Читать книгу Hadrosaurs - David A. Eberth - Страница 49

1. Maxillary tooth crowns are apicobasally elongate and characterized by the possession of a narrow, elevated primary ridge on the labial surface of the crown that is positioned slightly distal to the midline.

2. Dentary tooth crowns exhibit a clearly defined, but low, primary ridge that is offset distally to the midline.

3. In lingual view a shoulder-like edge is formed between the denticle that marks the tip of the primary ridge and the point of inflection that marks the start of the subvertical mesial margin of the crown.

4. The quadrate bears a well-defined semicircular notch on the central portion of the anterior margin of the jugal wing that is spanned anteriorly by the quadratojugal. With the quadratojugal in articulation a fully enclosed quadrate (paraquadratic) foramen is formed.

5. Predentary with divergent, ventrolateral processes.

6. The obturator process of the ischium is positioned proximally (separated by an embayment from the pubic peduncle of the ischium) on the anteromedial edge of the shaft of the ischium.

7. The ischial shaft has a subcircular cross section along almost its entire length and there is a small anterior expansion of the distal tip of the ischial shaft that forms a “boot.”

8. The femoral shaft displays an anterior intercondylar groove that is defined by a pair of ridges that form on the adjacent edges of the dorsal part of the distal articular condyles.

Comments There is a marked contrast between the style of morphological differentiation of the dentition in basal clypeodontans and that in dryomorphans, which suggests that a new morpho-functional trajectory (probably linked to oral food processing) had emerged. Additional characters, such as the development of a bilobate posteroventral processes on the predentary, probably served to reinforce a structurally weak dentary symphysis and may reflect alterations to jaw function. A ventrally bilobate predentary also appears sporadically in basal clypeodonts (rhabdodontids), but this is interpreted as an example of convergence (homoplasy) in the larger-bodied tenontosaurs and the very late appearing (Maastrichtian) rhabdodontids. The ischial characters (such as the positioning of the obturator process and curvature of the ischial shaft) also appear sporadically and probably convergently in Maastrichtian rhabdodontids.