Читать книгу Hadrosaurs - David A. Eberth - Страница 46

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 it recognizes a clade that includes Tenontosaurus tilletti and all taxa positioned closer to hadrosaurs (e.g., Parasaurolophus walkeri) and yet excludes Hypsilophodon and Thescelosaurus. The conventional node Iguanodontia (sensu Sereno, 2005) occurs immediately above the node here named Clypeodonta (see also discussion in Norman, in press).