Morphological Variation in the Hadrosauroid Dentary Morfologisk Variation I Det Hadrosauroida Dentärbenet

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Examensarbete vid Institutionen för geovetenskaper Degree Project at the Department of Earth Sciences ISSN 1650-6553 Nr 398

Morphological Variation in the Hadrosauroid Dentary Morfologisk variation i det hadrosauroida dentärbenet

D. Fredrik K. Söderblom

INSTITUTIONEN FÖR

GEOVETENSKAPER

DEPARTMENT OF EARTH SCIENCES

Examensarbete vid Institutionen för geovetenskaper Degree Project at the Department of Earth Sciences ISSN 1650-6553 Nr 398

Morphological Variation in the Hadrosauroid Dentary Morfologisk variation i det hadrosauroida dentärbenet

D. Fredrik K. Söderblom

ISSN 1650-6553

Morphological Variation in the Hadrosauroid Dentary D. Fredrik K. Söderblom

The near global success reached by hadrosaurid dinosaurs during the Cretaceous has been attributed to their ability to masticate (chew). This behavior is more commonly recognized as a mammalian adaptation and, as a result, its occurrence in a non-mammalian lineage should be accompanied with several evolutionary modifications associated with food collection and processing. The current study investigates morphological variation in a specific cranial complex, the dentary, a major element of the hadrosauroid lower jaw. 89 dentaries were subjected to morphometric and statistical analyses to investigate the clade’s taxonomic-, ontogenetic-, and individual variation in dentary morphology. Results indicate that food collection and processing became more efficient in saurolophid hadrosaurids through a complex pattern of evolutionary and growth-related changes. The diastema (space separating the beak from the dental battery) grew longer relative to dentary length, specializing food collection anteriorly and food processing posteriorly. The diastema became ventrally directed, hinting at adaptations to low-level grazing, especially in younger individuals. The coronoid process became anteriorly directed, and was relatively more elongate, resulting in increased moment arm length, with muscles being re-directed to pull the jaw more posteriorly, and mechanical advantage increasing. Although all hadrosauroid groups went through relative dental battery elongation during growth, by incorporating more teeth into each row, the dental battery became deeper in saurolophids. Previous research supports the interpretation that this is the result of more tooth rows being stacked vertically, allowing the dental battery to work as a shock-absorber during mastication, and allowed teeth to be replaced without interruption to food consumption. The increased anterior inclination of the coronoid process, and relative elongation of the diastema in saurolophids are herein suggested to have evolved through hypermorphosis, a version of peramorphosis where the growth trajectory in the descendant taxon extends beyond the ancestral state, whereas the relative elongation of the coronoid process, the relative deepening of the dental battery, and the increased ventral deflection of the diastema are the result of a novel juvenile condition.

Keywords: Hadrosauroidea, dentary, morphometric, heterochrony, peramorphosis, mastication

Degree Project E1 in Earth Science, 1GV025, 30 credits Supervisor: Nicolás E. Campione Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala (www.geo.uu.se)

ISSN 1650-6553Examensarbete vid Institutionen för geovetenskaper, No. 398, 2017

The whole document is available at www.diva-portal.org

Populärvetenskaplig sammanfattning

Morfologisk variation i det hadrosauroida dentärbenet D. Fredrik K. Söderblom

Hadrosauroider, även kallade ank-näbbs dinosaurier, var vanligt förekommande och nästan globalt distribuerade växtätare som levde under krita-perioden. Forskning har länge tytt på att deras framgång berott på deras förmåga att processera föda genom att tugga, en förmåga som länge ansågs exklusiv till däggdjur. Denna förmåga förbättrades sedan genom morfologiska förändringar som dök upp i de saurolophida hadrosauroidera. Dessa förändringar ledde till både effektivare födoinsamling och bearbetning. En serie analyser utfördes på 89 dentärben (ett av benen i underkäken) för att testa hur dessas form varierade inom Hadrosauroidea, beroende på gruppers släktskap, ålderstadie, samt individuell variation. Resultaten tyder på att diastemat, området som i hadrosauroider separerar predentärbenet från dental batteriet (liknande hur fram- och kindtänderna är separerade hos hästar) blev längre i saurolophider. Detta tillät ökad specialisering av predentärbenet till insamling av mat och i dental batteriet (rader av tätt sittande tänder placerade vertikalt på varandra) till att mala ned mat. Diastemats ventrala böjning avtog i alla grupper allt eftersom individen åldrades, men var i helhet mer böjt i saurolophider. Detta tolkas som möjlig anpassning till betande på lågt växande vegetation, som idag ses i vissa hovdjur (Ungulata). Koronoid processen, en vertikalt utstickande del av dentärbenet som är placerad långt bak, lutades framåt och förlängdes i saurolophider. Denna förändring riktade muskler till att dra käken mer bakåt, samt ökade hävstångsverkan, vilket ökade saurolophiders bitkraft. Alla hadrosauroider gick igenom förlängning av dental batteriet genom tillägg av tänder, vilket ökade tuggytans area. Saurolophiders dental batterier var djupare än sina föregångares, vilket har dokumenterats i litteratur som ännu fler vertikalt staplade rader. Fler tandrader betydde att stötar från starkare bitkraft kunde absorberas över en större yta. Dessutom så kunde tandrader därför bytas utan dröjsmål när den översta raden nötts ned, vilket tillät oavbruten födokonsumtion. Anledningen till den ökade lutningen i koronoid processen, och förlängningen av diastemat tros vara hypermorfos, vilket är när tillväxten av något som slutade växa i förfäderna istället fortsätter växa i ättlingarna. Den ökade längden på koronoid processen, det djupare dental batteriet, och det mer ventralt böjda diastemat i saurolophider uppstod p.g.a. att ett nytt juvenilt tillstånd utvecklades.

Nyckelord: Hadrosauroidea, dentärben, morfometrisk, heterokroni, peramorfos, tugga

Examensarbete E1 i geovetenskap, 1GV025, 30 hp Handledare: Nicolás E. Campione Institutionen för geovetenskaper, Uppsala universitet, Villavägen 16, 752 36 Uppsala (www.geo.uu.se)

ISSN 1650-6553, Examensarbete vid Institutionen för geovetenskaper, Nr 398, 2017

Hela publikationen finns tillgänglig på www.diva-portal.org

Table of Contents

1 Introduction ...... 1 2 Aims ...... 2 3 Background ...... 4 3.1 Cretaceous climate and environment ...... 4 3.2 Taxonomic conventions and the dentary’s use in phylogenetics ...... 4 3.3 Evolution of the feeding apparatus and jaw mechanics in hadrosauroids ...... 7 3.4 Examples of previous research on morphological variation in hadrosauroid dentaries ...... 9 3.4.1 Taxonomic variation ...... 9 3.4.2 Dentary growth patterns ...... 10 3.4.3 Individual variation, taphonomic-, and pathologic alteration ...... 11 4 Methods ...... 13 4.1 Data collection...... 13 4.2 Digitization ...... 13 4.3 Generalized procrustes analysis and semilandmarks sliding ...... 14 4.4 Principal component analysis, morphospace diagram and density plots...... 15 4.5 Normality tests and principal component group separation ...... 15 4.6 Bivariate allometry plots and confidence intervals ...... 16 5 Results ...... 17 5.1 Morphospace diagram and density plots ...... 17 5.1.1 Principal component 1 ...... 17 5.1.2 Principal component 2 ...... 19 5.2 Normality tests ...... 21 5.3 Tukey’s honest significant difference test ...... 22 5.3.1 Principal component 1 ...... 22 5.3.2 Principal component 2 ...... 24 5.4 Bivariate allometry plots and confidence intervals ...... 26 6 Discussion ...... 29 6.1 Evolution and growth of the hadrosauroid dentary ...... 29 6.1.1 Taxonomic variation ...... 29 6.1.2 Growth patterns and their taxonomic differences ...... 31 6.2 Implications for phylogenetic characters ...... 36 6.2.1 The diastema ...... 36 6.2.2 The coronoid process ...... 38 6.2.3 The dental battery ...... 39 7 Conclusions ...... 41 8 Acknowledgements ...... 42 9 References ...... 43 Table of Contents (continued)

Appendix ...... 49 Dataset ...... 49 Dataset references ...... 53

1 Introduction

Hadrosauroid dinosaurs (Hadrosauroidea von Huene 1954) were Cretaceous herbivores capable of processing their food through mastication (chewing) (Weishampel 1983; Norman & Weishampel 1985; Weishampel & Norman 1989; Horner, Weishampel & Forster 2004). Mastication was long thought to be unique to mammals, but it is now indicated to have been present in several in animal groups (e.g. mammals, edaphosaurids, dicynodonts, procolophonians, bolosaurids, ceratopsians, ankylosaurs, extant herbivorous turtles, some lizards and the eusuchian Iharkutosuchus) where it is used to mechanically break down food into smaller particles, thereby increasing its rate of chemical breakdown by increasing the area available for breakdown (Norman & Weishampel 1985; Reilly, McBrayer & White 2001; Ősi and Weishampel 2009; Tanoue et al. 2009; Ősi et al. 2014; Varriale 2016). This in turn increases the organism’s ability to meet metabolic demands. As vertebrates lack the enzymes to break down plant cell walls, they make use of micro-organisms to ferment their food. If food has been masticated before reaching the gut, fermentation becomes more efficient, and the energy-cost of herbivory as a foraging strategy is decreased (Reilly, McBrayer & White 2001). The first hadrosauroids appeared in the Early Cretaceous of Asia (You et al. 2003; Norman 2014; Shibata et al. 2015) and before they went extinct at the end of the Late Cretaceous hadrosauroids inhabited Asia, Europe, North- and South America, Antarctica (Horner, Weishampel & Forster 2004) and possibly Africa (Fanti et al. 2016). Their ability to successfully inhabit many continents and thrive has often been attributed to a highly efficient masticatory apparatus that evolved through several modifications of the skull (e.g. Weishampel 1984; Norman & Weishampel 1985; Erickson et al. 2012). Evolutionary differences in these morphological modifications are used in phylogenetic systematics, but the morphology of some evolutionary modifications can both arise due to changes in the way ontogeny (growth) progresses (i.e. heterochrony = an evolutionary pattern due to changes in rate, duration or timing of a trait’s development [Schoch 2014]) (e.g. the extreme cranial crest of Parasaurolophus [Farke et al. 2013]). The morphology of the evolutionary modifications change also during ontogeny (Maryańska & Osmólska 1981; Bell 2011; Campione & Evans 2011; Prieto-Márquez 2011, 2014; Farke et al. 2013; McGarrity, Campione & Evans 2013; Prieto-Márquez & Gutarra 2016), thereby placing juvenile specimens in a position further back in phylogenies than adult specimens of the same species (Tsuihiji et al. 2011; Campione et al. 2013; Prieto-Márquez 2014). To make informed decisions of which characters are suitable for use in phylogenetic studies and how they should be coded, it is important to know if the characters in question are ontogenetically variable. It is also important to understand which characters separate taxa from each other, including how character distribution varies in and between clades, as it allows for more resolved phylogenies to be constructed and in turn leads to a better understanding of evolution. Lastly, individual morphological variation is also important to consider as it can affect the perception of what characterizes taxa. Individual variation arises as the natural result of traits inherited from parents to

1 offspring that create minor differences among individuals (Campione & Evans 2011). On top of taxonomic, ontogenetic and individual variation, it is possible for pathology (illness or injury) (Godefroit, Bolotsky & Bolotsky 2012; Freedman Fowler & Horner 2015), and taphonomy (post- mortem deformation, e.g. crushing [Campione & Evans 2011]) to alter the morphology of hard tissues.

2 Aims

The aim of this study is to investigate taxonomic-, ontogenetic- and individual variation in hadrosauroid dentary morphology (figure 1). Based on previous research, it is expected that the results will show morphological changes in the dentary along the lineage leading from non-hadrosaurid hadrosauroids to saurolophids which amounted to more effective masticatory abilities, including: increased coronoid process inclination and dorsoventral length, increased relative diastema length and curvature, as well as increased dental battery depth (see dentary-related changes in section 3.3 for further details). Additionally, as previous research has shown that the morphology of skeletal elements in hadrosauroids can vary throughout ontogeny, it is assumed that major elements of the dentary also did, rendering some characters ontogenetically variable, meaning that character states for those characters obtained from non-adult individuals are unsuitable for use in phylogenetic analysis.

Figure 1. Hadrosauroid skull anatomy exemplified by E. annectens (formerly E. saskatchewanensis [Campione & Evans 2011]) specimen CMN 8509 (Canadian Museum of Nature, Ottawa, Ontario, Canada) in left lateral view. A, photograph of CMN 8509 courtesy of N. Campione; B, line drawing of CMN 8509 skull with skeletal elements indicated. Anatomical abbreviations—d, dentary; en, external naris; ex, exoccipital–opisthotic complex; f, frontal; j, jugal; l, lacrimal; m, maxilla; n, nasal; p, parietal; pd, predentary; pf, prefrontal; pm, premaxilla; po, postorbital; q, quadrate; qj, quadratojugal; sa, surangular; sq, squamosal.

3 Background 3.1 Cretaceous climate and environment Exact global temperatures during the Cretaceous are as of the present date not agreed upon, but studies mostly point to higher temperatures than present-day (an extended review in Hay 2017). It is currently debated if the North and South Poles were frozen and to what degree (Hay 2017), but fossil evidence of Antarctic pteridophytes (ferns), bennettitales, ginkgoes, bryophytes (mosses), hepatophytes (liverworts), lycophytes (e.g. clubmosses), conifers and angiosperms (flowering plants) in the late Early Cretaceous (Cantrill & Poole 2002; Nagalingum & Cantrill 2006; Vera 2015), and pteridophytes, bryophytes, bennettitales, cycads, angiosperms and conifers in the Late Cretaceous (Césari, Marenssi & Santillana 2001; Iglesias 2016; Kvaček & Vodrážka 2016), suggest a warmer global climate than today. Due to elevated temperatures and less frozen water, Cretaceous global sea- levels were higher than present and although the exact levels are still debated (Hay 2017), it is hypothesized that several continents, especially Africa, Europe and North America, were partially covered by shallow seas (Markwick & Valdes 2004; references in Müller et al. 2008).

3.2 Taxonomic conventions and the dentary’s use in phylogenetics It is generally agreed that Hadrosauridae is located within Hadrosauroidea (figure 2), however what taxa are included in Hadrosauridae and how they are distributed within the clade slightly differs depending on what character matrix is used. Two major matrices that are currently used to generate phylogenies were originally created by Prieto-Márquez (2010a), and Xing et al. (2014). Subsequently, the matrices were modified and used to generate phylogenetic trees that are more up-to-date (i.e. Prieto-Márquez, Erickson and Ebersole 2016a; Xing, Mallon & Currie 2017). The matrix by Prieto- Márquez, Erickson and Ebersole (2016a) has 273 characters, of which 24 (approximately 8.8%) characters are related to the dentary (14 regarding the dentary itself, 10 regarding the dentary teeth). The matrix by Xing, Mallon and Currie (2017) has 346 characters, of which 39 (approximately 11.3%) are related to the dentary (22 regarding the dentary itself, 17 regarding the dentary teeth). The dentary therefore has quite a big influence on phylogenetic analyses, considering that approximately 1/10 of the characters in both matrices are related to the dentary or its teeth. Although trees generated by the two matrices agree that Hadrosauridae Cope 1870 nests within Hadrosauroidea, they differ in the placement of Hadrosaurus foulkii Leidy 1858, and as such, the names and definitions of other clades within Hadrosauridae also change. Prieto-Márquez, Erickson and Ebersole (2016a) define Hadrosauridae as the clade stemming from the most recent common ancestor of Parasaurolophus walkeri Parks 1922 and Hadrosaurus foulkii (sensu Prieto-Márquez 2010a). In this tree Hadrosauridae includes Hadrosaurus foulkii, Eotrachodon orientalis and Saurolophidae. Saurolophidae (sensu Prieto-Márquez 2010a) is defined as the last common ancestor of Lambeosaurus lambei Parks 1923 and Saurolophus osborni Brown 1913 and all of its descendants. 4

Saurolophidae is divided into Saurolophinae and Lambeosaurinae. Saurolophinae (sensu Prieto- Márquez 2010a) is defined as Saurolophus osborni and all taxa more closely related to it than Lambeosaurus lambei or Hadrosaurus foulkii. Lambeosaurinae (sensu Prieto-Márquez 2010a) is defined as Lambeosaurus lambei and all taxa more closely related to it than or Edmontosaurus regalis Lambe 1917, Hadrosaurus foulkii or Saurolophus osborni. Xing, Mallon and Currie (2017) define Hadrosauridae as the least inclusive taxon containing Parasaurolophus and Saurolophus (sensu Sereno 1998). In their tree, Hadrosaurus places within what Prieto-Márquez, Erickson and Ebersole (2016a) call Saurolophinae, which changes the name to Hadrosaurinae Lambe 1918 (sensu Sereno 1998), defined by Xing, Mallon and Currie (2017) as all hadrosaurids closer to Saurolophus than to Parasaurolophus. Lambeosaurinae is not explicitly defined by Xing, Mallon and Currie (2017) but it is herein assumed that they follow the definition sensu Sereno (1998), in which Lambeosaurinae is defined as all hadrosaurids closer to Parasaurolophus than to Saurolophus. In their phylogeny, E. orientalis places in a polytomy just outside Hadrosauridae. As Hadrosauridae then only includes Hadrosaurinae and Lambeosaurinae, Saurolophidae equals Hadrosauridae. Hadrosauridae was named before Saurolophidae and as such takes precedence over it. This means that there are only two non- saurolophid hadrosaurid taxa in the tree by Prieto-Márquez, Erickson and Ebersole (2016a), and as such, the taxic composition of Saurolophidae is very similar to Hadrosauridae in Xing, Mallon and Currie (2017). The taxonomic scheme followed herein is largely based on that of Prieto-Márquez, Erickson and Ebersole (2016a), with positions of additional taxa that were not included in their analysis from other literature, including: Sirindhorna khoratensis, Altirhinus kurzanovi (Shibata et al. 2015), Shuangmiaosaurus gilmorei (Xing et al. 2014), Plesiohadros djadokhtaensis (Tsogtbaatar et al. 2014), Ugrunaaluk kuukpikensis (Mori, Erickson & Druckenmiller 2016), Gryposaurus alsatei (Lehman, Wick & Wagner 2016), and Amurosaurus riabinini (Xing, Mallon and Currie 2017) (figure 2). Taxa included in the taxonomic scheme are those analyzed and mentioned herein.

Figure 2. Taxonomic scheme used herein, based largely on that of Prieto-Márquez, Erickson and Ebersole (2016a) and additional sources (see text). Outgroup taxon is Iguanodon bernissartensis. Non-hadrosauroid iguanodontian taxa are marked with black-and-white lines. Non-hadrosaurid hadrosauroid taxa are marked with green lines. Non-saurolophid hadrosaurid taxa are marked with red lines. Saurolophine taxa are marked with yellow lines. Lambeosaurine taxa are marked with blue lines.

3.3 Evolution of the feeding apparatus and jaw mechanics in hadrosauroids The oldest fossils of hadrosauroids are from the late Early Cretaceous (Aptian) and belong to Sirindhorna khoratensis (Shibata et al. 2015). Unlike their ancestors, hadrosauroids had (1) more sophisticated teeth arranged into large grinding surfaces (dental batteries [dentary anatomy in figure 4 in the methods section]) to maximize the efficiency of mastication, (2) larger jaws to maximize the space for mastication that were separated into food gathering in the anterior (premaxillary- and predentary beaks) and food processing in the posterior (dental batteries), which allowed these elements to specialize in their respective tasks, (3) dentaries with more elongate coronoid processes that could generate more leverage and thereby increased bite force, and (4) explored large body sizes, with a posture to compensate for the increased weight (Norman 2014). Further evolution along the lineage, including hadrosaurid hadrosauroids which appeared during the Santonian (Late Cretaceous) (Prieto- Márquez 2010b; Prieto-Márquez, Erickson & Ebersole 2016a), would yield taxa with (5) more closely interlocking teeth in increased numbers of rows (Norman 2014), (6) dental batteries that elongated in a posterior direction medially past the coronoid processes to enable more surface for mastication, and (7) taller coronoid processes for increased leverage, whose apex was expanded in order to recruit more muscles for mastication (Norman 2014). Saurolophid hadrosaurids appeared shortly after non-saurolophid hadrosaurids in the late Santonian (Prieto-Márquez, Erickson & Ebersole 2016a). Saurolophidae encompasses the clades Saurolophinae and Lambeosaurinae, that had (8) teeth in which the pulp cavity of the functional tooth row was filled by dentine, which allowed the tooth root to be involved in grinding (LeBlanc et al. 2016), and (9) gained even further separation of food gathering and food processing areas in the jaws (Norman 2014). Saurolophids had dentition that was (10) further posteriorly expanded, that when combined with (11) an anteriorly inclined coronoid process created further increased leverage, and allowed even more efficient mastication (Kubota & Kobayashi 2009; Norman 2014). Additionally, (12) morphological differentiation of the beaks in different clades (wide with upturned margins in saurolophines, narrow with a droop-tipped shape in lambeosaurines) have been interpreted as adaptations that indicate different habitat choice and feeding ecology specialization (Carrano, Janis and Sepkoski 1999; Mallon & Anderson 2014). To cope with increased stresses on the skull roof associated with mastication, (13) the temporal region was strengthened (Norman 2014). The combined changes that took place in the hadrosauroid masticatory apparatus (especially those related to the coronoid process) meant that saurolophid bite force was increased relative to non-hadrosaurid iguanodontians (Nabavizadeh 2016), which would have increased the amount of food sources available for consumption. However, the relative bite forces of saurolophines and lambeosaurines were similar (Nabavizadeh 2016). As more efficient mastication led to increased food intake, (14) more vertebrae were incorporated into the sacrum to handle the increased mass (Norman 2014). The increase in mass eventually led to huge

7 forms in the Late Cretaceous such as the saurolophine Shantungosaurus giganteus with an estimated mass of approximately 17,000 kg (Benson et al. 2014). Pleurokinesis (figure 3A) was for over two decades accepted as the jaw mechanism model by which hadrosauroids masticated (Nabavizadeh 2016). It involved (1) adduction of the lower jaw, bringing the teeth of the dentaries and maxillae into occlusion, (2) continued adduction as the dentary teeth pushed the maxillary teeth dorsally, rotating several cranial elements laterally against other elements including the maxillae at the maxilla-premaxilla joints, the maxilla-jugal units at the connection with the lacrimals, and posterolateral rotation of the quadrates at the connection to the squamosals (Weishampel 1984; Norman & Weishampel 1985; Cuthbertson et al. 2012). Except for some secondary motions, this was followed by (3) medial rotation of the aforementioned cranial elements as the lower jaw moved ventrally to its original position (Weishampel 1984; Norman & Weishampel 1985; Cuthbertson et al. 2012). However, computer-modeling proved that pleurokinesis resulted in that several unlikely displacements between cranial elements, and as such, alternative methods of mastication were also investigated (Rybczynski et al. 2008; Cuthbertson et al. 2012). In an alternative model (figure 3B) the upper part of the skull is akinetic (cannot move), unlike the kinetic skull in pleurokinesis (Cuthbertson et al. 2012). Instead, (1) as the lower jaw is dorsally adducted and the teeth occlude, the lower jaw is also simultaneously retracted in a posterior direction, and (2) the dentaries are rotated along their anteroposterior axes at the jaw joint as well as the interface of the dentaries and predentary (Cuthbertson et al. 2012; Nabavizadeh 2014). The teeth grind against each other as the dentary is pulled posteriorly and rotated, acting both to shear food and move it posteriorly in the oral cavity (Nabavizadeh 2016). In addition to not creating displacements between several cranial elements, the alternative model also follows tooth facet wear patterns more closely than pleurokinesis (Nabavizadeh 2016).

Figure 3. Two hadrosauroid jaw mechanic mastication models. A, pleurokinesis in cross sectional view, including maxillae (above, middle grey) and dentaries (below, middle grey), with dental batteries (dark grey). B, alternative mastication model in left lateral view (top image), and cross sectional view (bottom image), including maxillae (above, middle grey) and dentaries (below, middle grey), with dental batteries (dark grey). All images based on those of Lambe (1920), and Nabavizadeh (2014).

3.4 Examples of previous research on morphological variation in hadrosauroid dentaries 3.4.1 Taxonomic variation Compared to non-hadrosauroid neoiguanodontians, non-hadrosaurid hadrosauroids had more elongate jaws (including dentary), and more extensive elongation of the jaws would not develop until saurolophids appeared (Norman 2014). The dental battery was situated anterior to the coronoid process in early hadrosauroids, however in later forms the dental battery’s posterior edge had migrated medially relative to the coronoid process (Norman 2014). Norman (2014) also mentioned that the length of the dentary diastema (the region between the dentary teeth and the predentary) was greater in non-hadrosaurid hadrosauroids than in non- hadrosauroid neoiguanodontians, and that further elongation did not take place until the appearance of

9 saurolophids. Kubota and Kobayashi (2009) stated that diastemata in Eolambia caroljonesa and Shuangmiaosaurus gilmorei were short or non-existent when compared to dentary length, whereas the diastema in Protohadros byrdi was quite elongate in comparison. The saurolophines Prosaurolophus maximus and Edmontosaurus were reported to have long diastemata (McGarrity, Campione & Evans 2013) which is in agreement with Kubota and Kobayashi (2009)’s general statement on the presence of longer diastemata in saurolophids; however Prieto-Márquez (2010c) found the saurolophine Gryposaurus to have a comparatively short diastema. Norman (2014) suggested that earlier non-hadrosaurid hadrosauroids (e.g. Probactrosaurus, Protohadros and Altirhinus) started to develop a downturned anterior region of the dentary, which became even more so in saurolophids. A statement made by Prieto-Márquez (2010c) is in line with Norman (2014)’s general statement in that Gryposaurus exhibits a short but strongly ventrally curved symphyseal process. A strong ventral curvature can also be seen in the dentary of lambeosaurines according to Godefroit, Bolotsky and van Itterbeeck (2004); however Evans (2010) mentioned that the lambeosaurine Parasaurolophus is an exception. Norman (2014) stated that non-hadrosaurid hadrosauroids possessed tall coronoid processes situated perpendicular to the dentary rami that in hadrosaurids are further elongated. Both Evans (2010) and Blanco, Prieto-Márquez and De Esteban-Trivigno (2015) stated that the coronoid processes in hadrosaurids are anteriorly inclined whereas Norman (2014) mentioned that anteriorly inclined coronoid processes are found in saurolophids (called Euhadrosauria in Norman 2014) and that the apex of the coronoid processes are more expanded. Examples of anteriorly inclined coronoid processes are evident in the lambeosaurines Arenysaurus and Blasisaurus (Blanco, Prieto-Márquez and De Esteban-Trivigno 2015). An odd combination of features pertaining to the coronoid processes are present in the non-saurolophid hadrosaurid Eotrachodon orientalis that possessed a vertically oriented coronoid process with a moderate expansion of the apexes (Prieto-Márquez, Erickson and Ebersole 2016b), although it should be noted that the individual died as a subadult and therefore might not closely reflect adult morphology.

3.4.2 Dentary growth patterns As outlined through morphometric analyses carried out on skulls of the saurolophine Edmontosaurus by Campione and Evans (2011), the skull of Edmontosaurus regalis changes height/length ratio when going from younger ontogenetic stages (i.e. formerly Thespesius edmontoni) to older ontogenetic stages. Campione and Evans (2011) also found that the skulls of Edmontosaurus annectens grew lower and more elongate in older ontogenetic stages (formerly Anatotitan copei) (likely reflecting similar changes in dentary morphology), although individual variation and taphonomic alteration might be additional influences. Prieto-Márquez (2014) stated that as the dentary of E. annectens grew larger, its ramus got deeper and longer, whereas the amount of teeth increased but remained the same size. Bell (2011) similarly stated that the number of tooth families (columns of teeth) increases as the 10 dental batteries of the saurolophines Edmontosaurus and Saurolophus elongate through ontogeny, which is also true for the non-hadrosaurid hadrosauroid E. caroljonesa according to Kirkland (1998). McGarrity, Campione and Evans (2013) were able to make a similar claim for P. maximus, which exhibited positive allometry in the length of the dentary, indicating that as P. maximus grew, the length of the dentary increased at a relatively faster rate. The same study could not reject isometry for the rate of growth in dentary height. Maryańska and Osmólska (1981) stated that the relative length of the dental battery in Saurolophus angustirostris occupied a larger portion of the mandible in adults than in juveniles. Maryańska and Osmólska (1981) have also mentioned that the coronoid process of S. angustirostris went from being directed dorsally in juveniles to more anteriorly in adults. Prieto-Márquez and Gutarra (2016) saw similar development of the coronoid process’ angle in Gryposaurus, and also included that the apex expanded in an anteroposterior direction through ontogeny. Unlike S. angustirostris and Gryposaurus, the coronoid process of Bactrosaurus johnsoni, a non-hadrosaurid hadrosauroid, remains vertical throughout ontogeny (Prieto-Márquez 2011). The symphyseal process in the dentary of B. johnsoni has been described as increasing in ventral curvature throughout ontogeny (Prieto-Márquez 2011). However, the same region in another non- hadrosaurid hadrosauroid, E. caroljonesa, does not show increase in symphyseal process curvature with increased growth (Kirkland 1998). In Edmontosaurus, the curvature of the symphyseal process slightly decreases through ontogeny (Prieto-Márquez 2014) whereas in Amurosaurus riabinini, a lambeosaurine, the ventral deflection increases with growth and moves posteriorly (Godefroit, Bolotsky & van Itterbeeck 2004). Kubota and Kobayashi (2009) measured the length of the diastema compared to the length of the dentary in several specimens and noticed that saurolophids had more elongate diastemata than non- hadrosaurid iguanodontians. The same study also found that saurolophines had an elongated diastema in the subadult stage, whereas lambeosaurines developed an elongated diastema transitioning from subadult to adult. Prieto-Márquez (2014), and Godefroit, Bolotsky and van Itterbeeck (2004) have expressed comparable opinions regarding the diastemata of E. annectens and A. riabinini, respectively, stating that they elongate relative to dentary length during ontogeny. McGarrity, Campione & Evans (2013) stated the same region in P. maximus shows isometric growth, which is contrary to what is interpreted in Kubota and Kobayashi (2009)’s figure 2, however unbeknownst to Kubota and Kobayashi (2009) as specimens belonging to Prosaurolophus blackfeetensis were found to be of a younger ontogenetic stage of P. maximus (McGarrity, Campione & Evans 2013).

3.4.3 Individual variation, taphonomic- and pathologic alteration Individual variation arises from that organisms naturally vary in morphology within the species as seen in any number of morphometric analyses (e.g. Campione & Evans 2011). Taphonomic alternation can modify the morphology of individual specimens, such as in the very large adult E. annectens 11 specimen AMNH 5730 (formerly A. copei) whose skull (including dentary) possibly was crushed dorsoventrally and therefore became more elongate (Campione & Evans 2011). Another way individual specimens can be secondarily altered is pathological alteration, as possibly seen in the case of an Olorotitan arharensis dentary, AEHM 2/845, in which the Meckelian groove is medially closed (Godefroit, Bolotsky & Bolotsky 2012). However, it has also been suggested that the unusually morphology might be due to advanced age (Godefroit, Bolotsky & Bolotsky 2012). It was noted that several other skeletal elements from the same individual had been crushed post-mortem, an aspect of taphonomy which might have affected the dentary as well (Godefroit, Bolotsky & Bolotsky 2012). Definite pathological alteration of dentary morphology can be seen in specimen MOR 2919, belonging to the saurolophine species Probrachylophosaurus bergei, which displays circular excavations in the dentaries (Freedman Fowler & Horner 2015).

4 Methods 4.1 Data collection Photographs of hadrosauroid dentary specimens in medial view along with information on image source, taxonomic affinity, and ontogenetic stage (juvenile, subadult, or adult) were obtained from the literature, and the personal digital libraries of N. Campione and A. Prieto-Márquez (table 9 in the appendix). The information was then compiled into a dataset consisting of 89 specimens representing 37 species (73 specimens), three genera indeterminate at the species level (ten specimens), two indeterminate hadrosauroids, two indeterminate hadrosaurids, one indeterminate saurolophid, and one indeterminate lambeosaurine. Medial view was chosen so the dental battery could be digitized, as it is not fully visible in lateral view. The choice of specimens included in the dataset was based on specimen completeness. Only specimens deemed to have a well-preserved coronoid process, outline of the dental battery, and portion of the dentary anterior to the dental battery were included. Specimens with minor damage were included if it was believed that missing data could be confidently approximated by knowledge of the anatomy of other specimens. If ontogenetic stage was not stated in literature, it was inferred based on the relative development of the crest or by comparing the size of the dentary following Evans (2010), who suggested an arbitrary division of ontogenetic stages where an individual is juvenile, subadult or adult if skull length is < 50%, 50-85%, or > 85%, respectively, of the maximum skull length in observed that taxon. Taxa known from a single dentary specimen that were not assigned to a particular ontogenetic stage were assumed to be adults due to the absence of comparative specimens, unless a closely related taxon of similar size could be compared with.

4.2 Digitization Specimens were compiled into a .tps file in tpsUtil (Rohlf 2017) and digitized in tpsDIG2 (Rohlf 2015) following a semilandmark configuration designed to efficiently capture the highly curved morphology of the hadrosauroid dentary, yet still allow for as many specimens as possible to be included (see figure 4 and table 1 for an explanation of semilandmark placement). The angular and splenial facets at the posterior of the dentary tend to be thin and often break off as a result of preservational processes. It was therefore decided to not include the posterior margin in the digitization process, thereby allowing specimens with missing angular and splenial facets to be included, thereby increasing the amount of specimens analyzed. Efforts, however, were made to include the posteroventral edge, the angular facet whose location could be easily interpreted based on other specimens. Interpreting the position of these landmarks enables comparison of the relative proportion of the dentary’s anteroposterior length with respect to other regions, such as the dental battery, the diastema and the coronoid process. The positions of missing semilandmarks along the posteroventral edge were estimated by hand. Semilandmarks were resampled equi-distantly to each other to allow the curves to be comparable across specimens (figure 4). The semilandmarks were then 13 converted to homologous landmarks in Notepad. tpsDIG2 was used to derive a scale factor from the amount of pixels contained along scale bars of predetermined length in digitized images. The scale factors for each image would be used in the calculation of centroid size during the generalized procrustes analysis.

Figure 4. Schematic drawing of the landmark configuration, with anatomy of the hadrosauroid dentary included. Dark blue lines represent equi-distant semilandmark curves. Dark blue numbers are the amount of semilandmarks present in each curve. Yellow points mark the ends of the curves that are homologous and hence do not slide. Yellow points are named using black numbers, and their positions are explained in table 1.

Table 1. Locations of the homologous landmarks and the ends of semilandmark curves (figure 4). No# Explanation 1 Posteriormost point of the dental battery. 2 Anteriormost point of the dental battery. 3 Anteriormost point of the symphyseal process. 4 Posteriormost point of the angular facet. 5 Intersection of the Meckelian fossa and the base of the anteroposteriorly expanded region of the coronoid process. 6 Anterior base of the anteroposteriorly expanded region of the coronoid process.

4.3 Generalized procrustes analysis and semilandmark sliding The digitized .tps file was used to run a generalized procrustes analysis (GPA) in the statistical computing program R (R Core Team 2016) by using the function gpagen() in the package geomorph (Adams & Otárola-Castillo 2013). A GPA removes the influence of isometric size, translates digitized images to a common centroid, and minimizes the effects of rotation to maximize the shape-related variation in the analysis. In the GPA, semilandmarks were slid along the curves as to match a reference outline (Bookstein et al. 2002) by using a sliders file that was created by hand in Microsoft

Excel. By sliding semilandmarks, the variation of individual semilandmarks was removed and curve shapes were averaged, making curves comparable between specimens (Bookstein et al. 2002). Centroid size was derived in the GPA using the distance of landmark coordinates from a mean value together with the scale factors. Centroid size was later used to generate bivariate allometry plots.

4.4 Principal component analysis, morphospace diagram and density plots Axes of major variation, also called principal components (PCs), were isolated in R using the output of the plotTangentSpace() function in the package geomorph. A morphospace diagram was plotted using PCs 1 and 2. The function summary() was used to obtain the proportions of variation (i.e. percentage of variation) explained by each PC axis. Proportions of variation were plotted onto corresponding PCs in the morphospace diagram. The function plotRefToTarget() from the package geomorph was used to generate thin-plate spline (TPS) grids of the shape extremes at the PCs minima and maxima, which added to the morphospace to visualize how certain aspects of morphology changed along the main axes of variance. Density plots separated by taxonomy and ontogenetic stage were generated using PCs with the function density() and were placed to match the principal component axes in morphospace to provide further clarification of groups’ distributions along the PCs.

4.5 Normality tests and principal component group separation To statistically assess the differences of major groups in morphospace, suitable tests had to be chosen. Normality tests were used to determine the suitability of conducting parametric tests (i.e. analysis of variance and Tukey’s honest significant difference test) on the groupings of the sample herein through use of the function parametric.tests() (function obtained from N. Campione). All normality tests are unable to reject normality, with the exception of non-hadrosaurid hadrosauroids along PC2 that were only weakly non-normal (W=0.8774280, p=0.04344031). As groups generally approach a normal distribution, standard parametric analyses of variance (ANOVA) and Tukey’s honest significant difference (TukeyHSD) tests were carried out on PC1 and PC2 using the functions aov() and TukeyHSD(), which compared mean values of groups, divided both by taxonomy and ontogenetic stage, and tested for significant differences. The aov() and TukeyHSD() functions were also run with the Edmontosaurus annectens specimen AMNH 5730 excluded to investigate its effects on the sample, as its position along PC1 in the morphospace diagram supported previous interpretations that it might have been dorso-ventrally crushed (Campione & Evans 2011). The same tests were also run with Eotrachodon orientalis as a non-hadrosaurid hadrosauroid, based on the phylogeny by Xing, Mallon and Currie (2017), to test the effects of a different taxonomic placement on the sample. Normality tests run when AMNH 5730 was removed and E. orientalis was assigned to a different taxonomic group indicated normal distributions in PC1 for both groups (W=0.9734956, p=0.4939943, and

W=0.9365358, p=0.30872298, respectively), and weak normal distributions in PC2 for both groups (W=0.9483623, p=0.0786448, and W=0.8937011, p=0.06376394, respectively).

4.6 Bivariate allometry plots and confidence intervals A number of bivariate plots were generated between centroid size (a proxy for dentary size, body size, and hence growth stage) and specific principal components. Some specimens were excluded from the allometry plots if scale bars were not included in the image, whereby scale factor could not be calculated. As some species in the analysis consisted of very few specimens, plots were generated by genus, to increase plot sample size. As a result, interpreted patterns should be considered as general growth patterns, rather than species-specific ontogenetic trajectories. The functions lm() and abline() were used to add a regression line to each plot and determine if growth patterns could be significantly differentiated from isometry and hence follow an allometric pattern. A slope coefficient that cannot be statistically differentiated from 0 suggests isometry, whereas any statistically significant deviation from 0 indicates allometry, respectively. Additionally, the confint() function was used on the regression lines of the Edmontosaurus and Ugrunaaluk/Edmontosaurus plots to investigate if it was possible to distinguish the growth patterns of these plots from each other by using confidence intervals.

5 Results 5.1 Morphospace diagram and density plots 5.1.1 Principal component 1 PC1 is responsible for 31.63% of the total variation. PC1 depicts the variation in inclination of the coronoid process and the relative length of the diastema (figure 5). Specimens closer to the negative end of PC1 have an anteriorly inclined coronoid process, and an elongated diastema relative to dentary length. Specimens closer to the positive end of PC1 have a dorsally inclined coronoid process, and a shortened diastema relative to dentary length.

Figure 5. Morphospace diagram of principal components 1 and 2 with density plots separated by clade for easier visualization. Major groups in morphospace are divided according to taxonomic affinity, which is visualized by overall color separation (green, red, blue, and yellow shades). Boxes show minimum and maximum extents of groups along respective principal components. Non-hadrosaurid hadrosauroids are shown in green colors and include Altirhinus kurzanovi (A), Bactrosaurus johnsoni (B), Eolambia caroljonesa (E), Jeyawati rugoculus (J), Plesiohadros djadokhtaensis (P), Probactrosaurus gobiensis (p), Protohadros byrdi (b), Shuangmiaosaurus gilmorei (S), Sirindhorna khoratensis (s) and Telmatosaurus transsylvanicus (T). Indeterminate juvenile hadrosauroids are marked by black question marks. Non-saurolophid hadrosaurids are shown in red color only include Eotrachodon orientalis (E). Indeterminate juvenile hadrosaurids are shown as grey question marks. Indeterminate hadrosaurids of unknown ontogenetic stage are shown as brown question marks. Juvenile saurolophids are marked by purple question marks. Lambeosaurines are shown in blue colors and include Arenysaurus ardevoli (A), Amurosaurus riabinini (a), Blasisaurus canudoi (B), Charonosaurus jiayinensis (j), Corythosaurus casuarius (c), Corythosaurus intermedius (i), Corythosaurus sp. (C), Hypacrosaurus stebingeri (H), indeterminate lambeosaurine (?), Lambeosaurus lambei (L), Olorotitan arharensis (O), Parasaurolophus tubicen (P), Parasaurolophus sp. (p), Sahaliyania elunchunorum (S), Tsintaosaurus spinorhinus (T) and

Velafrons coahuilensis (V). Saurolophines are shown in yellow colors and include Acristavus gagslarsoni (A), Brachylophosaurus canadensis (B), Edmontosaurus annectens (E), Edmontosaurus regalis (e), Gryposaurus alsatei (a), Gryposaurus latidens (l), Gryposaurus notabilis (n), Gryposaurus sp. (G), Kritosaurus navajovius (K), Maiasaura peeblesorum (M), Prosaurolophus maximus (P), Saurolophus angustirostris (S), Ugrunaaluk kuukpikensis (U) and Willinakaqe salitralensis (W). Ontogenetic stages are divided by shade of color (dark, medium and light) and line type (dotted, dashed and solid). Juveniles are denoted by dark shades and dotted lines, subadults by medium shades and dashed lines, and adults by light shades and solid lines.

Non-hadrosaurid hadrosauroids generally have dentaries with quite dorsally directed coronoid processes and relatively short diastemata compared to the saurolophids (saurolophines and lambeosaurines), with the exception of Plesiohadros djadokhtaensis (MPC-D100/745). Lambeosaurines display dentary morphologies with more anteriorly inclined coronoid processes as well as more elongate diastemata than non-hadrosaurid hadrosauroids. Saurolophines show the most extreme range of coronoid process angles and relative diastema lengths, even if the aberrant Edmontosaurus annectens specimen at the far negative side of PC1, AMNH 5730, is ignored. Saurolophinae also displays the most anteriorly inclined coronoid processes and elongate diastemata. When comparing ontogenetic stages, the juveniles generally display vertical to subvertical coronoid processes and relatively short diastemata (figure 5), the adults generally display anteriorly inclined coronoid processes and relatively long diastemata, and the subadults display intermediate coronoid process inclinations and relative diastema lengths, although all ontogenetic stages overlap partially. Lambeosaurine juveniles are an exception, having possessing coronoid processes that are more inclined and diastemata that are relatively longer than some lambeosaurine subadults. However, the restricted extent of the lambeosaurine juveniles along PC1 is perhaps due to low sample size (N= 4). The non-hadrosaurid hadrosauroid juveniles and lambeosaurine juveniles only show minor similarities to each other, but both are very similar to the saurolophine juveniles. The saurolophine juveniles display a morphological variation in the angle of the coronoid process and relative length of the diastemata equivalent to the entire range of non-hadrosaurid hadrosauroid grade (including juveniles, subadults, and adults). The lambeosaurine subadults display wider morphological variation than the saurolophine subadults in these aspects. The subadults of both clades have more anteriorly inclined coronoid process and relatively longer diastemata than the non-hadrosaurid hadrosauroid subadults, but partially overlap morphologically. The same pattern is also visible when the adult growth stages of the taxonomic groups are compared. The taxa with the most anteriorly inclined coronoid processes and relatively longest diastemata of the adult stages are Plesiohadros djadokhtaensis in non-hadrosaurid hadrosauroids, Tsintaosaurus spinorhinus and Olorotitan arharensis in the lambeosaurines, and Edmontosaurus in the saurolophines. The juveniles of all taxonomic groups in general possess dorsally inclined coronoid processes and short diastemata. The non-hadrosaurid hadrosauroid subadults have a slightly more anteriorly inclined coronoid process and lengthened diastema than the juveniles, though the non-hadrosaurid hadrosauroid subadults did not achieve the same degree of inclination and elongation as the saurolophid subadults. Although some

18 adult non-hadrosaurid hadrosauroids seem to have experienced further increase in coronoid process inclination and diastema elongation, this increase is minor compared to the increase in coronoid process inclination and relative diastema elongation experienced in saurolophids when transitioning from subadult to adult stage. Individual specimens of the non-hadrosaurid hadrosauroids Eolambia caroljonesa and Bactrosaurus johnsoni are confined to small stretches along PC1, which could be due to an actual pattern of limited individual variation, or due to small sample size. One indeterminate juvenile hadrosauroid is located further along the positive extent of PC1 than E. caroljonesa and B. johnsoni and therefore possesses the most vertical coronoid process and relatively shortest diastema in the entire sample. The only non-saurolophid hadrosaurid specimen in the sample belongs to a subadult Eotrachodon orientalis possessing a vertical coronoid process and a relatively short diastema, displaying a similar morphology to non-hadrosaurid hadrosauroids, and juvenile saurolophines. The lambeosaurine Corythosaurus shows a somewhat wide distribution of individuals, especially subadults. Lambeosaurus lambei is represented herein by three adult specimens that show a fairly wide individual variation. The two subadult specimens of Sahaliyania elunchunorum show similar coronoid process inclination and relative diastema length. Tsintaosaurus spinorhinus is represented by two adult specimens that display fairly different inclinations of the coronoid process and elongation of the diastema. The saurolophine juveniles of the taxon Willinakaqe salitralensis show a wide spread that spans approximately 2/3 of the growth stage’s group along PC1. Ugrunaaluk kuukpikensis displays approximately as much individual variation as W. salitralensis. Subadult Edmontosaurus display relatively little individual variation (but N=2), whereas subadult Brachylophosaurus specimens show moderate individual spread. Adult Edmontosaurus and Brachylophosaurus specimens display moderate individual variation (with AMNH 5730 excluded). Prosaurolophus maximus shows moderate individual variation in the subadult stage and little variation in the adult stage, although it is only represented by two subadult and two adult specimens. Subadult Gryposaurus specimens (two of three specimens belong to G. notabilis) show moderate individual variation, whereas three adult Gryposaurus specimens belonging to G. alsatei, G. latidens and G. notabilis are positioned within the individual variation of the subadults. As the adult Gryposaurus sample’s variation is composed of three different species, there is an uncertainty of whether the variation is individual or taxonomic in nature.

5.1.2 Principal component 2 PC2 is responsible for 26.63% of the total variation. The aspects of morphology that vary along PC2 include the relative dorsoventral elongation of the coronoid process, the overall dorsoventral extents of the dental battery, and the angle of the diastema relative to the main axis of the dentary (figure 5). Specimens closer to the negative end of PC2 have a tall coronoid process, a deep dental battery, and a 19 ventrally oriented diastema. Specimens closer to the positive end of PC2 have a short coronoid process, a dorso-ventrally shallow dental battery, and a diastema that is roughly in line with the main axis of the dentary. In general, non-hadrosaurid hadrosauroids have relatively short coronoid processes, shallow dental batteries, and anteriorly angled diastemata; whereas saurolophines and lambeosaurines have relatively tall coronoid processes, deeper dental batteries, and ventrally angled diastemata. Saurolophines and lambeosaurines are similar in these aspects, although saurolophines display slightly more extreme morphologies. The juveniles generally possess dentaries that are anteroposteriorly short and dorsoventrally elongate, as a result of relatively tall coronoid processes, deep dental batteries and ventrally angled diastemata, whereas adults generally possess more anteroposteriorly elongate and dorsoventrally short dentaries, as a result of relatively shorter coronoid processes, shallower dental batteries, and anteriorly angled diastemata. Subadults display intermediate dentary morphologies. Non-hadrosaurid hadrosauroid juveniles only display morphologies regarding these aspects that are present in non- hadrosaurid hadrosauroid subadults (but not the reverse), likely due to low sample size (N=5) and few taxa being included in the group (Bactrosaurus johnsoni and Eolambia caroljonesa). Lambeosaurine and saurolophine juveniles have similar relative coronoid process lengths, dental battery depths, and diastema angles, whereas the non-hadrosaurid hadrosauroid juveniles are dissimilar to the saurolophid juveniles. The subadults of all taxonomic groups display somewhat similar morphologies. However, the lambeosaurine subadults display morphologies more similar to the saurolophine subadults than to the non-hadrosaurid hadrosauroid subadults. The adults of all taxonomic groups are somewhat similar to each other, and the adults are also morphologically similar to the subadults of the respective taxonomic groups. The only taxonomic group where the adults display very different morphologies from the juveniles are the lambeosaurines, likely due to the juveniles’ low sample size (N=4). The only non-saurolophid hadrosaurid in the sample, Eotrachodon orientalis exhibits a somewhat, albeit not extremely elongate dentary, with a relatively short coronoid process, shallow dental battery, and anteriorly angled diastema, similar to that in subadult and adult non-hadrosaurid hadrosauroids, subadult and adult lambeosaurines, as well as adult saurolophines. Individual variation of the relative coronoid process length, dental battery depth and diastema angle in E. caroljonesa and B. johnsoni along PC2 is quite limited (figure 5), although the sample size is low (N=4 & 3). The lambeosaurine Corythosaurus shows slightly broader individual variation within separate growth stages than E. caroljonesa and B. johnsoni; however another lambeosaurine, Lambeosaurus lambei, shows slightly less variation. The individual variation of Sahaliyania elunchunorum and Tsintaosaurus spinorhinus is limited, but the two species are only represented by two subadult and two adult specimens, respectively. The saurolophine Willinakaqe salitralensis displays moderate individual variation, whereas Ugrunaaluk kuukpikensis displays minor individual variation. Subadult Edmontosaurus is represented by one E. annectens specimen and one E. regalis specimen, making it hard to differentiate the possibly moderate individual variation seen from possible

20 taxonomic variation. Subadult Brachylophosaurus is represented merely by two specimens that show minor to moderate variation. Both Brachylophosaurus and Edmontosaurus show somewhat wide individual variation in the adult stages. The Prosaurolophus maximus is limited in individual variation as well, both in subadult and adult stages, but is represented only by two subadults and two adults. Both subadult and adult Gryposaurus specimens show moderate individual variation.

5.2 Normality tests All normality tests proved insignificant, except for PC2 in the group containing all non-hadrosaurid hadrosauroids which proved weakly significantly different from normal (table 2). As there was only one slightly non-normal distribution, standard parametric ANOVA and TukeyHSD tests were conducted.

Table 2. Significance (p-value) obtained from normality tests on groupings by taxonomy, as well as taxonomy and ontogenetic stage combined for principal components (PC) 1 and 2. Significant p-values are in bold. Group PC1 p-value PC2 p-value All non-hadrosaurid 0.50257275 0.04344031 hadrosauroids All lambeosaurines 0.1635907 0.7189822 All saurolophines 0.1169364 0.2047141 Non-hadrosaurid 0.8189465 0.4216971 hadrosauroid juveniles Non-hadrosaurid 0.1846695 0.9966337 hadrosauroid subadults Non-hadrosaurid 0.1223226 0.4733782 hadrosauroid adults Lambeosaurine juveniles 0.335211145 0.642877209 Lambeosaurine subadults 0.4694408 0.2557978 Lambeosaurine adults 0.2409526 0.5447972 Saurolophine juveniles 0.9839272 0.2910490 Saurolophine subadults 0.2883999 0.1403135 Saurolophine adults 0.14193985 0.94603578

5.3 Tukey’s honest significant difference test 5.3.1 Principal component 1 Non-hadrosaurid hadrosauroids show significantly different dentary morphologies concerning coronoid process inclination, and relative diastema length to both saurolophines and lambeosaurines (table 3). Saurolophines and lambeosaurines, however, cannot be statistically differentiated, displaying overall similar morphologies.

Table 3. Significance (p-value) obtained from Tukey’s honest significant difference tests grouped by taxonomy for principal component (PC) 1. Significant p-values are in bold. PC1 All non-hadrosaurid All lambeosaurines All saurolophines hadrosauroids All non-hadrosaurid - 0.0417973 0.0000225 hadrosauroids All lambeosaurines 0.0417973 - 0.2136951 All saurolophines 0.0000225 0.2136951 -

Most of the significant differences along PC1 (table 4) relate to the average dentary morphology of saurolophine adults compared to that of all non-hadrosaurid hadrosauroids, lambeosaurine subadults, and saurolophine juveniles. Saurolophine adults are indistinguishable from that of lambeosaurine adults and saurolophine subadults. Additionally, saurolophine adults are significantly different to that of lambeosaurine juveniles, displaying only a weakly insignificant p-value (approximately 0.066). Non-hadrosaurid hadrosauroid juveniles display significant differences to saurolophine and lambeosaurine adults. When non-hadrosaurid hadrosauroid juveniles are compared to saurolophine subadults, only a weakly insignificant difference (approximately 0.091) is present. All other separations of groups along PC1 are insignificant. When the ANOVA and TukeyHSD test on PC1 grouped by ontogeny was re-run without the possibly taphonomically deformed Edmontosaurus annectens specimen AMNH 5730, some group- comparisons that were previously insignificant yielded significant p-values. These include: non- hadrosaurid hadrosauroid juveniles vs. saurolophine subadults (p=0.0306365), and non-hadrosaurid hadrosauroid adults vs. the lambeosaurine adults (p=0.0480906). When Eotrachodon orientalis was included into the non-hadrosaurid hadrosauroid groups along PC1, most p-values decreased, with only a few increasing, nevertheless, none changed from significant to insignificant or vice versa.

Table 4. Significance (p-value) obtained from Tukey’s honest significant difference tests grouped by ontogenetic stage within taxonomic groups for principal component (PC) 1. Non-had = Non-hadrosaurid hadrosauroid, Lam = Lambeosaurine, Sau = Saurolophine. Significant p-values are in bold. Asterisks (*) indicate group-comparisons that become significant when the specimen AMNH 5730 is removed. PC1 Non-had Non-had Non-had Lam Lam Lam Sau Sau Sau juveniles subadults adults juveniles subadults adults juveniles subadults adults Non-had - 0.9997940 0.9999982 0.9337304 0.7185403 0.0363428 0.9972201 0.0905686* 0.0000093 juveniles Non-had 0.9997940 - 1.0000000 0.9999794 0.9996859 0.5916222 1.0000000 0.7681311 0.0039094 subadults Non-had adults 0.9999982 1.0000000 - 0.9966482 0.9622939 0.1280366* 0.9999991 0.2716218 0.0000340 Lam juveniles 0.9337304 0.9999794 0.9966482 - 1.0000000 0.9829491 0.9999856 0.9970721 0.0662183 Lam subadults 0.7185403 0.9996859 0.9622939 1.0000000 - 0.8418496 0.9994886 0.9667614 0.0005076 Lam adults 0.0363428 0.5916222 0.1280366* 0.9829491 0.8418496 - 0.3523526 1.0000000 0.1373067 Sau juveniles 0.9972201 1.0000000 0.9999991 0.9999856 0.9994886 0.3523526 - 0.5899551 0.0001407 Sau subadults 0.0905686* 0.7681311 0.2716218 0.9970721 0.9667614 1.0000000 0.5899551 - 0.1111455 Sau adults 0.0000093 0.0039094 0.0000340 0.0662183 0.0005076 0.1373067 0.0001407 0.1111455 -

5.3.2 Principal component 2 Non-hadrosaurid hadrosauroids are significantly different from saurolophines along PC2. Non- hadrosaurid hadrosauroids are barely insignificant from the lambeosaurines (table 5). Lambeosaurines and saurolophines cannot be distinguished statistically.

Table 5. Significance (p-value) obtained from Tukey’s honest significant difference tests grouped by taxonomy for principal component (PC) 2. Significant p-values are in bold. PC2 All non-hadrosaurid All lambeosaurines All saurolophines hadrosauroids All non-hadrosaurid - 0.0503266 0.0000267 hadrosauroids All lambeosaurines 0.0503266 - 0.1981771 All saurolophines 0.0000267 0.1981771 -

Non-hadrosaurid hadrosauroid juveniles can be differentiated from non-hadrosaurid hadrosauroid adults and saurolophine juveniles, but not other groups, including non-hadrosaurid hadrosauroid subadults along PC2 (table 6). Non-hadrosaurid hadrosauroid subadults are significantly different from lambeosaurine juveniles, as well as saurolophine juveniles and subadults, but are indistinguishable from all other groups. Non-hadrosaurid hadrosauroid adults are significantly different from all other groups except for non-hadrosaurid hadrosauroid subadults. The lambeosaurine juveniles are along PC2 significantly different from non-hadrosaurid hadrosauroid subadults and adults, as well as lambeosaurine adults, but are indistinguishable from other groups, including lambeosaurine subadults. The lambeosaurine subadults are significantly different from non-hadrosaurid hadrosauroid adults and saurolophine juveniles, but are indistinguishable from all other groups, including lambeosaurine juveniles and adults. The lambeosaurine adults are significantly different from non-hadrosaurid hadrosauroid adults, lambeosaurine juveniles and saurolophine juveniles, but are indistinguishable from all other groups, including lambeosaurine subadults. The saurolophine juveniles are significantly different from all non-hadrosaurid hadrosauroid growth stages, lambeosaurine subadults and adults, as well as saurolophine adults, but are indistinguishable from lambeosaurine juveniles and saurolophine subadults along PC2. The saurolophine subadults are only significantly different from non-hadrosaurid hadrosauroid subadults and adults, but are indistinguishable from all other groups, including saurolophine juveniles and adults. The saurolophine adults are significantly different from non-hadrosaurid hadrosauroid adults and saurolophine juveniles, but not from other groups.

Table 6. Significance (p-value) obtained from Tukey’s honest significant difference tests grouped by ontogenetic stage within taxonomic groups for principal component (PC) 2. Non-had = Non-hadrosaurid hadrosauroid, Lam = Lambeosaurine, Sau = Saurolophine. Significant p-values are in bold. PC2 Non-had Non-had Non-had Lam Lam Lam Sau Sau Sau juveniles subadults adults juveniles subadults adults juveniles subadults adults Non-had - 0.6761508 0.0159610 0.6247033 1.0000000 0.9985762 0.0347774 0.9933408 1.0000000 juveniles Non-had 0.6761508 - 0.9792633 0.0068429 0.5849086 0.9506242 0.0000242 0.0355390 0.1800468 subadults Non-had 0.0159610 0.9792633 - 0.0000081 0.0027858 0.0284526 0.0000000 0.0000083 0.0000566 adults Lam juveniles 0.6247033 0.0068429 0.0000081 - 0.2866066 0.0366024 0.9991760 0.9789195 0.4983890 Lam subadults 1.0000000 0.5849086 0.0027858 0.2866066 - 0.9979780 0.0014063 0.8643521 0.9999479 Lam adults 0.9985762 0.9506242 0.0284526 0.0366024 0.9979780 - 0.0000171 0.1607324 0.6975326 Sau juveniles 0.0347774 0.0000242 0.0000000 0.9991760 0.0014063 0.0000171 - 0.1682581 0.0025212 Sau subadults 0.9933408 0.0355390 0.0000083 0.9789195 0.8643521 0.1607324 0.1682581 - 0.9888406 Sau adults 1.0000000 0.1800468 0.0000566 0.4983890 0.9999479 0.6975326 0.0025212 0.9888406 -

5.4 Bivariate allometry plots and confidence intervals Figure 6 consists of bivariate plots and regressions between axes of variation and size, in order to quantify growth patterns. All genera in the PC1 bivariate plots show negative slope coefficients except for the non-hadrosaurid hadrosauroid Eolambia (N=4) that shows a positive slope coefficient (figure 6A). All genera in the PC2 bivariate plots show positive slope coefficients except for the saurolophine Prosaurolophus (N=4) that shows a negative slope coefficient (figure 6N). Except for the two combined Ugrunaaluk/Edmontosaurus plots, regression line slope coefficients could not be statistically differentiated from 0 (table 7), and as such, isometry cannot be rejected as a possible growth pattern pertaining to the aspects of morphology varying along PC1 and PC2 for the genera in figure 6. The regression line slope coefficients in the combined Ugrunaaluk/Edmontosaurus plots display statistical significance, and therefore reject isometric growth in favor of allometry. Compared to the plots with only Edmontosaurus, the combined Ugrunaaluk/Edmontosaurus plots show slightly different slope coefficients (-0.1902329 and 0.1036075, vs. -0.1200599 and 0.09477978). If the taphonomically altered E. annectens specimen AMNH 5730 is removed from the plots, p-values only change slightly for the Edmontosaurus (0.1764 and 0.2526) and Ugrunaaluk/Edmontosaurus (0.0009144 and 0.0002597) plots, with no change in which plots display significant p-values. Additionally, the Edmontosaurus and Ugrunaaluk/Edmontosaurus plots’ slope coefficients then become even more similar (-0.1119478 and 0.08329709, vs. -0.09991728 and 0.08968447). Confidence intervals calculated for the Edmontosaurus and Ugrunaaluk/Edmontosaurus plots’ regression lines along the same PCs overlap (-0.4430343 to 0.06256844 and -0.03856708 to 0.2457821, vs. -0.1778981 to -0.06222179 and 0.06380037 to 0.1257592), making it impossible to distinguish between the growth patterns regardless if the Ugrunaaluk specimens are excluded or included. If confidence intervals are calculated with AMNH 5730 removed, the results are similar and still overlap (-0.3015227 to 0.07762713 and -0.08984103 to 0.2564352, vs. -0.1449758 to - 0.05485877 and 0.05632263 to 0.1230463).

Figure 6. Bivariate allometry plots of logged centroid size against principal components (PCs). Plots are generated by genera to increase sample size but divided to the species level when possible. Regression lines and corresponding slope coefficients were added for increased clarity. Taxonomic division is by color as in figure 5. Juveniles are denoted by dark shades, subadults by medium shades, and adults by light shades. Non-hadrosaurid hadrosauroids are shown in green and include Eolambia caroljonesa (E) seen in A (PC1) and B (PC2). Lambeosaurines are shown in blue and include Corythosaurus casuarius (c), C. intermedius (i) and Corythosaurus sp. (C) seen in C (PC1) and D (PC2). Saurolophines are shown in yellow and include Brachylophosaurus canadensis (B) seen in E (PC1) and F (PC2); Edmontosaurus annectens (E) and E. regalis (e) seen in G (PC1) and H (PC2); E. annectens (E), E. regalis (e) and Ugrunaaluk kuukpikensis (U) seen in I

(PC1) and J (PC2); Gryposaurus alsatei (a), G. latidens (l), G. notabilis (n) and Gryposaurus sp. (G) seen in K (PC1) and L (PC2); as well as Prosaurolophus maximus (P) seen in M (PC1) and N (PC2).

Table 7. Significance (p-value) of regression lines in allometry plots (figure 6), divided by genus and principal component (PC). Significant p-values are in bold. Genus PC1 p-value PC2 p-value Eolambia 0.374 0.6499 Corythosaurus 0.1831 0.2222 Brachylophosaurus 0.1973 0.2507 Edmontosaurus 0.1109 0.1199 Ugrunaaluk/Edmontosaurus 0.001127 0.00006904 Gryposaurus 0.2888 0.3754 Prosaurolophus 0.0922 0.5958

6 Discussion 6.1 Evolution and growth of the hadrosauroid dentary 6.1.1 Taxonomic variation The results of the morphospace analysis and statistical tests (figure 5; tables 3 and 5) add quantitative support to the previously published patterns stating that throughout the evolution of hadrosauroids the coronoid process increased in relative length and anterior inclination, the diastema became relatively more elongate and ventrally rotated, and the dental battery deepened (Evans 2010; Blanco, Prieto- Márquez & De Esteban-Trivigno 2015; Norman 2014). The relatively increased separation of food collection by the predentary and premaxillary beaks in the anterior, and food processing by the teeth of the dental battery in the posterior in saurolophids as is supported herein (figure 5; table 3), would have allowed both of these areas to become more specialized and effective in their respective tasks (Norman 2014). Functional specialization of the dentary’s anterior in saurolophids evidenced herein by increased curvature of the diastema (figure 5; table 5). This morphological change is suggestive of functional modifications regarding food gathering abilities (Norman 2014). Similarly to the suggestion that the ventrally extending rhamphoteca (keratinous beak) of the premaxilla allowed saurolophids to crop plants at ground level more efficiently without needing to bend the neck as extensively (Farke et al. 2013), the increased curvature of the diastema could have served a similar purpose by moving the anterior area of the jaws lower. This idea is further supported by several studies on ungulates indicating that a ventrally curved snout is indicative of low-level grazing (Spencer 1995; references in Mallon et al. 2013). If this was the case in hadrosauroids, then saurolophids display a diastema curvature indicative of a more grazing diet than non-hadrosaurid hadrosauroids (with lambeosaurines being very weakly insignificant to non-hadrosaurid hadrosauroids), and further, saurolophines were a little more adapted to low-level graze than lambeosaurines (figure 5), although the latter cannot be statistically recognized herein (table 5). Additionally, it has been discussed that saurolophines possessed adaptations for living in more open habitats, whereas lambeosaurines were morphologically more adapted to closed habitats (Carrano, Janis & Sepkoski 1999), which would lend credit to the idea that saurolophines were more adapted to low-level graze than lambeosaurines. The anteroposterior motion of the lower jaw (Cuthbertson et al. 2012) that worked to move food posteriorly and at the same time grind it was augmented in saurolophids, through relatively taller and more anteriorly inclined coronoid processes that resulted increased moment arm length. This re-directed muscles connecting the coronoid process to the skull’s posterior and posterodorsal areas to pull in a more posterior direction, and lead to increased mechanical advantage, which made mastication more effective (Nabavizadeh 2014, 2016; Norman 2014) (figure 5, tables 3 and 5). The increased depth of the dental battery in saurolophids compared to non-hadrosaurid hadrosauroids of the same ontogenetic stage is likely evidence of an increasing number of tooth rows along that lineage (Norman 2014). For example, three tooth rows were present in the non-hadrosaurid

29 hadrosauroid taxa Altirhinus (Norman 1998), Eolambia (Kirkland 1998), Equijubus (You et al. 2003), Probactrosaurus (Norman 2002) and Protohadros (Head 1998) whereas up to seven rows were present in some saurolophid hadrosaurids (Xing, Mallon & Currie 2017). Previous research has shown that the pulp in the teeth of the upper-most (functional) tooth row in at least saurolophid hadrosaurids was replaced with dentine to allow the entire tooth to be ground down through mastication and afterward be replaced by a new row of teeth (LeBlanc et al. 2016). It could be the case that the multiple adaptations mentioned herein that lead to more efficient mastication likely caused to increased tooth wear due to the increased force generated. Increased tooth wear meant that additional rows of teeth enabled saurolophids to eat throughout their lives and as such increased the individual’s lifespan and reproduction potential. This idea is supported by research stating that development of the dental battery in saurolophids started early; already being similar in embryonic and neonatal individuals to that found in adults (LeBlanc et al. 2016), and would additionally decrease the need for dental battery development needed after hatching. Additionally, saurolophid teeth were likely connected by periodontal (soft, non-mineralized) tissue and ligaments rather than (rigid and mineralized) cementum, which gave the teeth the ability to work as shock absorbers during dental occlusion (contact of the maxillary and dentary teeth during jaw closure) (LeBlanc et al. 2016). It has not been investigated how the teeth in non-hadrosaurid hadrosauroid were connected; however major differences are herein thought to be unlikely. A deeper dental battery would mean more space for tooth rows that worked to dissipate the force of chewing, as well as made the grinding of plant matter more efficient and allowed continuous food intake throughout life. The current results cannot resolve exactly when during hadrosauroid evolution that these morphological changes took place. E. orientalis’ phylogenetic placement would suggest that it should look more like a hadrosaurid, as it is just outside of Saurolophidae (Prieto-Márquez, Erickson & Ebersole 2016a). The fact that it doesn’t means that either (1) major shifts in the evolution of the lower jaw occur after Eotrachodon, within saurolophids, (2) major shifts occur before Eotrachodon which autapomorphically reverts to a more primitive state, or (3) the Eotrachodon specimen’s subadult nature makes it look like it has a more primitive condition. Alternatively, Xing, Mallon and Currie (2017)’s placement of E. orientalis as a non-hadrosaurid hadrosauroid could be considered affirmed by the placement of E. orientalis within the non-hadrosaurid hadrosauroids in morphospace, however when E. orientalis was added to the non-hadrosaurid hadrosauroid grade in the TukeyHSD tests, no significant change took place. If an adult E. orientalis were to be found and incorporated into the analysis performed herein it might help to shed light on the species’ development and hadrosauroid evolution. Morphometric analyses of other skeletal elements could also be performed in order to assess E. orientalis’ taxonomic placement.

6.1.2 Growth patterns and their taxonomic differences Non-hadrosaurid hadrosauroids experienced little-to-no change in coronoid process inclination and relative diastema length throughout growth (PC1). Even though their occupation in morphospace seems to expand with increasing growth stage (figure 5), statistical tests could not detect significant differences coronoid process inclination and relative diastema length between stages (table 4). Most of the non-hadrosaurid hadrosauroid taxa herein are represented only by one growth stage, and only two taxa are represented by both juvenile and adult stages (Bactrosaurus johnsoni and Eolambia caroljonesa), suggesting that there might be a taxonomical component to this pattern. Nevertheless, the constrained position of these two taxa reflect what was also seen in the statistical tests for the stages of the entire grade (table 4), and even though isometry cannot be rejected, nor proven, for non- hadrosaurid hadrosauroid individual taxa (table 7), the low slope coefficient of E. caroljonesa (slope=0.02, figure 6A) vaguely hints at very low amounts of change during growth for the taxon (but N=4), and possibly for the grade. Even though lambeosaurines seem to increase in coronoid process inclination and relative diastema length (figure 5), statistical tests cannot distinguish the stages’ shifts in morphospace from random (table 4). However morphospace occupation is quite narrow and sample size is quite low in the juvenile lambeosaurines (N=4), meaning that more extreme morphologies might have been excluded, which therefore alters the outcome of the statistical tests. In the saurolophines the increase in coronoid process inclination and relative diastema length (figure 5), is also reflected in the statistical tests (table 4), where the juveniles are significantly different from the adults, but the subadults cannot be distinguished from the juveniles, nor from the adults. This suggests a gradual increase in coronoid process inclination and relative diastema length in saurolophines (and possibly lambeosaurines which show a similar trend in morphospace occupation). Non-hadrosaurid hadrosauroids, lambeosaurines and saurolophines all show a trend toward increased relative dental battery length during growth (PC2), as seen from their morphospace occupation (figure 5). The statistical tests capture this morphological change, which is considered gradual between stages, as juveniles and adults display significant differences in dental battery length, but subadults and juveniles, as well as subadults and adults do not (table 6). This change in relative dental battery length is interpreted as increase in the amount of teeth in each row with growth, thereby increasing the available space to process food. All taxonomic groups seem to show shortening of the coronoid process, and shallowing of the dental battery. However, as these are changes relative to the elongation of the dental battery (and by extension the dentary, as at least in saurolophids, the diastema also relatively elongates), meaning that there is no absolute decrease in the dorsoventral length of the coronoid process or the depth of the dental battery. The diastema however, straightens during growth in all taxonomic groups suggesting a possible growth-related change in diet as the individual becomes larger and gains the ability to reach plants higher above ground level, rather than subsisting on low- level graze. This would possibly indicate ontogenetic dietary niche partitioning, which has been 31 suggested for lambeosaurines by Mallon and Anderson (2015), due to juveniles not being able to generate as much bite force as adults. Their results however, were statistically insignificant, possibly due to sample size. When coronoid process inclination and relative diastema length of the growth stages in a taxonomic group are compared to the same growth stages in the other taxonomic groups (figure 5; table 3), only the non-hadrosaurid hadrosauroid adults are significantly different from the lambeosaurine and saurolophine adults (when the taphonomically altered Edmontosaurus annectens specimen AMNH 5730 is excluded). As the lambeosaurine adults are insignificantly different from the saurolophine adults, it is interpreted that saurolophids went through additional increases in coronoid process inclination and relative diastema length in the transition from subadult to adult stage, compared to non-hadrosaurid hadrosauroids. The results therefore suggest possible long-term peramorphosis of these morphological aspects along the lineage leading from non-hadrosaurid hadrosauroids to saurolophids. Peramorphosis is defined as when the ontogenetic development of a trait in the descendant species extends beyond that of the ancestor species (Schoch 2014). The statement is meant to be very broad, as deviations from the overall trend are likely to have taken place through geological time. The statement is also made with caution, as the full sample size (89 specimens with AMNH 5730) is somewhat low. Even with the aforementioned shortcoming in mind, the results still fit with already published knowledge about taxonomic changes in coronoid process inclination and diastema elongation along the non-hadrosaurid hadrosauroid to saurolophid lineage (Norman 2014), suggesting that at least a general evolutionary pattern was still captured by the analyses herein, despite the sample size. Furthermore, there are three processes that lead to peramorphosis, all of which allow the descendant species to grow beyond that of the ancestral species. These include: (1) acceleration, whereby development is sped up, (2) hypermorphosis, whereby extension of the developmental trajectory takes place, and (3) pre-displacement, whereby developmental events take place earlier in growth (Alberch et al. 1979; Schoch 2014). The angle of the coronoid process and the relative length of the diastema in saurolophid juveniles and subadults are insignificantly different from that of non-hadrosaurid hadrosauroid juveniles and subadults (table 4), and thereby do not develop a difference in earlier stages, rejecting both acceleration and pre- displacement of development. That there is a significant difference between non-hadrosaurid hadrosauroid adults, and saurolophid adults (figure 5; table 4), suggests that the evolutionary increase coronoid process inclination and relative diastema length development in saurolophids relative to non- hadrosaurid hadrosauroids took place in the adult stage. This would be considered an extension of the developmental trajectory’s end, supporting hypermorphosis in coronoid process inclination and relative diastema length. Another instance of peramorphosis in saurolophids include halting of tooth replacement by acceleration of tooth formation, which allowed older generations (rows) of teeth to be retained, and as the pulp of the functional teeth were filled with dentine, they could be completely ground down, and a

32 new tooth row could erupt below it without any interruption to mastication (LeBlanc et al. 2016). Lambeosaurines overall are thought to have gone through pre-displacement of the cranial crest, and Parasaurolophus is thought to have gone through both pre-displacement and hypermorphosis of its cranial crest compared to other lambeosaurines (Farke et al. 2013). The lambeosaurine Hypacrosaurus and the saurolophines Brachylophosaurus and Maiasaura all show pre-displacement in the development of the supraacetabular process of the ilium (Guenther 2009). Peramorphosis is not unique to hadrosauroids however, as several instances have been found dinosaurs, e.g. hypermorphosis in the limbs and feet of sauropods compared to other saurischians (Lockley & Jackson 2008), pre- displacement in the bone micro-structure of the titanosaur Lirainosaurus astibiae compared to other sauropods (Company 2011), and possible acceleration within the skull of carcharodontosaurid theropods (Canale et al. 2014). Even though non-hadrosaurid hadrosauroids and saurolophids developed in a similar direction of morphological change from one growth stage to another in terms of the length of the coronoid process shortening and the dental battery shallowing relative to the straightening of the diastema and increasing length of the dental battery, overall saurolophids have more elongate coronoid processes and deeper dental batteries than non-hadrosaurid hadrosauroids of the same growth stage (figure 5). As saurolophid adults possessed similar relative coronoid process length and dental battery depth seen in non-hadrosaurid hadrosauroid juveniles and subadults but not that of non-hadrosaurid hadrosauroid adults, it at first resembles paedomorphosis, however this is likely not the case. Paedomorphosis is defined as when a trait in adults of a descendant species is similar to the same trait in juveniles of the ancestral species (Schoch 2014). Paedomorphosis can be achieved through (1) deceleration, whereby the developmental rate is slowed, (2) progenesis, whereby the developmental trajectory is stopped early, or (3) post-displacement, whereby the start of development is delayed (Alberch et al. 1979; Schoch 2014). As the distance between growth stages is approximately the same in morphospace (with the possible exception of the juveniles in the non-hadrosaurid hadrosauroids and lambeosaurines, due to low sample size), and the statistical tests show a similar pattern (juveniles are significantly different from adults, but subadults are insignificantly different from both juveniles and adults) for all taxonomic groups, they should all have changed morphologically in approximately equal amounts from one growth stage to another, ruling out both deceleration as growth stage separation is approximately the same, and also progenesis as no growth stage is truncated and remains in the same area of morphospace as an earlier ontogenetic stage. Post-displacement is not supported as rather than there being a delay in morphological change which would stack ontogenetic stages on top of each other, and also prevent saurolophid adults from reaching the same morphology as non-hadrosaurid hadrosauroid adults, the entire range of the saurolophid group has simply shifted negatively along PC2 (figure 5). This shift means that rather than that the morphology of the juveniles is retained for a longer time and then changed, instead a completely new juvenile condition appeared in saurolophids. This new juvenile condition is likely attributed to the accelerated development of teeth in embryonic

33 and neonatal saurolophids (LeBlanc et al. 2016), which created more tooth rows in saurolophids (Xing, Mallon & Currie 2017) than in non-hadrosaurid hadrosauroids (Head 1998; Kirkland 1998; Norman 1998, 2002; You et al. 2003), and therefore a deeper dental battery relative to its length. This is why in all three taxonomic groups, as growth progresses, the coronoid process looks shorter, and the dental battery looks shallower, but these are relative to the elongation of the dental battery. This means that relative dental battery elongation and diastema angle straightening throughout growth progressed in the same direction and extent in all three taxonomic groups, although the morphologies exhibited at the same growth stages were different in saurolophids and non-hadrosaurid hadrosauroid, and that is why saurolophids do not reach the juvenile non-hadrosaurid hadrosauroid morphology until subadult/adulthood. The combined growth- and heterochrony changes in hadrosauroids all suggest food selection- and processing-related adaptations. The increased anterior inclination of the coronoid process and the relatively longer diastema in saurolophids was achieved not merely by evolution alone but due to heterochrony, which increased mechanical advantage, directed muscles to pull posteriorly, and created functional specialization of the predentary and dental batteries (Nabavizadeh 2014, 2016; Norman 2014), in older growth stages compared to non-hadrosaurid hadrosauroids. All of the three taxonomic groups experienced relative elongation of the dental battery as well, indicative of increasing amounts of teeth per row, leading to larger surfaces for the grinding of plant matter which were appropriate considering the increase in size with advancing age. When this is considered with the increased relative diastema length in saurolophids, the resulting growth-related changes created adult herbivores with relatively very anteroposteriorly elongate dentaries. Furthermore, a growth-related straightening of the diastema hints at possible dietary niche partitioning between younger and older growth stages, similar to what was suggested by Mallon et al. (2013) on feeding height stratification. With a lower muzzle, young saurolophids would have reached low vegetation without needing to excessively bend their necks, whereas older (and therefore larger) saurolophids would have benefitted from the increased reach enabled by a straighter diastema when feeding from more elevated vegetation. This is further supported by the presence of multiple saurolophid pes prints, but absence of manus prints, around conifer and palm tree roots, and fallen logs in the Campanian-aged Blackhawk Formation of Utah, suggesting that saurolophids reared up against the trees to consume high-growing foliage (Mallon et al. 2013 and references therein). The specimens of Ugrunaaluk kuukpikensis included in the analysis herein belong to the Liscomb bonebed of the Prince Creek Formation in northern Alaska (Mori, Druckenmiller & Erickson 2016). Many saurolophine specimens that were found in the Prince Creek Formation were previously labeled as Edmontosaurus sp. (Gangloff & Fiorillo 2010), but they were determined as belonging to a new, closely related saurolophine genus and species, Ugrunaaluk kuukpikensis, in 2016 by Mori, Druckenmiller and Erickson. The authors used linear measurements to describe several ways in which the Prince Creek Formation specimens are different to Edmontosaurus regalis and Edmontosaurus

34 annectens, and therefore chose to assign them to a separate genus. Recently, Xing, Mallon and Currie (2017) declared Ugrunaaluk kuukpikensis a nomen dubium, as the Prince Creek Formation material in Mori, Druckenmiller and Erickson (2016)’s paper was solely composed of juveniles that were compared to adult E. annectens and E. regalis, meaning that Xing, Mallon and Currie (2017) considered the characters used by Mori, Druckenmiller and Erickson (2016) to differentiate the taxa as possibly being ontogenetically variable. As previous research has shown that using ontogenetically variable characters can cause non-adult specimens to place further back in phylogenies than adults of the same taxa (Tsuihiji et al. 2011; Campione et al. 2013; Prieto-Márquez 2014), it is herein considered that the decision made by Xing, Mallon and Currie (2017) to re-classify the Prince Creek Formation specimens as Edmontosaurus sp. was sensible, especially as no adult Prince Creek Formation specimens have been found to compare with adult Edmontosaurus specimens. To investigate similarities and differences in the dentaries of the Prince Creek Formation specimens to Edmontosaurus, the Prince Creek Formation specimens were added to bivariate allometry plots with Edmontosaurus. When the combined Ugrunaaluk and Edmontosaurus plots (figures 6I and 6J) are compared to the Edmontosaurus plots, they show that (1) inclusion of Ugrunaaluk specimens does not greatly change the slopes obtained from just Edmontosaurus specimens (only Edmontosaurus -0.19 and 0.10, vs. Ugrunaaluk/Edmontosaurus -0.12 and 0.09, for PC1 and PC2, respectively), and (2) by increasing the size range, isometry could be rejected in this group (p=0.001127 for PC1, and 0.00006904 for PC2). As Ugrunaaluk and Edmontosaurus specimens both plot very closely to the regression line, the growth trajectories with Ugrunaaluk included or excluded are similar and are impossible to distinguish from each other as confidence intervals overlap, it could be taken as an indication that the Ugrunaaluk specimens belong to Edmontosaurus, in which case the dentary Edmontosaurus would show an allometric growth pattern of increase in the inclination of the coronoid process, relative dental battery length, as well as relative diastema length and straightness. Xing, Mallon and Currie (2017) suggested that the Prince Creek Formation specimens might belong to a new northernmost extent of E. regalis as they display some similarities, but mentioned that there are no E. regalis specimens of similar size to the Prince Creek Formation specimens to compare with. If the Prince Creek Formation specimens belong to E. regalis, the taxon’s geological range would extend from the present Late Campanian into the Early Maastrichtian (Mori, Druckenmiller & Erickson 2016). Alternatively, the inclination of the coronoid process, relative elongation and straightening of the diastema, and the relative elongation of the dental battery progressed very similarly during growth in the clade encompassing both Edmontosaurus and Ugrunaaluk. Even though the results herein suggest close similarities in the dentaries of these taxa, analyses of other skeletal elements should be carried out to determine if they also show a similar pattern, before the status of Ugrunaaluk can be confidently supported or rejected. As Mori, Druckenmiller and Erickson (2016) in addition to the dentary used several characters of the

35 premaxilla, maxilla, jugal and quadratojugal to distinguish Ugrunaaluk from Edmontosaurus, they are worthy of further morphometric study.

6.2 Implications for phylogenetic characters The results presented here reveal much about the nature of variation in the hadrosauroid dentary and, as a result, have important implications for understanding the use of this structure in phylogenetic systematics. Characters associated with the dentary have been included in all hadrosauroid analyses, representing anywhere from 8.8% to 16.2% of the total number of characters (table 8). Given that these analyses assume variation to be solely taxonomic, it is important that the nature of variation be understood. In the following section I focus on the data sets of two recent phylogenetic analyses: Prieto-Márquez, Erickson and Ebersole (2016a), and Xing, Mallon and Currie (2017).

Table 8. Amount of dentary-related characters in literature. Percentage is of total amount of characters in that publication. Source Dentary teeth Dentary bone Dentary teeth + Total amount bone of characters Weishampel, 4 (10.8%) 2 (5.4%) 6 (16.2%) 37 Norman & Grigorescu 1993 Godefroit et al. 3 (5.4%) 2 (3.6%) 5 (8.9%) 56 2008 Evans & Reisz 5 (5.3%) 4 (4.3%) 9 (9.6%) 94 2007 (and Evans 2010) Prieto-Márquez 14 (3.8%) 21 (5.7%) 35 (9.5%) 370 2010a Xing et al. 2014 15 (4.3%) 21 (6.1%) 36 (10.4%) 345 Prieto-Márquez, 10 (3.7%) 14 (5.1%) 24 (8.8%) 273 Erickson and Ebersole 2016a Xing, Mallon 17 (4.9%) 22 (6.4%) 39 (11.3%) 346 and Currie 2017

6.2.1 The diastema When comparing the ratio between the diastema’s length, to the distance from the anteriormost tooth- position to the posterior margin of the coronoid process, Prieto-Márquez, Erickson and Ebersole (2016a), and Xing, Mallon and Currie (2017) use the same character descriptions (only that they are

36 characters 25 and 38, respectively) and character states that include (0) a ratio of less than 0.20, (1) ratio between 0.20 and 0.31, (2) ratio between 0.32 and 0.45, and (3) ratio greater than 0.45. Relative diastema length varies along PC1 and morphometric results support its taxonomic utility to separate non-hadrosaurid hadrosauroids from saurolophids (figure 5, table 3). More advanced growth stages occupy different areas of morphospace along PC1, and adult non-hadrosaurid hadrosauroids are significantly different from adult saurolophids (when the taphonomically altered AMNH 5730 is removed) (tables 3 and 4). However, most saurolophids (with the possible exception Amurosaurus riabinini and to some extent Gryposaurus) show that morphologies described by PC1 also vary as a result of growth, and it is, therefore, recommended that only adult saurolophids be used when coding the aforementioned character. When the large (possibly individual) variation in relative diastema length in juvenile saurolophid specimens (e.g. Ugrunaaluk, Willinakaqe) and the large ontogenetic changes they go through when transitioning to older ontogenetic stages is considered (e.g. Ugrunaaluk if it is considered to be Edmontosaurus sp., Corythosaurus), it becomes even more apparent that this aspect of morphology is ontogenetically variable in most saurolophid taxa herein. Prieto-Márquez, Erickson and Ebersole (2016a), and Xing, Mallon and Currie (2017) use the same character description and character states for the character (28 and 45, respectively) describing the shape of the diastema as being either (1) subtlety convex, subtlety concave or straight, or (2) very concave. Another character that’s applicable to describing the ventral deflection of the diastema is similarly described by Prieto-Márquez, Erickson and Ebersole (2016a), and Xing, Mallon and Currie (2017) (characters 26 and 41, respectively), where the anterior portion of the dentary is ventrally deflected at either (0) less than 17%, (1) 17-25%, or (2) greater than 25%. Both the ventral deflection and the shape of the diastema vary along PC2 (figure 5), whereby ventral deflection is strong and the diastema is rather straight in the negative end, whereas ventral deflection weak and concavity is pronounced in the positive end. As all taxonomic groups seem to vary in these aspects through growth, both of the characters are ontogenetically variable (figure 5; table 6) and as such make non-adult specimens unsuitable for use in coding these characters. However, between adults of different taxonomic groups, good separation is seen and as such it is a suitable character for separating non- hadrosaurid hadrosauroids from saurolophids. Exceptions to the ontogenetic variability are possibly Bactrosaurus johnsoni and Eolambia caroljonesa due to their restricted placement in morphospace and low slope coefficient (figures 5 and 6B), however sample size is restricted (N=3 and 4, respectively), so caution is advised. Additionally, some taxa show high individual variation in certain stages (e.g. Edmontosaurus, Brachylophosaurus, Willinakaqe, and Corythosaurus), and therefore these effects on character states should be investigated through measurements of several individuals of each taxon in order to assess the appropriate limits to the character states.

6.2.2 The coronoid process In the paper by Prieto-Márquez, Erickson and Ebersole (2016a), character 29 defines the direction of the coronoid process’ inclination in relation to the dorsal margin of the alveolar sulci of the dental battery, with (0) being posteriorly or subvertically inclined, and (1) being anteriorly inclined. In Xing, Mallon and Currie (2017)’s paper character 48 defines the same aspect of morphology but with (0) being posteriorly or vertically inclined, (1) being slightly inclined anteriorly at an angle between 70° to 85°, and (2) being obviously anteriorly inclined at an angle of less than 70°. Coronoid process inclination varies along PC1 and morphometric results support its taxonomic utility to separate non-hadrosaurid hadrosauroids from saurolophids (figure 5, table 3). Older growth stages occupy different areas of morphospace along PC1, and adult non-hadrosaurid hadrosauroids are significantly different from adult saurolophids (when the taphonomically altered AMNH 5730 is removed) (tables 3 and 4). However, most saurolophids (with the possible exception Amurosaurus riabinini and to some extent Gryposaurus) show that morphologies described by PC1 also vary as a result of growth, and it is, therefore, recommended that only adult saurolophids be used when coding the aforementioned characters. When the large (possibly individual) variation in inclination of the coronoid process in juvenile saurolophid specimens (e.g. Ugrunaaluk, Willinakaqe) and the large growth-related changes they go through when transitioning to older ontogenetic stages is considered (e.g. Ugrunaaluk if it is considered to be Edmontosaurus sp., Corythosaurus), it becomes even more apparent that these aspects of morphology are ontogenetically variable in most saurolophid taxa analyzed herein. Some specimens of the same saurolophid taxa (e.g. Brachylophosaurus canadensis, Edmontosaurus, Corythosaurus, Lambeosaurus lambei and Tsintaosaurus spinorhinus) display quite wide individual variation in coronoid process inclination during the adult stage (figure 5), meaning that they could possibly fit in both character state (1) or (2) for Xing, Mallon and Currie (2017)’s character 48, and as such Prieto-Márquez, Erickson, and Ebersole (2016a)’s character states seem better for including widely varying specimens of the same taxon into the same coronoid process inclination character state. Additionally, considering that non-hadrosaurid hadrosauroids mostly load positively on PC1 and possibly show low ontogenetic variability if E. caroljonesa and B. johnsoni are any indication, whereas saurolophids plot more negatively and are in general more ontogenetically variable in coronoid process inclination (and in most taxa are more inclined in older ontogenetic stages), it seems like the character states used by Prieto-Márquez, Erickson and Ebersole (2016)’s character 29 provide a better separation of non-hadrosaurid hadrosauroids to saurolophids than Xing, Mallon and Currie (2017)’s character 48. The character matrices of Prieto-Márquez, Erickson and Ebersole (2016a), and Xing, Mallon and Currie (2017) both lack any kind of character regarding the length of the coronoid process in relation to other elements. Considering that the length of the coronoid process changes along PC2, thereby being one of the second-most important sources of variance in the hadrosauroid dentary (at least in the elements of the dentary analyzed herein), and that previous research has stated that the coronoid 38 process became taller in hadrosaurids than non-hadrosaurid hadrosauroids (Norman 2014), it is curious that there is no phylogenetic character to describe such variation. It is herein suggested that a character describing the ratio between the length of the coronoid process and another dentary metric should be included in future character matrices. As seen from the results herein, such a character likely displays little-to-no variability in non-hadrosaurid hadrosauroids (at least in E. caroljonesa and B. johnsoni [figures 5 and 6B]), and therefore allows specimens of all ontogenetic stages to be used, whereas in many saurolophid taxa the character would be ontogenetically variable from subadult to adult (except possibly Gryposaurus [figure 6L]) and in some cases from juvenile to older ontogenetic stages (e.g. if Ugrunaaluk is Edmontosaurus sp., Corythosaurus) (figures 5, 6D and 6J), making it less suitable to use non-adult saurolophid specimens to describe this character.

6.2.3 The dental battery When it comes to the depth of the dental battery, the closest character described by Prieto-Márquez, Erickson and Ebersole (2016a) is (character 2) the minimum number of teeth per alveolus (minimum number of tooth rows) arranged dorsoventrally in the middle of the dental battery’s length, and include the character states (0) two, (1) three, (2) four, and (3) five or more. Xing, Mallon and Currie (2017) on the other hand describe it (character 3) as the maximum number of teeth per alveolus (maximum number tooth rows) of the dentary in adults and include the character states (0) two, (1) three, (2) four to six, (3) seven or more. As the analysis herein has only mapped the shape of the dental battery rather than the amount of tooth rows, nothing can be said with absolute certainty about the above-mentioned phylogenetic characters, however a deepening of the dental battery will, in general, reflect an increase in the number of teeth/tooth rows. All taxonomic groups pattern of dentary elongation with increasing ontogenetic stage that includes a more elongate dental battery. Increased relative length of the dental battery would indicate that there is more space for increasing amounts of teeth. Character 1 of Prieto-Márquez, Erickson and Ebersole (2016a), and Xing, Mallon and Currie (2017) describes the maximum number of tooth positions in the dentary dental battery, and Prieto-Márquez, Erickson and Ebersole (2016a)’s matrix includes the character states (0) 30 or less, (1) 31 to 42, and (2) more than 42, whereas Xing, Mallon and Currie (2017)’s matrix includes the character states (0) 30 or less, (1) 31 to 45, and (2) more than 45. As juvenile and adult E. caroljonesa and B. johnsoni display similar relative length of the dental battery, it is with cautious reservation suggested that all ontogenetic stages of the two taxa can be used to obtain phylogenetic characters, as they should have a similar amount of teeth per row. As an example, the adult E. caroljonesa holotype (CEUM 9758, included in the analysis) had fewer than 30 teeth per row, whereas a juvenile (OMNH 28511, not included herein) had 18 teeth per row (Kirkland 1998), and as such no matter which of the two matrices is used and no matter the ontogenetic stage, the character state is still 0. Prosaurolophus maximus also displays little variation in the relative length of the dental battery when transitioning from subadult to adult stage. However, a large adult specimen possessed at 39 least 40 teeth in one row (TMP 1984.001.0001; McGarrity, Campione & Evans 2013), which is very close to the limits between different character states, suggesting that for Prosaurolophus maximus it might be best to use adult specimens to describe this character. Remaining taxa analyzed herein show quite high ontogenetic variability in relative dental battery length, indicating that only adults should be used when assessing the character state. Furthermore, adult Brachylophosaurus, Edmontosaurus, and Corythosaurus display moderate to high individual variability in relative dental battery length, suggesting that, as defined, characters associated with the amount of teeth per tooth row do not accommodate for these levels of variation, which will introduce a certain level of noise to the analysis.

7 Conclusions

Results support several evolutionary trends mentioned in published literature, and indicate a highly intertwined pattern of growth-related and evolutionary changes taking place in the dentary during the transition between non-hadrosaurid hadrosauroids and saurolophids, resulting in enhanced food gathering and food processing abilities. Saurolophids had a relatively more elongate and ventrally angled diastema that allowed functional specialization of the predentary and dental battery (Norman 2014), as well as hints at possible adaptation to low-level grazing (Spencer 1995; references in Mallon et al. 2013). The coronoid process in saurolophids was relatively more elongate and more rostrally inclined, re-directing muscles to pull the jaw more caudally, and increasing mechanical advantage, which led to increased masticatory efficiency (Nabavizadeh 2014, 2016; Norman 2014). The dental battery became relatively deeper in saurolophids, accommodating an increased number of tooth rows (Head 1998; Kirkland 1998; Norman 1998, 2002; You et al. 2003; Xing, Mallon & Currie 2017) that both worked as shock-absorbers during mastication, and allowed teeth to be replaced continuously as the functional row was worn down (LeBlanc et al. 2016). The relative elongation of the diastema and increased rostral inclination of the coronoid process was achieved through hypermorphosis, an extension of the development of ancestral taxa seen in descendant taxa (Alberch et al. 1979; Schoch 2014). Saurolophids experienced a novel juvenile condition compared to non-hadrosaurid hadrosauroids, including a steeper ventral deflection of the diastema, a relatively taller coronoid process, and a deeper dental battery. Both non-hadrosaurid hadrosauroids and saurolophids went through relative dental battery elongation during growth, incorporating more teeth per row. Overall, the combination of these evolutionary and growth-related modifications to the dentary helped saurolophids become very abundant, and reach near global distribution in the Late Cretaceous. Finally, the supposed non-saurolophid hadrosaurid Eotrachodon orientalis was found to be more similar to non-hadrosaurid hadrosauroids than to saurolophids in dentary morphology, supporting Xing, Mallon and Currie (2017)’s placement of the taxon. Bivariate plots of Edmontosaurus only exhibit minor changes in regression line slope coefficient when the debated taxon Ugrunaaluk kuukpikensis is added, and additionally, confidence intervals are unable to distinguish between Edmontosaurus regression lines where Ugrunaaluk specimens are excluded or included, which supports the inclusion of U. kuukpikensis into the genus Edmontosaurus (Gangloff & Fiorillo 2010; Xing, Mallon & Currie 2017), that then displays allometric growth of several dentary aspects. Other skeletal elements of E. orientalis and U. kuukpikensis should be subjected to morphometric analyses to obtain more information regarding their systematics. Based on the results, the following main recommendations are made with regards to phylogenetic characters: (1) the use of characters pertaining to the anterior inclination of the coronoid process and the relative length of the diastema should only be coded for adult saurolophids due to large ontogenetic variability in these regions, (2) the use of characters describing the shape and ventral deflection of the diastema, and the amount of

41 teeth per tooth row can be used to distinguish between non-hadrosaurid hadrosauroids and saurolophids, but only using adult specimens as these regions are also ontogenetically variable in all taxonomic groups, and further investigation concerning individual variability is merited as it is potentially high in the adults of some taxa, and (3) the implementation of a character describing relative coronoid process length, as no such character is present in Prieto-Márquez, Erickson and Ebersole (2016a)’s or Xing, Mallon and Currie (2017)’s matrices, and it would provide good separation of non-hadrosaurid hadrosauroids from saurolophids.

8 Acknowledgments

I would like to thank Nicolás Campione for supervising my project, providing unavailable literature, for help with methodology, primary feedback, and for the knowledge he has conveyed to me throughout my university education. I am grateful to Albert Prieto-Márquez and Nicolás Campione for generously donating images of hadrosauroid dentaries from their personal digital libraries, as it allowed more specimens to be analyzed. I thank Anna Minnefors for providing preliminary feedback on early versions of the document, for feedback on the popular scientific summary, and for her incredible patience. I thank my thesis defense opponent Frida Hybertsen, my thesis reviewer Wendy den Boer, and my thesis examiner Lars Holmer for comments and feedback. Lastly, I thank Nicolás Campione, Alexander Paxinos and Christopher Freer for preparatory feedback on the thesis defense presentation.

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Appendix Dataset Table 9. Dataset used to conduct geometric morphometric analysis. References are included in a separate reference list below. A = Adult, Fam = Family, Had = Hadrosaurid hadrosauroid, indet = Indeterminate, J = Juvenile, L = Left dentary, Lam = Lambeosaurine, Non-had = Non-hadrosaurid hadrosauroid, Non-sid = Non-saurolophid hadrosaurid, Ogs = Ontogenetic stage, R = Right dentary, SA = Subadult, Sau = Saurolophine, Sid = Saurolophid hadrosaurid, Subfam = Subfamily, Unk = Unknown. Asterisks (*) denote information assumed by means described in the methods section, if the aforementioned information was not stated in the literature. If ontogenetic stage was not stated in image source but found in other literature, it is stated under the ontogenetic stage reference column. Institutional abbreviations—AEHM, Amur Natural History Museum, Blagoveshchensk, Russia; AMNH, American Museum of Natural History, New York, U.S.A; BMNH, The British Museum, London, UK; BP, Bernard Price Institute for Palaeontological Research, University of Witwatersrand, Johannesburg, South Africa; CEUM, College of Eastern Utah Prehistoric Museum, Price, Utah, USA.; CM, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA; CMN, Canadian Museum of Nature, Ottawa, Ontario, Canada; CPC, Colección Paleontológica de Coahuila, Saltillo, Coahuila, Mexico; FMNH, The Field Museum, Chicago, Illinois, USA; GMH, Geological Museum of Heilongjang Province, Harbin, China; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China; LACM, Natural History Museum of Los Angeles County, Los Angeles, California, USA; LPM, Liaoning Paleontology Museum, Shenyang Normal University, Shenyang, Liaoning Province, China; MCD, Museu de la Conca Dellà, Isona, Spain; MDE, Musée des Dinosaures d’Espéranza, Espéranza, France; MOR, Museum of the Rockies, Bozeman, Montana, USA.; MPC, Mongolian Paleontological Center, Academy of Sciences, Ulan Bataar, Mongolia; MPCA, Museo Provincial Carlos Ameghino, Cipoletti, Argentina; MPZ, Museo Paleontológico de la Universidad de Zaragoza, Gobiero de Aragón, Spain; MSC, McWane Science Center, Birmingham, Alabama, USA.; MSM, Mesa Southwest Museum (now Arizona Museum of Natural History), Mesa, Arizona, USA; MSNM, Museo Civico di Storia Naturale di Milano, Milano, Italy; NMMPH, New Mexico Museum of Natural History and Science, Albuquerque, New Mexico, USA.; NRRU, Northeastern Research Institute of Petrified Wood and Mineral Resources, Nakhon Ratchasima Rajabhat University, Thailand; OTM, Old Trail Museum, Chateau, Montana, USA; PIN, Palaeontological Institute, Moscow, Russia; ROM, Royal Ontario Museum, Toronto, Ontario, Canada; SMP, State Museum of Pennsylvania, Harrisburg, Pennsylvania, USA.; SMU, Shuler Museum of Paleontology, Southern Methodist University, Dallas, Texas, USA; TMM, Texas Memorial Museum, hosted at Jackson School of Geosciences Vertebrate Paleontology Laboratory, Austin, Texas, USA.; TMP, Royal Tyrrell Museum of Paleontology, Drumheller, Alberta, Canada; UALVP, University of Alberta Laboratory for Vertebrate Paleontology, Edmonton, Alberta, Canada;, University of Colorado Museum, Boulder, Colorado, USA; UCMP, University of California Museum of Paleontology, Berkeley, California, USA; USNM, United States National Museum (National Museum of Natural History), Washington DC, USA; YPM, Yale Peabody Museum of Paleontology, New Haven, Connecticut, USA; ZPAL, Instytut Paleobiologii, Polska Akademia Nauk, Warsaw, Poland. Specimen Jaw Fam Sub Genus Species Ogs Image source Page Image Ogs number side fam label reference MOR L Had Sau Acristavus gagslarsoni A A. Prieto- Freedman 1155 Márquez Fowler & Horner 2015. PIN R Non- Non- Altirhinus kurzanovi A Norman 1998 131 Figure 3386/7 had sid 16 Unk L Had Lam Amurosaurus riabinini SA A. Prieto- * Márquez AEHM 1- L Had Lam Amurosaurus riabinini A* A. Prieto- 12 Márquez MPZ L Had Lam Arenysaurus ardevoli A Blanco, 451 Figure Cruzado- 2008/258 Prieto- 3E Caballero Márquez & De et al. Esteban- 2014. Trivigno 2015 AMNH R Non- Non- Bactrosaurus johnsoni J A. Prieto- Prieto- 6581 had sid Márquez Márquez 2011. AMNH L Non- Non- Bactrosaurus johnsoni J A. Prieto- Prieto- 6580 had sid Márquez Márquez 2011. AMNH L Non- Non- Bactrosaurus johnsoni A A. Prieto- Prieto- 6553 had sid Márquez Márquez 2011. MPZ L Had Lam Blasisaurus canudoi J* A. Prieto- 99/665 Márquez FMNH L Had Sau Brachylophosau canadensis A A. Prieto- Prieto- 49

862 (PR rus Márquez Márquez 862) 2008. MOR L Had Sau Brachylophosau canadensis A* A. Prieto- 1071 rus Márquez CMN L Had Sau Brachylophosau canadensis A A. Prieto- Cuthberts 8893 rus Márquez on & Holmes 2010. MOR L Had Sau Brachylophosau canadensis A* A. Prieto- 1071-7- rus Márquez 13-99-93 MOR L Had Sau Brachylophosau canadensis SA A. Prieto- 1071-7- rus * Márquez 15-98-226 MOR R Had Sau Brachylophosau canadensis SA A. Prieto- 1071-8-1- rus * Márquez 99-313 MOR R Had Sau Brachylophosau canadensis A* A. Prieto- 1071-8- rus Márquez 15-98-574 GMH L Had Lam Charonosaurus jiayinensis A Godefroit, Zan 164 Plate 4, HljA40 & Jin 2001 Figure 1B ROM L Had Lam Corythosaurus casuarius A A. Prieto- Prieto- 1933 Márquez Márquez 2008. ROM 845 R Had Lam Corythosaurus intermedius A Dalla Vecchia 303 16.5H et al. 2014 CMN R Had Lam Corythosaurus sp. SA A. Prieto- 8676 * Márquez ROM L Had Lam Corythosaurus sp. SA A. Prieto- 1947 * Márquez USNM L Had Lam Corythosaurus sp. SA A. Prieto- 7948 * Márquez USNM L Had Lam Corythosaurus sp. A* A. Prieto- 11893 Márquez LACM L Had Lam Corythosaurus sp. A* A. Prieto- 53346 Márquez USNM R Had Lam Corythosaurus sp. J* A. Prieto- 16600 Márquez USNM R Had Lam Corythosaurus sp. SA A. Prieto- 16893 * Márquez TMP 82- L Had Lam Corythosaurus sp. SA A. Prieto- 37-01 * Márquez AMNH L Had Sau Edmontosaurus annectens A A. Prieto- 5879 Márquez LACM L Had Sau Edmontosaurus annectens A A. Prieto- Campione 23502 Márquez & Evans 2011. UCM R Had Sau Edmontosaurus annectens A* A. Prieto- 42764 Márquez CM 9970 R Had Sau Edmontosaurus annectens A A. Prieto- Márquez BMNH R Had Sau Edmontosaurus annectens A* A. Prieto- R4862 Márquez AMNH L Had Sau Edmontosaurus annectens S* A. Prieto- 5899 Márquez MOR 003 R Had Sau Edmontosaurus annectens A A. Prieto- Campione Márquez & Evans 2011. AMNH R Had Sau Edmontosaurus annectens A N. Campione 5730 CMN R Had Sau Edmontosaurus regalis A A. Prieto- Campione 2289 Márquez & Evans 50

2011. ROM 658 L Had Sau Edmontosaurus regalis SA A. Prieto- * Márquez CEUM R Non- Non- Eolambia caroljonesa J Garrison et al. 480 Figure 35714 had sid 2007 18 CEUM R Non- Non- Eolambia caroljonesa J Garrison et al. 480 Figure 35467 had sid 2007 18 CEUM R Non- Non- Eolambia caroljonesa A McDonald et 5 Figure 9758 had sid al. 2012 3B CEUM L Non- Non- Eolambia caroljonesa J McDonald et 6 Figure 34357 had sid al. 2012 4B MSC R Had Non- Eotrachodon orientalis SA Prieto- 35 Figure 7949 sid Márquez, 18E Erickson & Ebersole 2016 MDE- L Had Sid indet indet J* A. Prieto- Cas2-248 Márquez TMM R Had Sau Gryposaurus alsatei A Lehman, Wick 340 Figure 46033-1 & Wagner 8.3 2016 AMNH L Had Sau Gryposaurus latidens A Prieto- 521 Figure FARB Márquez 2012 8A 5465 TMP 80- R Had Sau Gryposaurus notabilis SA A. Prieto- 22-1 * Márquez MSNM R Had Sau Gryposaurus notabilis SA A. Prieto- V354 * Márquez AMNH R Had Sau Gryposaurus notabilis A Prieto- 850 Figure 5350 Márquez 2010 11 Unk L Had Sau Gryposaurus sp. SA A. Prieto- * Márquez TMP 92- R Had Unk indet indet J* A. Prieto- 36-724 Márquez TMP R Had Unk indet indet Unk A. Prieto- 1981-14- Márquez 32 BP-465 R Unk Unk indet indet J* A. Prieto- (MCD- Márquez 5007) CEUM R Unk Unk indet indet J McDonald et 7 Figure 34447 al. 2012 5B MOR 549 R Had Lam Hypacrosaurus stebingeri A A. Prieto- Evans Márquez 2010. TMP 88- L Had Lam Hypacrosaurus stebingeri SA A. Prieto- 151-88 * Márquez MSM R Non- Non- Jeyawati rugoculus SA A. Prieto- P4166 had sid * Márquez AMNH R Had Sau Kritosaurus navajovius A Prieto- 146 Figure 5799 Márquez 2014 14A UALVP R Had Lam indet indet J* A. Prieto- 11734 Márquez CMN 361 L Had Lam Lambeosaurus lambei A* A. Prieto- Márquez CMN 351 R Had Lam Lambeosaurus lambei A* A. Prieto- Márquez CMN R Had Lam Lambeosaurus lambei A Morris 1978 202 Plate 1, Prieto- 2869 Figure Márquez 5 2008. OTM R Had Sau Maiasaura peeblesorum SA A. Prieto- F138 * Márquez YPM-PU L Had Sau Maiasaura peeblesorum A* A. Prieto- 22405 Márquez AEHM L Had Lam Olorotitan arharensis A Godefroit, 539 Figure 2/845 Bolotsky & 10A1 51

Bolotsky 2012 SMP VP- R Had Lam Parasaurolophu sp. J Sullivan & 215 Figure 1090 s Bennett 2000 1B NMMNH L Had Lam Parasaurolophu tubicen SA A. Prieto- P-25100 s * Márquez MPC- L Non- Non- Plesiohadros djadokhtaensis A Tsogtbaatar et 123 7.14B D100/745 had sid al. 2014 PIN R Non- Non- Probactrosauru gobiensis SA Norman 2002 123 Figure 2232/42-1 had sid s * 12A MOR 447 L Had Sau Prosaurolophus maximus SA A. Prieto- * Márquez MOR 447 R Had Sau Prosaurolophus maximus SA A. Prieto- w-WSQ * Márquez 86 MOR L Had Sau Prosaurolophus maximus A* A. Prieto- 553S Márquez CMN R Had Sau Prosaurolophus maximus A A. Prieto- 8894 Márquez SMU L Non- Non- Protohadros byrdi SA A. Prieto- Head 74582 had sid Márquez 1998. GMH L Had Lam Sahaliyania elunchunorum SA Godefroit et 54 Figure W451 * al. 2008 6A1 GMH R Had Lam Sahaliyania elunchunorum SA Godefroit et 54 Figure W153 * al. 2008 6B1 ZPAL R Had Sau Saurolophus angustirostris SA A. Prieto- MgD-I * Márquez 162 LPM L Non- Non- Shuangmiaosau gilmorei A* You et al. 150 Figure 0166 had sid rus 2003 2B NRRU30 R Non- Non- Sirindhorna khoratensis A* Shibata et al. 13 Figure 01-167 had sid 2015 10B BMNH R Non- Non- Telmatosaurus transsylvanicu SA A. Prieto- R2967 had sid s * Márquez IVPP R Had Lam Tsintaosaurus spinorhinus A A. Prieto- V723 Márquez IVPP L Had Lam Tsintaosaurus spinorhinus A A. Prieto- Prieto- V725 Márquez Márquez 2008. UCMP R Had Sau Ugrunaaluk kuukpikensis J* A. Prieto- fld- Márquez UCMP L Had Sau Ugrunaaluk kuukpikensis J* A. Prieto- fld-- Márquez UCMP L Had Sau Ugrunaaluk kuukpikensis J* A. Prieto- fldAK-- Márquez UCMP R Had Sau Ugrunaaluk kuukpikensis J* A. Prieto- fldAK- Márquez 121-V-50 CPC-59 R Had Lam Velafrons coahuilensis SA A. Prieto- Gates et Márquez al. 2007. MPCA R Had Sau Willinakaqe salitralensis J A. Prieto- Cruzado- 774 Márquez Caballero (MPCA- & Coria Pv-SM4) 2016. MPCA L Had Sau Willinakaqe salitralensis J* A. Prieto- 775 Márquez (MPCA- Pv SM6) MPCA- R Had Sau Willinakaqe salitralensis J A. Prieto- Cruzado- Pv-SM9 Márquez Caballero & Coria 2016.

Dataset references Blanco, A., Prieto-Márquez, A. & De Esteban-Trivigno, S. (2015). Diversity of hadrosauroid dinosaurs from the Late Cretaceous Ibero-Armorican Island (European archipelago) assessed from dentary morphology. Cretaceous Research, vol. 56, pp. 447–457.

Campione, N. E. & Evans, D. C. (2011). Cranial growth and variation in Edmontosaurs (Dinosauria: Hadrosauridae): implications for latest Cretaceous megaherbivore diversity in North America.(Farke, A. A., Ed) PLoS ONE, vol. 6(9), e25186.

Cruzado-Caballero, P., Ruiz-Omeñaca, J. I., Gaete, R., Riera, V., Oms, O. & Canudo, J. I. (2014). A new hadrosaurid dentary from the latest Maastrichtian of the Pyrenees (north Spain) and the high diversity of the duck-billed dinosaurs of the Ibero-Armorican Realm at the very end of the Cretaceous. Historical Biology, vol. 26(5), pp. 619–630.

Cruzado-Caballero, P. & Coria, R. A. (2016). Revisiting the hadrosaurid (Dinosauria: Ornithopoda) diversity of the Allen Formation: a re-evaluation of Willinakaqe salitralensis from Salitral Moreno, Río Negro Province, Argentina. Ameghiniana, vol. 53(2), pp. 231–237.

Cuthbertson, R. S. & Holmes, R. B. (2010). The first complete description of the holotype of Brachylophosaurus canadensis Sternberg, 1953 (Dinosauria: Hadrosauridae) with comments on intraspecific variation. Zoological Journal of the Linnean Society, vol. 159(2), pp. 373–397.

Dalla Vecchia, F. M., Gaete, R., Riera, V., Oriol, O., Prieto-Márquez, A. & Garcia Sellés, A. (2014). The hadrosauroid record in the Maastrichtian of the eastern Tremp syncline (northern Spain). In: Eberth, D. A. & Evans, D. C. (Eds) Hadrosaurs.. Bloomington: Indiana University Press. (Life of the Past), pp. 298–314.

Garrison, J. R., Brinkman, D., Nichols, D. J., Layer, P., Burge, D. & Thayn, D. (2007). A multidisciplinary study of the Lower Cretaceous Cedar Mountain Formation, Mussentuchit Wash, Utah: a determination of the paleoenvironment and paleoecology of the Eolambia caroljonesa dinosaur quarry. Cretaceous Research, vol. 28(3), pp. 461–494.

Gates, T. A., Sampson, S. D., Jesús, C. R. D. de, Zanno, L. E., Eberth, D., Hernandez-Rivera, R., Martínez, M. C. A. & Kirkland, J. I. (2007). Velafrons coahuilensis, a new lambeosaurine hadrosaurid (Dinosauria: Ornithopoda) from the Late Campanian Cerro del Pueblo Formation, Coahuila, Mexico. Journal of Vertebrate Paleontology, vol. 27(4), pp. 917–930.

Godefroit, P., Zan, S. & Jin, L. (2001). The Maastrichtian (Late Cretaceous) lambeosaurine dinosaur Charonosaurus jiayinensis from north-eastern China. Bulletin de l’Institut Royal des Sciences Naturelles de Belgique Sciences de la Terre, vol. 71, pp. 119–168.

Godefroit, P., Shulin, H., Tingxiang, Y. & Lauters, P. (2008). New hadrosaurid dinosaurs from the uppermost Cretaceous of northeastern China. Acta Palaeontologica Polonica, vol. 53(1), pp. 47–74.

Head, J. J. (1998). A new species of basal hadrosaurid (Dinosauria, Ornithischia) from the Cenomanian of Texas. Journal of Vertebrate Paleontology, vol. 18(4), pp. 718–738.

McDonald, A. T., Bird, J., Kirkland, J. I. & Dodson, P. (2012). Osteology of the basal hadrosauroid Eolambia caroljonesa (Dinosauria: Ornithopoda) from the Cedar Mountain Formation of Utah. PLoS ONE, vol. 7(10), pp. 1–38.

Morris, W. J. (1978). Hypacrosaurus altispinus? Brown from the Two Medicine Formation, Montana a taxonomically indeterminate specimen. Journal of Paleontology, vol. 52(1), pp. 200–205.

Norman, D. B. (1998). On Asian ornithopods (Dinosauria: Ornithischia). 3. A new species of iguanodontid dinosaur. Zoological Journal of the Linnean Society, vol. 122, pp 291–348.

Norman, D. B. (2002). On Asian ornithopods (Dinosauria: Ornithischia). 4. Probactrosaurus Rozhdestvensky, 1966. Zoological Journal of the Linnean Society, vol. 136(1), pp. 113–144.

Prieto-Márquez, A. (2008). Phylogeny and historical biogeography of hadrosaurid dinosaurs. Diss. Florida: University of Florida. Available: http://linkinghub.elsevier.com/retrieve/pii/S0195667109000731. [8 July 2016].

Prieto-Márquez, A. (2010). The braincase and skull roof of Gryposaurus notabilis (Dinosauria, Hadrosauridae), with a taxonomic revision of the genus. Journal of Vertebrate Paleontology, vol. 30(3), pp. 838–854.

Prieto-Márquez, A. (2011). Cranial and appendicular ontogeny of Bactrosaurus johnsoni, a hadrosauroid dinosaur from the Late Cretaceous of northern China. Palaeontology, vol. 54(4), pp. 773–792.

Prieto-Márquez, A. (2012). The skull and appendicular skeleton of Gryposaurus latidens, a saurolophine hadrosaurid (Dinosauria: Ornithopoda) from the Early Campanian (Cretaceous) of Montana, USA. Canadian Journal of Earth Sciences, vol. 49(3), pp. 510–532.

Prieto-Márquez, A. (2014). Skeletal morphology of Kritosaurus navajovius (Dinosauria: Hadrosauridae) from the Late Cretaceous of the North American south-west, with an evaluation of the phylogenetic systematics and biogeography of Kritosaurini. Journal of Systematic Palaeontology, vol. 12(2), pp. 133–175.

Prieto-Márquez, A., Erickson, G. M. & Ebersole, J. A. (2016). Anatomy and osteohistology of the basal hadrosaurid dinosaur Eotrachodon from the uppermost Santonian (Cretaceous) of southern Appalachia. PeerJ, vol. 4, pp. 1–66.

Sullivan, R. M. & Bennett, III, G. E. (2000). A juvenile Parasaurolophus (Ornithischia: Hadrosauridae) from the Upper Cretaceous Fruitland Formation of New Mexico. Bulletin of the New Mexico Museum of Natural History and Science, vol. 17, pp. 215–220.

You, H., Ji, Q., Li, J. & Li, Y. (2003). A new hadrosauroid dinosaur from the mid-Cretaceous of Liaoning, China. Acta Geologica Sinica - English Edition, vol. 77(2), pp. 148–154.

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