An early juvenile specimen of Bolong yixianensis (Ornithopoda: Iguanodontia) from the Lower Cretaceous of Ningcheng County, Nei Mongol, China Wenjie Zheng a *, Xingsheng Jin a , Masateru Shibata b and Yoichi Azuma a,b a Zhejiang Museum of Natural History, Zhejiang 310014, P.R. China; b Fukui Prefectural Dinosaur Museum, Fukui 911-8601, Japan (Received 2 March 2013; final version received 21 May 2013; first published online 1 July 2013) We describe an early juvenile specimen (ZMNH M8812) of Bolong yixianensis from the Yixian Formation (Lower Cretaceous) of Ningcheng County, Nei Mongol, China. The specimen consists of an almost complete skeleton preserved two-dimensionally on a slab. The short and deep skull proportions and unfused neurocentral sutures in most preserved vertebrae suggest that the ZMNH M8812 is a juvenile individual. Osteohistological study confirms a very early developmental stage. The study reveals the ontogenetic changes of Bolong for the first time. The specimen revealed one additional autapomorphy for Bolong yixianensis: the lingual face of the maxillary crown is bounded by thickened mesial and distal margins and bisected by a prominent median principal ridge. The study revealed the following ontogenetic trends of Bolong: increased tooth rows in both maxilla and dentary, increased robustness of the jugal and scapula, the radius and ulna become more robust and shorter relative to the hindlimb and the metatarsals become proportionally shorter. ZMNH M8812 represents the first juvenile non-hadrosaurid iguanodontian specimen described from the Lower Cretaceous of eastern Asia. Keywords: Bolong yixianensis; juvenile; Iguanodontia; Lower Cretaceous; Yixian Formation; China Introduction Complete ontogenetic series currently exist for several Hadrosaurid dinosaurs (Horner and Currie 1994; Evans 2007; Brink et al. 2010; Evans 2010; Prieto-Ma ´rquez 2011). In contrast, the fossil material available for most non-hadrosaurid iguanodontians is insufficient for doc- umenting the full range of morphological changes through ontogeny (Prieto-Ma ´rquez 2011). Nevertheless, several specimens of juvenile non-hadrosaurid iguanodontians have contributed to ontogenetic studies (Forster 1990a, 1990b; Hu ¨bner and Rauhut 2010; Werning 2012). However, juvenile individuals, especially in early stages of development, and ontogenetic series of skeletal elements of non-hadrosauroid iguanodontians remain relatively rare. Although iguanodontian dinosaurs are very common in east-central Asia (Norman 1996, 1998, 2002; Wang and Xu 2001; You et al. 2003, 2005), no juvenile non-hadrosaurid iguanodontian from Asia has yet been described. Barrett et al. (2009) mentioned that several specimens possibly referable to Jinzhousaurus were excavated from Yixian County of Liaoning Province (Figure 1), but none of these have been described. Here, we describe an early juvenile specimen of Bolong yixianensis from Xitaizhi Village, Shantou Town, Ningcheng County, Nei Mongol Autonomous Region, People’s Republic of China (Figure 1). The specimen was collected by a local villager, and detailed information of the fossil locality remains unclear. It most probably belongs to the lower part of Yixian Formation (Wang et al. 2000, 2005; Liu et al. 2006). The specimen was prepared by Yuqing Zhang, under the supervision of Dr Junchang Lu ¨ in the Institute of Geology, Chinese Academy of Geological Sciences, Beijing. Dr Lu ¨ confirmed authen- ticity of the specimen. Systematic paleontology Dinosauria Owen, 1842 Ornithischia Seeley, 1887 Ornithopoda Marsh, 1881 Iguanodontia Dollo, 1888 Bolong yixianensis Wu, Godefroit and Hu, 2010 (Figures 2–15, Table 1) Material The specimen is a nearly complete skeleton housed in Zhejiang Museum of Natural History, Hangzhou, China (ZMNH M8812) (Figures 2–15, Table 1). Locality and horizon Xitaizhi Village, Shantou Town, Ningcheng County, Nei Mongol Autonomous Region, China. Lower Cretaceous, the lower part of Yixian Formation. q 2013 Taylor & Francis *Corresponding author. Email: [email protected]This article was originally published with an error. This version has been amended. Please see Corrigendum (DOI: http://dx.doi.org/ 10.1080/08912963.2014.899182) Historical Biology , 2014 Vol. 26, No. 2, 236–251, http://dx.doi.org/10.1080/08912963.2013.809347 Downloaded by [Harbin Institute of Technology] at 10:35 01 April 2014
Transcript
An early juvenile specimen of Bolong yixianensis (Ornithopoda: Iguanodontia) from the LowerCretaceous of Ningcheng County, Nei Mongol, China
Wenjie Zhenga*, Xingsheng Jina, Masateru Shibatab and Yoichi Azumaa,b
aZhejiang Museum of Natural History, Zhejiang 310014, P.R. China; bFukui Prefectural Dinosaur Museum, Fukui 911-8601, Japan
(Received 2 March 2013; final version received 21 May 2013; first published online 1 July 2013)
We describe an early juvenile specimen (ZMNH M8812) of Bolong yixianensis from the Yixian Formation (LowerCretaceous) of Ningcheng County, Nei Mongol, China. The specimen consists of an almost complete skeleton preservedtwo-dimensionally on a slab. The short and deep skull proportions and unfused neurocentral sutures in most preservedvertebrae suggest that the ZMNH M8812 is a juvenile individual. Osteohistological study confirms a very earlydevelopmental stage. The study reveals the ontogenetic changes of Bolong for the first time. The specimen revealed oneadditional autapomorphy for Bolong yixianensis: the lingual face of the maxillary crown is bounded by thickened mesial anddistal margins and bisected by a prominent median principal ridge. The study revealed the following ontogenetic trends ofBolong: increased tooth rows in both maxilla and dentary, increased robustness of the jugal and scapula, the radius and ulnabecome more robust and shorter relative to the hindlimb and the metatarsals become proportionally shorter. ZMNH M8812represents the first juvenile non-hadrosaurid iguanodontian specimen described from the Lower Cretaceous of eastern Asia.
Keywords: Bolong yixianensis; juvenile; Iguanodontia; Lower Cretaceous; Yixian Formation; China
Introduction
Complete ontogenetic series currently exist for several
Hadrosaurid dinosaurs (Horner and Currie 1994; Evans
2007; Brink et al. 2010; Evans 2010; Prieto-Marquez
2011). In contrast, the fossil material available for most
non-hadrosaurid iguanodontians is insufficient for doc-
umenting the full range of morphological changes through
ontogeny (Prieto-Marquez 2011). Nevertheless, several
specimens of juvenile non-hadrosaurid iguanodontians
have contributed to ontogenetic studies (Forster 1990a,
1990b; Hubner and Rauhut 2010; Werning 2012).
However, juvenile individuals, especially in early stages
of development, and ontogenetic series of skeletal
elements of non-hadrosauroid iguanodontians remain
relatively rare. Although iguanodontian dinosaurs are
very common in east-central Asia (Norman 1996, 1998,
2002; Wang and Xu 2001; You et al. 2003, 2005), no
juvenile non-hadrosaurid iguanodontian from Asia has yet
been described. Barrett et al. (2009) mentioned that several
specimens possibly referable to Jinzhousaurus were
excavated from Yixian County of Liaoning Province
(Figure 1), but none of these have been described.
Here, we describe an early juvenile specimen of
Bolong yixianensis from Xitaizhi Village, Shantou Town,
Ningcheng County, Nei Mongol Autonomous Region,
People’s Republic of China (Figure 1). The specimen was
collected by a local villager, and detailed information of
the fossil locality remains unclear. It most probably
belongs to the lower part of Yixian Formation (Wang et al.
2000, 2005; Liu et al. 2006). The specimen was prepared
by Yuqing Zhang, under the supervision of Dr Junchang
Lu in the Institute of Geology, Chinese Academy of
Geological Sciences, Beijing. Dr Lu confirmed authen-
ticity of the specimen.
Systematic paleontology
Dinosauria Owen, 1842
Ornithischia Seeley, 1887
Ornithopoda Marsh, 1881
Iguanodontia Dollo, 1888
Bolong yixianensis Wu, Godefroit and Hu, 2010
(Figures 2–15, Table 1)
Material
The specimen is a nearly complete skeleton housed in
Zhejiang Museum of Natural History, Hangzhou, China
(ZMNH M8812) (Figures 2–15, Table 1).
Locality and horizon
Xitaizhi Village, Shantou Town, Ningcheng County, Nei
Mongol Autonomous Region, China. Lower Cretaceous,
the lower part of Yixian Formation.
q 2013 Taylor & Francis
*Corresponding author. Email: [email protected] article was originally published with an error. This version has been amended. Please see Corrigendum (DOI: http://dx.doi.org/10.1080/08912963.2014.899182)
and Ouranosaurus, the anterior end of the dentary is also
slightly downturned; a strongly deflected dentary
symphysis is present in Altirhinus, Protohadros and
many hadrosaurids (Barrett et al. 2009). Anteriorly, the
dentary contacts the predentary. The right dentary shows
eight vertical tooth positions, fewer than that in the
holotype of Bolong (Wu and Godefroit 2012). The teeth
Figure 9. Dorsal vertebrae and ribs of ZMNH M8812. Abbreviations: drb, dorsal rib; dv, dorsal vertebra; il, ilium; pub, pubis; sv, sacralvertebra.
Figure 10. The photograph (A, C) and line drawing (B, D) of the caudal vertebrae of ZMNH M8812. Abbreviations: cdr, caudal rib; ch,chevron; isc, ischium; mt, metatarsal; ns, neural spine. Scale bar ¼ 2 cm.
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are bordered medially by the alveolar parapet. The
dentary tooth row is straight along its dorsal margin in
medial view. There is a diastema between the posterior
margin of the predentary and the first dentary tooth. The
coronoid process of right dentary is directed poster-
odorsally, identical to the holotype of Bolong (Wu and
Godefroit 2012). This morphology is similar to that seen
in Ratchasimasaurus (Shibata et al. 2011) and many non-
ankylopollexian ornithopods, such as Hypsilophodon
(Galton 1974), Orodromeus (Scheetz 1999) and Dysalo-
tosaurus (Hubner and Rauhut 2010). Most of the derived
iguanodontians possess an upright process (Hubner and
Rauhut 2010). The Meckelian groove below the tooth
alveoli extends along the whole length of the dentary, as
in most non-ankylopollexian ornithopods and Campto-
saurus dispar. In derived iguanodontians (including
hadrosaurs) the Meckelian groove tapers and ends well
posterior to the symphysis (Hubner and Rauhut 2010).
The overall morphology of dentary is very similar to that
in the holotype of Bolong (Wu and Godefroit 2012); as
Figure 11. The photograph (A) and line drawing (B) of humeri of ZMNH M8812. Abbreviations: dpc, deltopectoral crest; hm, humerus;L, left; R, right. Scale bar ¼ 2 cm.
Figure 12. The photograph (A, C) and line drawing (B, D) of forearms and manii. Abbreviations: mc, metacarpal; ph, phalanx; ra,radius; ul, ulna; I–V, digit numbers; 1–3, phalanx numbers; L, left; R, right. Scale bar ¼ 2 cm.
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in Hypacrosaurus, the embryonic dentary is similar in
morphology to the corresponding adult element (Horner
and Currie 1994).
Other parts of the skull, such as the surangular and
hyoid, are also present; however, they provide no useful
information.
Figure 13. The photograph (A) and line drawing (B) of pelvic girdle of ZMNH M8812. Abbreviations: app, anterior pubic process; fem,femur; il, ilium; ipi, iliac peduncle of the ischium; isc, ischium; obt, obturator process; pp, posterior pubic process; ppi, pubic peduncle ofthe ischium; pub, pubis; L, left; R, right. Scale bar ¼ 2 cm.
Figure 14. Femur (A. B), tibia and fibula (C, D) of ZMNH M8812. Abbreviations: fem, femur; fib, fibula; pub, pubis; tib, tibia; L, left; R,right. Scale bar ¼ 2 cm.
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Dentition
The maxillary dentition includes one active tooth per
alveolus (Figures 4–7); each maxillary alveolus holds one
active and one replacement tooth. The mesial and distal
margins of the teeth bear strong denticles, which become
smaller towards the apex. The shape of crown is
mesiodistally wide, namely a leaf shape, rather than
narrow. The enamel covers the entire lingual surface of the
maxillary teeth, a condition that suggests it is a basal
iguanodontian (Norman 2004). In the labial surface, the
primary ridge is offset distally, similar to that in the
holotype of Bolong (Wu and Godefroit 2012). The primary
ridge is not prominent, weaker than that in the holotype of
Bolong (Wu and Godefroit 2012). Up to six secondary
ridges are developed, most of them are positioned mesial
to the primary ridge. The secondary ridges are more
developed than that in the holotype of Bolong (Wu and
Godefroit 2012). Each ridge culminates in a denticle along
the margin of the crown. The ridges are perpendicular to
the wear facet, whereas in the holotype of Bolong, the
primary ridge is deflected distally (Wu and Godefroit
2012). The replacement crowns lie on the medial side of
the roots of functional teeth. The lingual face of the crown
is bounded by thickened mesial and distal margins and is
bisected by a prominent median principal ridge, which is
slightly offset distally. This character may represent an
autapomorphy of Bolong. The lingual (cutting) edge of the
occlusal surface displays a characteristic W-shaped view
in occlusal view.
As in all iguanodontians for which the dentition is
known, the dentary teeth are arranged in a series of
interlocking rows without spaces between the crowns. One
active and one replacement tooth occurs in each alveolus,
as in Probactrosaurus gobiensis (Norman 2002) and
Bactrosaurus (Godefroit et al. 1998). The crowns of
dentary teeth bear simple tongue-shaped denticles on the
mesial and distal margins (Figure 7(A)). The dentary
crowns are broadly expanded anteroposteriorly and their
overall proportion is close to the holotype of Bolong (Wu
and Godefroit 2012), it also resembles those seen in
Equijubus (You et al. 2003) and Altirhinus (Norman 1998).
All dentary teeth are visible in lingual view. The enamelled
lingual surface bears curved ridges running down from the
mesial and distal margins that meet basally, enclosing a
generally shield-shaped surface. The primary ridge is
offset distally on the lingual surface and divides it into two
asymmetrical areas. These areas are adorned by faint
parallel accessory ridges that extend as buttresses from
Figure 15. Photograph (A, C) and line drawing (B, D) of pes ZMNH M8812. Abbreviations: a, astragalus; c, calcaneum; fib, fibula; mt,metatarsal; ph, phalanx; tib, tibia; tib; L, left; R, right. Scale bar ¼ 2 cm.
Table 1. Selected measurements of ZMNH M8812 (in mm).
Right scapula Length 49.00Proximal width 13.51Minimum width 5.77
Distal width 9.02Right humerus Length ,42.9
Max. width 10.48Distal width 9.31
Left humerus Length ,41.30Max. width 9.55Distal width 10.68
Right ulna Length 34.10Left ulna Length 34.30Right radius Length 31.20Left radius Length 31.00Left metacarpal II Length 8.90Left metacarpal III Length 12.03Left metcarpal IV Length 11.83Left ischium Length 46.84Left femur Length 51.80Left tibia Length 50.50Left fibula Length 46.70Right metatarsal II Length 17.11Right metatarsal III Length 22.04Right metatarsal IV Length 18.22
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individual denticles on the crown margin in unworn
crowns (Figure 7). The primary ridge is more prominent
than on the maxillary teeth, contrary to the situation
observed in the holotype of Bolong (Wu and Godefroit
2012). The primary ridge is more prominent than that in
the holotype of Bolong (Wu and Godefroit 2012) and
Probactrosaurus (Norman 2002). As in the adult Bolong, a
secondary ridge is present mesial to the primary ridge,
bisects the larger mesial half of the crown and reaches the
upper part of the mesial margin; there may be a variable
number of more or less parallel subsidiary ridges, which
are extensions of the bases of the marginal denticles.
Vertebral column, ribs, chevrons
The posterior cervical vertebrae form an articulated series
(Figures 2 and 8). The cervicodorsal junction is obscured
by the presence of the overlying right scapula and ribs
(Figure 8). In lateral view, the main bodies of the centra
are almost square in outline. The centra are deep and
anteroposteriorly narrow. The centrum is compressed
ventrally and forms a thin keel. Dorsal to this keel, the
lower half of the centrum surface is concave, and develops
a horizontal ridge on the upper half. The parapophysis is
situated on the anterior end of this ridge. The anteriormost
preserved rib was treated as the cervical rib. The rib is
short bodied with the divergent rib head (Figures 2 and 8).
The morphology of the cervical vertebrae and rib is
generally similar to that of the holotype of Bolong (Wu and
Godefroit 2012).
Dorsal vertebrae are preserved in an articulated series
(Figures 2 and 9). The anterior series are largely obscured
by the overlying ribs and right scapula; the posterior dorsal
vertebrae are exposed in the ventrolateral view. The centra
are nearly equal in length. The ventral margin of the centra
bears a longitudinal groove bounded by low longitudinal
ridges. The centra are strongly expanded at the articular
ends. The centra of dorsal vertebrae in holotype of Bolong
are completely hidden by the scapula or the dorsal ribs,
only the neural spines can be observed (Wu and Godefroit
2012); this condition hampers the comparison between the
two specimens.
The dorsal ribs are relatively short anteriorly (Figure 8),
reach their maximal lengths in the anterior mid-trunk
region and gradually decrease in size posteriorly (Figure 2).
The posterior series of ribs are preserved in articulation
with the vertebrae, and the articular ends are covered by the
dorsal centra (Figure 9). The anterior series of ribs are
double headed (Figure 8), with a larger capitulum,
supported by a long capitular process, whereas the
tuberculum forms only a very short articular process that
extends in line with the rib shaft. The rib shaft is long and
curved.
The sacrodorsal and the sacrals are exposed in the
lateroventral view (Figure 2). The sacrum and adjacent
centra are unfused. There is no evidence of a ventral
groove or ridge on the ventral surface of the sacral centra.
The articular margins are expanded as in the preceding
dorsals. The three anterior sacral ribs are articulated with
the right ilium. Few other details of the sacral centra can be
observed.
The anterior and posterior series of caudal vertebrae
are preserved in articulation, and a break occurs between
the two sets (Figures 2 and 10). The articulated series are
visible in the right lateral view. At least 17 anterior neural
arches are unfused to their corresponding centra, whereas
the condition in the posterior series of caudals is unclear.
The 17 anterior caudal vertebrae are well preserved. In the
anteriormost two caudals, the prezygapophyses project
slightly beyond the anterior surface of the centrum. The
neural arches are displaced from the centra in the more
posterior elements of this series. Above the prezygapo-
physes, there is an elongated, slightly posteriorly inclined
and transversely compressed neural spine. The lateral face
of the centrum is slightly concave with the articular
margins transversely expanded. The centra become longer
and lower through the series. The caudal ribs are not fused
to the centra. The centrum heights of the 12 most anterior
caudals clearly exceed their lengths. In the more posterior
five caudals, the centra are slightly longer than high. The
neural spines decrease in dorsoventral height along the
series, and become increasingly posteriorly inclined. In the
posterior series, the centra become more elongated, and
the neural spines are more horizontally directed.
Eleven chevrons are preserved in articulation with the
caudal vertebrae (Figure 10). The posterior preserved
chevrons are formed by two arches that join proximally to
form the articular surface. The ventral rod tapers in both
lateral and anterior views to terminate in a rounded tip. The
chevrons are longer and more slender than the correspond-
ing neural spine. The anterior six chevrons direct more
caudally than the posterior chevrons. As in the holotype of
Bolong (Wu and Godefroit 2012), the caudoventral
orientation of posterior chevrons is apparently equivalent
to the caudodorsal angle of the corresponding neural spine.
The chevrons steadily become smaller along the series,
forming a mirror image of the neural spines.
Pectoral girdle and forelimbs
Both scapulae are present (Figures 2 and 8). The right
element is complete and exposed in lateral view and it
overlies the articulated series of the dorsal ribs. The left
scapula is partially exposed in the medial view, whereas
most of the elements are obscured by the overlying ribs. As
in the holotype of Bolong, the proximal end is expanded
dorsoventrally (distinctly deeper than the distal scapula) to
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support the glenoid and to provide a sutural surface for the
coracoid, the acromial process is directed dorsally and the
articular facet for the coracoid is extensive (Wu and
Godefroit 2012). The scapular shaft is slightly curved along
its length in the lateral view such that the dorsal margin of
the blade is gently convex and the ventral margin is slightly
concave. The curvature of the ventral margin is stronger
than that of the dorsal margin. The posterior margin of the
scapula is gently convex in the lateral view. The shaft is
slightly slender than that in the holotype of Bolong, the
ratio of scapular length to minimum width of the shaft is
about 8.49, whereas the ratio is only 7.61 in the holotype of
Bolong. Except this, the morphology of the scapula is
generally similar to that of the holotype of Bolong (Wu and
Godefroit 2012). Overall, the shaft is almost straight and
only modestly expanded distally, similar to that of
Probactrosaurus (Norman 2002), whereas the scapula of
Jinzhousaurus is more strongly curved along its length and
strongly expanded at the distal end (Wang et al. 2010).
The right coracoid is present and is unfused to the
scapula. In the lateral view, the anterodorsal margin is
subcircular; the anteroventral corner of the coracoid forms
a small hook-like process.
Both humeri are preserved in the anterior view
(Figures 2 and 11), and the humerus is approximately 81%
of the length of the femur. Overall, the shaft of the
humerus is slender, bowed and slightly sinuous in the
anterior view. The proximal end is broad and has a
shallowly concave anterior surface. The transverse width
of the proximal expansion exceeds that of the distal end.
The deltopectoral crest extends for approximately half of
the length of the humerus. The deltopectoral crest curves
anteriorly. The radial and ulnar condyles are separated by a
shallow intercondylar groove. The humerus in the
holotype of Bolong is also slender (Wu and Godefroit
2012).
The proximal end of the ulna is anteroposteriorly
expanded relative to the shaft, and the olecranon process is
incomplete (Figure 12). The ulna is longer and more robust
than the radius, The radius is slender, straight and
anteroposteriorly expanded at either end to form the
articular surfaces; it amounts ,74% of the length of the
humerus. Compared to the holotype of Bolong (Wu and
Godefroit 2012), the ulna and radius are more slender, the
ratio of proximal width to length is only 0.15 in ulna and
0.19 in radius, much smaller than that in the holotype of
Bolong (Wu and Godefroit 2012).
Both manii are incomplete with the dorsal surface is
extensively fractured (Figure 12). The elements of both
hands are mainly articulated, but the distal phalanges of
the left hand and the proximal elements of the right hand
are missing. The left metacarpals II–IV are closely
appressed to one another and exposed in the dorsal view.
Metacarpal IV is the longest, followed by metacarpals III
and II. Metacarpal V of the right hand is a small dumbbell-
shaped bone. The conical ungual on digit I is well
developed, and digit I is set at an oblique angle to the main
axis of the manus. The ungual phalanx is quite
characteristic of basal iguanodontoids: it is straight,
triangular, while in more derived iguanodontians the first
phalanx of digit I may be lacking (Norman 2004). Ungual
phalanx of digit II is large and subtriangular in the dorsal
view; unlike the holotype of Bolong (Wu and Godefroit
2012), the ungual II is smaller than ungual I. The left digit
V has two phalanges. Other phalanges are poorly
preserved. The morphology of the manus is similar to
the holotype of Bolong (Wu and Godefroit 2012), the first
digit has an enlarged, divergent, spine-like ungual, the
middle three digits form a rather compact unit and the fifth
digit is apparently long, flexible and opposable. This is
similar to the condition in Iguanodon (Norman 1986,
2004), and different from that of many basal iguanodon-
tians, such as Tenontosaurus (Forster 1990b) and Zalmoxes
(Weishampel et al. 2003).
Pelvic girdle and hindlimbs
Both ilia are preserved, but obscured by overlaying
remains. The anterior portions of preacetabular process of
both ilia are exposed in the medial view (Figures 2 and 13).
The process directs and tapers anteriorly; few anatomical
details can be ascertained from the material.
Both pubes are preserved in the medial view (Figures 2
and 13). The prepubic process of the pubis is slender,
concave along its dorsal margin and expanded at its distal
end. It is remarkably similar to those of Iguanodon
bernissartensis (Norman 1980) and Altirhinus (Norman
1998). The pubic shaft is short and tapering, it is probably
terminated about mid-way along the shaft of the ischium.
This is a derived character for a non-hadrosaurid
iguanodontian (Norman 2004).
Both ischia are preserved in the medial view (Figures 2
and 13). The acetabular margin is visible on the right
ischium and is gently concave in the medial view. In the
medial view, the ischial shaft is straight to slightly arched
dorsally and becomes deeper along its length. The
condition of the distal end is difficult to ascertain; it
looks like the distal end is expanded ventrally. The
obturator process is well developed on the ventral margin
of the shaft.
Both femora are present (Figures 2 and 14). The femur
is robust and straight. The fourth trochanter is pendant and
situated at approximately midlength of the shaft. Beneath
the fourth trochanter, the shaft is curved posteriorly. The
distal end of the femur is slightly expanded anteroposter-
iorly with respect to the shaft.
The tibia is straight and slightly shorter than the femur
(Figure 14, Table 1). The proximal end of the tibia is broad
and expanded. The distal end is transversely expanded to
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form the inner and outer malleoli. The outer malleolus is
separated from the inner by a step and projects more
distally. The distal fibula is articulated against the anterior
surface of the outer malleolus. The fibula is a straight,
elongated and rod-like bone attached to the lateral surface
of the tibia. Its proximal end is anteroposteriorly
expanded. The shaft tapers towards its distal end.
Metatarsals II–IV are tightly bound together into a
single broad metapodial unit (Figure 15). Metatarsal II is
laterally compressed and anteroposteriorly expanded. Its
mid shaft is slender, and the distal end is also
anteroposteriorly expanded. Metatarsal III is the longest
of the three elements, and straight. The proximal end of
metatarsal IV is expanded. About half-way down the shaft,
a vertical ridge is bound by ligaments to metatarsal III. Only
three phalanges of the right foot are preserved. They are
broad and have expanded proximal and distal ends. The
metatarsals are more slender than that in the holotype of
Bolong. The metatarsals are stout in YZH-001 with a ratio
of metatarsal III length to femur equal to 0.18; however, the
metatarsals are distorted (Wu and Godefroit 2012).
Discussion
Ontogenetic changes
The description of this juvenile specimen allows the study
of ontogenetic of Bolong for the first time. The body
length of this individual is ,50 cm. Osteohistological
study suggests a very early developmental stage of this
individual. In the holotype of Bolong, the caudal ribs are
not fused to the centra, indicating that sexual maturity
was probably not completely achieved. However, the
complete closure of the sutures between the neural arches
and centra throughout the vertebral column is suggestive
of skeletal maturity. The body length was estimated
approximately 3m for the holotype of Bolong (Wu and
Godefroit 2012). The body size is relatively small than
Jinzhousaurus, which has complete closure of the sutures
between the neural arches and centra throughout
the vertebral column, the body length is approximately
5–5.5m (Wang et al. 2010).
The number of tooth positions in the maxilla and
dentary increases during ontogeny as in other iguanodon-
tians (Weishampel et al. 2003; Horner et al. 2004; Hubner
and Rauhut 2010); the morphology of teeth is very similar
in juvenile and adult Bolong, as reported in basal
ornithopod Talenkauen (Egerton et al. 2013) and the
130) ordered. Starting treeswereWagner treeswith a random
seed of 1; 9999 replicates were used with 10 trees saved per
replication. Five OTUs (‘Camptosaurus’ valdensis, Draco-
nyx, NHMUK R8676, Delapparentia and Glishades) were
excluded (McDonald 2012; McDonald et al. 2012). The
analysis resulted in 10,750 most parsimonious trees (MPTs)
of 415 steps. The strict consensus tree was very poorly
resolved, with nearly the whole of Iguanodontia in an
W. Zheng et al.248
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unresolved polytomy. The 50%majority rule consensus tree
of 10,750MPTs (415 steps)was recovered from a traditional
search (Figure 16). Both ZMNH M8812 and YHZ-001 are
recovered as basal styracosternans, within a node that is very
poorly resolved. Bolong is basal to most other iguanodon-
tians from east-central Asia (such as Altirhinus, Equijubus,
Probactrosaurus or Jinzhousaurus), similar to the result in
Wu and Godefroit (2012). The phylogenetic characters used
by McDonald (2012) for inferring iguanodontian relation-
ships are not affected by ontogeny in Bolong, as in
Bactrosaurus that the juvenile and subadult specimens can
provide a substantial amount of reliable information for
phylogenetic inference (Prieto-Marquez 2011).
Conclusions
A juvenile specimen of Bolong yixianensis was described,
which has significantly increased the anatomical knowl-
Figure 16. The 50% majority rule consensus tree of 10,750 MPTs recovered from a traditional (heuristic) search in TNT. Figures belownodes represent the percentage of MPTs in which the node was recovered.
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edge of this basal iguanodontian ornithopod. The speci-
men revealed one additional autapomorphy for Bolong
yixianensis. The lingual face of the crown is bounded by
thickened mesial and distal margins and is bisected by a
prominent median principal ridge.
Osteohistologic studies of the femur indicate that
this individual was in the nestling stage when it died,
probably only several months after it hatched. Thus,
ZMNH M8812 is the first early juvenile iguanodontian
specimen reported from east Asia, as such, an important
specimen for future ontogenetic studies of basal
iguanodontians.
The comparison between the juvenile and holotype of
Bolong yixianensis revealed the following ontogenetic
variations in this taxon: the adult maxilla is deep and the
apex is more distally positioned, the adult jugal is more
robust, the tooth rows of both maxilla and dentary are less
in the juvenile individual, the scapula is more robust in the
adult, the radius and ulna are more robust in the adult and
became proportionally shorter relative to the hindlimb; the
adult metatarsals are proportionally shorter than the
juvenile. The following characters, previously used in the
diagnosis of Bolong yixianensis, are variable ontogeneti-
cally: the primary ridge is deflected distally on maxillary
crowns, ulna and radius are proportionally short and
robust, metatarsals are proportionally short.
Furthermore, this study suggests that, in Bolong
yixianensis, the phylogenetic characters used by McDo-
nald (2012) for inferring iguanodontian relationships are
not affected by ontogeny. Thus, juvenile specimens may
provide reliable information in phylogenetic analysis.
Acknowledgements
We are grateful to David Varricchio and Frankie Jackson(Montana State University, USA) for reading the manuscript andoffering many valuable suggestions. We thank Yuqing Zhangand Chaohe Yu for preparing the specimen and Dr Junchang Lu(Institute of Geology, Chinese Academy of Geological Sciences,Beijing) for helping in the process of preparation. The thinsections of the femur were made at Museum of the Rockies,Montana. We thank Anusuya Chinsamy-Turan (University ofCape Town, South Africa) for helpful comments in ourosteohistology study.
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