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SYNOPSIS Ornithischia is a familiar and diverse clade of dinosaurs whose global phylogeny has remained largely unaltered since early cladistic analyses in the mid 1980s. Current understanding of ornithischian evolution is hampered by a paucity of explicitly numerical phylogenetic analyses that consider the entire clade. As a result, it is difficult...
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... comprises a partial, disarticulated skull from the upper El- liot Formation of South Africa. Assignment of this speci- men to Lycorhinus (Thulborn 1970b, 1974Gow 1990) was not based on unique characters, but on general similarity. A number of subsequent authors criticised the referral of this specimen to Lycorhinus: Galton (1973a: caption to fig. 2) referred BMNH A100 to Heterodontosaurus sp.; Charig & Crompton (1974) considered BMNH A100 to be generic- ally distinct from both Heterodontosaurus and Lycorhinus;while Hopson (1975) provisionally referred BMNH A100 to ...
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... analysis recovered 3787 trees; filtering these trees so that only minimum length trees were retained resulted in 756 MPTs of 477 steps (Consistency Index (CI) = 0.505, Retention Index (RI) = 0.732, Rescaled Consistency Index (RC) = 0.370). Strict and 50% majority-rule component consensus trees (Fig. 2) and an Adams consensus tree, were calculated using PAUP * . The strict component consensus (SCC) tree contains two major polytomies and contains a much lower degree of resolution than the majority-rule or Adams consensus trees; the latter observation suggests that the low degree of resolution in the SCC tree results from a number of ...
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... support values are shown in Fig. 2. Most nodes have a decay index of +1, i.e. they are absent from the strict consensus of all trees of 478 steps or less. Stronger support is found only within the clades Thyreophora and Iguanodontia, but even here support is relatively low. However, it is possible that a few unstable 'wildcard' taxa, such as those identified by reduced ...
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... means that for each pseu- doreplicate data set the search for MPTs was truncated once 1000 trees had been found. Figure 2 shows the results of the bootstrap analysis. Bootstrap support is weak throughout much of the tree. ...
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... trees (MPTs) generated by the full analysis. The number above each node is a unique identifier used in the tree description (see Appendix 4). The number beneath a node represents the bootstrap proportion for that node (taken from the reduced bootstrap analysis). Note the increased levels of bootstrap support for a number of nodes when compared to Fig. 2. Abbreviations: ORN., Ornithischia; HETERODONT., Heterodontosauridae; GENA., Genasauria; NEORN., Neornithischia; ORNITH., Ornithopoda; MARG., Marginocephalia; CERAT., Ceratopsia; PACHY., ...
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... SCC tree (Fig. 2) recovered by this analysis posi- tions Pisanosaurus in an unresolved polytomy at the base of Ornithischia. However, the 50% majority-rule consensus tree (Fig. 2), maximum agreement subtree (Fig. 3) and the derivat- ive SRC tree (Fig. 4) all support the position of Pisanosaurus as the most basal known ornithischian. These consensus ...
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... SCC tree (Fig. 2) recovered by this analysis posi- tions Pisanosaurus in an unresolved polytomy at the base of Ornithischia. However, the 50% majority-rule consensus tree (Fig. 2), maximum agreement subtree (Fig. 3) and the derivat- ive SRC tree (Fig. 4) all support the position of Pisanosaurus as the most basal known ornithischian. These consensus trees additionally position heterodontosaurids as a monophyletic clade of non-genasaurians, close to the base of Ornithischia (discussed below). Pisanosaurus and ...
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... taxa (Heterodontosaurus tucki, Abrictosaurus con- sors, Echinodon becklesii, Lycorhinus angustidens) and one specimen (BMNH A100) previously referred to Heterodon- tosauridae were included in this analysis. These taxa do not form a clade in the SCC tree, but are included in a large polytomy at the base of Ornithischia (Fig. 2). However, re- duced consensus trees indicate that this basal polytomy is the result of the unstable and problematic taxon Echinodon (which is highly fragmentary, with 83% missing data); ex- clusion of Echinodon results in the remaining four OTUs forming a heterodontosaurid clade, while further exclusion of Lycorhinus (86% missing data) ...
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... analysis resolves Lesothosaurus as the most basal thyreophoran (Figs 2-4; see also Liu 2004 UCMP 130580). In all four taxa the ridge is prominent and transversely thickened, overhangs the lateral surface of the angular and laterally delimits a narrow, dorsally facing shelf. ...
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... possible most basal thyreophoran Lesothosaurus diagnos- ticus, discussed above) is well-established and uncontro- versial and the clade including Scutellosaurus lawleri, Emausaurus ernsti, Scelidosaurus harrisonii and Eurypoda (Ankylosauria + Stegosauria) is comparatively well- supported by total-evidence bootstrap proportions and Bremer support (Fig. 2). The characters that unambiguously support the monophyly of this clade (Appendix 4) have already been identified by Sereno (1986Sereno ( , 1999a, with the exception of the presence of cortical remodelling of cra- nial elements (Character 89, Appendix 2), which has been previously considered to be limited to ankylosaurs, but is also ...
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... monophyletic Ornithopoda is not recovered in the SCC tree (Fig. 2), but is present in the derivative SRC tree (Fig. 4) fol- lowing the a posteriori pruning of five unstable taxa, includ- ing the putative ornithopods Yandusaurus, Zephyrosaurus and Talenkauen. However, the content of the ornithopod clade recovered in the derivative SRC tree (Fig. 4) differs signific- antly from that suggested by ...
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... is closed in basal neornithischians (e.g. Agilisaurus, Peng 1992), ceratopsians (e.g. Zhao et al. 1999;You & Dodson 2004) and pachyceph- alosaurs (Marya´nskaMarya´nska et al. 2004), the prepubic process of the pubis is elongated in some basal neornithischians (Hexinlusaurus, He & Cai 1984), pachycephalosaurs (Marya´nskaMarya´nska et al. 2004: fig. 21.4C) and basal ceratopsians (e.g. Psit- tacosauridae, Sereno 1987; Archaeoceratops, You & Dodson 2003) and the obturator process is present on the ischium in basal neornithischians (Stormbergia, SAM-PK-K1105, BMNH R11000, Butler 2005;Agilisaurus, ZDM T6011;Hexinlusaurus, He & Cai 1984, ZDM T6001). Thus the ap- parently strong support ...
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... monophyletic Hypsilophodontidae was not recovered by this analysis (Figs 2-4). Although 'hypsilophodontids' are generally thought of as rather anatomically conservative, it is clear that they are actually relatively diverse, with some taxa (e.g. ...
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... monophyletic. This analysis finds Pachycephalosauria to be a monophyletic clade including Stenopelix valdensis, Wannanosaurus yansiensis, Goyocephale lattimorei, Homa- locephale calathocercos and Pachycephalosauridae; how- ever, Bremer support (+1) and total-evidence bootstrap val- ues (55%) show surprisingly low support for Pachyceph- alosauria (Fig. 2), although bootstrap support is stronger for more derived groupings within the clade. However, as with Ceratopsia, the real support for the clade may be ob- scured by the presence of one or more 'wildcard' taxa. The most obvious candidate is the problematic basal taxon Sten- opelix. To test this possibility, a backbone constraint was ...
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Citations
... The lateral surface of the maxilla is flat (Fig. 2d). The complete tooth row is inset medially from the lateral surface (Fig. 2f), similar to other stegosaurs and most other ornithischians 36,37 . The dorsolateral surface of the maxilla is laterally compressed, forming a thin lamina that forms the lateral wall of the nasal cavity (Fig. 2e). ...
Stegosaurs are a minor but iconic clade of ornithischian dinosaurs, yet due to a poor fossil record, their early evolution is poorly understood. Here, we describe a new stegosaur, Baiyinosaurus baojiensis, gen. et sp. nov. from the Middle Jurassic Wangjiashan Formation of the Pingchuan District, Baiyin City, Gansu Province, China. The frontal of Baiyinosaurus possesses a unique characteristic among Stegosauria: it is wider than long and contributes to both the medial and anterior margins of the supratemporal fenestra. The character combinations of dorsal vertebrae of Baiyinosaurus are also different to other stegosaurs: its neural arches are not greatly elongated, its parapophyses are well developed, and its neural spines are axially expanded in lateral. The features of the frontal and vertebrae of Baiyinosaurus are reminiscent of basally branching thyreophorans, indicating that Baiyinosaurus is transitional in morphology between early thyreophorans and early-diverging stegosaurs. Systematic analysis shows that Baiyinosaurus is an early-diverging stegosaur.
... Ornithischia is now known to include over 400 valid species from every continent, and to span from at least the earliest Jurassic until the Cretaceous-Paleogene Mass Extinction (Weishampel, 2004). While the origins and evolution of Ornithischia as an offshoot of Dinosauria are uncertain and controversial (Agnol ın & Rozadilla, 2018; Baron et al., 2017aBaron et al., , 2017bCabreira et al., 2016;Desojo et al., 2020;Langer et al., 2017;M€ uller & Garcia, 2020;Norman et al., 2022), the later diversification of Ornithischia into morphologically distinct armoured, plated, crested and horned forms is well established (Benson et al., 2018;Butler et al., 2008;Dodson et al., 2004;Galton & Upchurch, 2004;Horner et al., 2004;Madzia et al., 2021;Marya nska et al., 2004;Vickaryous et al., 2004). ...
... The use of Hypsilophodontidae as a natural group has largely fallen out of favour with recognition that 'hypsilophodontids' are united by plesiomorphies Scheetz, 1999;Weishampel et al., 2003;Winkler et al., 1997), but the clades Thescelosauridae, Dryosauridae and Elasmaria have all been erected to reunite some of these former 'hypsilophodontids' (Calvo et al., 2007;Milner & Norman, 1984;Rozadilla et al., 2016;Sternberg, 1937Sternberg, , 1940, and Hypsilophodontidae itself may have modern utility, albeit much restricted from its historical status (Dieudonn e et al., 2021;Madzia et al., 2021;Poole, 2022). There is much phylogenetic conflict of the relationships of these 'hypsilophodontids' or the 'hypsilophodont plexus' (Thulborn, 1971) of early ornithischian taxa, especially whether traditional genera fall within or outside Ornithopoda (Boyd, 2015;Brown et al., 2021;Butler et al., 2008;Herne et al., 2019;Madzia et al., 2021). The determinant in resolving the placement of 'hypsilophodontids' within Ornithopoda is the position of Marginocephalia. ...
... One of the earliest-diverging ornithischian clades is Heterodontosauridae (Kuhn, 1966). Heterodontosaurids once were considered members of either Ornithopoda or Marginocephalia (Sereno, 1986(Sereno, , 1999Xu et al., 2006), while most recent analyses recover the clade Heterodontosauridae in a more rootward position in Ornithischia (Baron et al., 2017a(Baron et al., , 2017b(Baron et al., , 2017cBoyd, 2015;Butler et al., 2008;Dieudonn e et al., 2016;Han et al., 2018;Herne et al., 2019;Langer et al., 2017;Sereno, 2012), a recent investigation contested this hypothesis. It proposed heterodontosaurids as earlydiverging branches of Marginocephalia, closely related to pachycephalosaurians, far from the base of Ornithischia (Dieudonn e et al., 2021). ...
... The presence of this ossification anteriorly to the premaxilla is an unambiguous synapomorphy of all ceratopsian dinosaurs (e.g. Sereno 1986, Butler et al. 2008). However, as indicated above, the morphology of the anteriormost portion of the upper jaw in Ajkaceratops differs markedly from the rostrals of ceratopsian dinosaurs. ...
At the climax of their evolutionary history in the latest Cretaceous, ceratopsian dinosaurs were among the most dominant components of North American and Asian land ecosystems. In other continental landmasses, however, ceratopsians were extraordinarily rare and the affinities of their proposed representatives often turned out to be inconclusive. Arguably the most significant evidence of Ceratopsia from outside North America and Asia is represented by Ajkaceratops kozmai from the Santonian (Upper Cretaceous) of Hungary. We provide a detailed osteological description of Ajkaceratops and highlight its bizarre anatomy. Ajkaceratops has been ‘traditionally’ interpreted to represent a Bagaceratops-like coronosaur, and its occurrence on the European islands was hypothesized to probably result from an early Late Cretaceous dispersal event from Asia. However, while the snout of Ajkaceratops may resemble that of some ceratopsians, closer inspection of the preserved elements indicates that these similarities are largely superficial. While it cannot be ruled out that Ajkaceratops represents a highly peculiar member of the clade, its placement is far from certain. Still, the discovery of Ajkaceratops exemplifies the importance and uniqueness of European dinosaur faunas.
... In the earliest diverging ornithischian clade [17,18,63], Heterodontosauridae, the replacement patterns have been studied in detail by CT analysis [47,64,65]. The holotype of Fruitadens includes a left maxilla containing six tooth positions with two replacement teeth and a right dentary containing eight tooth positions with three replacement teeth [64]. ...
Background
Tooth replacement patterns of early-diverging ornithischians, which are important for understanding the evolution of the highly specialized dental systems in hadrosaurid and ceratopsid dinosaurs, are poorly known. The early-diverging neornithischian Jeholosaurus , a small, bipedal herbivorous dinosaur from the Early Cretaceous Jehol Biota, is an important taxon for understanding ornithischian dental evolution, but its dental morphology was only briefly described previously and its tooth replacement is poorly known.
Results
CT scanning of six specimens representing different ontogenetic stages of Jeholosaurus reveals significant new information regarding the dental system of Jeholosaurus , including one or two replacement teeth in nearly all alveoli, relatively complete tooth resorption, and an increase in the numbers of alveoli and replacement teeth during ontogeny. Reconstructions of Zahnreihen indicate that the replacement pattern of the maxillary dentition is similar to that of the dentary dentition but with a cyclical difference. The maxillary tooth replacement rate in Jeholosaurus is probably 46 days, which is faster than that of most other early-diverging ornithischians. During the ontogeny of Jeholosaurus , the premaxillary tooth replacement rate slows from 25 days to 33 days with similar daily dentine formation.
Conclusions
The tooth replacement rate exhibits a decreasing trend with ontogeny, as in Alligator . In a phylogenetic context, fast tooth replacement and multi-generation replacement teeth have evolved at least twice independently in Ornithopoda, and our analyses suggest that the early-diverging members of the major ornithischian clades exhibit different tooth replacement patterns as an adaption to herbivory.
... Nevertheless, representatives of this clade, although well-known from the Late Jurassic and Cretaceous , are poorly known from the Middle Jurassic (Fig. 2). This is despite numerous predictions made by multiple phylogenetic analyses that ornithischians underwent a major radiation at this time (Boyd, 2015;Butler et al., 2008;Dieudonné et al., 2021;Raven et al., 2023). Moreover, many of the earliest fossils of the major ornithischian clades (e.g., Ornithopoda, Ankylosauria, Stegosauria) are rare specimens recovered from Middle Jurassic strata in Europe, east Asia, and north Africa (e.g., Dai et al., 2022;Maidment et al., 2020Maidment et al., , 2021Ruiz-Omeñaca et al., 2007). ...
... To aid morphological comparisons we make the following assumptions: the convex tooth surface is the labial surface in all cases; and any offset of the crown apex is in a posterior or lingual direction. We refer all the teeth described below to various clades in Ornithischia based on a combination of features: the mesiodistal expansion of the crown relative to the root; the presence of a cingulum; apically inclined and relatively large denticles; a low triangular crown in lingual/labial views (for cheek teeth); and asymmetry of the crown in labial/lingual view (Butler et al., 2008;Irmis et al., 2007;Sereno, 1986). Although many of these features are shared with other Mesozoic archosaurs, such as the Late Triassic aetosaur Revueltosaurus (Boyd, 2015;Irmis et al., 2007;Nesbitt et al., 2007;Parker et al., 2005), and (with the exception of a basal cingulum) cannot be considered to be unique synapomorphies of ornithischians (Butler et al., 2008;Nesbitt et al., 2007), the Middle Jurassic age of these teeth implies that the most parsimonious solution is to regard them as referrable to Ornithischia rather than other archosaur clades. ...
... We refer all the teeth described below to various clades in Ornithischia based on a combination of features: the mesiodistal expansion of the crown relative to the root; the presence of a cingulum; apically inclined and relatively large denticles; a low triangular crown in lingual/labial views (for cheek teeth); and asymmetry of the crown in labial/lingual view (Butler et al., 2008;Irmis et al., 2007;Sereno, 1986). Although many of these features are shared with other Mesozoic archosaurs, such as the Late Triassic aetosaur Revueltosaurus (Boyd, 2015;Irmis et al., 2007;Nesbitt et al., 2007;Parker et al., 2005), and (with the exception of a basal cingulum) cannot be considered to be unique synapomorphies of ornithischians (Butler et al., 2008;Nesbitt et al., 2007), the Middle Jurassic age of these teeth implies that the most parsimonious solution is to regard them as referrable to Ornithischia rather than other archosaur clades. Our morphological descriptions refer to the entirety of the sample for each morphotype, with the registered specimens used as exemplars. ...
... Despite their very long history of research, their interrelationships are still a matter of heated debate (e.g., Cruzado-Caballero et al., 2019;Dieudonné et al., 2020;Han et al., 2018;Madzia et al., 2018Madzia et al., , 2018Madzia et al., , 2021McDonald, 2012;Rozadilla et al., 2019Rozadilla et al., , 2020Xu et al., 2018). This systematic turmoil is a direct consequence of a more general paradigm shift in our comprehension of the inner relationships of Ornithischia, and started with the influential work of Butler et al. (2008). Regardless of the overall instability, usually Dryomorpha Milner and Norman, 1984, defined as the minimum clade containing Dryosaurus altus and Iguanodon bernissartensis (Madzia et al., 2021), has been consistently recovered by most workers. ...
Iguanodontia is a diverse clade of herbivorous ornithischian dinosaurs that were speciose and abundant during the Jurassic and Cretaceous. Although the monophyly of Iguanodontia is well supported, their internal relationships have sparked heated debate due to several phylogenetic paradigm shifts. Late Jurassic basally branching iguanodontians in particular are not well understood in terms of their systematic affinities and evolutionary relevance. Their fossil record in Europe is meager compared with North America, with only a few species currently recognized. Two taxa are currently known from the Upper Jurassic of England, the basally branching styracosternan Cumnoria prestwichii and the putative dryosaurid Callovosaurus leedsi. In the Upper Jurassic of Portugal, the styracosternan Draconyx loureiroi and the dryosaurid Eousdryosaurus nanohallucis are presently the only described basally branching iguanodontians. Here we report a new species of early diverging iguanodontian from the Upper Jurassic Lourinhã Formation of western-central Portugal. The new species is clearly distinguished from all other coeval taxa by an exclusive combination of characters that include a tibia with a cnemial crest that is directed craniolaterally and a fibular condyle that is angled at 90°with respect to the proximal epiphysis, a fibula with symmetrical proximal margins, and a reduced metatarsal I. The phylogenetic relationships of the Lourinhã iguanodontian were explored using maximum parsimony and Bayesian inference. The two analyses recover the Lourinhã iguanodontian as an indeterminate dryomorphan, with more precise affinities precluded due to the current available material. Body size is estimated between 3 and 4 meters for the holotype specimen, adding to the diversity of small ornithopods already recognized in the paleoichnological record of the Lourinhã Formation.
... In their phylogenetic analysis, Weishampel et al. (2003) recovered it as the sister-clade to Iguanodontia. Since then, the Rhabdodontidae has been consistently placed at the base of the iguanodontian radiation (Butler et al. 2008;McDonald 2012;Ősi et al. 2012;Boyd 2015;Dieudonné et al. 2016Dieudonné et al. , 2021Verdú et al. 2018Verdú et al. , 2020Madzia et al. 2018;Bell et al. 2018Bell et al. , 2019Yang et al. 2020;Poole 2022;Augustin et al. 2022). Such a basal phylogenetic position within Iguanodontia, combined with their fossil record being limited to the later Late Cretaceous (Santonian-Maastrichtian), indicates an exceptionally long ghost lineage for rhabdodontids. ...
The Rhabdodontidae was one of the most important dinosaur groups inhabiting the Late Cretaceous European Archipelago. Currently, the clade comprises nine species within six genera, which have been found in southern France, northern Spain, eastern Austria, western Hungary and western Romania, ranging from the Santonian to the late Maastrichtian. Phylogenetic analyses consistently place the Rhabdodontidae at the very base of the iguanodontian radiation, whereas the in-group relationships of rhabdodontids are relatively poorly understood; nevertheless, the clade seems to have had a rather complicated biogeographical history. Generally, rhabdodontids were small- to medium-sized, probably habitually bipedal herbivores, characterised by a rather stocky build and a comparatively large, triangular skull. Several lines of evidence suggest that they were presumably gregarious animals, as well as selective browsers that fed on fibrous plants and occupied different ecological niches than sympatric herbivorous dinosaur clades. Moreover, the sympatry of at least two rhabdodontid taxa was rather common and can be explained, at least in some instances, by niche partitioning. While rhabdodontids disappeared prior to the K/Pg extinction event in Western Europe, they survived close to the end of the Cretaceous in Eastern Europe, where they were amongst the last non-avian dinosaurs still present before the end of the Cretaceous. In this paper, we provide an overview of the rhabdodontid taxonomic history, diversity, phylogenetic relationships and palaeobiogeographic history, as well as palaeoecology and extinction. In addition, we also highlight still open questions on each of these topics and suggest potential future research directions.
... Among early-diverging avemetatarsalians, osteoderms may be present in the dinosauriform Lewisuchus admixtus (Romer 1972b, Bittencourt et al. 2015, Agnolín et al. 2022), but no other avemetatarsalian taxon has the extensive covering of osteoderms seen in Mambachiton fiandohana, except within Dinosauria, where osteoderms occur in ornithischians (e.g. Butler et al. 2008), sauropodomorphs (Currie- Rogers et al. 2011), and rarely in theropods (Hendrickx et al. 2022). Although the osteoderm shape, number per vertebra, and arrangement are unique for Mambachiton fiandohana among archosauriforms, they more closely resemble those of stem archosaurs (e.g. ...
Understanding the evolution of the earliest avemetatarsalian (bird-line) archosaurs and inferring the morphology of the last common ancestor of Archosauria are hampered by a poor fossil record in critical temporal intervals. Here we describe an early-diverging avemetatarsalian from the ?Earliest Late Triassic (~235 Ma) ‘basal Isalo II’/Makay Formation of Madagascar, which helps bridge these gaps. This taxon, Mambachiton fiandohana gen. et sp. nov., is represented by well-preserved postcranial material and possibly a postfrontal bone. Features of the neck region include anteroposteriorly elongated vertebrae with laterally expanded dorsal ends of the neural spines with three pairs of osteoderms per cervical vertebra, lying dorsal to those vertebrae. Inclusion of Mambachiton in a phylogenetic analysis of archosauromorphs recovers it at the base of Avemetatarsalia, outside of the aphanosaur + ornithodiran clade. This new specimen indicates that osteoderms were present in the earliest avemetatarsalians, but were lost in more crownward lineages. The plesiomorphic morphology of the taxon also underscores the difficulty of identifying early avemetatarsalians from incomplete skeletons. This early-diverging avemetatarsalian occurring together with a lagerpetid and silesaurid in the ‘basal Isalo II’/Makay Formation of Madagascar documents the co-occurrence of multiple non-dinosaurian avemetatarsalian clades in Gondwana near the Middle–Late Triassic transition. Translated abstract ( Malagasy and French) is provided in the Supplementary information.
... Neornithischia is a clade of herbivorous dinosaurs including ornithopods, marginocephalians and a diversity of small bipedal basal forms that were historically referred to as hypsilophodontids [1][2][3]. The classification of basal neornithischians was presented and debated by many palaeontologists [4][5][6][7][8]. ...
... The systematics of basal neornithischians is still problematic [1,2]. A variety of basal forms were traditionally referred to as hypsilophodontids [1][2][3]. ...
... The systematics of basal neornithischians is still problematic [1,2]. A variety of basal forms were traditionally referred to as hypsilophodontids [1][2][3]. Many taxa, which were once considered early members of ornithopods, have been transferred into the basal neornithischians [6,8] (see Table 1). ...
An exceptional articulated skeleton of a new basal neornithischian dinosaur, Minimocursor phunoiensis gen. et sp. nov., was discovered in the Late Jurassic Phu Kradung Formation at the Phu Noi locality, Kalasin Province, Thailand, a highly productive non-marine fossil vertebrate locality of the Khorat Plateau. It is one of the best-preserved dinosaurs ever found in Southeast Asia. Minimocursor phunoiensis gen. et sp. nov. shows a combination of both plesiomorphic and apomorphic characters resembling those of Late Jurassic to Early Cretaceous small-bodied ornithischians from China: a low subtriangular boss is projected laterally on the surface of the jugal, the brevis shelf of the ilium is visible in lateral view along its entire length, a distinct supraacetabular flange is present on the pubic peduncle of the ilium, the prepubis tip extends beyond the distal end of the preacetabular process of the ilium, and the manus digit formula is ?-3-4-3-2. The phylogenetic analysis shows that this
dinosaur is among the most basal neornithischians. This study provides a better understanding of the early evolution and taxonomic diversity of ornithischians in Southeast Asia.
... The early evolution of Ornithopoda and its divergence from other neornithischian clades remains one of the most contentious and poorly resolved areas of Ornithischian research (e.g., [28,102]. Tenontosaurus has long featured as a transitional representative between "hypsilophodonts" and later diverging ornithopods (dryomorphs) (e.g., [90,[102][103][104]) a position it later came to share with a variety of early-diverging taxa from the Gondwanan landmasses (e.g., elasmarians, Australian ornithopods)(e.g., [105,106]) and a recently expanding ...
... The early evolution of Ornithopoda and its divergence from other neornithischian clades remains one of the most contentious and poorly resolved areas of Ornithischian research (e.g., [28,102]. Tenontosaurus has long featured as a transitional representative between "hypsilophodonts" and later diverging ornithopods (dryomorphs) (e.g., [90,[102][103][104]) a position it later came to share with a variety of early-diverging taxa from the Gondwanan landmasses (e.g., elasmarians, Australian ornithopods)(e.g., [105,106]) and a recently expanding ...
Intensifying macrovertebrate reconnaissance together with refined age-dating of mid-Cretaceous assemblages in recent decades is producing a more nuanced understanding of the impact of the Cretaceous Thermal Maximum on terrestrial ecosystems. Here we report discovery of a new early-diverging ornithopod, Iani smithi gen. et sp. nov., from the Cenomanian-age lower Mussentuchit Member, Cedar Mountain Formation of Utah, USA. The single known specimen of this species (NCSM 29373) includes a well-preserved, disarticulated skull, partial axial column, and portions of the appendicular skeleton. Apomorphic traits are concentrated on the frontal, squamosal, braincase, and premaxilla, including the presence of three premaxillary teeth. Phylogenetic analyses using parsimony and Bayesian inference posit Iani as a North American rhabdodontomorph based on the presence of enlarged, spatulate teeth bearing up to 12 secondary ridges, maxillary teeth lacking a primary ridge, a laterally depressed maxillary process of the jugal, and a posttemporal foramen restricted to the squamosal, among other features. Prior to this discovery, neornithischian paleobiodiversity in the Mussentuchit Member was based primarily on isolated teeth, with only the hadrosauroid Eolambia caroljonesa named from macrovertebrate remains. Documentation of a possible rhabdodontomorph in this assemblage, along with published reports of an as-of-yet undescribed thescelosaurid, and fragmentary remains of ankylosaurians and ceratopsians confirms a minimum of five, cohabiting neornithischian clades in earliest Late Cretaceous terrestrial ecosystems of North America. Due to poor preservation and exploration of Turonian–Santonian assemblages, the timing of rhabdodontomorph extirpation in the Western Interior Basin is, as of yet, unclear. However, Iani documents survival of all three major clades of Early Cretaceous neornithischians (Thescelosauridae, Rhabdodontomorpha, and Ankylopollexia) into the dawn of the Late Cretaceous of North America.
