the theropod Tawa (Long & Murry
1995; Nesbitt et al. 2009).
It is possible to infer that the bilobate astragalus in dis-
tal view and the subquadrangular and robust ascending
process in lateral view are features unknown in most dino-
sauriforms, and may be regarded as autapomorphic for
Pisanosaurus (Fig. 21).
The calcaneum in Pisanosaurus has a complex articula-
tion for the fibula. In lateral view it is proximally concave,
whereas in medial view it is convex (Novas 1989;
Fig. 13). This complex articulation resembles that of basal
dinosauriforms such as Asilisaurus (Nesbitt et al. 2010)
and Silesaurus (Dzik 2003), contrasting with dinosaurs in
which the proximal articulation is concave (Nesbitt 2011).
The calcaneum of Pisanosaurus is disc-like, which differs
from the complex subtriangular condition of silesaurids
and basal saurischians such as Herrerasaurus and Eorap-
tor (Novas 1994; Sereno et al. 2013). The calcaneum of
Pisanosaurus superficially resembles those of Heterodon-
tosaurus and Fruitadens in being disc-shaped (Sereno
2012; Butler et al. 2012; Galton 2014). However, these
taxa show important anatomical differences. In Hetero-
dontosaurus and Fruitadens, the calcaneum is notably
proximally extended and the proximal end is hook-shaped
and medially tilted (Butler et al. 2012; Galton 2014). Fur-
thermore, in heterodontosaurids the calcaneum is strongly
anteroposteriorly compressed and dorsoventrally tall in
lateral view, with a flat and subhorizontally oriented
Figure 20. Comparison between right tibia in posterior view. A,
Pisanosaurus mertii;B, Silesaurus opolensis (modified from
Dzik 2003 based on personal observations of specimens); C,
Stormbergia dangershoeki (modified from Butler 2005 based on
personal observations of specimens). Abbreviation: om, outer
malleolus. Not to scale.
Phylogenetic reassessment of a Late Triassic basal dinosauriform 19
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fibular facet (Butler et al. 2012; Galton 2014). This com-
bination of characters is totally absent in Pisanosaurus.
On the other hand, Pisanosaurus is similar to sauri-
schians and basal dinosauriforms in having overlapping
proximal metatarsals (Novas 1996; Ezcurra 2010), differing
from the non-overlapping condition in ornithischians
(Butler et al. 2008;Fig. 22). In Pisanosaurus, metatarsals
are strongly transversely compressed proximally (espe-
cially metatarsal II) and are appressed against each other
along almost all of their lengths (Fig. 22). In these features,
the metatarsals of Pisanosaurus are very similar to those of
Sacisaurus and Silesaurus (Langer & Ferigolo 2013).
Figure 21. Pisanosaurus mertii holotype. A–D, right astragalus in anterior (A), posterior (B), lateral (C) and medial (D) views. E, distal
view of articulated astragalus and calcaneum articulated; arrow indicates the anterior direction. Abbreviations: astr, astragalus; ap,
ascending process; cc, calcaneum; ct, calcaneal tuber; lc, lateral condyle; mc, medial condyle. Scale bar D1 cm.
20 F. L. Agnol
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To summarize the comparisons made above, Pisano-
saurus lacks features that unambiguously position it
among ornithischians and even dinosaurs, while possess-
ing several traits that are shared with basal dinosauriforms
of the clade Silesauridae. A few brief comparisons are
made here between Pisanosaurus and the ornithischian
clade Heterodontosauridae, to which it was referred by
Bonaparte (1976) and other authors (Cooper 1981; Weish-
ampel 1984; Sereno 1991,2012). Sereno (2012) recently
defended the heterodontosaurid affinities for Pisanosau-
rus on the basis of some shared features, including the
absence of replacement foramina in available dentary
bones, also present in Laquintasaura (Barrett et al. 2014);
a broad external mandibular fossa; and a disc-shaped cal-
caneum. However, the absence of replacement foramina
in tooth-bearing bones and broad external mandibular
fossa are features shared with some silesaurids (Dzik &
Sulej 2007; Nesbitt et al. 2010). A disc-shaped calcaneum
is unknown in any basal dinosauriform or dinosaur, with
the exception of heterodontosaurids (e.g. Heterodontosau-
rus and Tianyulong: Sereno 2012), Tawa (Nesbitt et al.
2009) and Pisanosaurus. However, as indicated above,
the morphology of the calcaneum in these taxa is strongly
different, suggesting that a disc-like morphology was
probably acquired independently in heterodontosaurids
and Pisanosaurus.
In spite of the similarities noted above, and as also
pointed out by other authors, Pisanosaurus differs
strongly from remaining heterodontosaurids in a large
number of characters. As indicated by Galton (1974) and
Sereno (2012) most typical heterodontosaurid features
reside almost entirely in the skull and especially in the
form of the teeth. In this regard, Pisanosaurus differs
from Heterodontosaurus and other heterodontosaurids
(e.g. Fruitadens and Echinodon) in having lingually
inclined teeth, that lack strong carinae and denticles on
the crown, very low crown proportions, and teeth that do
not conform to a continuous masticatory surface (Weish-
ampel & Witmer 1990;B

aez & Marsicano 2001;Fig. 23).
Clear wear facets from tooth-to-tooth occlusion are not
present in Pisanosaurus and, as was pointed out above,
masticatory surfaces vary in angle from tooth to tooth,
and tooth margins are not squared for close apposition.
With respect to the postcranium, Sereno (2012) consid-
ered that the open acetabulum of Pisanosaurus was an
additional feature shared with heterodontosaurids. How-
ever, reinterpretation of the pelvis suggests that the ace-
tabulum was closed or nearly so, thus weakening this
proposal.
The hind limb of Pisanosaurus also shows several fea-
tures differing from heterodontosaurids, and other dino-
saurs, as indicated above. For example, most
heterodontosarids, including Fruitadens,Tianyulong,
Abrictosaurus and Heterodontosaurus, show co-ossified
proximal tarsals, fibula and tibia (Thulborn 1974;Santa
Luca 1980; Zhen et al. 2009; Butler et al.2012; Sereno
2012; Galton 2014;Fig. 23). In contrast, in Pisanosaurus
these elements are not ossified but are closely connected.
Sereno (2012) listed a single potential postcranial synapo-
morphy for Heterodontosauridae: the reduced shaft and dis-
tal end of the fibula. In heterodontosaurids (e.g. Fruitadens,
Tianyulong,Abrictosaurus and Heterodontosaurus:Thul-
born 1974; Santa Luca 1980; Zhen et al.2009; Butler et al.
2012; Galton 2014)andalsoLesothosaurus (Sereno 1991),
Figure 22. Comparison between the right foot of selected dinosauriforms in anterior view. A, Pisanosaurus mertii;B, Marasuchus lil-
loensis (modified from Sereno & Arcucci 1994); C, Silesaurus opolensis (modified from Dzik 2003 based on personal observations of
specimens); D, Lesothosaurus diagnosticus (modified from Sereno 1991). Not to scale.
Phylogenetic reassessment of a Late Triassic basal dinosauriform 21
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the shaft and distal end of the bone are very slender (less
than 25% of the transverse width of adjacent parts of the
tibia). In contrast, as recognized by Sereno, this feature is
absent in Pisanosaurus, in which the shaft and distal end of
the fibula are not reduced, and the latter has a minimum
shaft diameter that is plesiomorphic in being approximately
70% that of the tibia (see also Sereno 1991).
Phylogenetic position of Pisanosaurus revisited
Basedondetailedobservationsoftheholotype,andonthe
novel phylogenetic analysis, Pisanosaurus is considered a
stem Dinosauriformes of the Silesauridae clade. Dinosauri-
form synapomorphies present in Pisanosaurus are numerous
and include the presence of a pubis with articular surfaces of
ilium and ischium separated by a gap, ischium with articula-
tions for ilium and pubis separated by a fossa, straight cne-
mial crest on proximal tibia, astragalus with acute
craniomedial corner, a cranial ascending process of astraga-
lus, reduced calcaneum, subtriangular-shaped distal tarsal 4,
and tightly bunched elongate metatarsals (Novas 1989,
1996;Ezcurra2006; Nesbitt 2011;Langeret al. 2013).
On the other hand, Pisanosaurus retains the plesiomor-
phic states for two unambiguous dinosaur apomorphies:
(1) tibia with craniolaterally curved cnemial crest (straight
and relatively low cnemial crest in Pisanosaurus as Irmis
et al. (2007) indicated); and (2) absence of a calcaneal
tuber (character 269-1; present and relatively well devel-
oped in Pisanosaurus, as demonstrated by Bonaparte
(1976); retained in some basal dinosaurs such as Herrera-
saurus: Novas (1989)). These, together with several char-
acters reported above, support a stem-dinosauriform
phylogenetic position for Pisanosaurus.
The present phylogenetic analysis nests Pisanosaurus
within a polytomy including all silesaurids. Pisanosaurus
shows some derived traits that resulted as unambiguous
synapomorphies of the Silesauridae clade: (1) reduced to
absent denticles on maxillary and dentary teeth (character
93-3: see Langer & Ferigolo (2013); the reduction of the
denticles is a feature shared by Pisanosaurus with Silesau-
rus and Asilisaurus, in contrast to Diodorus, Sacisaurus
and Technosaurus, which show relatively well-developed
denticles: Chatterjee 1984; Ferigolo & Langer 2007;
Kammerer et al.2012); (2) sacral ribs shared between two
sacral vertebrae (character 139-1; see Langer & Benton
2006); (3) lateral side of proximal tibia with a fibular
flange (present also in heterodontosaurids and several sau-
rischians; character 248-1: see Nesbitt et al.2010; Langer
& Ferigolo 2013; Galton 2014); (4) dorsoventrally flat-
tened ungual phalanges (character 289-1: see Nesbitt
2011); and (5) ankylothecodonty, teeth partially fused to
maxilla and dentary bone (character 293-1: see Nesbitt
2011). As discussed above, characters 2, 3 and 4 are also
present in ornithischians (although character 3 is strongly
different from ornithischians), whereas characters 1 and 5
are lacking in ornithischians.
Some silesaurid synapomorphies reported by previous
authors were not found as unambiguous diagnostic traits
of this clade in our phylogenetic analysis, although they
are shared between Pisanosaurus and remaining silesaur-
ids, including apicobasally short tooth crowns and proxi-
mally extended ridges surrounding the popliteal fossa of
femur (Nesbitt et al. 2010; Nesbitt 2011; Langer et al.
2013). All other proposed synapomorphies shared by sile-
saurids and Pisanosaurus cannot be corroborated due to
the incomplete nature of the one available specimen (Sup-
plemental Appendix 3). Because of this, the relationships
of Pisanosaurus within silesaurids are unresolved.
Phylogenetic and evolutionary implications
The inclusion of Pisanosaurus within Silesauridae implies
that this taxon does not constitute the oldest ornithischian
Figure 23. Comparison between Pisanosaurus and selected
ornithischians. A, B, left dentary in lateral view of A, Pisanosau-
rus mertii and B, Echinodon becklesii (modified from Sereno
2012). C–E, right ankle in anterior view of C, Pisanosaurus mer-
tii,D, Heterodontosaurus tucki (modified from Galton 2014) and
E, Scutellosaurus lawleri (modified from Colbert 1981). Abbre-
viations: as, astragalus; ca, calcaneum; cp, coronoid process; fo,
fossa; mc, medial condyle; mf, mandibular fenestra; pa, ascend-
ing process; pb, postdentary bones. Not to scale.
22 F. L. Agnol
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representative, as advocated by most authors. Pisanosaurus
was the only late Carnian–Early Norian ornithischian
known to date (Rogers et al. 1993), and implied that the
two main dinosaurian lineages (Ornithischia and Sauri-
schia) were present by 228 million years ago (Rogers et al.
1993;Irmiset al. 2007). This also implies a significant gap
of more than 30 million years between Pisanosaurus and
the first records of ornithischians such as Eocursor (Irmis
et al. 2007). However, we note that Olsen et al. (2011),
after a detailed analysis of Triassic fossil localities, con-
cluded that the age of Eocursor is questionable, and that it
may well belong to the Early Jurassic (Hettangian).
The only other Triassic ornithischian from South Amer-
ica is a fragmentary tooth-bearing dentary from the Upper
Triasic Laguna Colorada Formation of Argentina,
reported by B
aez & Marsicano (2001). Several characters,
such as closely packed, columnar teeth with basal vertical
ridges lingually curved and having wear facets, suggest it
is probably a Heterodontosauridae (B
aez & Marsicano
2001; Irmis et al. 2007). Although this specimen was
interpreted as Norian in age by B
aez & Marsicano (2001),
Olsen et al. (2011) considered it highly probable that the
age of the Laguna Colorada Formation is Early Jurassic.
Because supposed Triassic ornithischians are represented
by three specimens from the Southern Hemisphere, they
were regarded as exceptionally rare faunal components dur-
ing the Triassic and restricted geographically to the middle–
high latitudinal regions of southern Pangaea (Irmis & White-
side 2011).BasedonananalysisbyOlsenet al. (2011),
Pisanosaurus would stand as the the only unambigously
dated Upper Triassic ornithischian in the world, with the
potential to shed light on early ornithischian origin and radia-
tion. However, our hypothesis that Pisanosaurus be
excluded from Ornithischia implies that the oldest unambig-
uous record of the clade dates back to Lower Jurassic
Laquintasaura and Lesothosaurus, which may be dated as
Hettangian in age (Barrett et al.2014). Further, the scarce or
absent record of Triassic ornithischians blurs the hypothesis
of a large pre-Jurassic radiation for several clades (B
aez &
Marsicano 2001;Irmiset al.2007). In this regard, the poor
record of Triassic ornithischians invites us to consider that
this clade may have experienced its major diversification in
Early Jurassic times. This agrees with previous interpreta-
tions (e.g. Sereno 1999; Irmis et al. 2007; Butler et al. 2007)
proposing that ornithischian radiation occurred after the Tri-
assic–Jurassic boundary, when several groups of herbivorous
synapsids and archosauromorphs disappeared (Olsen & Sues
1986), allowing ornithischians to fill empty ecological
niches. In fact, Barrett et al.(2014) pointed out that the Early
Jurassic ornithischian record is richer than that of the Trias-
sic, being represented by several basal ornithischians, hetero-
dontosaurids and early armoured dinosaurs in Europe, North
America, South America and Africa. Exclusion of Pisano-
saurus from Ornithischia reinforces such hypotheses.
Figure 24. Comparative silhouette of Pisanosaurus mertii based on available material. A, modified from Bonaparte (1976); B, present
study. Not to scale. Bones in grey indicate non-preserved elements.
Phylogenetic reassessment of a Late Triassic basal dinosauriform 23
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Comment on the validity of Pisanosauridae
Casamiquela, 1967
Casamiquela (1967) established the name Pisanosauridae
based on the unique anatomical characters that distinguished
Pisanosaurus from the remaining ornithischians. On the
other hand, Langer et al.(2010) created the clade Silesauri-
dae to include Silesaurus and its kin. In the phylogenetic
analysis undertaken here, we recovered Pisanosaurus within
a monophyletic clade with Silesaurus and its kin. Applying
the principle of priority, Pisanosauridae should take priority
over Silesauridae, the latter being a junior synonym. How-
ever, because Pisanosauridae has not been employed by the
great majority of authors in the last five decades, and because
the name Silesauridae is widely accepted by most (if not all)
authors, we opt to retain the latter for the sake of clarity, fol-
lowing Article 23 (23.9.1.2) of the International Code of
Zoological Nomenclature. Further, it is probably best not to
abandon Silesauridae at least until further evidence defini-
tively shows that Pisanosaurus belongs within this clade.
Conclusions
The analysis and revision of the only known specimen of
Pisanosaurus mertii, the oldest purported ornithischian,
results in the reinterpretation of several anatomical fea-
tures (Fig. 24) and implies a new phylogenetic position
for Pisanosaurus. We find that Pisanosaurus lacks a large
number of dinosaurian features on its postcranium, and is
here regarded as a stem dinosaur, probably belonging to
the herbivorous basal dinosauriform clade Silesauridae.
Because of the uniqueness of several morphological fea-
tures, especially in the astragalus and calcaneum, Pisano-
saurus may also belong to a still poorly known clade of
stem dinosauriforms. However, this hypothesis is not sus-
tained by the phylogenetic analysis performed here.
Our results suggest that Pisanosaurus should be removed
from the base of Ornithischia and that it may no longer be
considered the oldest representative of this dinosaurian
clade. This is consistent with previous interpretations pro-
posing that ornithischian radiation occurred after the Trias-
sic–Jurassic boundary and, as Olsen et al.(2011)pointed
out, ornithischians were very scarce in Triassic times.
The identification of fragmentary specimens with
ornithischian-like dentitions should be undertaken with
caution, a fact reinforced by the discovery of a large num-
ber of non-dinosaurian archosauromorphs with herbivo-
rous dentition (Dzik 2003; Parker et al. 2005; Irmis et al.
2007; Flynn et al. 2010).
Acknowledgements
We thank the late Jaime Powell, Rodrigo Gonz
alez, Caro-
lina Madozzo Ja
en and Pablo Ort
ız (Instituto Miguel
Lillo) for their help during the revision of the collections
of the Instituto Miguel Lillo. We also thank F. Novas, M.
Ezcurra, G. Lio, F. Briss
on Egli and N. Chimento (Museo
Argentino de Ciencias Naturales) for comments and dis-
cussion on the phylogenetic relationships of basal ornithi-
schians and Pisanosaurus. We are especially indebted to
J. F. Bonaparte for fruitful discussions and sharing unpub-
lished data on the Pisanosaurus finding, and to Trevor
Worthy for his help with the English language. We thank
reviewers R. J. Butler, B. Parker and P. M. Barrett for
helpful and enlightening comments.
Supplemental material
Supplemental material for this article can be accessed at:
https://doi.org/10.1080/14772019.2017.1352623
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