2.24. The strict consensus tree (based on 12 MPTs) generated by running the data matrix with characters unweighted and coded as unordered.
CHARACTER DISTRIBUTION AND PHYLOGENETIC IMPLICATIONS
The degree of resolution seen in the tree generated through this analysis (Fig. 2.26) provides a topology that is available for morpho-phylogenetic interpretation within this taxonomic subset of the ornithopod lineage, and forms a framework that highlights the anatomical acquisitions that resulted in the attainment of true hadosaurian morphology (represented by the clade Euhadrosauria [sensu Weishampel et al., 1993]).
2.25. (Left) Adams consensus tree based upon 12 MPTs generated using characters coded as unordered; (right) The 50% majority rule tree resulting from analysis of the same 12 MPTs.
Basal Ornithopod Taxa
Basal, or “hypsilophodontian-grade” ornithopods sensu lato (Norman et al., 2004; see also Butler et al., 2008), are generally small–medium sized (1–3 m long), bipedal cursors with simple leaf-shaped teeth in both upper and lower jaws; their dental morphology is similar in morphology to that described by Thulborn (1970) and is typical of all basal ornithischians. At the base of the tree, the outgroup is polarized (in this analysis, crudely) against what appear to be more derived taxa in the absence of a consideration of the substantially greater diversity of hypsilophodontian-grade ornithopods (see Butler et al., 2008).
Clypeodonta clade nov.
Phylogenetic Definition The stem-based definition of Clypeodonta (“shield-tooths”) is Parasaurolophus walkeri and all taxa positioned more closely to P. walkeri than to Thescelosaurus neglectus. However, the topology of more basal stem taxa falls outside the scope of this account, and Clypeodonta in this analysis occurs at the node that includes Hypsilophodon foxii, Parasaurolophus walkeri, their common ancestor, and all of its descendants.
Character Acquisition at This Node
ACCTRAN: 19, 20, 45, 47, 50, 52, 53, 54, 55, 56, 59, 60, 65, 79, 80, 85.
DELTRAN: 20, 45, 47, 50, 52, 53, 54, 55, 56, 59, 60, 79, 80.
Condensed Diagnosis
1. Dentary crowns laterally compressed, asymmetrical, and shield shaped in lingual aspect only. The lingual surface of the crown is demarcated from the root by an oblique cingulum.
2. Dentary teeth are curved apicobasally along their length and describe an arc (a convexity lingually) as they emerge from the alveolus during growth.
3. Crowns of dentary and maxillary teeth display an asymmetrical distribution of enamel (dentary crowns have thicker enamel lingually, and maxillary crowns have thicker enamel labially).
4. Dentary crowns bear a prominent primary ridge on the lingual surface that is flanked by a variable number of less prominent subsidiary (accessory, or tertiary) ridges.
2.26. The strict consensus tree (based upon three MPTs) generated by running the data matrix with characters unweighted and coded as ordered.
5. Maxillary crowns are transversely compressed, asymmetrical and shield shaped, but their thickly enameled labial surface bears a variable number of subsidiary ridges, and a prominent primary ridge is not present.
Comments Clypeodontans (and more basal ornithopods) – Hypsilophodon, rhabdodontids (Zalmoxes spp., Rhabdodon, Mochlodon), and tenontosaurs – are skeletally conservative. However, the specialized modifications seen in clypeodontan teeth exclude a substantial diversity of basal ornithopod taxa – including Jeholosaurus, Othnielia, Gasparinisaura, Orodromeus, Parksosaurus, Thescelosaurus, and Bugenasaura (Butler et al., 2008). Tenontosaurs exhibit larger size and some graviportal adaptations that converge upon those seen in more derived taxa such as Camptosaurus, but the latter taxon and more derived forms exhibit a fundamentally different dental morphology.
Clypeodontan dentary crowns are laterally compressed, are inclined labially, and have denticulate margins; the crowns sit upon curved roots and, as a consequence, the teeth move along a lingual-to-labial arc as they emerge from the dental alveolus, instead of rising vertically from the alveolus as is the case in basal ornithopods, and basal ornithischians more generally. When viewed lingually, dentary crowns are broad and exhibit a clearly defined (“shield-like” surface), and are more thickly enameled than on their labial surfaces (Norman et al., 2004:fig. 18.3). The lingual crown surface exhibits incomplete, oblique ledges (sometimes referred to as a “cingulum”) that define a V-shaped junction between crown and root. The enameled crown surface bears a prominent primary ridge flanked by a variable number of much less prominent subsidiary ridges. Maxillary teeth are also transversely compressed, but it is the labial surface of the crown that is more thickly enameled and traversed (apicobasally) by a variable number of low ridges; a primary ridge is not present. The roots of maxillary teeth do not appear to display the lengthwise curvature seen in dentary teeth.
It may also be noted that the singular combination of a prominent primary ridge on the dentary crown and no primary ridge on the maxillary crown may contribute to character combinations that define a more restrictively defined clypeodontan clade – which includes Rhabdodon, Zalmoxes spp., Mochlodon, and Tenontosaurus spp. – and that this clade represents a sister clade to the Dryomorpha. In addition, the unusually specialized basal ornithischians known as heterodontosaurids (Butler et al., 2008) and most notably Heterodontosaurus tucki (Norman et al., 2011) homoplasticly exhibit specialized, superficially shield-like dentary and maxillary tooth crown morphologies, with asymmetrical enamel distribution and prominent enamel ridges. However, these teeth are straight rooted, and the detailed structure of these teeth is quite distinct from that described in clypeodontans. Conventional usage of the name Iguanodontia (sensu Sereno, 1986, 2005) is becoming increasingly problematic because