Читать книгу Hadrosaurs - David A. Eberth - Страница 102

Holotype The holotype (NHMUK R496) is an isolated, well-preserved, and unworn left dentary tooth, lacking only the basal part of the root. In lingual view (Fig. 6.1A), the tooth crown is apicobasally elongate, mesiodistally expanded with respect to the root, and asymmetrically diamond shaped in outline, with the apex inclined slightly distally. The medial crown margin describes an almost continuous smooth curve from base to apex, while the distal margin is divided into apical and basal portions by a distinct break in slope that occurs at mid-crown height. The apical portion of the distal margin is straight, while the basal portion is gently concave. The crown apex is bluntly rounded and the crown basal margin is gently convex in an apical direction, forming a shallow notch for the reception of a replacement tooth. Only the lingual surface of the crown bears enamel. The crown is 23.5 mm long and 12mm wide, giving a height-to-width ratio of ~1.96. In mesial or distal view (Fig. 6.1B, D), the crown surface is labially inclined, so that the enameled lingual surface faces dorsolingually. This also gives the tooth a very gently bowed appearance in mesial or distal view. In apical view, the crown has a subtriangular cross section, with the apex of this triangle directed labially.

6.2. Holotype tooth of ‘Iguanodon’ hillii (GSM 1966). (A) ?lingual view; (B) detail of marginal denticles, coated in ammonium chloride, with arrows highlighting the bifurcated rows of mammillae. Scale bars equal 10 mm (A) and 5 mm (B).

A sharp, very slightly distally positioned, and prominent primary ridge divides the lingual surface of the crown into two subequal areas (Fig. 6.1A). There are no secondary ridges and the lingual crown surface is otherwise smooth and unornamented, although the enamel has a fine granular texture. Small denticles are present on the apical halves of both the mesial and distal crown margins, with approximately 22 present along the distal margin and a minimum of 19 along the mesial margin (Fig. 6.1A, E). (The true number along the mesial margin is likely to have been substantially higher than that present distally, as approximately one-quarter of the denticulate part of the mesial margin is slightly damaged, which has removed the denticles from this area). The majority of the denticles are subtriangular in outline in lingual view, bear mammillae, and are oriented at approximately 45° to the apicobasal axis of the crown. However, those denticles adjacent to the crown apex are reduced to small rounded nubbins of enamel, project apically, and lack mammillae.

The broken base of the root has an arch-shaped cross section, which has a concave lingual margin, straight mesial and distal margins, and a convex labial margin. The lingual concavity is created by a pronounced groove that would have housed a replacement tooth. This groove extends along the lingual surface of the root, terminating at the base of the crown where it creates the notched basal margin of the crown (see above). In labial or lingual view (Fig. 6.1A, C), the root and unenameled part of the crown merge into each other and exhibit two well-defined vertical facets, which would have accommodated the margins of adjacent tooth crowns. The lingual-most of these is small and triangular in outline and covers the area positioned immediately labial to the lingual groove. A second larger facet, which covers most of the unenameled crown/root surface, is situated labial to the first and is divided from the lingually positioned facet by a distinct change in slope, which forms a low apicobasally extending ridge. The labial boundary of this second facet is formed by the mesiodistally convex labial surface of the tooth. These facets demonstrate that the unerupted tooth would have been in contact with at least five other teeth, set within a complex dental battery.

Referred Material Lydekker (1888b:245) provisionally referred other specimens from the Cambridge Greensand to “Trachodon cantabrigiensis”: a ?pedal first phalanx (NHMUK OR33884); a more distally positioned phalanx (mentioned as a second or third phalanx, not referred to either the manus or pes: NHMUK OR33885); and two pedal ungual phalanges (NHMUK OR33886 [suggested to be from digit 3]; NHMUK OR33887). Lydekker (1888b) provided no justification for these referrals, was unable to present exact locality information, and gave no indication that they were associated with either each other or with the holotype tooth. Nopcsa (1923:193–194, pl. 7, fig. 3) also referred two unassociated, poorly provenanced phalanges to this taxon, an ungual (CAMSM B55744) and non-ungual phalanx (CAMSM B55398). All of these elements are partially encrusted in matrix and/or heavily abraded, obscuring many salient anatomical features. For convenience, all will be described as though the foot was held in a plantigrade stance.

The first pedal phalanx (NHMUK OR33884) is poorly preserved and it cannot be determined which digit it belonged to, or to which side of the body. In dorsal view, the proximal and distal ends of the phalanx are obviously expanded with respect to the shaft, but are so abraded that the true extent of this expansion cannot be ascertained. The shaft has a subtriangular cross section at midlength, as one surface of the phalanx (here termed lateral, for convenience) is anteroposteriorly expanded relative to the medial surface. In dorsal view, the surface of the shaft is bounded laterally by a low, prominent ridge (representing the dorsal margin of the expanded lateral surface), and the rest of the surface is either gently concave mediolaterally (proximal to shaft midlength), or gently convex (distal to shaft midlength). In lateral view, the dorsal margin of the phalanx is almost straight and slopes ventrally from the proximal to the distal ends; in medial view, the dorsal margin is gently concave. In either lateral or medial view, the ventral margin of the phalanx is arched dorsally. A prominent boss of bone is present on the ventral surface, positioned just distal to the proximal articular surface, and is also visible in both lateral and medial views. In proximal view, the phalanx has a subtrapezoidal outline. The articular surfaces of both the proximal and distal ends are too poorly preserved to warrant description.

The other non-ungual pedal phalanges (NHMUK OR33885, CAMSM B55398) are anteroposteriorly shorter than they are wide mediolaterally, with length-to-width ratios of approximately 1.9 and 2.3, respectively. In dorsal view, the proximal and distal margins of the phalanges are subequal in mediolateral width and there is no distinct shaft. The proximal articular surfaces are concave, both dorsoventrally and mediolaterally, whereas the distal articular surfaces are mediolaterally concave and dorsoventrally convex (saddle shaped). In both phalanges, the ventral surface is mediolaterally and anteroposteriorly concave, while the dorsal surface is mediolaterally convex and anteroposteriorly flat, sloping slightly ventrally from the proximal to the distal end. Collateral ligament pits are absent, and the distal articulations are not developed into distinct ginglymi. All of the articular surfaces are wider than tall.

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.