The phylogenetic nomenclature of ornithischian dinosaurs

Abstract

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.

Keywords: Dinosauria; International Code of Phylogenetic Nomenclature; Ornithischia; PhyloCode; Phylogenetic definition; Phylogenetic nomenclature.

Figure 1. Specifier-based phylogeny of Ornithischia.

Subclade topologies reflect those of the primary reference phylogenies: Ankylosauria (Figure 11 of Arbour & Currie, 2016; Figure 5 of Rivera-Sylva et al., 2018a), Hadrosauridae (Figure 25 of Prieto-Márquez et al., 2013; Figure 18 of Prieto-Márquez, Wagner & Lehman, 2020), Marginocephalia (Figure 27 of Schott & Evans, 2017; Figure 9 of Chiba et al., 2018; Figure 9a of Fowler & Freedman Fowler, 2020; Figure 10 of Morschhauser et al., 2019; Figure 4 of Yu et al., 2020), non-ankylosaur Thyreophora (Figure 16 of Han et al., 2018), non-cerapod Neornithischia (Figure 4 of Madzia, Boyd & Mazuch, 2018), non-genasaur Ornithischia (Figure 4 of Madzia, Boyd & Mazuch, 2018), non-hadrosaurid Ornithopoda (Figure 2.26 of Norman, 2014; Figure 4 of Madzia, Boyd & Mazuch, 2018; Figure 12 of Madzia, Jagt & Mulder, 2020). Abbreviations: Ch. – Chasmosaurinae; Ni. – Neoiguanodontia; Pd. – Pachycephalosauridae; Pn. – Pachycephalosaurinae; Pr. – Pachyrostra; Rh. – Rhabdodontomorpha; Rd. – Rhabdodontidae; and Sh. – Shamosaurinae. Majority of the silhouettes were obtained from phylopic.org: Ankylosaurinae (Andrew A. Farke, CC BY 3.0), Camptosauridae (Tasman Dixon, public domain), Centrosaurinae (Andrew A. Farke, CC BY 3.0), Chaoyangsauridae (Andrew A. Farke, CC BY 3.0), Chasmosaurinae (Jagged Fang Designs, public domain), Dryosauridae (Gereth Monger, CC BY 3.0), Heterodontosauridae (Scott Hartman, CC BY 3.0), Iguanodontidae (Tasman Dixon, public domain), Lambeosaurinae (Dmitry Bogdanov, CC BY 3.0), Nodosaurinae (Scott Hartman, public domain), Polacanthinae (FunkMonk, public domain), Protoceratopsidae (Andrew A. Farke, CC BY 3.0), Rhabdodontidae (Scott Hartman, CC BY 3.0), Stegosauria (Scott Hartman, CC BY 3.0). We have further added silhouettes for Elasmaria (Victoria M. Arbour, CC BY 4.0), Pachycephalosauria (Victoria M. Arbour, CC BY 4.0), Saurolophinae (Victoria M. Arbour, CC BY 4.0), and Thescelosauridae (Victoria M. Arbour, CC BY 4.0).

Figure 2. The phylogenetic nomenclature of ornithischian dinosaurs using the topology of Madzia, Boyd & Mazuch (2018: 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 Lagosuchus talampayensis (Agnolin & Ezcurra, 2019).

Figure 3. The phylogenetic nomenclature of ornithischian dinosaurs using the topology of Dieudonné et al. (2020: Figs. 1 and 2).

Note that Dieudonné et al. (2020) followed Carpenter & Lamanna (2015) in placing aphanoecetes within Camptosaurus. Owing to the results of recent phylogenetic analyses (e.g., Madzia, Jagt & Mulder, 2020; Verdú et al., 2020), aphanoecetes is placed here within Uteodon McDonald, 2011. Additionally, the name Psittacosaurus major was changed to Psittacosaurus lujiatunensis (following Hedrick & Dodson, 2013), and Ankylosauria and Stegosauria of Dieudonné et al. (2020) were placed in quotation marks to highlight that these names have not been necessarily used by the authors as defined in the present study. Note also that the extent of Ornithischia is difficult to indicate on the tree because Chilesaurus diegosuarezi may represent a theropod (see ‘Discussion’). Abbreviation: An. – Ankylosauria.

Figure 4. The phylogenetic nomenclature of ornithischian dinosaurs using the topology of Yang et al. (2020: Fig. 12).

Ankylosauria and Stegosauria of Yang et al. (2020) were placed in quotation marks to highlight that these names have not been necessarily used by the authors as defined in the present study. In turn, Psittacosauridae of Yang et al. (2020) was placed in quotation marks because the name has not been formally defined yet. Abbreviation: St. – Stegosauria.

Figure 5. Specifier-based phylogeny of Hadrosauridae showing alternative placements of Hadrosaurus foulkii.

The silhouette of Lambeosaurinae was obtained from phylopic.org (Dmitry Bogdanov, CC BY 3.0). The silhouette of Hadrosaurinae/Saurolophinae was prepared by Victoria M. Arbour (CC BY 4.0).