The phylogenetic nomenclature of ornithischian dinosaurs

Ornithischians form a large clade of globally distributed Mesozoic dinosaurs, and represent one of their three major radiations. Throughout their evolutionary history, exceeding 134 million years, ornithischians evolved considerable morphological disparity, expressed especially through the cranial and osteodermal features of their most distinguishable representatives. The nearly two-century-long research history on ornithischians has resulted in the recognition of numerous diverse lineages, many of which have been named. Following the formative publications establishing the theoretical foundation of phylogenetic nomenclature throughout the 1980s and 1990s, many of the proposed names of ornithischian clades were provided with phylogenetic definitions. Some of these definitions have proven useful and have not been changed, beyond the way they were formulated, since their introduction. Some names, however, have multiple definitions, making their application ambiguous. Recent implementation of the International Code of Phylogenetic Nomenclature (ICPN, or PhyloCode) offers the opportunity to explore the utility of previously proposed definitions of established taxon names. Since the Articles of the ICPN are not to be applied retroactively, all phylogenetic definitions published prior to its implementation remain informal (and ineffective) in the light of the Code. Here, we revise the nomenclature of ornithischian dinosaur clades; we revisit 76 preexisting ornithischian clade names, review their recent and historical use, and formally establish their phylogenetic definitions. Additionally, we introduce five new clade names: two for robustly supported clades of later-diverging hadrosaurids and ceratopsians, one uniting heterodontosaurids and genasaurs, and two for clades of nodosaurids. Our study marks a key step towards a formal phylogenetic nomenclature of ornithischian dinosaurs.

INTRODUCTION (e.g., within the context of Art. 11.10 of the ICPN). With that respect, relevant comments are provided in the 'Comments' sub-section of the name entries. The five new clade names introduced in the present study are provided with their etymologies. Additionally, owing to the fact that the phylogenetic relationships of ornithischian dinosaurs are intensively researched, each clade name entry could be supplemented with numerous reference phylogenies. Rather than list all of the relevant phylogeny reconstructions available, we decided to refer to a subset of the more recent tree topologies that justify the 'conversion' of the taxon name in accordance with the ICPN.
We have not followed any strict approach while selecting primary reference phylogenies. Instead of providing references to studies that represent, for example, the most recent iterations of some datasets, we preferred to refer to studies that we have been either directly involved in, and are therefore familiar with the original data used for phylogeny inference, or consider to cover relevant data sampling.
With respect to the clade 'Composition', we list only those subtaxa that are included in the primary reference phylogeny. It is essential to realize that some of the clades for which names are provided have insufficiently explored origins and their basal branching is expressed through polytomies (this applies especially, but not exclusively, to nonhadrosaurid ornithopod subclades). In such cases, the actual extent may not be certain and some of the taxa listed in the 'Composition' subsection may in fact fall outside the clades. Note also that some of the selected primary reference phylogenies do not show the placements of all taxa used as specifiers in the definitions of the names to be defined. In such cases, the phylogenetic positions of these specifiers are discussed in the 'Comments'.
We also realize that the list of taxon names provided in 'Synonyms' is not exhaustive and does not list all historically used approximate synonyms. When discussing names that may be considered synonymous with those whose application is preferred here, we have focused especially on those names that have been used for the same or very similar contents in recent years, or those that have been used interchangeably with those that we define (e.g., Iguanodontidae and Iguanodontoidea, Thescelosauridae and Parksosauridae). Therefore, the names that have not been in use for a long time were mostly omitted.
Further, Article 8.1 of the ICPN states that, "(i)n order for a name to be established under [the ICPN], the name and other required information must be submitted to the registration database for phylogenetically defined names (see Art. 22.2). A name may be submitted to the database prior to acceptance for publication, but it is given only a temporary registration number at that time. The registration number will become permanent after the author notifies the database that the paper or book in which the name will appear has been published, provides a full reference to the publication, and confirms that the definition in the database is identical to that in the publication". We have therefore registered all names, whose phylogenetic definitions are established in the present study, to the database of phylogenetically defined names, the RegNum (ICPN: Art. 22; Appendix A), and obtained registration numbers that are included in the clade name entries.
Finally, we follow the ICPN in that all scientific names are italicized (ICPN: Recommendation 6.1A.) and that names are attributed to the earliest author(s) to spell them rather than according to the Principle of Coordination (ICPN: Note 9.15A.3).

Phylogenetic definitions
The names of ornithischian clades are defined using the following two types of definitions: (a) minimum-clade definition, known previously as 'node-based' definition (ICPN: Art. 9.5) and (b) maximum-clade definition, known previously as 'branch-based' or 'stembased' definition (ICPN: Art. 9.6). We refer to the aforementioned Articles of the ICPN for details.
Adopted conventions for abbreviated definitions. We abbreviate the definitions using the following conventions (as per Notes 9.4.1 and 11.12.1 of the ICPN): max = the largest; min = the smallest; ∇ = clade; () = containing; & = and; ∨ = or;~= but not (in trivial maximum-clade definitions) or it does not (while using a qualifying clause); | = on the condition that. See also Note 9.6.2 of the ICPN for explanation of differences between the use of '&' and '∨' in the definitions. Additionally, we apply the set theory symbols ∈, that means "belongs to", and ∉, meaning "not element of", to indicate that a name is applied within or outside another clade, respectively (see Euornithopoda, Jeholosauridae, Orodrominae, and Polacanthinae for some examples).
Selection of specifiers. Specifiers are selected following Art. 11 of the ICPN. Numerous names pertaining to ornithischian clades have been informally defined in the past and these definitions can still be considered applicable. We have attempted to formalize most of these definitions, providing only the changes that were necessary to reflect all currently inferred phylogenies and to comply with the Articles of the ICPN. However, in some cases we have decided to replace certain specifiers with taxa that we consider to be more appropriate candidates. For example, we have replaced Parasaurolophus walkeri Parks, 1922 in some definitions with Iguanodon bernissartensis Boulenger in Beneden, 1881 (designated as the type species of Iguanodon Mantell, 1825 by the International Commission on Zoological Nomenclature (2000)), provided that this taxon has always been considered part of the clade (when selected as an internal specifier) or outside the clade (when selected as an external specifier) whose name is being defined. I. bernissartensis is known based on multiple complete or near-complete individuals of different ontogenetic stages and has been extensively researched (e.g., Norman, 1980;Verdú et al., 2017). It has also been frequently used as the specifier in previous, informal phylogenetic definitions, and was recently included as the internal specifier of Dinosauria . It is further essential to note that some taxa had to be used as internal specifiers despite their suggested dubious taxonomic status. For example, Ceratops montanus Marsh, 1888 is the name-bearer of Ceratopsia, Ceratopsoidea, Ceratopsidae, and Ceratopsinae (the last name is not converted to a clade name in the present study). At the same time, however, the taxon is generally considered to lack diagnostic features and is commonly treated as a nomen dubium (e.g., Dodson, Forster & Sampson, 2004;Mallon et al., 2016). Following Article 11.10 of the ICPN (which specifies that "(w)hen a clade name is converted from a preexisting name that is typified under a rank-based code or is a new or converted name derived from the stem of a typified name, the definition of the clade name must use the type species of that preexisting typified name or of the genus name from which it is derived (or the type specimen of that species) as an internal specifier."), Ceratops montanus must be the internal specifier (or among the internal specifiers) in the definitions of the names in question.

PHYLOGENETIC NOMENCLATURE OF ORNITHISCHIAN CLADES
For the sake of clarity, all clade names are provided in alphabetical order. The definitions are summarized in Table 1. The extent of all clade names is further depicted on Fig. 1 that shows the relationships of taxa included in the present study as specifiers (both, internal as well as external) and additionally on Figs. 2-4 that represent selected ornithischian-wide phylogenies published within recent years: Madzia, Boyd & Mazuch (2018: Fig. 4B), Dieudonné et al. (2020: Figs. 1 and 2), and Yang et al. (2020: Fig. 12).            Reference phylogeny. Figure 11 of Arbour & Currie (2016) is treated here as the primary reference phylogeny. Additional reference phylogenies include Figure 3 of Thompson et al. (2012), Figure 1 Fig. 4B). Note that Nanosaurus agilis has been analyzed by Madzia, Boyd & Mazuch (2018) as 'Othnielosaurus'. The name was changed here following Carpenter & Galton (2018). Additionally, the name Marasuchus lilloensis was placed in quotation marks to highlight that the taxon may not be distinct from  Synonyms. The name Ankylosauromorpha Carpenter, 2001 has been recently used under an alternative systematic scheme for the same branch as Ankylosauria, as defined herein (Norman, 2021;see 'Discussion'). No other taxon names are currently in use for the same or approximate clade. Comments. The name Ankylosauria has been (informally) defined before (Carpenter, 1997;Sereno, 1998;Sereno, 2005). These definitions were maximum-clade and used Ankylosaurus (Carpenter, 1997;Sereno, 1998) or Ankylosaurus magniventris (Sereno, 2005) as the internal specifier and Stegosaurus (Carpenter, 1997;Sereno, 1998) or Stegosaurus stenops (Sereno, 2005) as the external specifier. Since Ankylosauria has been 'traditionally' used in this sense (though, see also 'Discussion'), we formalize this definition. Note that Norman (2021)  Reference phylogeny. Figure 11 of Arbour & Currie (2016) is treated here as the primary reference phylogeny. Additional reference phylogenies include Figure 3 of Thompson et al. (2012), Figure 1  Composition. Under the primary reference phylogeny, Ankylosauridae comprises Ahshislepelta minor, Aletopelta coombsi, Cedarpelta bilbeyhallorum, Chuanqilong chaoyangensis, Gastonia burgei, Liaoningosaurus paradoxus, and members of the clades Shamosaurinae and Ankylosaurinae.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Ankylosauridae has been (informally) defined before by Sereno (1998Sereno ( , 2005 who applied a maximum-clade definition and used Ankylosaurus magniventris as the internal specifier and Panoplosaurus mirus as the external specifier. Considering that Ankylosauridae has been traditionally used as a sister taxon to Nodosauridae (see, e.g., Thompson et al., 2012 for details), we use a definition that incorporates Nodosaurus textilis as the external specifier. Note that N. textilis is not included in the primary reference phylogeny. Both, A. magniventris and N. textilis were analyzed by, and their relationship is indicated in, Rivera-Sylva et al. (2018a).
Synonyms. No other taxon names are currently in use for the same or approximate clade.

Reference phylogeny.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Synonyms. The name Maiasaurini Sereno, 2005 is an approximate synonym of Brachylophosaurini. To our knowledge, the name was used only in two recent papers ) that attributed the name to Horner (1992). However, this attribution was due to the adherence of the authors to the Principle of Coordination, as Horner (1992) used the name Maiasaurinae. Nevertheless, all recent phylogenetic studies consistently use Brachylophosaurini (e.g., Freedman Fowler & Horner, 2015;Cruzado-Caballero & Powell, 2017;Kobayashi et al., 2019;Zhang et al., 2019;Prieto-Márquez, Wagner & Lehman, 2020;Zhang et al., 2020;Kobayashi et al., 2021;McDonald et al., 2021). No other taxon names are currently in use for the same or approximate clade.
Comments. The name Brachylophosaurini has been (informally) defined before (Gates et al., 2011;Freedman Fowler & Horner, 2015). These definitions were maximum-clade and used Brachylophosaurus, Maiasaura, and Acristavus (Gates et al., 2011) or Brachylophosaurus, Probrachylophosaurus, Maiasaura, andAcristavus (Freedman Fowler &Horner, 2015) as the internal specifiers and Gryposaurus and Saurolophus as the external specifiers. The composition of Brachylophosaurini and the relationships of the clade to other hadrosaurids have been stable across studies since the introduction of the name. Therefore, using more than one internal specifier is unnecessary. We use a definition that ensures Brachylophosaurini does not cover taxa 'traditionally' comprised within Edmontosaurini, Kritosaurini, and Saurolophini. Composition. Under the primary reference phylogeny, Camptosauridae comprises Camptosaurus dispar and Cumnoria prestwichii. Under alternative hypotheses, however, Camptosauridae includes only a single unequivocal member, Camptosaurus dispar (e.g., Madzia, Jagt & Mulder, 2020: Fig. 12).
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Camptosauridae was first (informally) defined by Sereno (1998: 62) who used the maximum-clade definition and selected Camptosaurus as the internal specifier and Parasaurolophus as the external specifier. We prefer to use Iguanodon bernissartensis as the external specifier to maintain the 'node-branch triplet' ('node-stem triplet' of Sereno (1998: 52-54)) comprising Ankylopollexia, Camptosauridae, and Styracosterna (all formally defined in the present paper). The inclusion of a different external specifier does not change the extent of Camptosauridae under any of the published phylogeny inferences.
Synonyms. No other taxon names are currently in use for the same or approximate clade. Although Ceratops montanus may fall within the largest clade containing Centrosaurus apertus but not Chasmosaurus belli and Triceratops horridus as well, the name Ceratopsinae Abel, 1919 has not been associated with the same contents as Centrosaurinae in the past. Therefore, Ceratopsinae is not considered to be an approximate synonym of Centrosaurinae. In any case, C. montanus does not seem to be diagnostic beyond Ceratopsidae at present (Dodson, Forster & Sampson, 2004;Mallon et al., 2016). Therefore, its position within the clade is uncertain. Lucas et al. (2016: 202) have argued that Pachyrhinosaurinae von Huene, 1950 has priority over Centrosaurinae under the Article 61 of the ICZN (International Commission on Zoological Nomenclature, 1999). However, the name Pachyrhinosaurinae has not been used in the literature recently and even Lucas et al. (2016) used Centrosaurinae for the clade in question.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name was first (informally) defined by Ryan et al. (2017) who applied the maximum-clade definition and used Centrosaurus apertus as the internal specifier and Pachyrhinosaurus canadensis as the external specifier. We formalize this definition. Cerapoda Sereno, 1986 (converted clade name)
Reference phylogeny. Composition. Under the primary reference phylogeny, Cerapoda comprises members of the clades Ornithopoda and Marginocephalia.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Composition. Under the primary reference phylogeny, Ceratopsia comprises Psittacosaurus spp. and members of the clades Chaoyangsauridae and Neoceratopsia.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Ceratopsoidea has been (informally) defined before by Sereno (1998Sereno ( , 2005 who applied a maximum-clade definition and used Triceratops horridus as the internal specifier and Protoceratops andrewsi as the external specifier. We include an additional internal specifier, the mandatory Ceratops montanus. Chaoyangsauridae Zhao, Cheng & Xu, 1999 (converted clade name)
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Chaoyangsauridae has been (informally) defined before by Han et al. (2015) who applied a maximum-clade definition and used Chaoyangsaurus youngi as the internal specifier and Triceratops horridus and Psittacosaurus mongoliensis as the external specifiers. We formalize this definition.
Synonyms. The taxon Ceratops montanus may also fall within the largest clade containing Chasmosaurus belli and Triceratops horridus but not Centrosaurus apertus (see, e.g., Mallon et al., 2016). In such case, Ceratopsinae Abel, 1919 would be an approximate synonym. Though the name has been advocated to be the proper name for the clade (it has been (informally) defined by Sereno, 1998 andSereno, 2005), it was actually introduced 4 years later than Chasmosaurinae. Note that the Principle of Coordination, which would make Ceratopsinae attributable to Marsh (1888), rather than to Abel (1919), does not apply under the ICPN (see Note 9.15A.3). Therefore, Ceratopsinae would not have priority over Chasmosaurinae under the ICPN. Anyway, C. montanus does not seem to be diagnostic beyond Ceratopsidae at present (Mallon et al., 2016), and its position within the clade is thus uncertain.
Comments. The name Chasmosaurinae has been (informally) defined before by Dodson, Forster & Sampson (2004) who applied a maximum-clade definition and used Triceratops as the internal specifier and Centrosaurus as the external specifier. We apply the name Chasmosaurinae for the same known contents; adopting Triceratops horridus and the mandatory Chasmosaurus belli as the internal specifiers and Centrosaurus apertus as the external specifier. Composition. Under the primary reference phylogeny, Clypeodonta comprises a clade formed by Hypsilophodon foxii, Rhabdodontidae, and Tenontosaurus spp., and a clade uniting Dryosauridae and Ankylopollexia (termed Iguanodontia in Norman, 2015). However, see 'Comments' below for discussion of potential alternative composition of Clypeodonta.
Synonyms. No other taxon names are currently in use for the same or approximate clade. Iguanodontia, as reconstructed, for example, by Madzia, Jagt & Mulder (2020) covers a similar taxic composition; though the topology of Madzia, Jagt & Mulder (2020) differs from that of the primary reference phylogeny of Clypeodonta significantly.
Comments. The name Clypeodonta was claimed as being new in two different studies (Norman, 2014: 29;Norman, 2015: 102), although Norman (2015: 170) also cites Norman (2014) as the establishing reference. The use of the name Clypeodonta differed across studies. Originally, Norman (2014Norman ( , 2015 intended to use it for a subclade of Ornithopoda that (approximately) comprises Hypsilophodon foxii and its relatives, and ornithopods later-diverging than H. foxii, and (informally) defined the name as pertaining to either, the branch of "Parasaurolophus walkeri and all taxa more closely related to P. walkeri than to Thescelosaurus neglectus" (Norman, 2014: 29) or the node of "Hypsilophodon foxii, Edmontosaurus regalis, their most recent common ancestor, and all of its descendants" (Norman, 2015: 170). In both these studies, Clypeodonta is said (Norman, 2014: 29) or figured (Norman, 2015: Fig. 50) to cover the same known contents although neither of the studies included taxa in their analyses that would fall outside the clade (except for Lesothosaurus diagnosticus). Madzia, Boyd & Mazuch (2018) followed the definition of Norman (2015). In their phylogenetic analysis, however, the name covers a much broader contents as one of the internal specifiers of Clypeodonta, Hypsilophodon foxii, is reconstructed outside Cerapoda in that study (Madzia, Boyd & Mazuch, 2018: Fig. 4). Still, Madzia, Boyd & Mazuch (2018: Appendix 1) stated that as Clypeodonta was a relatively new name with no 'traditional' meaning, they saw no reason for its redefinition. They also noted, though, that "given the unstable position of H. foxii among neornithischians, the name might have only limited utility" (Madzia, Boyd & Mazuch, 2018: Appendix 1).
Here we define the name Clypeodonta using the minimum-clade definition of Norman (2015). However, by including the part "within Ornithopoda" in the definition, we restrict the use of Clypeodonta only when H. foxii represents an ornithopod (see Article 11.14 of the ICPN), following the original intent of Norman (2014Norman ( , 2015. Composition. Under the primary reference phylogeny, Coronosauria comprises members of the clades Protoceratopsidae and Ceratopsoidea.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Coronosauria has been (informally) defined before by Sereno (1998Sereno ( , 2005 who applied the minimum-clade definition and used Triceratops horridus and Protoceratops andrewsi as the internal specifiers. We formalize this definition.
Reference phylogeny. Figure 18 of Prieto-Márquez, Wagner & Lehman (2020) is treated here as the primary reference phylogeny. Additional reference phylogenies include Figure  Composition. Under the primary reference phylogeny, Corythosauria comprises members of the clades Lambeosaurini and Parasaurolophini.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Corythosauria is established for the well-supported node uniting Lambeosaurini and Parasaurolophini, two lambeosaurine clades characterized by their distinctive, 'crested' crania. Composition. Under the primary reference phylogeny, Dryomorpha comprises members of the clades Dryosauridae and Ankylopollexia.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Dryomorpha was first (informally) defined by Sereno (2005) who attributed the name to "(t)he most inclusive clade containing Dryosaurus altus (Marsh, 1878) and Parasaurolophus walkeri Parks, 1922". However, due to the use of 'most', rather than 'least', such definition makes the name inapplicable within Ornithischia. Boyd (2015) later corrected the wording and proposed a minimum-clade definition using the same taxa as the internal specifiers. Here we use the same type of definition but replace P. walkeri with I. bernissartensis. This taxon has always been considered a part of Dryomorpha.
Reference phylogeny. Figure 12   Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. Dryosauridae was first (informally) defined by Sereno (1998: 61) who used the maximum-clade definition and Dryosaurus altus as the internal specifier and Parasaurolophus walkeri as the external specifier. Here we use the same type of definition but replace P. walkeri with I. bernissartensis. This taxon has always been considered outside Dryosauridae.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Edmontosaurini has been (informally) defined before (Sereno, 2005;Xing et al., 2014). Sereno (2005) applied the maximum-clade definition and used Edmontosaurus regalis as the internal specifier and Maiasaura peeblesorum and Saurolophus osborni as the external specifiers. In turn, Xing et al. (2014) applied a minimum-clade definition, with Edmontosaurus and Kerberosaurus as the internal specifiers. We formalize a maximum-clade definition similar to that of Sereno (2005) Gilmore, 1913).
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Elasmaria has been (informally) defined before (Calvo, Porfiri & Novas, 2007;Herne et al., 2019). The definition proposed by Calvo, Porfiri & Novas (2007) was minimum-clade, while the definition of Herne et al. (2019) was maximum-clade. However, both studies used Talenkauen santacrucensis and Macrogryphosaurus gondwanicus as the internal specifiers. Herne et al. (2019) proposed to add Iguanodon bernissartensis and Hypsilophodon foxii as the external specifiers to maintain the use of the name Elasmaria to the 'traditional' contents under a hypothesis in which one of the internal specifiers was reconstructed, for example, closer to iguanodontians. We keep the use of a minimum-clade definition (as first proposed for the name). However, even though all phylogenetic analyses consistently reconstruct close relationships between T. santacrucensis and M. gondwanicus, we follow Herne et al. (2019) in that the unsettled placement of elasmarians on the neornithischian phylogenetic tree warrants addition of external specifiers. We include Iguanodon bernissartensis and Hypsilophodon foxii as the external specifiers (following Herne et al., 2019) and further add a third external specifier, Thescelosaurus neglectus, to reflect that elasmarians were already inferred as a clade within Thescelosaurinae, as the sister taxon to Thescelosaurus spp. (Boyd, 2015 Sternberg, 1950).
Reference phylogeny. Composition. Under the primary reference phylogeny, Eucentrosaura comprises members of the clades Centrosaurini and Pachyrhinosaurini.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name was first (informally) defined by Chiba et al. (2018) who applied the minimum-clade definition and used Centrosaurus apertus and Pachyrhinosaurus canadensis as the internal specifiers. We formalize this definition.

Euceratopsia (new clade name)
Registration number: 610 Definition. The smallest clade containing Leptoceratops gracilis Brown, 1914b, Protoceratops andrewsi Granger & Gregory, 1923, and Triceratops horridus Marsh, 1889. This is a minimum-clade definition. Abbreviated definition: min ∇ (Leptoceratops gracilis Brown, 1914b& Protoceratops andrewsi Granger & Gregory, 1923& Triceratops horridus Marsh, 1889. Etymology. Derived from the Greek eu-(true) and formed to show its association to members of Ceratopsia. Note that Euceratopsia does not derive from the name Ceratops Marsh, 1888, and, as such, the taxon does not have to be the internal specifier in the used definition. Composition. Under the primary reference phylogeny, Euceratopsia comprises members of the clades Leptoceratopsidae and Coronosauria.
Synonyms. The name Coronosauria Sereno, 1986 covers the same contents under the topology of You & Dodson (2004). However, see 'Comments'. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Euceratopsia is established for the well-supported node uniting the three latest-diverging clades of ceratopsians -Leptoceratopsidae, Protoceratopsidae, and Ceratopsoidea. The monophyly of the grouping is supported by all recently published phylogenies that infer Euceratopsia to branch into two cladesleptoceratopsids and coronosaurs (protoceratopsids + ceratopsoids). Both these clades comprise representatives that are very close or survived to the Cretaceous/Paleogene mass extinction event (Fowler, 2017:   Composition. Under the primary reference phylogeny, Euhadrosauria comprises members of the clades Saurolophinae and Lambeosaurinae. Synonyms. The name Hadrosauridae Cope, 1869 is an approximate synonym of Euhadrosauria. If Hadrosaurus foulkii nests within the smallest clade containing Saurolophus osborni and Lambeosaurus lambei, and within the 'Saurolophus branch' of the clade (see the entry for the name Saurolophinae), the name Hadrosauridae is used for the node instead, and Euhadrosauria becomes inapplicable. Additionally, the name Saurolophidae has been used for the same contents as well (see 'Comments').
Comments. The history and application of Euhadrosauria is complicated and has been thoroughly described and discussed by Madzia, Jagt & Mulder (2020: 14-16). We therefore refer to that study for details.
Reference phylogeny. Figure 13 of Coria & Salgado (1996) is treated here as the primary reference phylogeny.
Composition. Under the primary reference phylogeny, Euiguanodontia comprises Gasparinisaura and members of the clades Dryosauridae and Ankylopollexia.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Euiguanodontia is applicable only on the condition that G. cincosaltensis, D. altus, and C. dispar form a clade exclusive of T. tilletti, as originally used by Coria & Salgado (1996). We follow the definition advocated by Madzia, Boyd & Mazuch (2018: Appendix 1) and refer to that study for additional comments. Note also that Euiguanodontia must be a subclade of Iguanodontia under the proposed definition because T. tilletti is an internal specifier in the definition of the name. Finally, note that the internal specifiers Dryosaurus altus and Camptosaurus dispar are not included in the primary reference phylogeny. The former belongs to Dryosauridae (e.g., Madzia, Boyd & Mazuch, 2018), while the latter is part of Ankylopollexia (see, e.g., Madzia, Jagt & Mulder, 2020). Both these clades are indicated on Figure 13 of Coria & Salgado (1996).
Reference phylogeny. Figure 1 of Sereno (1999) is treated here as the primary reference phylogeny.
Composition. Under the primary reference phylogeny, Euornithopoda comprises Tenontosaurus spp. and members of the clades Ankylopollexia, Dryosauridae, and Hypsilophodontidae.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Euornithopoda has been (informally) defined before (Sereno, 1998;Sereno, 2005). These definitions were maximum-clade and used Parasaurolophus as the internal specifier and Heterodontosaurus tucki, Pachycephalosaurus wyomingensis, Triceratops horridus, and Ankylosaurus magniventris (Sereno, 2005) as the external specifiers. Here we define the name Euornithopoda using a similar maximum-clade definition as that of Sereno (1998) but replace Parasaurolophus with Iguanodon bernissartensis. Also, by including the part "within Ornithopoda" in the definition, we restrict the use of Euornithopoda to the branch only when Heterodontosaurus tucki represents an ornithopod (see Article 11.14 of the ICPN), thus maintaining the 'traditional' use (Sereno, 1998;Sereno, 2005  Composition. Under the primary reference phylogeny, Eurypoda comprises members of the clades Ankylosauria and Stegosauria.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Eurypoda has been (informally) defined before by Sereno (1998) who used Ankylosaurus and Stegosaurus as the internal specifiers. Since Eurypoda has never been proposed an alternative use, we formalize this definition. Note that the internal specifier Stegosaurus stenops is not included in the primary reference phylogeny. The taxon is most closely related to the clade comprising the operational taxonomic units (OTUs) Stegosaurus armatus (nomen dubium according to Galton, 2010; S. armatus has long been the type species of Stegosaurus but was replaced by S. stenops as the type through an ICZN ruling (International Commission on Zoological Nomenclature, 2013)) and Huayangosaurus taibaii (see, e.g., Maidment et al., 2020).
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Reference phylogeny. Figure 18 of Prieto-Márquez, Wagner & Lehman (2020) is treated here as the primary reference phylogeny. Additional reference phylogenies include  Reference phylogeny. Figure 12  Composition. Under the primary reference phylogeny, Hadrosauriformes comprises members of the clades Iguanodontidae and Hadrosauroidea.
Synonyms. If Hypselospinus fittoni nests within the smallest clade containing Hadrosaurus foulkii and Iguanodon bernissartensis, the name Hadrosauriformes is a potential heterodefinitional synonym of Neoiguanodontia (see the name entry). In such case, the name Hadrosauriformes should have priority. The name Iguanodontoidea Hay, 1902 has been also used as an approximate synonym (Sereno, 1986;Norman, 2002). Note that Norman (2002) used Iguanodontoidea for a clade "(s)erially more derived than Camptosaurus" (Norman, 2002: 138) and defined it as "Iguanodon and all iguanodontians more closely related to Edmontosaurus than to Camptosaurus". Such definition would make Iguanodontoidea applicable for the same clade as Styracosterna (see the name entry). However, Figure 35 of Norman (2002) shows that the name does not cover Lurdusaurus, which should be included within the clade under such maximum-clade definition. Since Norman (2002) considers Iguanodontoidea to be a synonym of Hadrosauriformes of Sereno (1997Sereno ( , 1998Sereno ( , 1999, it is apparent that Norman (2002) concept of Iguanodontoidea would be more similar to that of Hadrosauriformes rather than Styracosterna.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Huayangosauridae was first (informally) defined by Galton & Upchurch (2004: 358)  Reference phylogeny. Figure 50 of Norman (2015) is treated here as the primary reference phylogeny.
Composition. Under the primary reference phylogeny, Hypsilophodontia comprises Hypsilophodon foxii, Tenontosaurus spp., and members of the clade Rhabdodontidae. However, see 'Comments' below for discussion of potential alternative composition of Clypeodonta.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Here we define the name Hypsilophodontia using a similar minimum-clade definition as that of Norman (2015) but by including the part "within Ornithopoda" in the definition, and adding an external specifier, we restrict the use of Hypsilophodontia to the node only when H. foxii represents an ornithopod (see Article 11.14 of the ICPN) and when Hypsilophodon foxii and Tenontosaurus tilletti are more closely related to each other than either is to I. bernissartensis, following the original intent of Norman (2015). Note that the internal specifier Tenontosaurus tilletti is not indicated in the primary reference phylogeny. The taxon is the type species of Tenontosaurus Ostrom, 1970 and is comprised there within the 'tenontosaurs'. Reference phylogeny. Figure 2 of Dieudonné et al. (2020) is treated here as the primary reference phylogeny.
Synonyms. The name Parksosaurinae has been recently for the same contents (Yang et al., 2020), and attributed (apparently following the Principle of Coordination) to Buchholz (2002). No other taxon names are currently in use for the same or approximate clade.
Comments. Hypsilophodontidae was first (informally) defined by Sereno (1998: 61) who used the maximum-clade definition and Hypsilophodon foxii as the internal specifier and Parasaurolophus walkeri as the external specifier. Here we use the same type of definition but replace P. walkeri with I. bernissartensis. This taxon has always been considered outside Hypsilophodontidae. Additionally, we include Rhabdodon priscus as a second external specifier to prevent the inclusion of Rhabdodontidae within Hypsilophodontidae under the topology of Norman (2015: Fig. 50 Reference phylogeny. Figure 12  Composition. Under the primary reference phylogeny, Iguanodontia comprises members of the clade Rhabdodontomorpha, Tenontosaurus spp., and Dryomorpha. Synonyms. No other taxon names are currently in use for the same or approximate clade. Clypeodonta, as reconstructed by Norman (2015) covers a similar taxic composition; though the topology of Norman (2015) differs from that of the primary phylogeny of Iguanodontia significantly.
Comments. The application of Iguanodontia has been described and discussed by Madzia, Boyd & Mazuch (2018: Appendix 1) and Madzia, Jagt & Mulder (2020: Table 1). We therefore refer to these studies for details. Our definition differs from that advocated by Madzia, Boyd & Mazuch (2018) and Madzia, Jagt & Mulder (2020) in that the name is newly applicable only if it is used for a clade that does not include Hypsilophodon foxii (e.g., it becomes inapplicable under the topology of Norman, 2015: Fig. 50). Reference phylogeny. Figure 13  Composition. Under the primary reference phylogeny, Iguanodontidae comprises Barilium dawsoni, Iguanodon bernissartensis, Iguanodon galvensis, and Lurdusaurus arenatus.

Iguanodontidae Bonaparte, 1850 (converted clade name)
Synonyms. The name Iguanodontoidea Hay, 1902 is an approximate synonym of Iguanodontidae (see, e.g., Figure 20 of Verdú et al., 2018). Both these names have been used for various sets of taxa thought or reconstructed to be more closely related to Iguanodon bernissartensis than to hadrosaurids. Considering that significant differences exist between phylogeny reconstructions of Iguanodon-grade ornithopods (e.g., Madzia, Boyd & Mazuch, 2018;Verdú et al., 2018;Madzia, Jagt & Mulder, 2020;McDonald et al., 2021), it is difficult to link either of the names to a certain, stable composition. Here, we prefer to apply the name Iguanodontidae because it is more frequent in the literature and because it was coined 52 years before Iguanodontoidea. It is worth noting that the name Iguanodontoidea has been also used as an approximate synonym of Hadrosauriformes (see the name entry).
Comments. The name Iguanodontidae was first (informally) defined before (Sereno, 1998;Sereno, 2005;Santos-Cubedo et al., 2021). These definitions were maximum-clade and used Iguanodon bernissartensis as the internal specifier and Parasaurolophus walkeri (Sereno, 1998;Sereno, 2005)   , the work does not meet the general requirements for establishing Iguanodontidae as a phylogenetically defined clade name (see Articles 7 of the ICPN), nor it provides anything that would indicate such intention. Specifically, the name Iguanodontidae is not explicitly designated as a converted clade name, no bibliographic citations demonstrating prior application of the name to a taxon approximating the clade for which it is being established have been provided (including the authorship of the preexisting name), and no evidence is provided that the required information has been submitted to the registration database for phylogenetically defined names, the RegNum (registration number is missing). The study specifies the phylogenetic information, such as the placement of the clade on the ornithopod tree and the distribution of apomorphies supporting the existence of the clade, and presents the hypothesized composition of the clade. This information alone, however, would not be sufficient for the name Iguanodontidae to be established as a converted clade name, as required by the ICPN. Reference phylogeny. Synonyms. The name Jeholosaurinae has been used recently for the same contents (Yang et al., 2020), and attributed (apparently following the Principle of Coordination) to Han et al. (2012). No other taxon names are currently in use for the same or approximate clade.
Comments. We use a maximum-clade definition similar to that of Han et al. (2012), which is the only definition (informally) used for Jeholosauridae. Our definition differs in that we replaced the original representative of Ceratopsia (Protoceratops andrewsi) with a taxon that is widely used in phylogenetic definitions of ornithischian clade names (Triceratops horridus). Additionally, our definition prevents the use of Jeholosauridae under the potential hypotheses in which Jeholosaurus is inferred as part of Hypsilophodontidae or Thescelosauridae. Note that the internal specifiers Pachycephalosaurus wyomingensis and Triceratops horridus are not included in the primary reference phylogeny. The former belongs to Pachycephalosauria (see, e.g., Dieudonné et al., 2020), while the latter is part of Ceratopsia (e.g., Morschhauser et al., 2019), both within Marginocephalia that is indicated on Figure 25 Brown, 1912).
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
(2019) preferred a maximum-clade definition with T. horridus and P. wyomingensis as the internal specifiers and Parasaurolophus walkeri as the external specifier, arguing that "(previous) definitions (were) not complementary with present definitions of Cerapoda and Ornithopoda within a node-stem triplet arrangement of clades" and that "re-definition of Marginocephalia as a stem now mirrors its sister stem clade, Ornithopoda, within a node-based Cerapoda. As a result, this stabilization of definition allows for the definitive assignment of all cerapodan OTUs either as ornithopods or marginocephalians" (Herne et al., 2019: Supplemental Text S1: 4). However, Marginocephalia has never formed such 'triplet'. When its use in a 'node-branch triplet' is considered, it is more closely tied with Ceratopsia and Pachycephalosauria rather than with Cerapoda and Ornithopoda.
Here, the internal specifiers in the definition of Marginocephalia are used from among the taxa representing the two major subclades -Ceratopsia (Ceratops montanus and Triceratops horridus) and Pachycephalosauria (Pachycephalosaurus wyomingensis).
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name was first (informally) defined by Ryan et al. (2017) who applied the maximum-clade definition and used Nasutoceratops titusi as the internal specifier and Centrosaurus apertus as the external specifier. We formalize this definition.
Reference phylogeny. Figure 10 of Morschhauser et al. (2019) is treated here as the primary reference phylogeny. Additional reference phylogenies include Figure 16 of Han et al. (2018), Figure S1 of Knapp et al. (2018), and Figure 4 of Yu et al. (2020).
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Neoceratopsia has been (informally) defined before by Sereno (1998Sereno ( , 2005 who applied a maximum-clade definition and used Triceratops horridus as the internal specifier and Psittacosaurus mongoliensis as the external specifier. We further include a second external specifier, Chaoyangsaurus youngi, to ensure that Chaoyangsauridae, a clade usually reconstructed as some of the earliest-diverging ceratopsians (e.g., Han et al., 2018;Knapp et al., 2018;Yu et al., 2020), are maintained outside Neoceratopsia.  Parks, 1922).
Reference phylogeny. Composition. Under the primary reference phylogeny, Neoiguanodontia comprises Hypselospinus fittoni and members of the clade Hadrosauriformes.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Nodosauridae has been (informally) defined before by Sereno (1998Sereno ( , 2005 who used Panoplosaurus mirus (Sereno, 1998) or Panoplosaurus mirus and Nodosaurus textilis Sereno (2005) as the internal specifiers and Ankylosaurus magniventris as the external specifier. Considering that all phylogeny reconstructions that include P. mirus and N. textilis indicate that these taxa are more closely related to each other than either is to A. magniventris (or placed outside the Ankylosauridae + Nodosauridae node), we use a definition that incorporates Nodosaurus textilis as the sole internal specifier. Reference phylogeny. Composition. Under the primary reference phylogeny, Nodosaurinae comprises Acantholipan gonzalezi, Ahshislepelta minor, Niobrarasaurus coleii, Nodosaurus textilis, Peloroplites cedrimontanus, Sauropelta edwardsi, Silvisaurus condrayi, Taohelong jinchengensis, Tatankacephalus cooneyorum, members of the clades Panoplosaurini and Struthiosaurini, and the specimen CPC 273.

Nodosaurinae Abel, 1919 (converted clade name)
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Synonyms. No other taxon names are currently in use for the same or approximate clade. Though not in use, the name Predentata Marsh, 1894 is being occasionally recalled as an approximate synonym (e.g., Butler, Upchurch & Norman, 2008;Langer & Ferigolo, 2013).
Comments. The name Ornithischia has been (informally) defined before (Padian & May, 1993;Sereno, 1998;Weishampel, 2004;Norman, Witmer & Weishampel, 2004;Sereno, 2005;Baron, Norman & Barrett, 2017a). These definitions were maximum-clade and used Triceratops horridus (Padian & May, 1993;Sereno, 1998;Weishampel, 2004;Sereno, 2005;Baron, Norman & Barrett, 2017a) or Iguanodon bernissartensis (Norman, Witmer & Weishampel, 2004) as the internal specifiers. In turn, "birds" (Padian & May, 1993), Neornithes (Sereno, 1998), Tyrannosaurus (Weishampel, 2004), Cetiosaurus (Norman, Witmer & Weishampel, 2004), Passer domesticus and Saltasaurus loricatus (Sereno, 2005), and Passer domesticus and Diplodocus carnegii (Baron, Norman & Barrett, 2017a) were used as the external specifiers. Although both, I. bernissartensis and T. horridus, are clearly 'traditional' members of Ornithischia, we have selected the former as the internal specifier and Allosaurus fragilis and Camarasaurus supremus as the external specifiers. These specifiers are preferred because (a) they represent deeply nested taxa within their respective clades (Ornithischia, Theropoda, and Sauropodomorpha), (b) they have been historically associated with these clades, thus being their 'traditional' members, and (c) their phylogenetic placements are stable across studies. Two external specifiers, instead of one, are used due to the alternative topologies of dinosaur relationships (see, e.g., Baron, Norman & Barrett, 2017a;Langer et al., 2017). Additionally, Iguanodon bernissartensis was used as the internal specifier in the formal definition of Dinosauria , considered therein as a 'traditional' representative of Ornithischia, and the external specifier in the formal definition of Sauropodomorpha , again considered therein as a 'traditional' representative of Ornithischia; A. fragilis was used as the internal specifier in the formal definitions of Theropoda (Naish et al., 2020) and Saurischia , and as the external specifier in the formal definition of Sauropodomorpha , considered in these contributions as a 'traditional' representative of Theropoda; and Camarasaurus supremus was used as the internal specifier in the formal definition of Saurischia  and considered therein as a 'traditional' representative of Sauropodomorpha.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Ornithopoda has been (informally) defined before (Sereno, 1998;Sereno, 2005;Butler, Upchurch & Norman, 2008;Herne et al., 2019). Two of these definitions were minimum-clade (Sereno, 1998;Sereno, 2005) and used Parasaurolophus walkeri and Heterodontosaurus tucki as the internal specifiers. Sereno (2005) further restricted the name to a hypothesis in which P. walkeri and H. tucki were more closely related to each other than either was to Pachycephalosaurus wyomingensis, Triceratops horridus, and Ankylosaurus magniventris. We selected a definition that follows Herne et al. (2019) in that it includes two external specifiers (T. horridus and P. wyomingensis, representatives of two clades closely related to ornithopods; i.e., Ceratopsia and Pachycephalosauria, respectively). However, we prefer to use Iguanodon bernissartensis as the internal specifier rather than P. walkeri, because the former is among the few taxa that have been considered a part of Ornithopoda when the name was being introduced in the literature (e.g., Marsh, 1882). The inclusion of a different internal specifier does not change the extent of Ornithopoda under any of the published phylogeny inferences. Note that the external specifiers Pachycephalosaurus wyomingensis and Triceratops horridus are not included in the primary reference phylogeny. The former belongs to Pachycephalosauria (see, e.g., Dieudonné et al., 2020), while the latter is part of Ceratopsia (e.g., Morschhauser et al., 2019), both within Marginocephalia that is indicated on Figure 4 of Madzia, Boyd & Mazuch (2018 Gilmore, 1913).
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Orodrominae has been (informally) defined before Boyd, 2015). Both these definitions were maximum-clade and used Orodromeus makelai as the internal specifier and Thescelosaurus neglectus  or Thescelosaurus neglectus and Parasaurolophus walkeri (Boyd, 2015) as the external specifiers. Considering the 'traditional concept' of Orodrominae, as a subclade of Thescelosauridae/'hypsilophodonts', and keeping in mind the unstable phylogenetic position of H. foxii (e.g., Madzia, Boyd & Mazuch, 2018), we apply Orodrominae only when it is inferred either within Thescelosauridae or Hypsilophodontidae (see Article 11.14 of the ICPN).
Reference phylogeny. Figure 27 of Schott & Evans (2017) is treated here as the primary reference phylogeny. Additional reference phylogenies include Figure 5 of Evans et al. (2013), Figure 16 of Han et al. (2018), and Figure 1 of Dieudonné et al. (2020).
Composition. Under the primary reference phylogeny, Pachycephalosauria comprises Wannanosaurus yanshiensis and members of the clade Pachycephalosauridae.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Pachycephalosauridae has been (informally) defined before by Sereno (1998Sereno ( , 2005 who applied the minimum-clade definition and used Pachycephalosaurus wyomingensis and Stegoceras validum as the internal specifiers. This definition is followed here though we also include a qualifying clause that excludes H. tucki from Pachycephalosauridae. Even though no phylogenetic analysis has ever reconstructed H. tucki or any other 'traditional' heterodontosaurid to be within the smallest clade containing P. wyomingensis and S. validum, Heterodontosauridae were inferred to be early-diverging pachycephalosaurs . The addition of a qualifying clause that excludes H. tucki from Pachycephalosauridae will therefore ensure that the name will never comprise Heterodontosauridae. Note that H. tucki is not included in the primary reference phylogeny. See Figure 1 of Dieudonné et al. (2020) for its potential placement with respect to pachycephalosaurids.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Pachycephalosaurini has been (informally) defined before by Sereno (2005) who applied a minimum-clade definition and used Pachycephalosaurus wyomingensis and Stygimoloch spinifer as the internal specifiers. Even though such definition is congruent with the original intent of Sullivan (2003) who established the taxon name to unite S. spinifer and P. wyomingensis, it has since been hypothesized that S. spinifer and P. wyomingensis may represent different growth stages of a single taxon (P. wyomingensis) rather than two distinct taxa (Horner & Goodwin, 2009). Nevertheless, the name Pachycephalosaurini may still be considered useful as recent studies indicate close relationships between P. wyomingensis and Alaskacephale gangloffi (Longrich, Sankey & Tanke, 2010;Evans et al., 2013;Williamson & Brusatte, 2016;Schott & Evans, 2017;Woodruff et al., 2021). Owing to the unsettled phylogenetic ties between the latest Cretaceous pachycephalosaurs, we prefer to establish a maximum-clade definition for Pachycephalosaurini to enable the name to be used for a wider range of late-diverging members of Pachycephalosauridae.
Reference phylogeny. Composition. Under the primary reference phylogeny, Pachyrhinosaurini comprises Einiosaurus procurvicornis and members of the clade Pachyrostra.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name was first (informally) defined by Fiorillo & Tykoski (2012) who applied the maximum-clade definition and used Pachyrhinosaurus canadensis as the internal specifier and Centrosaurus apertus as the external specifier. We formalize this definition.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name was first (informally) defined by Fiorillo & Tykoski (2012) who applied the minimum-clade definition and used Achelousaurus horneri and Pachyrhinosaurus canadensis as the internal specifiers. We formalize this definition. Etymology. Derived from the stem of Panoplosaurus Lambe, 1919, the name of an included taxon, which combines the Greek words pan (all), hoplon (type of shield), and sauros (lizard, reptile).
Synonyms. The name Panoplosaurinae Nopcsa, 1929 has been recently suggested for the same clade (e.g., Rivera-Sylva et al., 2018a; see also 'Comments' below). Additionally, Bakker (1988) coined the name Edmontoniinae for Edmontonia rugosidens, Edmontonia longiceps, and Denversaurus schlessmani and Edmontoniidae to include Edmontoniinae and Panoplosaurinae; no phylogenetic definition was proposed for either and neither clade name has been widely used since.
Comments. The grouping, here covered under the name Panoplosaurini, has previously been suggested to be named Panoplosaurinae (Rivera-Sylva et al., 2018a).
No (informal) phylogenetic definition for Panoplosaurinae has ever been published in the peer-reviewed literature, though Burns (2015) proposed "all Late Cretaceous nodosaurids more closely related to Panoplosaurus than to Pawpawsaurus" in his dissertation, and the name itself has not been widely used. Bakker (1988) provided a diagnosis of Panoplosaurinae, as nodosaurids with lumpy armor and expanded internarial bridges, which contained the two species of Panoplosaurus he recognized (Panoplosaurus mirus and Panoplosaurus sp. 1, represented by ROM 1215). Alpha taxonomic reviews of the Campanian-Maastrichtian North American nodosaurids generally recognize Panoplosaurus mirus, Edmontonia rugosidens, and Edmontonia longiceps as valid taxa (e.g., Carpenter, 2001) and these typically form a clade (e.g. Kirkland, 1998, Vickaryous, Maryanska & Weishampel, 2004Thompson et al., 2012;Yang et al., 2013), sometimes with additional taxa such as Texasetes (Arbour, Zanno & Gates, 2016;Rivera-Sylva et al., 2018a) or Animantarx (Hill, Witmer & Norell, 2003). Rivera-Sylva et al. (2018a) noted that the grouping Animantarx, 'Denversaurus', Edmontonia, Panoplosaurus, Texasetes, and an unnamed Argentinian ankylosaur could bear the name Panoplosaurinae. In several recent analyses Edmontonia and Panoplosaurus are found as the sister clade to a clade containing Struthiosaurus (Arbour, Zanno & Gates, 2016;Brown et al., 2017;Rivera-Sylva et al., 2018a), here named Struthiosaurini (see the name entry). Owing to the fact that the 'Panoplosaurus clade' is nested within Nodosaurinae, we prefer to use a name that implies a lesser inclusiveness. The suffix -inae (as in Panoplosaurinae) is typically associated with the rank of 'subfamily' under the ICZN. Therefore, the use of the suffix '-inae' for the 'Panoplosaurus clade', without discussing the phylogenetic context, may suggest that Panoplosaurinae represents a clade outside Nodosaurinae. When the widely used suffix -ini (typically associated with the rank of 'tribe') is applied, such confusion is eliminated.
Synonyms. Jaekel (1910) introduced the name Polacanthidae to include ankylosaurs that appeared intermediate between Ankylosauridae and Nodosauridae. Kirkland (1998) was the first to assess 'polacanthids' using cladistic methods and found them to be a clade of early-diverging ankylosaurids, and as such should preferably be called Polacanthinae rather than Polacanthidae, to eliminate the possible confusion that Ankylosauridae and Polacanthidae refer to mutually exclusive clades. Carpenter (2001) argued that Polacanthidae was instead valid and defined the name to cover all ankylosaurs closer to Gastonia than to Edmontonia or Euoplocephalus.
Comments. The name Polacanthinae was (informally) defined before by Yang et al. (2013), who used Polacanthus foxii as the internal specifier and Ankylosaurus magniventris and Panoplosaurus mirus as the external specifiers. Kirkland (1998) diagnosed Polacanthinae as comprising ankylosaurs with an ankylosaurid-like skulls, nearly straight and parallel tooth rows, long basipterygoid processes, well-developed acromion arising from dorsal margin of scapula, ventrally flexed ischia, coossified pelvic osteoderms forming pelvic shield, posteriorly grooved and elongate pectoral osteoderms, and caudal osteoderms large, elongate laterally directed, and with hollow bases. Kirkland (1998) initially found Polacanthinae at the base of Ankylosauridae including Gastonia, Polacanthus, and Mymoorapelta and also referred Hoplitosaurus and Hylaeosaurus to the clade. Arbour, Zanno & Gates (2016), Rivera-Sylva et al. (2018a), andZheng et al. (2018) inferred what could be called Polacanthinae at the base of Nodosauridae, including Polacanthus foxii and Hoplitosaurus marshi. Polacanthinae is poorly supported in most phylogenetic analyses yet frequently referenced in the literature. Taxa typically referred to as 'polacanthines' most often form a grade of early-diverging nodosaurids (e.g., Thompson et al., 2012;Brown et al., 2017). Additional taxonomic and phylogenetic revisions are needed to provide an assessment of Polacanthinae. We define the name here to ensure that it is applicable either within Ankylosauridae or Nodosauridae. If the 'Polacanthus clade' is reconstructed outside the Ankylosauridae + Nodosauridae node, the name Polacanthinae becomes inapplicable and the preferred name for the grouping should probably be Polacanthidae (not defined here).
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Protoceratopsidae has been (informally) defined before by Sereno (1998Sereno ( , 2005 who applied a maximum-clade definition and used Protoceratops andrewsi as the internal specifier and Triceratops horridus as the external specifier. We include two additional external specifiers Ceratops montanus and Leptoceratops gracilis. C. montanus was added because the name Protoceratopsidae has been traditionally applied to the sister taxon of Ceratopsoidea, and L. gracilis was included to ensure that Protoceratopsidae and Leptoceratopsidae remain mutually exclusive clades. Rhabdodontidae Weishampel et al., 2003 (converted clade name) Registration number: 648 Definition. The smallest clade containing Rhabdodon priscus Matheron, 1869 and Zalmoxes robustus (Nopcsa, 1900). This is a minimum-clade definition. Abbreviated definition: min ∇ (Rhabdodon priscus Matheron, 1869 & Zalmoxes robustus (Nopcsa, 1900)).
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The application of Rhabdodontomorpha has been described, and (informally) proposed definitions have been discussed, by Madzia, Boyd & Mazuch (2018: Appendix 1) and Madzia, Jagt & Mulder (2020: Table 1). We therefore refer to these studies for details. Our formalized maximum-clade definition is similar to that of Madzia, Jagt & Mulder (2020) in that it uses Rhabdodon priscus as the internal specifier and Iguanodon bernissartensis as the external specifier. We have further added a second external specifier, Hypsilophodon foxii, to prevent its inclusion to Rhabdodontomorpha under phylogenies similar to that of Norman (2015: Fig. 50). Etymology. Derived from the Greek safis (clear, definite) and formed to include all members of Ornithischia whose placement within the clade is well established.
Composition. Under the primary reference phylogeny, Saphornithischia comprises Pisanosaurus mertii and members of the clades Heterodontosauridae and Genasauria.
Synonyms. Following the widespread application of the Principle of Coordination, under which Hadrosaurinae has to be attributed to Cope (1869), the name Hadrosaurinae is generally considered to have priority over Saurolophinae, even though the latter was coined 4 years earlier (Saurolophinae Brown, 1914a;Hadrosaurinae Lambe, 1918).
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Reference phylogeny. Figure 11 of Arbour & Currie (2016) is treated here as the primary reference phylogeny. Additional reference phylogenies include Figure 1  Composition. Under the primary reference phylogeny, Shamosaurinae comprises Gobisaurus domoculus and Shamosaurus scutatus.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. Tumanova (1987) described Shamosaurinae based on a list of diagnostic features: shamosaurines were ankylosaurids with narrow anterior snouts, angle of the orbital plane with the skull axis less than 25 , anterior wall of the pterygoid inclined posteriorly, occipital condyle a wide oval, pterygoids fused with the basisphenoid, small interpterygoid fenestra, and orbits at the midlength of the skull. Shamosaurinae is not reconstructed in all recent phylogenetic analyses, as Shamosaurus and Gobisaurus are sometimes inferred as successive outgroups to Ankylosaurinae rather than as a clade (e.g., Thompson et al., 2012;Wiersma & Irmis, 2018 Brown, 1908).
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Reference phylogeny. Figure 12 of Maidment et al. (2020) is treated here as the primary reference phylogeny. Additional reference phylogenies include Figure 11 of Maidment et al. (2008) and Figure 1 of Raven & Maidment (2017 Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Stegosauridae was first (informally) defined by Sereno (1998Sereno ( , 2005 who used the maximum-clade definition and selected Stegosaurus stenops as the internal specifier and Huayangosaurus taibaii as the external specifier. We formalize this definition. Struthiosaurini (new clade name) Etymology. Derived from the stem of Struthiosaurus Bunzel, 1871, the name of an included taxon, which combines the Latin word struthio (ostrich) and Greek sauros (lizard, reptile).
Synonyms. The name Struthiosaurinae Nopcsa, 1923 has been recently used for an approximate clade (Kirkland et al., 2013;Blows & Honeysett, 2014;Villanueva-Amadoz et al., 2015). No other taxon names are currently in use for the same or approximate clade.

Comments.
A grouping similar to that covered here under the name Struthiosaurini has previously been named Struthiosaurinae (Kirkland et al., 2013). The name Struthiosaurinae was (informally) defined by Kirkland et al. (2013) who applied the maximum-clade definition and used Europelta as the internal specifier and Cedarpelta, Peloroplites, Sauropelta, and Edmontonia as the external specifiers. Struthiosaurinae was considered to represent the clade of Late Cretaceous European nodosaurids. However, Kirkland et al. (2013) did not include a character matrix or phylogenetic analysis in their study and have not yet published a follow-up paper with results indicating the extent of their Struthiosaurinae. They provided, however, a list of diagnostic characters. According to Kirkland et al. (2013), Struthiosaurinae includes nodosaurid ankylosaurs with narrow predentaries, nearly horizontal and unfused quadrates, quadrate condyles that are 3 times transversely wider than long, premaxillary teeth and dentary teeth that are near the predentary symphysis, dorsally arched sacra, an acromion process dorsal to the midpoint of the scapulocoracoid suture, straight ischia with a straight dorsal margin, long slender limbs, a sacral shield, and erect sacral osteoderms with flat bases. This suite of characters was considered to unite Anoplosaurus, Europelta, Hungarosaurus, and Struthiosaurus, but many of these characters have a broad distribution in Ankylosauria and Nodosauridae (Ősi, 2015 As was the case with Panoplosaurinae, owing to the fact that the 'Struthiosaurus clade' is nested within Nodosaurinae, we prefer to use a name that implies a lesser inclusiveness (that is, -ini rather than -inae). The use of Struthiosaurinae, without discussing the phylogenetic context, may suggest that Struthiosaurinae and Nodosaurinae are mutually exclusive clades. When the suffix -ini is applied, such confusion is eliminated. Note that the recent use of Struthiosaurinae has been largely limited to mentions of Kirkland et al. (2013) application of the name (Blows & Honeysett, 2014;Villanueva-Amadoz et al., 2015).
Reference phylogeny. Figure 12  the sister taxon to a Thescelosaurinae + Orodrominae node. Even though no such phylogenetic hypothesis has been proposed, the placement of taxa 'traditionally' dubbed the 'hypsilophodonts' is highly pliable across studies (Han et al., & Weishampel, 1988).
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Synonyms. No other taxon names are currently in use for the same or approximate clade.
Comments. The name Thyreophora has been (informally) defined before (Sereno, 1998;Sereno, 2005;Norman, 2021). All these definitions were maximum-clade. The definitions of Sereno (1998Sereno ( , 2005 used Ankylosaurus (Sereno, 1998) or Ankylosaurus magniventris (Sereno, 2005) as the internal specifier, and Triceratops (Sereno, 1998) or Triceratops horridus, Parasaurolophus walkeri, and Pachycephalosaurus wyomingensis (Sereno, 2005) as the external specifiers. In turn, Norman (2021) defined Thyreophora using Euoplocephalus and Stegosaurus as the internal specifiers and Hypsilophodon as the external specifier. In order to maintain the 'traditional' concept of Genasauria as a clade comprising Neornithischia and Thyreophora, the internal specifiers in the definition of Thyreophora are used from among the taxa representing the two major subclades -Ankylosauria (Ankylosaurus magniventris) and Stegosauria (Stegosaurus stenops)and the external specifiers are used from among the taxa representing the neornithischian clades Ornithopoda (Iguanodon bernissartensis) and Marginocephalia (Triceratops horridus). Note that the internal specifier Ankylosaurus magniventris and the external specifier Triceratops horridus are not included in the primary reference phylogeny. The former belongs to Ankylosauria within Thyreophora (see, e.g., Thompson et al., 2012), while the latter is part of Ceratopsia (e.g., Morschhauser et al., 2019).
Synonyms. No other taxon names are currently in use for the same or approximate clade.

DISCUSSION
Phylogeny reconstructions of some ornithischian clades currently face challenges that have an impact on the construction of the phylogenetic definitions of several taxon names. Below, we provide discussion of some topological conflicts.
The phylogeny of early-diverging ornithischians The early evolution of Ornithischia and the phylogenetic relationships of taxa nested near the base of the clade are currently contentious, particularly with respect to the potential Triassic members of the clade. Ornithischians have been 'traditionally' represented by a single undisputed Triassic taxon, Pisanosaurus mertii Casamiquela, 1967.
Recent reassessments of the type specimen of P. mertii showed, however, that the morphological features of the taxon are rather difficult to interpret and that it may represent either a non-dinosaur dinosauriform from the clade Silesauridae (Agnolín & Rozadilla, 2018, Baron, 2019 or an early-diverging ornithischian (Desojo et al., 2020). Even if P. mertii turns out to be a silesaurid, however, it may still represent an early-diverging ornithischian dinosaur as a few studies have proposed that silesaurids, a group of Anisian-?Rhaetian (Middle and Late Triassic) dinosauriforms that are usually inferred to be the sister group to dinosaurs (e.g., Nesbitt et al., 2010;Peecook et al., 2013;Ezcurra, 2016;Cau, 2018;Ezcurra et al., 2020), may form an early clade of ornithischians (Langer & Ferigolo, 2013;Cabreira et al., 2016;Pacheco et al., 2019) or a paraphyletic assemblage of early-diverging ornithischians (Müller & Garcia, 2020). Such placement of the silesaurid taxa, especially the oldest known members referred to the group, would have considerable implications for the early evolution of dinosaurs as a whole because neither of the two other major dinosaur clades, Theropoda and Sauropodomorpha, are known from the Middle Triassic.
Pending additional studies more focused on the basal dinosauriform-dinosaur transition, we do not define neither Silesauridae Langer et al., 2010 nor the recently proposed name Sulcimentisauria Martz & Small, 2019. If formal definitions for the names are to be proposed in the future, the definitions should comply with all recently proposed phylogenies, including the possible paraphyletic 'dissolution' of Silesauridae (Müller & Garcia, 2020) that would make Sulcimentisauria, as (informally) defined by Martz & Small (2019), applicable to a clade containing the vast majority of 'traditional' silesaurids and all 'core' ornithischians. One option is to restrict the use of Sulcimentisauria for a clade only when inferred within Silesauridae (e.g., 'max ∇ ∈ Silesauridae (Silesaurus opolensis Dzik, 2003~Asilisaurus kongwe Nesbitt et al., 2010'), as originally intended by Martz & Small (2019).
Recently, Baron & Barrett (2017, 2018 reconstructed the enigmatic dinosaur Chilesaurus diegosuarezi from the Tithonian (uppermost Jurassic) of Central Patagonian Cordillera in Chile to represent the earliest-diverging ornithischian, which was in striking contrast to the original inference of the taxon at the base of Tetanurae, within Theropoda (Novas et al., 2015). Although the proposed placement of Chilesaurus among early-diverging ornithischians does not have any impact on the use of particular clade names (except when the extent of Ornithischia is to be indicated on some recently inferred phylogenies; see Fig. 3), it is perhaps appropriate to express some skepticism towards this inference. As already noted by Müller et al. (2018), the originally proposed tetanurine affinities have not been tested by Baron & Barrett (2017, 2018, nor by Baron (2019), or Müller & Dias-da-Silva (2019), all of whom have also reconstructed C. diegosuarezi at the earliest-diverging position within Ornithischia. It is worth noting that Dieudonné et al. (2020) included C. diegosuarezi in their data matrix as well; however, despite being considered an ornithischian by the authors, its placement at the very base of their tree does not indicate ornithischian affinities for the taxon. Dieudonné et al. (2020) did not include any theropods and/or sauropodomorphs in their analysis and, as such, they did not explore the placement of C. diegosuarezi among dinosaurs. In turn, the studies of Baron & Barrett (2017, 2018, Baron (2019), and Müller & Dias-da-Silva (2019) have all been based on a dataset modified from the one first published by Baron, Norman & Barrett (2017a) that was constructed to test the relationships of rootward dinosaurs (ornithischians, theropods, and sauropodomorphs), especially Late Triassic and Early Jurassic forms (though some younger taxa of Ornithischia were included as well) and their closest pan-avian relatives. We are of the opinion that, at present and with the evidence provided, the placement of Chilesaurus, a latest Jurassic taxon with mosaic features, at the very base of a clade that originated in the Triassic or at the Triassic/Jurassic boundary interval, may be best interpreted as being indicative of inadequate/inappropriate data sampling. In other words, the dataset of Baron, Norman & Barrett (2017a) and its more recent versions are most likely unable to actually test the phylogenetic placement of Chilesaurus.
Regardless of which of the hypotheses will gain further support in subsequent studies, the definition of the name Heterodontosauridae needs to reflect each of them. Therefore, the applied phylogenetic definition of the name includes representatives of all major ornithischian lineages, Ceratopsia (Triceratops horridus), Ornithopoda (Iguanodon bernissartensis), Pachycephalosauria (Pachycephalosaurus wyomingensis), and Thyreophora (Stegosaurus stenops).
Following his thorough redescription of Scelidosaurus harrisonii (Norman, 2020a(Norman, , 2020b(Norman, , 2020c, Norman (2021) assessed the phylogenetic relationships of early-diverging thyreophorans and reconstructed E. ernsti, Sce. harrisonii, and Scu. lawleri as the earliest-diverging ankylosauromorphs (Ankylosauria sensu this study), restricting the name Ankylosauria to a smaller clade, approximately comprising ankylosaurids and nodosaurids (two definitionsone minimum-clade and one maximum-cladewere provided; both applying the name to the same known contents). Norman (2021: 70) further noted that the node comprising ankylosaurids and nodosaurids "has the potential to become the new taxon Euankylosauria but this additional clade name is neither essential nor particularly desirable".
When applying a minimum-clade definition (e.g., 'min ∇ (Ankylosaurus magniventris Brown, 1908& Nodosaurus textilis Marsh, 1889)'), the name Euankylosauria may indeed be useful in the future, especially if further studies support the placement of some taxa, such as Mymoorapelta maysi and Kunbarrasaurus ieversi (as in Arbour & Currie, 2016), or E. ernsti, Sce. harrisonii, and Scu. lawleri (as in Norman, 2020c), as non-ankylosaurid/ non-nodosaurid ankylosaurs. However, there is no need to replace Ankylosauria with Ankylosauromorpha as the name for the largest clade containing A. magniventris but not Stegosaurus stenops. The branch has long been named Ankylosauria and it has always been expected that it may contain taxa with characters that are absent in 'traditional' ankylosaurs (i.e., ankylosaurids and nodosaurids). We suggest that the name Ankylosauromorpha is abandoned.

Problematic clades within Ankylosauria
Comprehensive alpha taxonomic reviews and phylogenetic analyses of Ankylosauridae in recent years have clarified many of the interrelationships within this clade (e.g., Arbour & Currie, 2013;Arbour & Currie, 2016). However, similar reviews for Nodosauridae have not been undertaken in recent years, and phylogenetic resolution within Nodosauridae is often poor and inconsistent between different phylogenies (e.g., Thompson et al., 2012;Arbour, Zanno & Gates, 2016;Brown et al., 2017), in part because many recent ankylosaur phylogenetic analyses are modified from Arbour & Currie (2016) which was designed to test relationships within Ankylosauridae, not Nodosauridae. Additionally, many names for clades within Nodosauridae have been introduced by various authors based on proposed diagnostic characters rather than phylogenetic hypotheses, and have not been defined phylogenetically. In particular, the validity of Polacanthidae or Polacanthinae, Sauropeltinae, Struthiosaurinae, and Stegopeltinae, and the contents of Edmontiniinae or Panoplosaurinae, are unclear. In this manuscript we provide a formal definition of Polacanthinae, and discuss the use of Struthiosaurinae and Panoplosaurinae, as the names have been mentioned recently with some frequency and have had informal definitions proposed previously. Ford (2000) introduced the names Sauropeltinae and Stegopeltinae and provided diagnostic characters but did not test their contents phylogenetically; Sauropeltinae included Sauropelta edwardsorum and Silvisaurus condrayi and Stegopeltinae included Aletopelta coombsi, Glyptodontopelta mimus, and Stegopelta landerensis. Sauropelta and Silvisaurus do not form a clade in any recent analyses, nor do Stegopelta, Glyptodontopelta, and Aletopelta. As such, we do not provide formal definitions for Sauropeltinae or Stegopeltinae at this time.

The origin of Ornithopoda
The understanding of the origin and early evolution of Ornithopoda is tightly connected with the knowledge of the character distribution among rootward neornithischians. With that respect, the basal neornithischian-ornithopod transition is among the poorest known stages of the ornithischian evolutionary history, as recent phylogenetic studies that focused on that particular tree segment provided strikingly conflicting topologies (Boyd, 2015;Dieudonné et al., 2016;Han et al., 2018;Madzia, Boyd & Mazuch, 2018;Andrzejewski, Winkler & Jacobs, 2019;Herne et al., 2019;Dieudonné et al., 2020;Yang et al., 2020).
Substantial conflicts are apparent especially with regards to the phylogenetic placements of taxa 'traditionally' dubbed the 'hypsilophodonts' (compare, e.g., Boyd, 2015;Han et al., 2018;Herne et al., 2019), including Hypsilophodon foxii itself (e.g., Madzia, Boyd & Mazuch, 2018). Phylogeny reconstructions of ornithopods provide more stable results around the node marking the origin of Iguanodontia (e.g., Madzia, Boyd & Mazuch, 2018;Madzia, Jagt & Mulder, 2020), although alternative hypotheses of early iguanodontian phylogenetic relationships exist as well (e.g., Norman, 2015). The names of non-cerapod neornithischian and rootward ornithopod clades are defined here to reflect these uncertainties though we recognize that some potential topologies may still render issues. For example, if Hypsilophodon forms a clade with thescelosaurids but falls outside the Thescelosaurus + Orodromeus node, Hypsilophodontidae would cover Thescelosauridae if the latter name was defined using a minimum-clade definition (as in Brown et al., 2013 andMadzia, Boyd &Mazuch, 2018). We do not include T. neglectus as an external specifier in the definition of Hypsilophodontidae because under the scenario, in which H. foxii would be inferred within the Thescelosaurus + Orodromeus node, the names Thescelosauridae, Thescelosaurinae, and Orodrominae would be all inapplicable, while Hypsilophodontidae could effectively remain in use only for H. foxii. The definitions we propose ensure that if H. foxii is component of the Thescelosaurus + Orodromeus clade, Thescelosauridae becomes inapplicable, while Thescelosaurinae and Orodrominae still remain in use. The potential issue with Hypsilophodontidae covering Thescelosauridae under a topology in which Hypsilophodon is the sister taxon to the Thescelosaurus + Orodromeus node was solved by providing Thescelosauridae with a maximum-clade definition that makes it inapplicable under such scenario.
Arenysaurini here. If future studies support the results of Longrich et al. (2021), Arenysaurini should probably be defined so that it becomes inapplicable if inferred within Lambeosaurini. The easiest way to do so would be to define Arenysaurini through a maximum-clade definition using Arenysaurus ardevoli and at least one other internal specifier that would make the name applicable only in the case Arenysaurus is inferred outside Lambeosaurini. The taxon Adynomosaurus arcanus is a possible candidate, if such a solution is preferred. In turn, Blasisaurus canudoi should be avoided as this taxon has been inferred as the sister taxon of A. ardevoli in some analyses (e.g., Cruzado-Caballero, Pereda-Suberbiola & Ruiz-Omeñaca, 2010; Cruzado- Prieto-Márquez et al., 2019;Prieto-Márquez, Wagner & Lehman, 2020;Gates, Evans & Sertich, 2021). Another option is to apply a clause similar to that used in the definitions of Clypeodonta, Euornithopoda, Hypsilophodontia, Orodrominae, and Thescelosaurinae. That is, by using the set theory symbol ∉, meaning "not element of", the name Arenysaurini could be applicable only under the condition that the clade for which the name was intended was reconstructed outside Lambeosaurini and Parasaurolophini. Such definition could be abbreviated as follows: max ∇ ∉ Lambeosaurini & Parasaurolophini (Arenysaurus ardevoli Pereda-Suberbiola et al., 2009~Lambeosaurus lambei Parks, 1923& Parasaurolophus walkeri Parks, 1922.

CONCLUSIONS
Ornithischian dinosaurs were a major clade of globally distributed Mesozoic archosaurs that achieved substantial taxic diversity and apparent morphological disparity, expressed especially through their cranial features and the body armor of some of their most distinctive members. Throughout their two-century-long research history, ornithischians have been thoroughly assessed both taxonomically and phylogenetically, which has led to the recognition of numerous clades.
Following the pivotal studies establishing the theoretical foundation of the phylogenetic nomenclature in the 1980s and early 1990s, many names for the ornithischian clades have been provided phylogenetic definitions, some of which have proven useful and have not been changed since their introduction.
However, following the 2020 establishment of the International Code of Phylogenetic Nomenclature (ICPN), or the PhyloCode, all of the definitions proposed before the implementation of the Code are treated as formally ineffective.
We have reconsidered the utility of previously proposed phylogenetic definitions of established ornithischian taxon names and provide definitions for 81 names of ornithischian clades, five of which are newly proposed here, as specified by the Articles of the ICPN, thus marking a key step towards a formal phylogenetic nomenclature of ornithischian dinosaurs.