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Clypeodonta Clypeodontans exhibit a unique combination of features involved in the differentiation of the dentary and maxillary dentitions (partially integrated opposing occlusal surfaces and the creation of incipient dental magazines of smaller teeth). Dryomorphans link these dental modifications with others that stabilize the anterior end of the lower jaw (the ventrally bilobed predentary). The restructuring of the pelvis, notably through the development of a structurally dominant, J-shaped ischium (functionally replacing the elongate, but slender, posterior pubic ramus) suggests the need to support a more massive and by implication more complex gut, which reflects the increasing dietary sophistication implied from the structure of the jaws and teeth. Furthermore, the specialization of the knee joint, through the development of a defined anterior intercondylar groove, may be correlated with increasing strength of the joint (to cope with the more massive gut) and at the same time maintaining or even improving joint mechanics and locomotor ability.

2.29. Cladogram generated by the analysis of Wu and Godefroit (2012). Modified in order to present the topology that reflects the taxa considered in this account. Clade names and positions indicated as in the original version.

Neoiguanodontia Neoiguanodontians are notable for the development of larger and longer ( jaw-dominated) skulls with larger and increasingly structurally sophisticated teeth. Teeth form incipient dental magazines that show increasing signs of integration into mutually supportive arrangements, creating the equivalent of single functional megatooth in each jaw. The leverage exerted by the jaw muscles is augmented by the development of a tall, perpendicular coronoid process, and there are a number of lineages that appear to experiment with the functional separation and specialization of food gathering (the premaxillary and predentary beak) and food processing (cheek teeth) through the development of a diastema. It is very notable that some of the more derived taxa tend to develop a broader, down-turned snout (Altirhinus, Protohadros, Probactrosaurus), which anticipates a feature seen among euhadrosaurs and suggests that these animals were functionally modifying their food gathering abilities. Whether this trend can be linked to vegetational changes is uncertain at present. The skeletons of neoiguanodontians are generally large (6–12 m in body length), their guts are clearly very large and heavy, and their limbs become increasingly stout and graviportally adapted. Linked to the generally pedestrian build of their bodies, these forms develop a specialized spike-like pollex that is anchored to an enlarged and fused, or partially fused, set of carpometacarpals. These anatomical features correlate with stoutly constructed pectoral and forelimb skeletons that were evidently adapted for weight support and locomotion. The pollex may have had multiple functions, but as a potential stilleto-like weapon of defense, wielded at close quarters, this may well have proved advantageous in animals that lacked obvious cursorial abilities.

Hadrosauromorpha Rather unexpectedly, the taxa that form successional sister taxa to the clade Euhadrosauria exhibit relatively conservative anatomies, insofar as their skulls are concerned. Within their jaws, the fully integrated and interdentally cemented dentary magazine (megatooth) with interlocked, diamond-shaped dentary teeth and multiple successional crowns is firmly established. The coronoid process is both tall and expanded apically, suggesting that both the volume of musculature that can be recruited to jaw closure and its lever-arm mechanics have been augmented. The dental magazine also begins to migrate distally along the jaw, medial to the coronoid process. Counterintuitively, given the recognition (above) of a diffuse trend focusing upon elaboration of the shape and proportions of the lower jaw in derived neoiguanodontians, there is little evidence for elongation, deepening, or curvature of the dentary. The diastema is comparatively abbreviated and the specialized functional separation between the cropping (beak) and food processing (dental battery) regions in the jaw that had become apparent in (for example) Mantellisaurus, Ouranosaurus, Altirhinus, and Protohadros is far less evident.

The most derived non-hadrosauromorphan neoiguanodontian in this analysis (Probactrosaurus) exhibits the onset of a trend that culminates in the loss of both the substantial carpometacarpus and manus digit I (the latter taxon being still characterized by its offset, small conical pollex ungual). As a functionally linked consequence, hadrosauromorphans show a reduction in robustness of the antebrachium and manus, which had previously been associated with the specialized locomotor and weight-supporting structures, consistently seen in more basally positioned neoiguanodontians.

No hadrosauromorphan exhibits a pollex spine or the heavily co-ossified carpus (even though modest-sized hoof-like unguals are present on digits II and III of the manus), and the forelimb and manus when considered as a whole present comparatively gracile proportions combining a short, sinuous humerus with elongation and slenderness in the more distal elements (antebrachium and metacarpals). The pectoral girdle shows a reduction in the size of the coracoid and modification to the structure of the scapular acromion that appears to correlate with the gracility of the forelimb. The pelvis is modified primarily by the elaboration of the dorsal margin of the ilium to create a discrete pendule and a postacetabular process that is bar-like and laterally compressed. Since the dorsal margin of the ilium and the brevis shelf of the postacetabular process are areas for the origin of significant hindlimb musculature (Norman, 1986; Maidment et al., this volume), these anatomical changes are suggestive of changes in hindlimb functionality. The hindlimb is characterized by a columnar (straight) femoral shaft, a globular femoral head (which lacks the femoral head notch seen in more basal forms), and a completely enclosed (tunnel-like) anterior intercondylar groove. The pedal unguals become short and remarkably hoof shaped, rather than elongate with truncated tips and claw grooves as seen in more basally positioned neoiguanodontians.

2.30. Summary cladogram calibrated against the geological timescale (Ma ages listed along right side of timescale). New clade names discussed in the text.

Euhadrosaurians Derived hadrosauromorphans (euhadrosaurians) represent a culmination of the general trends seen through more primitive neoiguanodontians and hadrosauromorphans. The elongation of upper and lower jaws creates a prominent diastema and an undoubted functional separation between the cropping and food processing components of jaw action. Linked to this functional differentiation, the muzzle/beak (and its ensheathing rhamphotheca) can be seen to become increasingly diverse in morphology (ranging from a droop-tipped and comparatively narrow morphology common among lambeosaurines to extreme transverse expansions bordered by upturned margins seen in some hadrosaurines/saurolophines). These variations in muzzle form are suggestive of ecological separation (niche partitioning) with respect to feeding guilds within euhadrosaurians (Carrano et al., 1999). The dental magazines are highly integrated and migrate distally (posteriorly) along the jaw so that they come to lie partly behind the elevated and anteriorly curved coronoid process; the effect of these changes is to increase the lever-arm mechanics (and hence efficiency) of the jaw muscles by promoting the development of a “bent first-order lever” from the traditional diapsid low-efficiency third-order lever mechanics that are associated with most non-mammalian vertebrate jaws. In addition to the jaw anatomy changes, the architecture of the roof of the skull becomes modified in a variety of ways: the temporal region becomes robust in order to withstand the stresses imposed upon the skull roof by the large and complex jaw adductors, and modifications of the nasal vestibule and dorsally projecting crests have been linked to a variety of biological functions. These include olfaction, sound production, and visual recognition (Ostrom, 1961; Hopson, 1975; Weishampel, 1981) that are suggestive of increasingly complex social interactions between euhadrosaurs. The postcranial skeleton differs little from that seen in hadrosauromorphans, except that the sacrum incorporates many more sacrals and produces an extremely strong region to support the stresses generated through weight-support using the hind limbs almost exclusively. It is also worth noting that some euhadrosaurs are the largest of all ornithopods (15+ m long) and yet retained a facultatively bipedal locomotor strategy. As noted above, the forelimbs are comparatively slender and distally elongate, but the unguals of digits II and III are hoof shaped. It is presumed their forelimbs were used for support while feeding upon low browse or when moving slowly and cautiously when feeding or moving in, for example, crowded colonial nesting sites, or indulging in nest building and related activities. So it appears that the mechanical efficiency of their hindlimb support and locomotor system was considerable. In contrast it is the case that among more basal neoiguanodontians an upper size range (~11 m long) is accompanied by a consistent tendency to become specialized by becoming secondarily obligate quadrupeds (Norman, 1980).

The evidence based upon times of occurrence in the fossil record (Prieto-Márquez, 2010:fig. 10) suggests that the pattern of diversification of euhadrosaurians displays a significant lag phase during the Coniacian–Santonian before a log-phase diversification in the Campanian and an equilibration during the Maastrichtian. Whether this pattern is an artifact of preservation in the fossil record, or represents some element of “bottle-necking” associated with the process of assembly of euhadrosaurian anatomy (and implicit biology), cannot yet be resolved satisfactorily.