Figure 7.19. Cladogram of basal Ornithopoda. Derived characters include: at 1, subcircular external antorbital fenestra, distal offset to apex of maxillary crowns, strongly constricted neck to the scapular blade, ossification of sternal ribs, hypaxial ossified tendons in the tail; at 2, rectangular lower margin of the orbit, widening of the frontals, broadly rounded predentary, dentary with parallel dorsal and ventral margin, absence of premaxillary teeth, 10 or more cervical vertebrae, 6 or more sacral vertebrae, presence of an anterior intercondylar groove, inflation ofthe medial condyle of the femur.
Figure 7.20. Cladogram of basal Iguan-odontia. Derived characters include: at 1, premaxilla with a transversely expanded and edentulous margin, reduction of the antorbital fenestra, denticulate margin of the predentary, deep dentary ramus, loss of sternal rib ossification, loss of a phalanx in digit III of the hand, compressed and blade-shaped prepubic process; at 2, strong offset of premaxilla margin relative to the maxilla, peg-in-socket articulation between maxilla and jugal, development of a pronounced diastema between the beak and mesial dentition, mammillations on marginal denticles of teeth, maxillary crowns narrower and more lanceolate than dentary crowns, closely appressed metacarpals II—IV, deep triangular fourth trochanter, deep extensor groove on femur.
somewhat larger, more robust forms (Parksosaurus and Thescelosaurus), and the diverse clade Iguanodontia (Figure 7.20), residence of such dinosaurian luminaries as Camptosaurus (Figure 7.21), Iguanodon, and all the hadrosaurids. In general, these animals tended to reach their apogee in the Late Jurassic-Early Cretaceous interval.
Hadrosaurids are among the best-known of all dinosaurs, with a fabulous fossil record that allows, as we have seen, insights into their behavior. Two major clades within hadrosaurids - Lambeosaurinae and Hadrosauridinae - constitute most of Hadrosauridae (Figure 7.22), with a few forms left whose relationships within Hadrosauridae are uncertain.
Several interesting evolutionary trends are present within Ornithopoda. It is perhaps no coincidence that ornithopod diversity seems to parallel gymnosperm and angiosperm
Figure 7.22. Cladogram of Hadrosauridae. Derived characters include: at 1, three or more replacement teeth per tooth position, posterior extension of the dentary tooth row to behind the apex of the coronoid process, absence of the surangular foramen, absence or fusion of the supraorbital to the orbit rim, long coracoid process, dorsoventrally narrow proximal scapula, very deep, often tunnel-like intercondylar extensor groove; at 2, absence of the coronoid bone, reduction in surangular contribution to coronoid process, double-layered premaxillary oral margin, triangular occiput, eight or more sacral vertebrae, reduced carpus, fully open pubic obturator foramen, absence of distal tarsals II and III; at 3, maxilla lacking an anterior process but developing a sloping dorsal shelf, groove on the posterolateral process of the premaxilla, low maxillary apex, a parietal crest less than half the length of the supratemporal fenestrae; at 4, presence of a caudal margin on the circumnarial fossa.
diversity (see Figure 13.9), suggesting that these dinosaurs and plants may have been involved in a kind reciprocal pas de deux: as gymnosperms developed ways to discourage predation, ornithopods developed more and more efficient ways of extracting nutrients. The reciprocal evolution culminated in the highly efficient pleurokinetic hadrosaurid jaw, with its well-developed, integrated packages of teeth in continuously replacing dental batteries. Overall patterns within the Late Cretaceous of North America and Asia, at least, suggest that hadrosaurids ecologically replaced large non-hadrosaurid iguanodontians. Hadrosaurids arguably evolved the art of chewing to levels of sophistication unparalleled in the history of life.
Along with the specializations associated with chewing, ornithopod evolution may have also been characterized by a greater and greater investment of parents in their young. We have seen that Orodromeus produced relatively precocial offspring; its basal position within Euornithopoda (see Figure 7.19) suggests that precocity may be primitive for at least Euornithopoda. As the diversity of Euornithopoda increased, altricial behavior likely evolved in more derived euornithopods some time prior to the origin of Hadrosauridae, which all are thought to have given birth to altricial young.
Was this article helpful?