BOX

Hypotheses that didn't go the distance

In the history of the study of ornithopods, habitats and anatomy conspired to put some of these animals in exotic places and give them unusual locomotor skills. For example, hadrosaurids were once regarded as amphibious, in part because the tail was long and deep (great for sculling in the water), the hand appeared to be webbed, and jaws were deemed too weak to handle anything but soft aquatic vegetation. Not true in all three cases. In a similar fashion, for over 100 years, Hypsilophodon was regarded as a tree-dweller. Upon close scrutiny by R M. Galton, however; this animal was found to have no specializations for this particularly demanding mode of life.

The combination of a strongly seasonal African habitat and some basic heterodontosaurid anatomy created a dilemma - and ultimately a solution - for R.A.Thulborn in 1978. Heterodontosaurids, he believed, chewed by moving the lower jaw forwards and backwards relative to the upper jaw.Yet, evidence of tooth replacement that he expected (given that heterodontosaurids fed on very abrasive food) simply did not exist.To replace the teeth gradually would have impaired their ability to feed, he reasoned, so the teeth could only have been replaced en masse. How could this be accomplished? Thulborn argued that heterodontosaurids must have estivated (lain dormant), most likely during the dry season. While dormant, the formerly functional teeth fell out and were replaced, to be worn down while the animal was active and feeding during each wet season.

Several years afterThulborn's estivation hypothesis appeared, J. A. Hopson re-examined heterodontosaurid jaw mechanics and tooth replacement patterns. As it turns out, heterodontosaurids chewed transversely, not forward and backward, so that tooth replacement was reduced, but not lost, in these animals.The combination of these two aspects of heterodontosaurid feeding are mutually compatible and certainly do not call for periods of dormancy to accommodate rapid tooth replacement.Thus anatomical support forThulborn's hypothesis disappeared.There is no compelling reason to believe that heterodontosaurids engaged in estivation during the harshness of the southern African climate of the Early Jurassic.

Iguanodon, may have engaged in extensive quadrupedal locomotion. These forms have a solidly built wrist and hand that clearly was capable of considerable weight support. Interestingly, for these quadrupedal ornithopods, juvenile individuals of the same species may have been more bipedal than their adult counterparts. The same appears to have been true of Tenontosaurus. Here we have an indication of shifts in locomotion with age. By and large, the larger iguanodontians were also primarily terrestrial bipeds, although not as fast running as the smaller ornithopods.

In all cases, the tail was long, muscular, strengthened by ossified tendons, and held at or near horizontal, making an excellent counterbalance for the front of the animal. In general, the powerful hindlimbs tend to be at least and sometimes more than twice the length of the forelimbs.

How fast could these dinosaurs have traveled? Larger iguanodontians such as hadrosaurids may have been able to reach 15 to 20 km/h during a sustained run, but upward of 50 km/h over a short distance. Quadrupedal galloping appears to have been unlikely, given the rigidity of the vertebral column and the limited of movement of the shoulder

Iguanodon Thumb Claw

Figure 10.

Iguanodon. thumb.

5. The hand of Note the spiked

Figure 10.

Iguanodon. thumb.

5. The hand of Note the spiked against the ribcage and sternum. For smaller ornithopods, running speeds were higher. Maximum speeds were probably on the order of 60 km/h.

Fast running, maybe, but were they smart? According to J. A. Hopson, yes, they were. In fact, Hopson suggested that they were as smart or smarter than might be expected of living crocodilians if they were scaled up to dinosaur size. For example, Leaellynasaura, a basal euornitho-pod from Victoria, Australia, was apparently quite brainy and had acute vision, as suggested by prominent optic lobes.1 In general, ornithopod braininess may relate to greater reliance on acute senses for protection that, in the absence of extensive defensive structures, may have been their only recourse. Moreover, brain size in these dinosaurs may relate to their complex behavioral repertoire, which we will discuss below.

What did the dominantly bipedal ornithopods do with their hands? To the degree that we can tell (based on proportions of limb elements, development of muscle scars, presumed habitat, etc.), Heterodontosaurus may have used its powerful forelimbs and clawed hands to grab at vegetation or to dig up roots and tubers. The forelimbs and hands of many basal euornithopods appear to have been less powerful than those of either heterodontosaurids or iguanodontians. Nevertheless, because they were not usually used in weight support, the hands were free to grasp at leaves and branches, bringing foliage closer to the mouth so that it could be nipped off by the toothed beak. By contrast, Iguanodon, Altirhinus, Ouranosaurus, and to a degree Camptosaurus have a very specialized hand as compared with those of other ornithopods (Figure 10.5). The first digit is conical and sharply pointed, likely to have been used as a stiletto-like, close-range weapon or for breaking into seeds and fruits. In contrast, the outer finger, digit V, was capable of some degree of opposition against the middle three digits, all of which were hoofed. By curling around to face the palm, the fifth digit was opposable (very much as the thumb is in humans2). The same cannot be said for hadrosaurids. Their reduced hands, with three hoofed fingers joined together in a thickened pad, hardly had any way to function other than as a support while the animal was standing on all fours. Manual dexterity was not a hadrosaurid specialty.

Feeding and food

No other group of dinosaurs has been the subject of as much research on feeding as ornithopods. Not only are their skulls marvels of intricate mechanics, but also in some cases (the so-called hadrosaurid "mummies") stomach contents have been fossilized within the abdominal region. These spectacular specimens apparently dried before burial and replacement (see Chapter 1). Preserved are beautiful skin impressions, dried, stretched tendons and muscles, and fossilized remnants of the last supper in the gut. Another reason why the feeding

1 The animal had an estimated encephalization quotient (EQ; see Box 15.4) of 1.8; Hopson estimated that the average EQ of other ornithopods is about 1.5.

2 Human success is sometimes ascribed to an opposable thumb, but, as you can see in this book dinosaurs invented opposable digits at least twice: once in ornithopods and once in theropods.

mechanisms of ornithopods have been so extensively studied is that the group obviously had some unique and remarkably sophisticated ways of processing food. Best of all, these have left a tangible, easily preserved imprint in the design of the teeth and jaws. Suggestions about how ornithopods chewed their food were made as early as 1895 and several detailed studies of ornithopod jaw mechanics have been published since 1900. In addition, it was ornithopods that gave us our first clues about the presence of fleshy cheeks, of great utility to animals that chew their food and, as we know by now, a condition that is found in all genasaurian ornithischians.

So what did ornithopods eat? The hadrosaurid "mummies," at least in life, ate twigs, berries, and coarse plant matter. This correlates nicely with their size: ornithopods are thought to have been active foragers on ground cover and low-level foliage from conifers and in some cases from deciduous shrubs and trees of the newly evolved angiosperms (Box 16.3).

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Figure 10.6. Left lateral view of the skuil of (a) Heterodontosaurus, (b) Hypsilophodon, (c) Yandusaurus, (d) Zephyrosaurus, (e) Tenontosaurus, (f) Dryosaurus, (g) Camptosaurus, (h) Iguanodon, and (i) Ouranosaurus.

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Figure 10.6. Left lateral view of the skuil of (a) Heterodontosaurus, (b) Hypsilophodon, (c) Yandusaurus, (d) Zephyrosaurus, (e) Tenontosaurus, (f) Dryosaurus, (g) Camptosaurus, (h) Iguanodon, and (i) Ouranosaurus.

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Browsing on such vegetation appears to have been concentrated within the first meter or two above the ground, but the larger animals must have been capable of reaching vegetation as high as 4 m above the ground.

Eating coarse, fibrous food requires some no-nonsense equipment in the jaw to extract enough nutrition for survival, and ornithopods had what it took (Figures 10.6, 10.7, and 10.8). In general, the group came equipped with a beak in the front for cropping vegetation, a well-developed block of teeth (the dental battery) for shearing coarse plant matter (Figure 10.9), a large, robust coronoid process for serious chewing muscles, and, as we have seen, a tooth row that was deeply set in, indicating that large fleshy cheeks were present. But beyond these basics, different ornithopods had different modifications of the jaw, and different lands of jaw motion are believed to have been used for the processing of food.

The first modern treatment of ornithopod jaw mechanics in ornithopods was an extensive study of the cranial anatomy - including

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Figure 10.7. Left lateral view of the skull of (a) Telmatosaurus, (b) Maiasaura, (c) Gryposaurus, (d) Brachylophosaurus, (e) Prosaurolophus, (f) Saurolophus, and (g) Edmontosaurus.

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Figure 10.7. Left lateral view of the skull of (a) Telmatosaurus, (b) Maiasaura, (c) Gryposaurus, (d) Brachylophosaurus, (e) Prosaurolophus, (f) Saurolophus, and (g) Edmontosaurus.

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Figure 10.8. Left lateral view of the skull of (a) Parasaurolophus, (b) Hypacrosaurus, (c) Corythosaurus, and (d) Lambeosaurus.

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Figure 10.9. Upper tooth of (a) Lycorbinus, (b) upper tooth of Hypsilophodon, (c) three upper teeth of Iguanodon, and (d) lower dental battery of Lambeosaurus.

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Figure 10.8. Left lateral view of the skull of (a) Parasaurolophus, (b) Hypacrosaurus, (c) Corythosaurus, and (d) Lambeosaurus.

skeletal, as well as muscular, vascular, and nervous - of North American hadrosaurids published in 1961 by J. H. Ostrom. Using these four anatomical perspectives, which provided the basis for reconstructing the pattern of chewing in these Late Cretaceous ornithopods, Ostrom suggested that hadrosaurids chewed back to front - in what is called propalinal jaw movement - on both sides of the mouth at the same time.

Other dinosaur paleontologists have suggested otherwise, at least for different ornithopods. P. M. Galton noted that Hypsilophodon may have chewed in much the same way as many mammals do today - side-to-side on one side of the mouth at a time. R. A. Thulborn regarded chewing in heterodontosaurids as similar to what Ostrom suggested for hadrosaurids: bilateral propalinal jaw movement.

Figure 10.9. Upper tooth of (a) Lycorbinus, (b) upper tooth of Hypsilophodon, (c) three upper teeth of Iguanodon, and (d) lower dental battery of Lambeosaurus.

Hypsilophodon Jaw Mechanics

Figure 10.10. (a) Jaw mechanics in Euornithopoda, showing lateral mobility of the upper jaws (pleurokinesis), and (b) in Heterodontosauridae, showing medial mobility of the lower jaws.

More recently, the ways in which these herbivores chewed their food and how these jaw mechanisms evolved have been the focus of considerable research by D. B. Norman, D. B. Weishampel, A. W. Crompton, and J. Attridge. These studies have been based not only on comparisons of ornithopod skulls and teeth, but also on computer analyses of cranial mobility that might translate into special lands of movement between chewing teeth.

What emerges from these studies is yet again more ornithopod diversity, this time at the level of feeding and foodstuffs. In the most primitive ornithopods, the very front of the cornified beak was relatively narrow and lacked teeth, suggesting a somewhat selective cropping ability. Iguanodontians, by contrast, lose their front teeth, broaden their snouts, and even develop a strongly serrate margin to their rhampho-theca. These animals were not selective feeders; instead, they hacked at and severed leaves and branches without much regard for what they were talcing in. Whereas basal ornithopods were careful nibblers, most iguanodontians were lawn-mowers.

Once these gulp-fulls of leaves had passed the rhamphotheca into the mouth, all ornithopods chewed their food. Yet how they solved the problem of combining bilateral occlusion (where the teeth meet on both sides of the jaws at the same time) with chewing is one of the most intriguing aspects of dinosaur feeding, for both heterodontosaurids and euornithopods evolved different solutions to this problem, solutions that both parallel those "invented" by ungulate mammals (such as sheep or horses) but remain uniquely distinct from them and from each another.

On the basis of their skull architecture, patterns of tooth wear, and computer modeling, we know that heterodontosaurids chewed by combining vertical movement of the lower jaws with a slight degree of rotation of the mandible about their long axes (Figure 10.10b). In this way, they were able to move their upper and lower teeth in a transverse direction and thus break up the bits of plant food that the tongue had placed between them. Naturally, the fleshy cheeks prevented most of the food from falling out of the corners of the mouth.

Euornithopods, on the other hand, evolved a distinctly different pattern of skull movement in order to solve the problem of having bilateral occlusion and still chewing from side to side. Instead of loosening up the lower jaws to rotate about their long axes, euornithopods mobilized their upper jaws. This kind of mechanism, which Norman called pleurokinesis, involved a slight rotation of portions of the upper jaw, especially the maxilla (the bone that contains the upper teeth), relative to the snout and skull roof (Figure 10.10a). When the upper and lower teeth were brought into contact on both right and left sides, the upper jaws rotated laterally and the opposing surfaces of the teeth sheared past one another to break up plant food in the mouth. Unlike humans, in which the bones of the skull are solidly fused and locked together, an adaptation such as this requires flexibility at the joints between bones of the skull. In hadrosaurids, the complex occlusal surfaces afforded by the development of a dental battery would have made short work of virtually all foliage. Like the situation in heterodon-tosaurids, pleurokinesis represented an important advance for euor-nithopods, providing them the ability to chew a variety of plant foods, including those with a great deal of fiber.

As in all of the other ornithischians that have been discussed, once the food was properly chewed, it was swallowed and quicldy passed into a capacious gut, which was present in all ornithopods and appears to have been relatively larger in the absolutely larger iguanodontians. Between the extensive chewing of food in the mouth and fermentation in the large gut, it is very likely that all ornithopods were well suited for a subsistence diet of low-quality, high-fiber vegetation.

Social behavior From the time of their discovery, ornithopods of all kinds have attracted a good deal of attention, particularly for their oddly appearing ornamentation. The apparently outlandish crests on the heads - many of them hollow and highly chambered - of hadrosaurids, the tusks of heterodontosaurids, and the lumps on the forehead of Ouranosaurus, have called out for an explanation. It is safe to say that virtually all of these features - like those odd bumps and horns of cer-atopsians, and for that matter the antlers of deer and horns of cows and antelope - hint at sophisticated social behavior.

Hadrosaurids have attracted the most attention, in large part because they clearly stand out from the crowd with their wild headgear. Once thought to relate to the aquatic habits of the group (see Box 10.1) or to the olfactory (sense of smell) function of the nasal cavity, much of the discussion about the functional significance of hadrosaurid ornamentation now centers on combat, display, and their reproductive consequences. In 1975, J. A. Hopson suggested that the unusual cranial features - principally involving the nasal cavity - that we see in hadrosaurids probably evolved in the context of social behavior among members of the same species. In particular, Hopson regarded the special cranial features in hadrosaurids as indicative of either intra- or interspecific aggression, more especially in the case of both solid and hollow crests in visual and vocal display In order for crests to function as good signals to convey information about what species, what sex, and even rank an individual might be, they must be both visually and vocally distinctive. Only then can they be regarded as promoting successful matings by informing the consenting adults.

So how are we ever to make sense of these suggestions about unfos-silizable behavior? Hopson made five predictions that link the fossil record of hadrosaurids to the social behaviors he anticipated were driven by sexual selection. First, to interpret incoming display information, hadrosaurids must have had both good hearing and good vision. These are qualities that cannot be measured directly in extinct vertebrates, but all hadrosaurids have large eye sockets, often with sclerotic rings that would have encircled the outer region of the eye. In all cases, eye size was quite large and so sight must have been reasonably acute. Similarly, we

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Figure 10.1 I. Growth and sexual dimorphism in lambeosaurine hadrosaurids. (a) Juvenile and (b) adult Corythosaurus. (c) Male and (d) female Lambeosaurus.

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Figure 10.1 I. Growth and sexual dimorphism in lambeosaurine hadrosaurids. (a) Juvenile and (b) adult Corythosaurus. (c) Male and (d) female Lambeosaurus.

have evidence of the hearing via preserved middle and inner ear structures, also indicative of reasonable hearing across a wide range of frequencies in these animals.

Secondly, if the crest serves for visual display and as a vocal resonator, then its shape need not necessarily closely follow the shape of the cavities contained within. That is, the external shape of the crest may have been as important as its internal structure if it was to act in visual display. Again, this prediction is upheld by hadrosaurid fossils: in virtually all cases, the profile of the crest is much more elaborate or extensive than the walls of the internal plumbing.

If crests acted as visual signals (prediction 3), then they should be species specific in size and shape, and they should also be sexually dimorphic. This is amply upheld in large part thanks to studies by P. Dodson on the growth and development in lambeosaurine hadrosaurids (Figure 10.11). Using a variety of statistical techniques, Dodson was able

10 cm to show that crests become most prominent when an animal approached sexual maturity. In addition, he demonstrated that each lambeosaurine species was dimorphic, particularly in terms of crest size and shape. Could these "morphs" be male and female? It certainly fits well with Hopson's prediction.

The last two predictions have to do with hadrosaurids in time and space. When several species occur together in the same area, they should exhibit great differences in the shape of their crests. Sameness would create a great deal of confusion among closely related hadrosaurids living in the same place, but distinctiveness in display structures would prevent such confusion, an obvious advantage during breeding season. Are crests more distinctive as the number of hadrosaurids living together goes up? The answer is "Yes." At Dinosaur Provincial Park in Alberta, Canada, where the number of hadrosaurids that have been found in the Dinosaur Park Formation (and thus thought to have lived together) is high, there are three distinctively crested lambeosaurines, one solid-crested form (Prosaurolophus) and two other species of hadrosaurine, each distinctive in its own right. In contrast, elsewhere where hadrosaurid diversity is lower, the variety of flamboyant headdresses is decreased.

The last prediction, that crests should become more distinctive through time as a consequence of sexual selection (see Chapter 8), is not at all well supported. Hopson depended on the older, small-crested Prosaurolophus and the younger, large-crested Saurolophus being closely related, but this no longer seems to be the case (see below). In addition, lambeosaurine crests arguably become less distinctive over time.

If these supracranial contraptions were used for species recognition, intraspecific combat, ritualized display, courtship, parent-offspring

Figure 10.12. Brachylophosaurus, a solid-crested hadrosaurid from western North America.

famil'iy11»"»»'""""""-

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Figure 10.13. The circumnarial depression (indicated by cross-hatched region) which may have supported an inflatable flap of skin in hadrosaurines such as Gryposaurus (a). Highly modified nasal cavity housed within the hollow crest on top of the head of Lambeosaurus (b).

communication, and social ranking, the accentuated nasal arch and stout cranial crests seen in Gryposaurus, Maiasaura, and Brachylophosaurus were probably used for broadside or head-pushing during male-male combat (Figure 10.12). Hopson suggested that inflatable flaps of skin covered their nostrils and surrounding regions (Figure 10.13); these would have been blown up and used for visual display, as well as to make some noise - a kind of Mesozoic bagpipe. In Prosaurolophus and Saurolophus, this sac would have extended onto the solid crest that extended above the eyes, while in Edmontosaurus, where the nasal arch is not accentuated nor is there a crest, the complexly excavated nostril region may have housed an inflatable sac (Figure 10.14). With such an exceptional development of sacs around the nostrils and up and down the crest, ritualized combat with accompanying vocal and visual display became the norm.

When it came to display, none did it better than the lambeosaurines. In these animals, the hollow crests perched atop the head must have provided for instant recognition. This could have been achieved visually and by low honking tones produced in the large resonating chamber within the crest (Figures 10.13 and 10.15). Either way, by sight and/or through vocal cacophony, the crests of lambeosaurines would have functioned well as species-specific display organs.

Here we have a compelling case for hadrosaurid social behavior, but what of other ornithopods? Although the results are not as conclusive, it appears that the evolution of canine-like teeth of hetero-dontosaurids may have had something to do with intraspecific display and combat. Thulborn and R. E. Molnar independently suggested that, since these teeth are present only in mature "males," they would have been used not only in gender recognition, but also for intraspecific combat, ritualized display, social ranking, and possibly even courtship. A modern analogue is the tusked tragulids, living artiodactyls related to deer. Similarly, the development of a jugal boss in heterodontosaurids might also be interpreted as a form of visual display.

Figure 10.14. Edmontosaurus, a flat-headed hadrosaurid from the western USA.

Likewise, the low, broad bumps on top of the head of Ouranosaurus and the arched snout of Muttaburrasaurus and Altirhinus may well have similar behavioral significance (Figure 10.16). Perhaps these bumps aided individuals in the recognition of members of the same species, or members of the opposite sex. Or perhaps they were used in ritualized head-butting contests. Ouranosaurus was also equipped with extremely high spines on the vertebrae, which formed a high, almost sail-like ridge down its back. Like the case of Stegosaurus (see Chapter 6), it is possible that these long spines were covered with skin and used as a radiator or solar panel, to warm up or cool down. Alternatively (and not mutually

Figure 10.15. Corythosaurus, a hollow-crested hadrosaurid from the Late Cretaceous of western Canada. (Photograph courtesy of the Royal Ontario Museum.)

exclusively), they may have had a display function, providing the animal with a greater side profile than it would otherwise have had.

Display behavior in many ornithopods begins to make even more sense when considered with other aspects of their lifestyles. Consider communication between adults and between grown-ups and juveniles. There are several examples of single-species bonebeds - for example, Dryosaurus, Iguanodon, Maiasaura, Hypacrosaurus, and others - that support the notion that these animals were not only common but may have formed herds of

Figure 10.16. The Early Cretaceous iguanodontian Ouranosaurus from Niger
Figure 10.17. Right lateral view of the skull and skeleton of a hatchling hadrosaurid Maiasaura.

both youngsters and adults. It has even been suggested that such large aggregations required migratory movement, most likely seasonal, in order to meet the energy demands of the members of the herd.

Intraspecific social behavior is one thing, but the secrets of dino-saurian reproductive behavior are beginning to be told. For example, Horner has produced abundant evidence that ornithopods had different ways of "bringing up baby." For example, hatchlings of Orodromeus, a relatively small basal euornithopod, had well-developed limb bones, with fully formed joint surfaces, indicating that these young could walk, run, jump, and forage for themselves as well as any adult. With the young well-formed and capable self-starters from the outset, parental care is assumed to have been minimal, if present at all. Once out and about, young Orodromeus appear to have stayed in groups, where mutual protection and a degree of communication have an advantage. It is not known whether these groups split up or remained together later in life, simply because these age groups have yet to be found in the fossil record.

Not all ornithopods took such a laissez-faire attitude toward their children. Maiasaura, Hypacrosaurus, and probably other hadrosaurids likewise, nested in colonies, digging a shallow hole in soft sediments and laying up to 17 eggs in each nest. These nests were a mother's body size apart from the next, strongly suggestive that nests were regularly tended by a parent (Mom?). Vegetation probably covered the eggs to keep them warm. Hadrosaurid hatchlings (Figure 10.17) tend to be found within their nests, having wreaked havoc on the eggs that once housed them. Consequently, we have an abundance of eggshell fragments but very poor information on complete hadrosaurid eggs. It is clear that hatchlings remained in the nest for extended periods of time, perhaps upward of eight or nine months. During this nest-bound time, the offspring were literally helpless. With poorly developed limbs, they could hardly have foraged far from the nest and must have depended on their parents to provision them with food and protection. In their dependent state, hadrosaurid hatchlings had to take real advantage of parental generosity. If our estimates of the length of the nest-bound period are correct, then it appears that hatchling growth rates were exceedingly fast, well within the range of fast-growing mammals and birds at approximately 12 cm in length per month (see Chapters 14 and 15). This means that hatchlings must have channeled virtually all of the food that their parents brought them into growth.

Once these hatchlings left the nest, they appear to have stayed together as a small cohort. But at least during the breeding season, if not for longer periods of time, these animals gathered into exceedingly large herds, which were capable of producing the large, single-species bone-beds that have been discovered. Horner estimated that a single herd could have exceeded 10,000 individuals, rivalling the multikilometer-sized bison herds that roamed the Great Plains of North America. Hadrosaurid life, therefore, probably involved much opportunity for interaction: within herds, as breeding pairs, and as families. All of this is nicely correlative with the visual and vocal communication we postulated earlier, and suggests complex social behavior.

With ornithopods of all lands in and out of the nest, and demanding or rejecting parental care, we enter perhaps one of the most elusive aspects of the fossil record: life history strategies. These strategies detail the ways in which particular organisms grow, reproduce, and die. Consider the mosquito, the blood-sucking blight of a warm summer day. These animals produce enormous numbers of eggs that result in thousands of offspring, the vast majority of which do not survive to reproduce themselves, even during their incredibly short lifespans. No parental care here - too many children for one thing and the bugs are not programmed that way anyway. Now consider us, with much longer lifespans, fewer offspring, and lots of parental care (too much, some say, when stuck with 30-somethings returning to the parental abode). In the former case, species survival is based on saturation - with so many mosquitoes some are bound to survive. This land of life history is referred to as an r-strategy, the symbol for the unrestricted, intrinsic rate of increase of individuals in a population. Because these organisms must fend for themselves, we regard them as precocial, which means that the young are rather adult-like in their behavior. The word "precocious," always used when referring to young geniuses like Mozart (who, even as a youngster, wrote music as an adult would), describes this condition to a "T." In contrast, human survival is thought to depend in large part on parental care of only a few, often slow-growing offspring. Instead of being r-strategists, we employ a K-strategy, named after the symbol for the carrying-capacity of an environment. Because the young are delaying their maturation - thereby requiring the extra input of care by parents - we refer to this condition as being altricial. So no matter how ... like ... totally and maximally mature some of our young teenagers would have us believe they are, we are biologically all altricial, as are virtually all mammals and most birds.

How do those ornithopods for which we have information conform to either of these two contrasting strategies? With their ability to fend for themselves like adults, we consider Orodromeus to have been closer to an r-strategist, a claim that is based on the inferred precocial nature of these dinosaurs. In contrast, Maiasaura, Hypacrosaurus, and perhaps other hadrosaurids that had nest-bound hatchlings requiring parental care, all appear to have been altricial and thereby K-strategists. Gazing elsewhere among ornithopods, it appears that precocity maybe primitive for at least Euornithopoda, while altricial behavior probably evolved for the first time within the clade sometime prior to the origin of Hadrosauridae.

Whatever the broader meaning of these changes might be, we - and ornithopods - cannot escape from the effects of family. For ornithopods such as Orodromeus, parenting must have been easy - no provisioning or protection of the kids. But the toll to be paid was reduced survival of these offspring - wherever it was that they wandered off to. From a hadrosaurid perspective, however, it was a good thing to take care of the kids. For no matter how loud, squawky, and hard to handle these hatch-lings might have been, Mom and Dad played a direct part in increasing their survival.

The evolution of Ornithopoda is defined as all the descendants of the common ancestor Ornithopoda a mon°phyletic clade that includes Heterodontosauridae and Euornithopoda (Figure 10.18) within this clade. Heterodontosauridae and Hadrosauridae are themselves monophyletic. Also containing a host of

Figure 10.18. Cladogram of Genasauria, emphasizing the monophyly of Ornithopoda. Derived characters include: at I pronounced ventral offset of the premaxillary tooth row relative to the maxillary tooth row, crescentic paroccipital processes, strong depression of the mandibular condyle beneath the level of the upper and lower tooth rows, elongation of the lateral process of the premaxilla to contact the lacrimal and/or prefrontal; at 2 high-crowned cheek teeth, denticles on the margins restricted to the terminal third of the tooth crown, caniniform tooth in both the premaxilla and dentary; at 3 scarf-like suture between postorbital and jugal, inflated edge on the orbital margin of the postorbital, deep postacetabular blade on the ilium, well-developed brevis shelf, laterally swollen ischial peduncle, elongate and narrow prepubic process.

Figure 10.18. Cladogram of Genasauria, emphasizing the monophyly of Ornithopoda. Derived characters include: at I pronounced ventral offset of the premaxillary tooth row relative to the maxillary tooth row, crescentic paroccipital processes, strong depression of the mandibular condyle beneath the level of the upper and lower tooth rows, elongation of the lateral process of the premaxilla to contact the lacrimal and/or prefrontal; at 2 high-crowned cheek teeth, denticles on the margins restricted to the terminal third of the tooth crown, caniniform tooth in both the premaxilla and dentary; at 3 scarf-like suture between postorbital and jugal, inflated edge on the orbital margin of the postorbital, deep postacetabular blade on the ilium, well-developed brevis shelf, laterally swollen ischial peduncle, elongate and narrow prepubic process.

basal forms such as Thescelosaurus, Hypsilophodon, Gasparinisaura, and Agilisaurus, as well as Dryosauridae and forms sequentially more-closely related to Hadrosauridae, Ornithopoda is diagnosed on the basis of a number of derived features, among them pronounced ventral offset of the premaxillary tooth row relative to the maxillary tooth row, crescentic paroccipital processes, strong depression of the mandibular condyle beneath the level of the maxillary and dentary tooth rows, and elongation of the lateral process of the premaxilla to contact the lacrimal and/or prefrontal bones.

Where do ornithopods reside among the dinosaurs? They and margino-cephalians form a monophyletic group called Cerapoda. Successively larger groups in the hierarchy are Thyreophora and Lesothosaurus (see introductory text to Part II: Ornithischia) to form Ornithischia. The next larger inclusive node on the cladogram is Saurischia, which is the basal division within Dinosauria.

Given the high diversity (and phylogenetic complexity) of ornithopods, our aim in this section is to provide a basic overview of the phylogeny of this important group of ornithischians. We will therefore discuss the general shape of the ornithopod cladogram, noting the major divisions and the features that support these clades. Much of what follows is based on a wealth of discussion on ornithopod phylogeny, including the 1984 studies by P. C. Sereno, Norman, and A. Milner and Norman, as well as more recent work by Sereno, Norman, and Weishampel and colleagues.

Heterodontosauridae Basally, Ornithopoda are divided into Heterodontosauridae and Euor-

nithopoda. The first of these groups, Heterodontosauridae is defined as all the descendants of the common ancestor of Heterodontosaurus and Lanasaurus. Basally in the history of this clade, which also contains Lycorhinus and Abrictosaurus, these heterodontosaurids evolved high-crowned teeth, each bearing a chisel-shaped crown ornamented with denticles. In addition, and the principal basis for the name "hetero-dontosaurid," a large caninelike tooth is present in both upper and lower jaws. These "canines" are not the true canine teeth that characterize mammals.

Euornithopoda Euornithopoda constitutes the remaining ornithopod clade (Figure 10.19). Defined as all the descendants of the common ancestor of Agilisaurus and Iguanodontia, euornithopods are characterized by features of the orbit (eye socket: scarf-like suture between postorbital and jugal, an inflated edge on the orbital margin of the postorbital bone), the pelvis (deep postacetabular blade on the ilium, well-developed brevis shelf, laterally swollen ischial peduncle, elongate and narrow prepubic process, tab-like obturator process on the ischial shaft), and femur (deep pit on the femoral shaft adjacent to the fourth trochanter).

This large euornithopod clade consists of a host of often relatively small, agile ornithopods such as Hypsilophodon, Agilisaurus, and Gasparinisaura, as well as a few somewhat larger, more robust forms (Parksosaurus,

'nithopoda Ornithopoda

Figure 10.19. Cladogram of basal Euornithopoda, with more distant relationships with Heterodontosauridae and Marginocephalia. Derived characters include: at I 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 of the medial condyle of the femur

'nithopoda Ornithopoda

Figure 10.19. Cladogram of basal Euornithopoda, with more distant relationships with Heterodontosauridae and Marginocephalia. Derived characters include: at I 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 of the medial condyle of the femur

Thescelosaurus), and Iguanodontia, residence of such dinosaurian luminaries as Camptosaurus (Figure 10.20), Iguanodon, and the hadrosaurids. Many of the smaller basal euornithopods had been placed in a group called Hypsilophodontidae, thought at that time to be a natural (mono-phyletic) group. However, with the discovery of new material, the recognition of new taxa, and the use of cladistic analyses to test the relationships of both new and earlier-known forms, Hypsilophodontidae has not stood up to scrutiny and therefore is now abandoned. In its place is the serial arrangement of Agilisaurus, Hypsilophodon, Gasparinisaura, Thescelosaurus, Zalmoxes, and other less well-known taxa more closely related to Iguanodontia. The evolution of these taxa successively entails modifications of the facial skeleton (reduction in size and rounding of the external antorbital fenestra, loss of contact between the lacrimal and pre-maxilla, reduction in length and repositioning of the palpebral), teeth (development of asymmetrical crowns, distinct cingulum), ribcage (ossification of sternal ribs), the development of ossified hypaxial tendons, and shoulder (distinct neck on scapula, angular deltopectoral crest).

How the many other basal members of Euornithopoda fit into this phylogeny remains to be seen. They may "shoe-horn" in nicely or they may

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Responses

  • Helen Richardson
    Are the Hadrosauridae smart?
    9 years ago
  • SOPHIA
    Why is it said that ornithopods are the deer and antelopes of the mesozoic?
    8 years ago

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