Читать книгу Hadrosaurs - David A. Eberth - Страница 104

Holotype of “Trachodon cantabrigiensis” NHMUK R496 possesses a melange of dental features seen in hadrosaurids and basal hadrosauroids. The proportions of the “Trachodon cantabrigiensis” tooth are similar to those of Eolambia and Protohadros dentary teeth (with crown height-to-width ratios of approximately 2.1 and 1.8, respectively; Head, 1998; Kirkland, 1998), but are less elongate of those than Levnesovia, Probactrosaurus, Telmatosaurus, or hadrosaurids, which usually have crown height-to-width ratios >3.0 (e.g., Norman, 2002; Dalla Vecchia, 2006; Sues and Averianov, 2009). Distal recurvature of the tooth apex is present in NHMUK R496, strongly in Telmatosaurus (Weishampel et al., 1993; Dalla Vecchia, 2006), and some teeth of Bactrosaurus (Godefroit et al., 1998), but is absent in Probactrosaurus (Norman, 2002), Protohadros (Head, 1998), and hadrosaurids (e.g., Lull and Wright, 1942; Ostrom, 1961; Horner et al., 2004; Sues and Averianov, 2009).

A prominent primary ridge (or carina) is ubiquitous among basal hadrosauroids and hadrosaurids (e.g., Lull and Wright, 1942; Ostrom, 1961; Head, 1998; Kirkland, 1998; Norman, 2002; Horner et al., 2004; Prieto-Márquez, 2010a). Secondary ridges are present on the lingual surfaces of the tooth crowns in Bactrosaurus (Godefroit et al., 1998), Levnesovia (Sues and Averianov, 2009), Probactrosaurus (Norman, 2002), Protohadros (Head, 1998), Telmatosaurus (Weishampel et al., 1993), and subadult hadrosaurids (Hall, 1993). These features are absent or weakly developed in adult hadrosaurids (e.g., Lull and Wright, 1942; Ostrom, 1961) and NHMUK R496. The slightly distal offset of the primary ridge in NHMUK R496 is similar to that seen in the majority of basal hadrosauroids (Weishampel et al., 1993; Godefroit et al., 1998; Head, 1998; Norman, 2002; Sues and Averianov, 2009), differing from the more centrally positioned carinae of hadrosaurids (Prieto-Márquez, 2010a). In “T. cantabrigiensis” the carina lacks the sinuous curve present in some lambeosaurine hadrosaurids (Prieto-Márquez, 2010a).

The retention of small marginal denticles positioned around the apical half of the crown margins is also common among basal hadrosauroids and hadrosaurids (e.g., Weishampel et al., 1993; Head, 1998; Kirkland, 1998; Norman, 2002; Horner et al., 2004; Dalla Vecchia, 2006; Sues and Averianov, 2009), although the denticles are lost in some deeply nested hadrosaurid lineages (Prieto-Márquez, 2010a). Mammillae are present on the marginal denticles of “T. cantabrigiensis” and “basal hadrosauroids” such as Probactosaurus (Norman, 2002), Protohadros (Head, 1998), and Telmatosaurus (Dalla Vecchia, 2006), but are generally lost in hadrosaurids (Norman, 2002; Prieto-Márquez, 2010a).

Referred Material of “Trachodon cantabrigiensis” The narrow, closely packed, and straight alveolar grooves of the poorly preserved maxillary fragment (CAMSM B55527) do resemble those of hadrosaurids (e.g., Ostrom, 1961; Horner et al., 2004), but are also similar to those of a variety of other basal hadrosauroids and basal iguanodontians, including Mantellisaurus (Norman, 1986), Probactrosaurus (Norman, 2002), and Telmatosaurus (Weishampel et al., 1993; Dalla Vecchia, 2006).

All of the isolated ungual phalanges referred to “T. cantabrigiensis” lack the distinctive spade-shaped outline seen in Bactrosaurus (Godefroit et al., 1998) and hadrosaurids (Lull and Wright, 1942; Horner et al., 2004). Three of these (CAMSM B55744, NHMUK OR33886–7) are similar to those of non-hadrosaurid ornithopods such as Iguanodon (Norman, 1980), Mantellisaurus (Norman, 1986), and Probactrosaurus (Norman, 2002). The remaining two (CAMSM B55400, B55430) are strongly abraded and could be referable to either a non-hadrosaurid iguanodontian or an ankylosaur, whose pedal phalanges are somewhat similar (see Pereda-Suberbiola and Barrett [1999] for descriptions of ankylosaur unguals from the Cambridge Greensand Member). The isolated dorsal rib (CAMSM B55390) and non-ungual phalanges (NHMUK OR33884–5, CAMSM B55397–99) bear no distinctive features that allow them to referred to any particular dinosaur clade with confidence and these should all be regarded as Dinosauria indet.

‘Iguanodon’ hillii The fragmentary nature of the holotype specimen of ‘Iguanodon’ hillii precludes detailed comparison. Many features of the tooth are shared with the majority of basal hadrosauroids and hadrosaurids, including a strong primary ridge, the presence of mammillate marginal denticles, and the absence of secondary ridges (see comparative comments on “Trachodon cantabrigiensis,” above). Although the tooth crown is incomplete basally, the absence of secondary ridges in ‘I.’ hillii is probably genuine, as in those basal hadrosauroids that possess secondary ridges these normally extend onto the apical part of the tooth (e.g., Head, 1998; Norman, 2002). In addition, this clearly prevents referral of the specimen to Iguanodon or other similar iguanodontian taxa, such as Mantellisaurus. The distal offset of the primary ridge distinguishes the tooth of ‘I.’ hillii from those of hadrosaurids (e.g., Prieto-Márquez, 2010a), although the lack of distal recurvature is more similar to the hadrosaurid condition than that seen in “T. cantabrigiensis” (see above) or Telmatosaurus (Weishampel et al., 1993; Dalla Vecchia, 2006). Bifurcated rows of mammilae similar to those of ‘I.’ hillii are also present in some specimens of Telmatosaurus (Dalla Vecchia, 2006).