Two of the ungual phalanges (NHMUK OR33886, CAMSM B55744) are hoof shaped in dorsal view. The proximal articular surface supports a short “neck” that expands mediolaterally to form the main body of the ungual, which is offset from the proximal neck by a distinct notch both laterally and medially, and has a broad, subtriangular outline that tapers to a bluntly rounded distal margin. The dorsal surface of the ungual is mediolaterally and proximodistally convex and shallow attachment grooves extend from each of the notches for a short distance. In proximal view, the articular surface is wider than it is tall, with a subelliptical outline. It is concave mediolaterally and dorsoventrally and is divided into subequal lateral and medial portions by a low ridge that extends dorsoventrally along the approximate midline of the articular surface. In lateral view, the dorsal margins of the unguals are convex, while the ventral margins are concave. A subtle midline ridge extends along the ventral surface of CAMSM B55744, which is absent in NHMUK OR33886. The third ungual phalanx (NHMUK OR33887) is similar to the other two, but lacks the development of the neck and notches separating the proximal articular surface from the rest of the phalanx, and so has an elongate, sub-triangular overall outline in dorsal view.
Other Cambridge Greensand “Trachodontids” A partial abraded maxilla (listed as an indeterminate dinosaur by Seeley [1869:23]: CAMSM B55527) was regarded as a “Trachodontid” by Nopcsa (1923:193). It is broken into two parts, which comprise a short section from the anterior portion of a maxilla, with eight, narrow parallel dental grooves. These grooves are closely packed and lack the impressions of individual tooth crowns. The tooth row is medially inset, creating a rounded buccal recess. No teeth are preserved, and no other anatomical details can be determined.
Several other specimens in the collections of the Sedgwick Museum are cataloged as “trachodontids” following identifications made by Nopcsa during his visit to the collections, including a heavily abraded ungual phalanx (CAMSM B55434), two non-ungual phalanges (CAMSM B55397, B55399), and the proximal portion of a large dorsal rib (CAMSM B55390), although Nopcsa did not provide formal referrals for these specimens.
‘Iguanodon’ hillii Newton, 1892
Unfortunately, only the apical part of the holotype tooth crown is preserved (GSM 1966: Fig. 6.2) and it is too incomplete to determine whether it was a maxillary or dentary tooth. For convenience, the tooth is described as a left dentary tooth in order to standardize terminology and facilitate comparisons. The labial crown surface is partially embedded within matrix.
The preserved portion of the crown has a subtriangular outline in lingual view, tapering apically (Fig. 6.2A). The unenameled labial surface of the crown is strongly convex mesiodistally, giving the tooth a D-shaped cross section in basal view. The lingual surface of the tooth is enamelled and bears a strong, slightly distally offset primary ridge that divides the crown surface into a large mesial and smaller distal portion. There are no secondary ridges. The enamel has a fine granular texture and faint parallel wrinkles extend horizontally across the crown surface mesiodistally (although these are not as prominent as in the figure provided by Newton [1892:fig. 1A]). The crown apex is unworn and the mesial and distal crown margins bear numerous denticles.
The mesial crown margin bears 16 large denticles, which are separated from each other by deep sulci. Basally, the denticles are oriented at approximately 45° to the apicobasal axis of the crown, but this angle changes progressively towards the crown apex, with more apically positioned denticles oriented at 80–90°. These denticles are subequal in size and are separated from the tooth apex by a group of 6 very small apically extending denticles, which are effectively small nubbins of enamel that are more like mammillae than true denticles. The largest, basal-most denticles each bear a row of mammillae (with around 4–5 mammillae per denticle). However, progressing apically around the crown margin the tips of each denticle bifurcate, to produce a forked structure: the apically positioned part of this fork bears a row of mammalliae (consisting of 2–4 mammillae), whereas the basally positioned part is comprised of a single, tongue-like process (Fig. 6.2B). This bifurcation of the denticle tips is subtle in lingual view, but more apparent in mesial view.
The denticles on the distal surface of the tooth are poorly preserved, but appear to follow similar trends in terms of size and orientation (12 large denticles are preserved, with a minimum of 2–3 small nubbin-like denticles just distal to the crown apex). Unfortunately, the poorer preservation of the apical-most distal denticles precludes examination of their tips, and it cannot be determined if they were bifurcated. However, the basal-most denticles possess single rows of mammillae that are identical to those on the basally positioned mesial denticles.
DISCUSSION
Morphological Comparisons
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”