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Heterodonty and double occlusion in Manidens condorensis: a unique adaptation in an Early Jurassic ornithischian improving masticatory efficiency

Authors:
Marcos Gabriel Becerra at Centro de Investigaciones en Ciencias de la Tierra (CICTERRA)
Marcos Gabriel Becerra
  • Centro de Investigaciones en Ciencias de la Tierra (CICTERRA)
Diego Pol at Museo Argentino de Ciencias Naturales "Bernardino Rivadavia"
Diego Pol
Gertrud E. Rössner at Bavarian Natural History Collections
Gertrud E. Rössner
Oliver W M Rauhut at Bayerische Staatssammlung für Paläontologie und Geologie
Oliver W M Rauhut
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Abstract

New materials of the ornithischian dinosaur Manidens condorensis highlight a strong heterodonty between the upper and lower dentitions and reveal a novel occlusion type previously unreported in herbivorous dinosaurs. The diamond-shaped maxillary teeth have prominent cingular entolophs in a V- to Z-shaped configuration that are absent in dentary teeth. These cingular entolophs bear denticles and serrations with vertical wear that is coplanar with the apical wear facets, supporting their involvement in chewing. The separated apical and basal wear in dentary teeth is consistent with the apical and cingular wear in maxillary teeth, indicating an alternate occlusion, an orthal jaw motion, and shearing interactions between marginal and cingular edges in a double occlusion. Measurements of the length and wear area along the marginal and cingular edges indicate that the latter are functionally equivalent to adding eight teeth to a maxillary toothrow of ten, almost doubling the lengths of cutting edges and the degree of intraoral processing, while maintaining a plesiomorphic skull anatomy, an adaptation to herbivory unique in Ornithischia.
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SHORT COMMUNICATION
Heterodonty and double occlusion in Manidens condorensis: a unique
adaptation in an Early Jurassic ornithischian improving
masticatory efficiency
Marcos G. Becerra
1
&Diego Pol
1
&Gertrud E. Rössner
2
&Oliver W. M. Rauhut
2
Received: 27 March 2018 /Revised: 31 May 2018 /Accepted: 2 June 2018
#Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract
New materials of the ornithischian dinosaur Manidens condorensis highlight a strong heterodonty between the upper and lower
dentitions and reveal a novel occlusion type previously unreported in herbivorous dinosaurs. The diamond-shaped maxillary
teeth have prominent cingular entolophs in a V- to Z-shaped configuration that are absent in dentary teeth. These cingular
entolophs bear denticles and serrations with vertical wear that is coplanar with the apical wear facets, supporting their involve-
ment in chewing. The separated apical and basal wear in dentary teeth is consistent with the apical and cingular wear in maxillary
teeth, indicating an alternate occlusion, an orthal jaw motion, and shearing interactions between marginal and cingular edges in a
double occlusion. Measurements of the length and wear area along the marginal and cingular edges indicate that the latter are
functionally equivalent to adding eight teeth to a maxillary toothrow of ten, almost doubling the lengths of cutting edges and the
degree of intraoral processing, while maintaining a plesiomorphic skull anatomy, an adaptation to herbivory unique in
Ornithischia.
Keywords Manidens condorensis .Ornithischia .Maxillary dentition .Heterodonty .Jaw mechanics
Introduction
The diversity of ornithischian dinosaurs was low before the
Middle Jurassic in comparison with sauropodomorph and the-
ropod clades (Irmis et al. 2007; Barrett et al. 2009). In contrast,
ornithischians occupied herbivorous niches at low to interme-
diate highs of the vertical tiering during the second half of the
Mesozoic and evolved the most sophisticated jaw mechanics
among archosaurs (Cuthbertson et al., 2012; Williams et al.
2009; Sereno et al. 2010; Ösi et al. 2014; and references
therein). Common craniomandibular adaptations of ornithis-
chians to herbivory include a horny beak, an increase in num-
ber and decrease in size of teeth, the integration of individual
teeth into sophisticated tooth batteries, medial displacement of
the toothrow, ventral displacement of the jaw articulation, and
increase of jaw adductor musculature and bite force
(Nabavizadeh 2016; Strickson et al. 2016; MacLaren et al.
2017; and references therein). Heterodontosauridae represents
the first radiation of ornithischians and possibly the first
adapted to herbivory (Butler et al. 2008), including basal spe-
cies with low, triangular, and well-spaced denticulated tooth
crowns (Sereno 2012) and derived forms with tall and
diamond-shaped teeth in a closely packed dentition, with
oblique wear facets (Norman et al. 2011). Manidens
condorensis from the late Early Jurassic of Argentina has been
interpreted as a morphologically intermediate stage in the evo-
lution of Heterodontosauridae in terms of its dentition (Pol et
al. 2011; Becerra et al. 2014). Here, we report new materials
that preserve its maxillary dentition and reveal an unreported
occlusion pattern interpreted as an adaptation to herbivory that
increases masticatory efficiency.
Communicated by: Sven Thatje
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s00114-018-1569-6) contains supplementary
material, which is available to authorized users.
*Marcos G. Becerra
mbecerra@mef.org.ar
1
CONICET- Museo Paleontológico Egidio Feruglio, Fontana 140,
U9100GYO Trelew, Chubut, Argentina
2
SNSB, Bayerische Staatssammlung für Paläontologie und Geologie,
andDepartmentforEarthandEnvironmentalSciencesand
GeoBioCenter, LMU München, Richard-Wagner-Str. 10,
80333 Munich, Germany
The Science of Nature (2018) 105:41
https://doi.org/10.1007/s00114-018-1569-6
Materials and methods
All materials were recovered from the Cañadón Asfalto
Formation (Pol et al. 2011) and are housed at Museo
Paleontológico Egidio Feruglio (Trelew, Chubut). Micro-
tomography in MPEF-PV 3211 and 3809 was conducted at
Staatliche Naturwissenschaftliche Sammlungen Bayerns in
Munich. DICOM datasets processing to 3D reconstructions
was made with 3DSlicer version 4.3.0 (Fedorov et al. 2012).
The measurement of chewing margins and wear facets in
MPEF-PV 3809 was made with ImageJ (Schneider et al.
2012). More details are presented at the Online Resource 2.
Results
Teeth The dentary teeth of Manidens have an asymmetrical
diamond-shaped outline, with the length of the mesial carina
about 60% the distal carina, one to two denticles mesially and
four to six distally, showing a mesial cavity and crenulated
edges of the denticles (Online Resource 1). The dentary
toothrow is notably heterodont in terms of size and shape
including the presence of a large caniniform in the first tooth
position (Pol et al. 2011).
The maxillary dentition of the holotype of Manidens
(MPEF-PV 3211) was 3D reconstructed based on micro-CT
data (Fig. 1a, b; Online Resource 1). A new isolated maxilla
(MPEF-PV 3809) preserves ten maxillary teeth and replace-
ment teeth (Fig. 1c–e; Online Resource 1) and can be referred
to Manidens condorensis on the basis of autapomorphies in
tooth morphology.
Wear has erased the apical features of the anterior maxillary
teeth in MPEF-PV 3211 (M2–M5) and 3809 (M3–M4), but
these are present in MPEF-PV 3818, an isolated, apicobasally
tall, and slender crown (Fig. 1b, d–f). Some teeth in MPEF-PV
3211 (M6–M8) and 3809 (M5–M10) show two to three den-
ticles symmetrically distributed around the apical denticle,
each forming a rather low apex (Fig. 1b–e). The mesial cavity
is present from M5 onwards in both specimens (Fig. 1c,
Online Resource 1), although all the maxillary dentition is
closely packed, in contrast to the dentary dentition (Becerra
et al. 2014). In both specimens, an oblique distal ectoloph is
evident from position M4 onwards and an oblique distal
entoloph from M3 onwards (Fig. 1d, e; Online Resource 1).
A mesial entoloph appears from positions M5–M6, obliquely
oriented in M6 and almost horizontally in M7–M8 in MPEF-
PV 3211, and similarly oblique for M5–M7 and horizontal for
M8–M10inMPEF-PV3809(Fig.1b, d–e). Cingular
entolophs are apically oriented and extend over half of the
crown, gradually changing from a V- to a Z-shaped configu-
ration; their mesiodistal length and medial prominence in-
crease posteriorly (Fig. 1d, e). Additionally, the cingular
entolophsin MPEF-PV 3809and isolated maxillary teeth bear
ornamentations, with large denticles at the mesial entolophs
(two to three denticles in M5–M6), and small serrations at the
distal entolophs (M5–
M7). Maxillary and dentary cheek den-
titions share the same size heterodonty (Becerra et al. 2014).
Maxillary teeth have thickened enamel over the cutting edge
of the denticles instead of crenulated margins, evident in iso-
lated teeth (Online Resource 1). Isolated teeth show up to
seven denticles in their mesial entoloph and an extra mesial
ectoloph that morphologically varies from a rounded promi-
nence to a denticulated edge (Fig. 1g, h), features possibly
increasing posteriorly along the toothrow (Online Resource 1).
Manidens differs from other heterodontosaurids and ornith-
ischians in the marked morphological differentiation between
the upper and lower toothrows (Fig. 1i, j). The symmetric
distribution of denticles of the diamond-shaped maxillary den-
tition of Manidens is shared with Echinodon,whiletheasym-
metrichand-shapeddentaryteetharesimilartothoseof
Pegomastax (Sereno 2012). Cingular protuberances were re-
ported in other heterodontosaurids: distal ectolophs in
Lycorhinus angustidens, and slight ectolophs in Tianyulong,
the posterior teeth of Echinodon, and the largest crowns of
Fruitadens (Sereno 2012, and references therein). Outside
Heterodontosauridae, some species of Thyreophora, basal
Ornithopoda, Pachycephalosauria, and basal Ceratopsia have
evolved cingular edges in their dentition (exhaustive compar-
ison in Online Resource 2). Nevertheless, upper and lower
tooth rows in these species are morphologically similar and
specularly arranged (Weishampel et al. 2004).
Wear facets Wear on dentary teeth of Manidens is character-
ized by a near vertical orientation of distinct apical and basal
facets (Becerra et al. 2014), identified in the type specimen
and isolated teeth (Online Resource 1). Apical wear facets
face posteriorly in distal denticles and the crown apex, and
anteriorly in the facets of mesial denticles. Basal wear facets
are also located either distally or mesially on the base of the
crown and are coplanar with the apical facets.
Wear facets on maxillary teeth are present at the apical and
lingual cingular margins. As in dentary teeth (Becerra et al.
2014), the facets are almost vertically oriented (67°/77° for
apical/cingular wear with respect to the horizontal plane)
and are flat to concave, depending on the degree of develop-
ment (Fig. 1d, e). Anterior teeth have apical wear facets larger
than the cingular facets, but in posterior teeth, the apical facets
are smaller than those of the cingular entolophs. The wear
facets show a step at their basal enamel-dentine boundary
(trailing edge) and a continuous transition at their apical limit
(leading edge) (Online Resource 1). As in dentary teeth, apical
and basal wear facets located on the distal half of the crown
face slightly posteriorly, while those on the mesial half face
anteriorly (Fig. 2a–f). Additional wear is present at the non-
occlusal surface of some of the dentary and maxillary teeth
(Online Resource 1).
41 Page 2 of 5 Sci Nat (2018) 105:41
Discussion
Scratch orientation in wear facets of maxillary (apical and
cingular) and dentary (apical) teeth (with angles ranging
around 80–105° and variable lengths), together with the lead-
ing and trailing edges located apically and basally in these
facets, support a simple orthal jaw closure (Fig. 2e, f,
Online Resource 1). Wear at the non-occlusal surface of teeth,
with smoothened boundaries and few randomly oriented
scratches, is likely due to food abrasion (Fig. 2a,
Online Resource 1). No wear was identified within the
paracingular fossa (Fig. 2a), indicating that the apex of the
antagonistic crown did not fit within this fossa (as the cusp-
basin occlusion of tribosphenic mammals). The different
orientation of wear facets in mesial and distal regions in
maxillary and dentary crowns indicates an imbricating,
and, probably, precise occlusion of the opposing toothrows
(Fig. 2e). The coplanar orientation of apical and basal wear
facets indicates that the apical carinae of dentary teeth oc-
cluded with the apical and cingular margins of maxillary
teeth, forming a double sequential occlusion (Fig. 2f).
Thus,inadditiontotheshearingmotionofapex-apexcon-
tact, the apical carinae of dentary teeth formed a second
shearing interaction with the cingular entolophs of maxil-
lary teeth during the same masticatory cycle, a feature pre-
viously unknown in other dinosaurs.
Fig. 1 a,c3D reconstructions of
MPEF-PV 3211 and MPEF-PV
3809 in right and medial views
highlighting the functional
(orange/darker gray) and
replacement maxillary teeth
(yellow/brighter gray). b
Maxillary teeth M7–M8 of
MPEF-PV 3211 in lingual view.
d,eDentition of MPEF-PV 3809
in lingual view. fMPEF-PV 3818
in lingual view. g,hMPEF-PV
3820 in labial and lingual views,
respectively. Additional isolated
maxillary teeth in
Online Resource 1.i,jHypothetic
reconstruction of dentition in
Manidens condorensis in qlabial
and rlingual views, based on this
work and unpublished informa-
tion (MPEF-PV 3808, gray
caniniform; Becerra et al., in
prep.). Abbreviations: awf, apical
wear facet; cwf, cingular wear
facet; dec, distal ectoloph; den,
distal entoloph; mcd, mesial
cingular denticle; mec, mesial
ectoloph; men, mesial entoloph;
nwf, wear at the non-functional
face
Sci Nat (2018) 105:41 Page 3 of 5 41
The double shearing mechanism of Manidens improves the
masticatory efficiency of the toothrow. The length of shearing
apical margins of the upper toothrow is subequal to the length
of shearing margins of the entoloph (21.76 versus 19.83 mm;
Fig. 2b, c). Similarly, the area of the apical wear facets is
3.615 mm
2
and that of the cingular wear facets is
2.895 mm
2
(80.1% of the apical wear; Fig. 2b, d). Thus,
both the length and area of the occlusal surface provided by
the cingular entolophs in Manidens almost double the mas-
ticatory efficiency by adding a shear power equivalent to
6.4 to 8.2 additional teeth (based on the sum of marginal
area or length) if only the apical carina was functional
(Online Resource 3).
The unique characteristics of the jaw mechanics of
Manidens (plesiomorphic orthal occlusion and a double
cutting edge) were achieved through minor morphological
modifications that almost double the masticatory efficiency,
ultimately reducing the energy spent in tooth formation and
mandibular motion. This deviates from previous evolutionary
trends in Ornithischia (Sereno 1997;Belletal.2009;Erickson
et al. 2012; and references therein), in which improvements in
mastication efficiency were commonly achieved through the
development of dental batteries, the increase in rostral length
of the skull, or by acquiring a complex jaw motion. These
new data of the dentition of Manidens reveal previously
unrecognized functional and morphological adaptations in
Heterodontosauridae during the first radiation of ornithis-
chians in the Early Jurassic, adding to the already diverse
array of herbivorous adaptations known in ornithischian
dinosaurs.
Fig. 2 aSEM imaging of MPEF-
PV 3820 in occlusal view. lines
indicate the interaction facets with
two antagonistic teeth, dots
indicate food abrasion facets, and
hues differentiate apical from
cingular wear. bSchematics of
the dentition of MPEF-PV 3809,
lines and dark areas representing
the length and wear facet
measurements as they were taken.
c,dPlots depicting measured
values of length (c) and wear area
(d) on each tooth position, with
hypothetic values for M2. e
Hypothetical staggered tooth
occlusion based on wear
development (contact of different
opposing tooth in blue, red, and
yellow or hues of gray), with f
two shearing interactions at the
same masticatory cycle in a
double occlusion
41 Page 4 of 5 Sci Nat (2018) 105:41
Acknowledgements Dr. Y. Herrera (UNLP), the institution and stafffrom
the Staatliche Naturwissenschaftliche Sammlungen Bayerns (Munich),
and the reviewers are thanked.
Funding information The Deutscher Akademischer Austauschdienst
(DAAD) financed the travel for M.G.B. to Germany (ID 57130097);
fieldwork and research were supported by Agencia Nacional de
Promoción Científica y Técnica to D.P. (ANPCyT, PICT 1288 and
0808), Deutsche Forschungsgemeinschaft to O.W.M.R. (DFG,
RA1012/9-1), and National Science Fundation to Dr. G. Rougier (NSF,
DEB 0946430 and DEB 1068089).
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of
interest.
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Supplementary resource (1)

Supplementary Online Material - Heterodonty and double occlusion in Manidens condorensis.pdf
Data
August 2021
Marcos Gabriel Becerra · Diego Pol · Gertrud E. Rössner · Oliver W M Rauhut
... Heterodontosaurus sp. from the Lower Jurassic of the Laguna Colorada Formation (El Tranquilo Group, Argentina; Báez & Marsicano, 2001;Pol et al., 2021), and undescribed heterodontosaurid remains from the Kayenta Formation (Sinemurian-Pliensbachian, Arizona; Sereno, 2012). Regardless of the phylogenetic relationship among heterodontosaurids, the clade includes taxa possessing a plesiomorphic craniodental anatomy, including Echinodon, Fruitadens, and Tianyulong, as well as taxa featuring specialized craniodental traits, including Abrictosaurus, Heterodontosaurus, Lycorhinus, Pegomastax, and Manidens (Pol et al., 2011;Butler et al., 2012;Sereno, 2012;Becerra et al., 2018). The best-preserved skulls in Heterodontosauridae to date are known from Heterodontosaurus Norman et al., 2011;Sereno, 2012;Radermacher et al., 2021). ...
... The heterodontosaurid Manidens condorensis, known from one relatively complete individual and other fragmentary remains, represents the most complete ornithischian from the Lower Jurassic of South America (Pol et al., 2011). Manidens combines plesiomorphic (incipient development of wear in a vertical orientation) and derived (sub-hypsodont crowns closely packed in the mid-posterior dentition) craniodental features, as well as multiple autapomorphies (Pol et al., 2011;Becerra et al., 2018Becerra et al., , 2021. Although the dentition of Manidens has been studied in detail (Becerra et al., 2014(Becerra et al., , 2018(Becerra et al., , 2021Becerra & Pol, 2020), its craniomandibular anatomy has only briefly been described. ...
... Manidens combines plesiomorphic (incipient development of wear in a vertical orientation) and derived (sub-hypsodont crowns closely packed in the mid-posterior dentition) craniodental features, as well as multiple autapomorphies (Pol et al., 2011;Becerra et al., 2018Becerra et al., , 2021. Although the dentition of Manidens has been studied in detail (Becerra et al., 2014(Becerra et al., , 2018(Becerra et al., , 2021Becerra & Pol, 2020), its craniomandibular anatomy has only briefly been described. In addition, Manidens comes from the Pliensbachian-Callovian gap in the worldwide ornithischian fossil record. ...
Craniomandibular osteology of Manidens condorensis (Ornithischia: Heterodontosauridae) from the upper Lower Jurassic of Argentina
Article
  • Apr 2023
Heterodontosauridae is a clade that appears early in the ornithischian fossil record, and includes small-bodied, highly specialized species characterized by an unusual heterodont dentition. Although known from relatively few taxa, the early representation of the clade and unsolved phylogenetic relationships within heterodontosaurids and among early ornithischians implies that novel information has a marked effect on broader phylogenetic hypotheses and our understanding of early diversification patterns within Ornithischia. This paper describes the cranial osteology of the heterodontosaurid Manidens condorensis based on computed micro-tomographic scans of MPEF-PV 3211 and MPEF-PV 3809. This enabled more detailed descriptions of previously recognized bones, corrections to the literature, and the identification of undescribed elements. We present a new skull reconstruction and propose an emended diagnosis in light of novel anatomical information. Areas of jaw muscle attachment were identified and compared with Heterodontosaurus and Lesothosaurus, and mandibular function among heterodontosaurids is discussed. Our results indicate that diverse skull morphologies and functions existed among Early Jurassic ornithischians, with Manidens being intermediate between the plesiomorphic cranial shape and function associated with a generalist diet in ornithischians such as Tianyulong and Lesothosaurus, and the more derived cranial construction specialized for herbivory identified in heterodontosaurids from South Africa such as Heterodontosaurus.
View
... The sectioned isolated crowns of Manidens condorensis does not represent the entirely dentition, as there is no evidence of enamel morphology in the still unknown premaxillary dentition and the dentary caniniform only preserved at the holotype specimen MPEF-PV 3211. In addition, the maxillary and dentary dentition of Manidens were recently reported as extremely heterodont (Becerra et al., 2018). The descriptions performed in the main text only reach to the schmelzmuster level for Manidens, although was performed also a comparison between dentitions in the matter of enamel structure, thickening and ordering, due that the morphologic differences between opposing dentitions also involves strong enamel disparities. ...
... Mesiodistally and apicobasally, all sections can be orientated following three main features of Manidens dentition: (i) the occlusal face of the crowns correspond to the apicobasally higher face (dentary teeth) or the shorter face with an enlarged cingular entoloph (maxillary teeth); (ii) the non-occlusal face corresponds to the shorter one (dentary teeth) or the higher one with a slight change in orientation near the base corresponding to the oblique cingular ectoloph; (iii) and that the lingual crest limiting the mesial cavity goes further mesially if compared to the lingual crest of the same cavity for both maxillary and dentary teeth (Becerra et al., 2014(Becerra et al., , 2018. ...
... Nonetheless and as mentioned in the main text, several lines of research describe and optimize that an additional process of enamel thickening and development of enamel asymmetry occurs within Heterodontosauridae, counting in both cases as the fourth process of this kind in Ornithischia. Summarizing, these authors (Sereno, 2012;Becerra et al., 2014Becerra et al., , 2016Becerra et al., , 2018 detail that enamel is symmetric in basal species if compared with the basal species Fruitadens, Echinodon and Tianyulong (Butler et al., 2012;Sereno, 2012;Becerra et al., 2014). If the change of enamel distribution is mapped in the more complete sampling performed by Becerra et al. (2016) in Heterodontosauridae and basal Ornithischia, the optimized states indicates that instead of being ambiguous, the symmetric enamel corresponds to the ancestral state at least for Ornithischia, Genasauria and Tyreophora (nodes affected in the optimization by the ambiguous state of Manidens). ...
View
... Manidens condorensis, a heterodontosaurid ornithischian represented by skull, postcranial remains and isolated teeth, was found in the latest Early Jurassic Cañadón Asfalto Formation (Central Patagonia, Argentina) (Pol et al. 2011;Becerra et al. 2018). Heterodontosauridae are phylogenetically and chronologically the first diversification of Ornithischia, predating the diversification of Genasauria (Thyreophora + Neornithischia). ...
... The following isolated teeth morphologically referred to Manidens were sectioned: dentary teeth MPEF-PV 10862, 10863 and 10865; maxillary teeth MPEF-PV 3821, 10823 and 10864 ( Fig. 1A 1 -F 1 ). Tooth morphology, as described by Pol et al. (2011), and Becerra et al. (2014, 2018, allows relating each isolated tooth to either the maxillary or dentary tooth rows (see SOM, Supplementary Online Material available at http://app.pan.pl/SOM/app65-Becerra_Pol_SOM.pdf). In addition, the degree of development of the cingular edges (maxillary teeth), the height/width proportion of each tooth (maxillary and dentary teeth), the presence of a mesial cavity (maxillary and dentary teeth excepting MPEF-PV 10823), and the variation of these features within both tooth rows allow relating all teeth to a mid-posterior position of their corresponding tooth rows (Becerra et al. 2014(Becerra et al. , 2018. ...
... Tooth morphology, as described by Pol et al. (2011), and Becerra et al. (2014, 2018, allows relating each isolated tooth to either the maxillary or dentary tooth rows (see SOM, Supplementary Online Material available at http://app.pan.pl/SOM/app65-Becerra_Pol_SOM.pdf). In addition, the degree of development of the cingular edges (maxillary teeth), the height/width proportion of each tooth (maxillary and dentary teeth), the presence of a mesial cavity (maxillary and dentary teeth excepting MPEF-PV 10823), and the variation of these features within both tooth rows allow relating all teeth to a mid-posterior position of their corresponding tooth rows (Becerra et al. 2014(Becerra et al. , 2018. However, the degree of damage of these teeth precludes a more accurate inference of their possible position, and thus the following enamel description, although based in mid-posterior teeth, is generalized to each dentition. ...
The enamel microstructure of Manidens condorensis: New hypotheses on the ancestral state and evolution of enamel in Ornithischia
Article
Full-text available
  • Mar 2020
Previous studies on enamel microstructure in Ornithischia have focused on derived lineages of this clade based on species from the northern hemisphere. Here we describe the enamel microstructure of Manidens condorensis from the late Early Jurassic of Argentina that belongs to Heterodontosauridae (interpreted as the basal-most clade of Ornithischia). Enamel microstructure in the cheek teeth lacks a basal unit layer, presents incipient divergent crystallite as the dominant enamel type and parallel crystallite enamel type (with or without incrementing lines). Enamel of maxillary and dentary teeth differs from each other in enamel distribution (asymmetric vs. symmetric), structure (presence vs. absence of tubules, and less vs. more abundant parallel crystallite enamel with incrementing lines) and ordering (regular ordering of enamel types vs. in patches). The enamel microstructure of Manidens is the simplest of all known Ornithischia, and is more similar to that of the sauropodomorph Plateosaurus than to the one reported for the basal theropod Coelophysis. Similarities within Ornithischia are present with pachycephalosaurids and, to a lesser extent, with ankylosaurs. Phylogenetic optimization of enamel characters in Ornithischia allows the inference of new ancestral states for the internal nodes of the major lineages and to highlight evolutionary transformations: (i) absence of a basal unit layer and presence of parallel crystallite and incipient divergent columnar enamel as the ancestral state for Ornithischia; (ii) the shared presence of incipient divergent columnar units or poorly developed divergent columnar enamel in Pachycephalosauridae and Thyreophora represents a retention of the plesiomorphic ornithischian condition; (iii) the wavy enamel of Dryomorpha evolved from the ancestral incipient divergent columnar units present in Ornithopoda and; (iv) enamel thickness and asymmetry has independently evolved at least four times in Ornithischia.
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... mammals and ornithischian dinosaurs), which increase dental complexity, sauropodomorphs are hypothesized to have simplified their teeth and increased replacement rates in association with gigantism (Sander et al. 2011;Melstrom et al. 2021). Accordingly, tooth shape should be generally linked to sauropodomorph evolution, making teeth an important proxy for inferring ecological and taxonomic diversity (Buffetaut et al. 2006;Vullo et al. 2007;Vullo & Néraudeau 2010;Ősi et al. 2017;Becerra et al. 2018;Frauenfelder et al. 2020;Poropat et al. 2022). Taxonomic identification of sauropodomorph teeth has previously relied on a combination of two indices: the slenderness index (SI; Upchurch 1998) and the compression index (CI;Díez Díaz et al. 2013). ...
... Strictly speaking, heterodonty refers to variation in dental function along the tooth row that allows animals to compartmentalize food processing (such as chewing, grasping, etc.) thereby increasing the efficiency with which nutrients are extracted (Shimada 2001;Smith 2005;D'Amore et al. 2019). Such functional differences are most often associated with omnivorous, insectivorous, and herbivorous vertebrates (Edmund 1969;Hungerbühler 2000;Smith 2005;Butler et al. 2009;Reichel 2010;Norman et al. 2011;Zanno & Makovicky 2011;D'Amore 2015;Becerra et al. 2018;Clack et al. 2019;Melstrom & Irmis 2019), with the notable exception of sauropod sauropodomorphs, in which heterodonty is purportedly absent (Chure et al. 2010). However, heterodonty has been described for most non-sauropod sauropodomorphs (e.g. ...
Revealing the use of dental indices to infer taxonomic variation in sauropod dinosaurs
Article
  • Oct 2024
Sauropod teeth are commonly categorized taxonomically by two well‐established measurement indices: slenderness index (SI; apicobasal height/mesiodistal width), which quantifies breadth, and compression index (CI; labiolingual width/mesiodistal width), which quantifies cross‐sectional circularity. Although both indices are used to infer high‐level taxonomic affinities, little is known about the linear relationships between the constituent measurements or how the indices vary intra‐cranially and at lower taxonomic levels. Here, we evaluate these relationships using a novel dataset of sauropod teeth (N = 898) spanning all major sauropod groups. Results for both indices indicate significant differential scaling within Sauropodomorpha, both in slope and elevation. Broad‐crowned sauropods mostly display positive allometry in SI compared to isometry in narrow‐crowned sauropods. However, this distinction is less clear with CI as most sauropods display isometry, whilst non‐sauropod sauropodomorphs (e.g. Plateosauridae) display positive allometry. An anova reveals SI varies significantly with genus and tooth position. Specifically, jaw type (maxilla and dentary) is significant within Plateosauridae, whilst tooth position is significant within titanosaurs. Overall, variation within CI is restricted to genus. Our findings reveal that whilst indices have taxonomic utility, there are caveats. The measurements used to calculate the indices exhibit significant allometry, indicating that index values are size‐dependent. Furthermore, the indices may not accurately reflect size heterodont conditions present among early‐branching sauropodomorphs. Our study highlights the importance of quantifying taxonomic relationships of measurement data, which can be used to inform hypotheses regarding the physiological and palaeoecological drivers influencing tooth shape evolution.
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... The presence of heterodontosaurids in the Middle Jurassic is still disputed. Manidens condorensis (Pol et al., 2011) from the basal levels of the Cañadón Asfalto Formation (Argentina) is considered to be either Early (Becerra et al., 2018(Becerra et al., , 2021 or Middle Jurassic in age (Pol et al., 2011;Sereno, 2012) with recent radiometric dating supporting a middle-late Toarcian age (Becerra et al., 2022;Pol et al., 2020). The age of Tianyulong from the Middle-Upper Jurassic Daohugou Beds (Yanliao Biota) in northeastern China is controversial because of stratigraphic uncertainties surrounding the placement of volcanic rocks within the sequence used to obtain radiometric dates (Sullivan et al., 2014;Xu et al., 2016) with recent work considering it to be either Early Cretaceous (Liu et al., 2012) or Middle Jurassic (Callovian) (Becerra et al., 2022). ...
... Stegosaur teeth are broadly similar in morphology: however, stegosaur marginal denticles are usually rounded in lateral view compared with the more pointed form commonly seen in ankylosaurs (Barrett, 2001;Galton & Upchurch, 2004). A denticulate cingulum (as in NHMUK PV R 38816 and PV R 38817) occurs in several ornithischian taxa (Becerra et al., 2018), including ankylosaurs ( Fig. 13) such as Niobrarasaurus, Sauropelta, Panoplosaurus, and 'Priodontognathus' (Arbour & Currie, 2016;Carpenter et al., 1995;Galton, 1980c;Mallon & Anderson, 2014), the stegosaur Isaberrysaura (Salgado et al., 2017), the heterodontosaurid Manidens (Becerra et al., 2013), the Late Jurassic neornithischian Nanosaurus (Carpenter & Galton, 2018), Late Cretaceous ornithischians such as Pachycephalosaurus (Brown & Schlaikjer, 1943), some Late Triassic archosaurs (Parker et al., 2005), and isolated ornithischian teeth from Portugal and Spain (Rauhut, 2001(Rauhut, , 2002Thulborn, 1973). STEGOSAURIA Marsh, 1877 Gen. et sp. ...
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... Despite such omnivory in the earliest ornithischians, a highly efficient craniodental apparatus, including dental batteries, evolved in Early Jurassic clades such as heterodontosaurids (55,64), indicating a shift to herbivory in some early ornithischian lineages. Among heterodontosaurids, Manidens has been described as having intermediate craniodental traits (41,48), with an incipient dental battery compared to Heterodontosaurus (64) but with an efficient jaw apparatus to process plant material (65). Our prediction based on tooth morphology is in line with this evidence, although Manidens is here classified as a carnivore based on tooth mechanics. ...
... While we think this emerges from analyzing isolated teeth, it must be noted that occasional faunivory has been proposed for heterodontosaurids based on their enlarged caniniform teeth (66). Nonetheless, the complex jaw apparatus and mechanics of heterodontosaurids (64,65), and even the tooth occlusion of Lesothosaurus (55,61), indicate that the earliest ornithischians were more efficient at plant processing than most non-sauropodan sauropodomorphs. This, however, did not translate into a greater evolutionary success of ornithischians; on the contrary, sauropodomorphs were more diverse and abundant during the Late Triassic and Early Jurassic (2). ...
Dental form and function in the early feeding diversification of dinosaurs
Article
Full-text available
  • Dec 2022
Dinosaurs evolved a remarkable diversity of dietary adaptations throughout the Mesozoic, but the origins of different feeding modes are uncertain, especially the multiple origins of herbivory. Feeding habits of early dinosaurs have mostly been inferred from qualitative comparisons of dental morphology with extant analogs. Here, we use biomechanical and morphometric methods to investigate the dental morphofunctional diversity of early dinosaurs in comparison with extant squamates and crocodylians and predict their diets using machine learning classification models. Early saurischians/theropods are consistently classified as carnivores. Sauropodomorphs underwent a dietary shift from faunivory to herbivory, experimenting with diverse diets during the Triassic and Early Jurassic, and early ornithischians were likely omnivores. Obligate herbivory was a late evolutionary innovation in both clades. Carnivory is the most plausible ancestral diet of dinosaurs, but omnivory is equally likely under certain phylogenetic scenarios. This early dietary diversity was fundamental in the rise of dinosaurs to ecological dominance.
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... For example, pacu, the herbivorous relatives of piranhas, have both incisiform and molariform teeth that are used to distinguish genera (Berkovitz 1980, Kolmann et al. 2019. The evolution of morphological heterodonty is consistently used in systematics often to infer differences in prey processing (Becerra et al., 2018, Kenne 1991. We explicitly do not assume that size and shape are the same as functioninstead we pose the question: do teeth that look alike actually function in the same way (Cohen et al. 2020, Hulsey et al. 2020, Mihalitsis & Bellwood 2019? ...
The Evolutionary Continuum of Functional Homodonty to Heterodonty in the Dentition of Halichoeres Wrasses
Article
Full-text available
  • Sep 2020
  • INTEGR COMP BIOL
Vertebrate dentitions are often collapsed into a few discrete categories, obscuring both potentially important functional differences between them and insight into their evolution. The terms homodonty and heterodonty typically conflate tooth morphology with tooth function, and require context-dependent subcategories to take on any specific meaning. Qualifiers like incipient, transient, or phylogenetic homodonty attempt to provide a more rigorous definition but instead highlight the difficulties in categorizing dentitions. To address these issues, we recently proposed a method for quantifying the function of dental batteries based on the estimated stress of each tooth (inferred using surface area) standardized for jaw out-lever (inferred using tooth position). This method reveals a homodonty-heterodonty functional continuum where small and large teeth work together to transmit forces to a prey item. Morphological homodonty or heterodonty refers to morphology, whereas functional homodonty or heterodonty refers to transmission of stress. In this study, we use Halichoeres wrasses to explore how functional continuum can be used in phylogenetic analyses by generating two continuous metrics from the functional homodonty-heterodonty continuum. Here we show that functionally heterodont teeth have evolved at least three times in Halichoeres wrasses. There are more functionally heterodont teeth on upper jaws than on lower jaws, but functionally heterodont teeth on the lower jaws bear significantly more stress. These nuances, which have functional consequences, would be missed by binning entire dentitions into discrete categories. This analysis points out areas worth taking a closer look at from a mechanical and developmental point of view with respect to the distribution and type of heterodonty seen in different jaws and different areas of jaws. These data, on a small group of wrasses, suggest continuous dental variables can be a rich source of insight into the evolution of fish feeding mechanisms across a wider variety of species.
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... This combination of traits allows for a mostly vertically oriented vector for a broad fan of temporal musculature with a relatively moderate mechanical advantage (Nabavizadeh, 2016). These taxa are primarily orthal feeders, raising the lower jaw isognathously (i.e., to occlude on both sides at once), with only slight hemimandibular long-axis rotation against the predentary and, in some cases, a slight palinal movement of the jaw (e.g., heterodontosaurids, stegosaurs, and nonhadrosaurid iguanodontians; Weishampel, 1984;Crompton and Attridge, 1986;Galton, 1986;Weishampel and Norman, 1989;Norman and Weishampel, 1991;Barrett, 2001;Galton and Upchurch, 2004;Porro, 2009;Norman et al., 2011;Nabavizadeh, 2016Nabavizadeh, , 2018Nabavizadeh and Weishampel, 2016;Virág and } Osi, 2017;Becerra et al., 2018;Woodruff et al., 2019). Some of these taxa also possess transversely broadened temporal regions (e.g., stegosaurs, basal ornithopods, and nonhadrosaurid iguanodontians) for greater relative mAME muscle body attachment and strength at occlusion. ...
Cranial Musculature in Herbivorous Dinosaurs: A Survey of Reconstructed Anatomical Diversity and Feeding Mechanisms
Article
Full-text available
Herbivorous dinosaurs exhibited diverse cranial feeding mechanisms. Although osteological, microwear, and biomechanical research has revealed some of this diversity, the evolutionary reorientation of cranial musculature throughout nonavian herbivorous Dinosauria and its influence on feeding mechanisms requires more study. Here, cranial muscle reconstructions in herbivorous dinosaurs are reviewed and informative anatomical characters are compared across 142 dinosaur genera (84 ornithischians, 36 sauropodomorphs, and 22 herbivorous nonavian theropods), both through examination of specimens and literature. Traits include those relating to the temporal region, adductor chamber, palate, and mandibular attachments, such as the coronoid elevation and retroarticular process. Findings reveal many combinations of anatomical traits influencing a diversity of feeding mechanisms. Some primarily more orthal feeders, including herbivorous theropods, nonsauropod sauropodomorphs, basal ornithischians, and derived stegosaurs (which also show varying degrees of coinciding slight palinal motion and long‐axis hemimandibular rotation), possess traits indicative of more prominent temporal musculature and moderately sized palatal musculature. However, orthal feeding sauropods and pachycephalosaurs possess traits indicative of greatly reduced, low‐angled temporal musculature, and enhanced palatal musculature producing a primarily vertical, orthal feeding vector. Among ankylosaurs, hadrosaurids, and neoceratopsians, a rostrolabial temporal muscle expansion is present (with a tall coronoid elevation in hadrosaurids and ceratopsids) for greater temporal muscle support and mechanical advantage for complex palinal feeding motions. This also aids in long‐axis hemimandibular rotation against the predentary in hadrosaurs and ankylosaurs. This diversity in cranial muscle architecture provides an informative spectrum of numerous adaptations acquired given the evolution of various anatomical constraints in the skull. Anat Rec, 303:1104–1145, 2020. © 2019 American Association for Anatomy
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Tooth replacement in Manidens condorensis: baseline study to address the replacement pattern in dentitions of early ornithischians
Article
Full-text available
  • Oct 2020
Dental replacement in Heterodontosauridae has been debated over the last five decades primarily on indirect evidence, such as the development of wear facets and the position of erupted teeth. Direct observation of unerupted teeth provides unambiguous data for understanding tooth replacement but this has been done only for Heterodontosaurus and Fruitadens. This study addresses dental replacement in Manidens condorensis based on the positioning of functional and replacement teeth using microcomputed tomography data, differential wear along the dentition and the differences in labiolingual/apicobasal level of functional teeth. Dental replacement in Manidens condorensis was continuous in an anterior-to-posterior wave pattern, with asynchronous tooth eruption and the addition of new teeth posteriorly to the toothrow during ontogeny. Manidens shows the first evidence of dental replacement for the large dentary caniniform in Heterodontosauridae, which possibly had replacement timing distinct from the cheek dentition. Newly erupted teeth imbricate in a mesial cavity-distal crown base relationship during eruption, so that imbrication of the mid-posterior dentition remains unaltered during tooth replacement. The presence/absence of a small caniniform tooth in the D3 position of several specimens suggests possible intraspecific dimorphism in Manidens. On longitudinal sections of isolated crowns, the histological features such as Howship's lacunae and odontoclast spaces are similar in size to extant reptiles. The differential wear decreasing posteriorly and hypothetical Z-spacing below 2.3 in Manidens are similar to basal ornithischians. Tooth replacement in Heterodontosauridae (and other early ornithischians) provides key information for understanding the dynamics of jaw function and craniomandibular specialization to herbivory.
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Repeated Evolution of Divergent Modes of Herbivory in Non-avian Dinosaurs
Article
  • Dec 2019
  • CURR BIOL
The importance of adaptation [1-4] versus organizational constraints [5-7] in shaping common macroevolutionary trends remains unclear [8]. The fossil record is key to this problem, as it provides data on repetitive trait evolution between lineages [4, 8]. However, quantitative analyses investigating these dynamics with fossil data are rare [8]. Herbivory evolved multiple times within Mesozoic dinosaurs [9, 10], allowing analysis of common phenotypic responses to dietary evolution. Whereas repeated patterns of character acquisition [9] and functional changes [11-13] are observed between some herbivorous dinosaur clades, biomechanical studies resolve significant differences between morphologically similar taxa [12-14]. However, previous biomechanical analyses have not accounted for phylogenetic non-independence (e.g., [13-16]) or been restricted to individual clades (e.g., [11, 12, 16]). Here, we use multivariate analysis of biomechanical characters, within a robust phylogenetic context, to investigate functional pathways to herbivory in a large sample of non-avian dinosaurs. Results demonstrate multiple solutions to herbivory. Notably, two fundamentally different modes are observed to evolve independently multiple times, with morphofunctional changes in the skull co-varying with digestive strategy. These modes distinguish between gut-processing sauropodomorphs and theropods tending toward gracile crania and low bite forces and ornithischian taxa exhibiting character complexes associated with extensive oral processing. Although convergence within these subsets of taxa is common, it is not observed between them due to functional constraints imposed during the early evolution of each group. This highlights the hierarchical nature of evolution, with adaptation driving convergence within regions of morphospace delimited by phylogenetic contingency.
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Herbivorous dinosaur jaw disparity and its relationship to extrinsic evolutionary drivers
Article
Full-text available
  • Feb 2017
Morphological responses of nonmammalian herbivores to external ecological drivers have not been quantified over extended timescales. Herbivorous nonavian dinosaurs are an ideal group to test for such responses, because they dominated terrestrial ecosystems for more than 155 Myr and included the largest herbivores that ever existed. The radiation of dinosaurs was punctuated by several ecologically important events, including extinctions at the Triassic/Jurassic (Tr/J) and Jurassic/Cretaceous (J/K) boundaries, the decline of cycadophytes, and the origin of angiosperms, all of which may have had profound consequences for herbivore communities. Here we present the first analysis of morphological and biomechanical disparity for sauropodomorph and ornithischian dinosaurs in order to investigate patterns of jaw shape and function through time. We find that morphological and biomechanical mandibular disparity are decoupled: mandibular shape disparity follows taxonomic diversity, with a steady increase through the Mesozoic. By contrast, biomechanical disparity builds to a peak in the Late Jurassic that corresponds to increased functional variation among sauropods. The reduction in biomechanical disparity following this peak coincides with the J/K extinction, the associated loss of sauropod and stegosaur diversity, and the decline of cycadophytes. We find no specific correspondence between biomechanical disparity and the proliferation of angiosperms. Continual ecological and functional replacement of pre-existing taxa accounts for disparity patterns through much of the Cretaceous, with the exception of several unique groups, such as psittacosaurids that are never replaced in their biomechanical or morphological profiles.
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Taxonomy, morphology, masticatory function and phylogeny of heterodontosaurid dinosaurs
Article
Full-text available
  • Oct 2012
Heterodontosaurids comprise an important early radiation of small-bodied herbivores that persisted for approximately 100 My from Late Triassic to Early Cretaceous time. Review of available fossils unequivocally establishes Echinodon as a very small-bodied, late-surviving northern heterodontosaurid similar to the other northern genera Fruitadens and Tianyulong. Tianyulong from northern China has unusual skeletal proportions, including a relatively large skull, short forelimb, and long manual digit II. The southern African heterodontosaurid genus Lycorhinus is established as valid, and a new taxon from the same formation is named Pegomastax africanus gen. n., sp. n. Tooth replacement and tooth-to-tooth wear is more common than previously thought among heterodontosaurids, and in Heterodontosaurus the angle of tooth-to-tooth shear is shown to increase markedly during maturation. Long-axis rotation of the lower jaw during occlusion is identified here as the most likely functional mechanism underlying marked tooth wear in mature specimens of Heterodontosaurus. Extensive tooth wear and other evidence suggests that all heterodontosaurids were predominantly or exclusively herbivores. Basal genera such as Echinodon, Fruitadens and Tianyulong with primitive, subtriangular crowns currently are known only from northern landmasses. All other genera except the enigmatic Pisanosaurus have deeper crown proportions and currently are known only from southern landmasses.
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Dynamics of dental evolution in ornithopod dinosaurs
Article
Full-text available
  • Jul 2016
Ornithopods were key herbivorous dinosaurs in Mesozoic terrestrial ecosystems, with a variety of tooth morphologies. Several clades, especially the ‘duck-billed’ hadrosaurids, became hugely diverse and abundant almost worldwide. Yet their evolutionary dynamics have been disputed, particularly whether they diversified in response to events in plant evolution. Here we focus on their remarkable dietary adaptations, using tooth and jaw characters to examine changes in dental disparity and evolutionary rate. Ornithopods explored different areas of dental morphospace throughout their evolution, showing a long-term expansion. There were four major evolutionary rate increases, the first among basal iguanodontians in the Middle-Late Jurassic, and the three others among the Hadrosauridae, above and below the split of their two major clades, in the middle of the Late Cretaceous. These evolutionary bursts do not correspond to times of plant diversification, including the radiation of the flowering plants, and suggest that dental innovation rather than coevolution with major plant clades was a major driver in ornithopod evolution.
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The dentition of Manidens condorensis (Ornithischia; Heterodontosauridae) from the Jurassic Cañadón Asfalto Formation of Patagonia: morphology, heterodonty and the use of statistical methods for identifying isolated teeth
Article
Full-text available
  • Apr 2013
  • Ichnos
The recently described Manidens condorensis is one of the most completely known taxa of the family Heterodontosauridae from the southern landmasses. However, some dental aspects are not well known due to preservational problems in the type material. This contribution reports new isolated teeth found in the Cañadón Asfalto Formation (Early-Middle Jurassic). These teeth are referred to Manidens condorensis based on the presence of autapomorphic characters of the unusual dentition of this taxon, such as the highly asymmetric tooth crowns and small crenulations on each denticles. The isolated crowns are well preserved and reveal the presence of undescribed and new autapomorphical features, including apical and basal wear facets on the occlusal surface of isolated crowns and a wear surface also in the caniniform tooth. We carried out statistical analyses (including morphogeometrical and discriminant analyses), using the holotype crowns as a morphological starting point, for characterising shape variation of the crowns along the toothrow and for identifying the position of isolated crowns. These analyses allow defining morphological regions within the postcaniniform toothrow and produce a metrically based discriminant function to predict the hypothetical position of future discoveries, providing a methodological framework that could be applied to other extinct heterodont dinosaurs.
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Taxonomy, morphology, masticatory function and phylogeny of heterodontosaurid dinosaurs
Article
Full-text available
  • Oct 2012
Heterodontosaurids comprise an important early radiation of small-bodied herbivores that persisted for approximately 100 My from Late Triassic to Early Cretaceous time. Review of available fossils unequivocally establishes Echinodon as a very small-bodied, late-surviving northern heterodontosaurid similar to the other northern genera Fruitadens and Tianyulong. Tianyulong from northern China has unusual skeletal proportions, including a relatively large skull, short forelimb, and long manual digit II. The southern African heterodontosaurid genus Lycorhinus is established as valid, and a new taxon from the same formation is named Pegomastax africanus gen. n., sp. n. Tooth replacement and tooth-to-tooth wear is more common than previously thought among heterodontosaurids, and in Heterodontosaurus the angle of tooth-to-tooth shear is shown to increase markedly during maturation. Long-axis rotation of the lower jaw during occlusion is identified here as the most likely functional mechanism underlying marked tooth wear in mature specimens of Heterodontosaurus. Extensive tooth wear and other evidence suggests that all heterodontosaurids were predominantly or exclusively herbivores. Basal genera such as Echinodon, Fruitadens and Tianyulong with primitive, subtriangular crowns currently are known only from northern landmasses. All other genera except the enigmatic Pisanosaurus have deeper crown proportions and currently are known only from southern landmasses.
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Complex Dental Structure and Wear Biomechanics in Hadrosaurid Dinosaurs
Article
Full-text available
  • Oct 2012
  • SCIENCE
A Toothy Problem Large mammalian herbivores such as horses and bison are well known to possess a complex, grinding dentition that facilitates processing of their tough, cellulose-rich plant diet. Hadrosaurid, or duck-billed, dinosaurs also possessed complex teeth, but how this was achieved has been unknown because reptiles typically possess simple teeth. Erickson et al. (p. 98 ) show how Hadrosaurs evolved teeth composed of six tissues, which allowed for the development of tooth complexity rivaling, or exceeding, that of modern herbivorous mammals.
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The Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962: Cranial anatomy, functional morphology, taxonomy, and relationships
Article
Full-text available
  • May 2011
  • ZOOL J LINN SOC-LOND
The cranial anatomy of the Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962 is described in detail for the first time on the basis of two principal specimens: the holotype (SAM-PK-K337) and referred skull (SAM-PK-K1332). In addition several other specimens that have a bearing on the interpretation of the anatomy and biology of Heterodontosaurus are described. The skull and lower jaw of Heterodontosaurus are compact and robust but perhaps most notable for the heterodont dentition that merited the generic name. Details of the cranial anatomy are revealed and show that the skull is unexpectedly specialized in such an early representative of the Ornithischia, including: the closely packed, hypsodont crowns and ‘warping’ of the occlusal surfaces (created by progressive variation in the angulation of wear on successive crowns) seen in the cheek dentition; the unusual sutural relationships between the bones along the dorsal edge of the lower jaw; the very narrow, deeply vaulted palate and associated structures on the side wall of the braincase; and the indications of cranial pneumatism (more commonly seen in basal archosaurs and saurischian dinosaurs). Evidence for tooth replacement (which has long been recognized, despite frequent statements to the contrary) is suggestive of an episodic, rather than continuous, style of tooth replacement that is, yet again, unusual in diapsids generally and particularly so amongst ornithischian dinosaurs. Cranial musculature has been reconstructed and seems to conform to that typically seen in diapsids, with the exception of the encroachment of M. adductor mandibulae externus superficialis across the lateral surface of the temporal region and external surface of the lower jaw. Indications, taken from the unusual shape of the occlusal surfaces of the cheek dentition and jaw musculature, are suggestive of a novel form of jaw action in this dinosaur. The taxonomy of currently known late Karoo-aged heterodontosaurids from southern Africa is reviewed. Although complicated by the inadequate nature of much of the known material, it is concluded that two taxa may be readily recognized: H. tucki and Abrictosaurus consors. At least one additional taxon is recognized within the taxa presently named Lanasaurus and Lycorhinus; however, both remain taxonomically problematic and their status needs to be further tested and may only be resolved by future discoveries. The only other named taxon, Geranosaurus atavus, represents an invalid name. The recognition of at least four distinct taxa indicates that the heterodontosaurids were speciose within the late Karoo ecosystem. The systematics of Heterodontosaurus and its congeners has been analysed, using a restricted sample of taxa. A basal (nongenasaurian) position within Ornithischia is re-affirmed. There are at least four competing hypotheses concerning the phylogenetic placement of the Heterodontosauridae, so the evidence in support of the various hypotheses is reviewed in some detail. At present the best-supported hypothesis is the one which places Heterodontosauridae in a basal (non-genasaurian) position; however, the evidence is not fully conclusive and further information is still needed in respect of the anatomy of proximate outgroups, as well as more complete anatomical details for other heterodontosaurids. Heterodontosaurids were not such rare components of the late Karoo ecosystem as previously thought; evidence also suggests that from a phylogenetic perspective they occupied a potentially crucial position during the earliest phases of ornithischian dinosaur evolution. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011.
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Evolutionary Trends in the Jaw Adductor Mechanics of Ornithischian Dinosaurs
Article
  • Dec 2015
  • Anat Rec
Jaw mechanics in ornithischian dinosaurs have been widely studied for well over a century. Most of these studies, however, use only one or few taxa within a given ornithischian clade as a model for feeding mechanics across the entire clade. In this study, mandibular mechanical advantages among 52 ornithischian genera spanning all subclades are calculated using 2D lever arm methods. These lever arm calculations estimate the effect of jaw shape and difference in adductor muscle line of action on relative bite forces along the jaw. Results show major instances of overlap between taxa in tooth positions at which there was highest mechanical advantage. A relatively low bite force is seen across the tooth row among thyreophorans (e.g., stegosaurs and ankylosaurs), with variation among taxa. A convergent transition occurs from a more evenly distributed bite force along the jaw in basal ornithopods and basal marginocephalians to a strong distal bite force in hadrosaurids and ceratopsids, respectively. Accordingly, adductor muscle vector angles show repeated trends from a mid-range caudodorsal orientation in basal ornithischians to a decrease in vector angles indicating more caudally oriented jaw movements in derived taxa (e.g., derived thyreophorans, basal ornithopods, lambeosaurines, pachycephalosaurs, and derived ceratopsids). Analyses of hypothetical jaw morphologies were also performed, indicating that both the coronoid process and lowered jaw joint increase moment arm length therefore increasing mechanical advantage of the jaw apparatus. Adaptive trends in craniomandibular anatomy show that ornithischians evolved more complex feeding apparatuses within different clades as well as morphological convergences between clades. This article is protected by copyright. All rights reserved.
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The Dinosauria
Book
  • Jun 2004
This revised edition of this book continues in the same vein as the first but encompasses recent spectacular discoveries that have continued to revolutionize this field. A thorough scientific view of current world research, the volume includes comprehensive coverage of dinosaur systematics, reproduction, and life history strategies, biogeography, taphonomy, paleoecology, thermoregulation, and extinction. It contains definitive descriptions and illustrations of these magnificent Mesozoic beasts. The first section of the book begins with the origin of the great clade of these fascinating reptile ... More This revised edition of this book continues in the same vein as the first but encompasses recent spectacular discoveries that have continued to revolutionize this field. A thorough scientific view of current world research, the volume includes comprehensive coverage of dinosaur systematics, reproduction, and life history strategies, biogeography, taphonomy, paleoecology, thermoregulation, and extinction. It contains definitive descriptions and illustrations of these magnificent Mesozoic beasts. The first section of the book begins with the origin of the great clade of these fascinating reptiles, followed by separate coverage of each major dinosaur taxon, including the Mesozoic radiation of birds. The second part of the volume navigates through broad areas of interest. Here we find comprehensive documentation of dinosaur distribution through time and space, discussion of the interface between geology and biology, and the paleoecological inferences that can be made through this link. This revised edition of this book continues in the same vein as the first but encompasses recent spectacular discoveries that have continued to revolutionize this field. A thorough scientific view of current world research, the volume includes comprehensive coverage of dinosaur systematics, reproduction, and life history strategies, biogeography, taphonomy, paleoecology, thermoregulation, and extinction. It contains definitive descriptions and illustrations of these magnificent Mesozoic beasts. The first section of the book begins with the origin of the great clade of these fascinating reptile ... More
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Wear Pattern, Dental Function, and Jaw Mechanism in the Late Cretaceous Ankylosaur Hungarosaurus
Article
Feeding in thyreophoran dinosaurs is poorly understood. Although the group existed for over 130 million years, only the Early Jurassic basal thyreophoran Scelidosaurus harrisonii and the Late Cretaceous ankylosaurid Euoplocephalus tutus have been studied from this perspective in detail. In contrast to the earlier, conservative hypothesis of a simple “orthal pulping” feeding mode with no or limited tooth–tooth contact, recent studies have demonstrated precise dental occlusion with differing jaw mechanisms in these two species. Here, we describe the first detailed study of feeding related characters in a nodosaurid ankylosaur, Hungarosaurus tormai, from the Late Cretaceous of Hungary. Dental wear patterns comprising small, apical, and low-angled facets on the maxillary and steep, extended, and bowl-like facets on the dentary teeth reveal sophisticated tooth–tooth contact in this basal nodosaurid. The presence of two different scratch generations (vertical and low-angled) on the dentary teeth unambiguously demonstrate a multiphasic powerstroke, which is further supported by the morphology of the quadrate-articular and mandibular symphyseal joints and by the architecture of the reconstructed jaw adductors. Chewing started with an initial slicing phase associated with orthal movement that was followed by a retractive powerstroke with significant occlusal contact. Because of the curved tooth rows, these movements were probably facilitated by some mediolateral translation and/or axial rotation of the mandibles to produce precise shearing along the whole tooth row. These results demonstrate that complex jaw mechanisms and dental occlusion were more widespread among thyreophorans than thought previously and that palinal movement was present in at least two ankylosaurian lineages. Anat Rec, 2014. © 2014 Wiley Periodicals, Inc.
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