FIGURE 11.9 The hadrosaurid Maiasaura with its nest and juveniles, based on associated body and trace fossil material from the Late Cretaceous Two Medicine Formation of Montana. Fernbank Museum of Natural History, Atlanta, Georgia; restoration on loan from the Museum of the Rockies, Bozeman, Montana.

FIGURE 11.9 The hadrosaurid Maiasaura with its nest and juveniles, based on associated body and trace fossil material from the Late Cretaceous Two Medicine Formation of Montana. Fernbank Museum of Natural History, Atlanta, Georgia; restoration on loan from the Museum of the Rockies, Bozeman, Montana.

to 4000 cm3. In the absence of in-situ embryos, connecting egglayers with dinosaur eggs is always fraught with risk, but a few ornithopod examples provide well-supported identifications. The most complete information regarding eggs, embryos, nest structures, and associated juveniles for an ornithopod species is represented by the Late Cretaceous hadrosaurine Maiasaura pebblesorum of western North America (Fig. 11.9).

Clutch sizes for Maiasaura consisted of a minimum of 11 eggs. However, the arrangement of the clutch and whether the eggs in a nest structure actually represent a single egg-laying episode are still uncertain. These trace fossils were sedimentary structures distinct from the surrounding sediment, so they were probably mound nests. Fossil evidence for vegetation in the nests, such as carbonized plant material or leaf impressions, is currently lacking. Some paleontologists, however, have inferred that vegetation must have been present for protection and keeping eggs at near-constant incubation temperatures. No information has conclusively shown that Maiasaura, or any other ornithopods, actively brooded their clutches by sitting on their nests, as proposed for smaller theropods such as Oviraptor and Troodon. Maiasaura must have done nest building through digging, piling, and compacting sediment into the described structures, presumably using their limbs but they may also have put their shovel-shaped

Nest structures occur at the same horizons in some locations, suggesting that at least some of them were contemporaneous. This coincidence implies communal nesting grounds for this species. Interestingly, no root structures or other evidence of freest structures associated with Maiasaura eggs and juvenile remains are circular bowl-shaped depressions about 2 meters wide, and 0.8 meters deep, with slopes of nearly 30° in places.

snouts to good use.

in-situ vegetation is noticeable between nest structures, which probably means that the mounds were made in open areas without substantial tree cover. The nest structures are about seven meters apart from one another, which is the length of an adult Maiasaura. This distance may point toward a respect for living space by different Maiasaura parents. For example, body-length spacing (at a minimum) would have helped to prevent adults from stumbling over or into nests belonging to other adults. The recurrence of nesting horizons on different stratigraphic levels but in the same area argues persuasively for site fidelity, where the maiasaurs returned to the same area year after year for nesting and brooding. Such site fidelity for dinosaur nesting is also interpreted for Late Cretaceous titanosaurids in Argentina (Chapter 10).

The most startling hypothesis that emerged from detailed examination of these nesting grounds is that the adults took care of their young for extended periods of time after they hatched. Data supporting this hypothesis were based on the following observations and inferences:

■ Juvenile remains of as many as 15 individual Maiasaura also occur in several of the nest structures. These skeletons reveal that some individuals were as much as 3 meters long while still in the nest, so they may have been several years old.

■ Eggshell fragments are in the nest structure with the bones and have the appearance of having been broken by more than simple hatching. The small fragments could be a result of daily trampling by the juveniles. Alternatively, this circumstance may also have a non-biological cause, such as compaction by overlying sediments.

■ Limb joints of the smaller juveniles show little evidence of ossified bone, meaning that they were still cartilaginous and thus not well suited for supporting their weight (Chapter 5). This undeveloped state implies a limited mobility, so the juveniles were altricial, in contrast to some theropod juvenile limbs that show cursorial abilities at an early age (precocial).

■ The large size of some juveniles indicates that they were eating, despite dwelling in or near nest structures located in an area that had no evidence of nearby vegetation. The lack of nearby food sources combined with their altricial condition meant that food was brought to the juveniles, probably by their parents, so that they were able to thrive. Even if the juveniles became semi-precocial and were able to move out of the nest, they would have had little incentive to leave home while they were still being fed.

Another ornithopod, Hypacrosaurus, probably also had nesting grounds. A horizon in the Late Cretaceous of Montana associated with Hypacrosaurus remains, consisting of an overwhelming number of eggshell fragments, can be followed for 3 km. This egg horizon represents the richest known concentration of dinosaur egg material in North America. Unfortunately, the sequential evidence of egg/nest structure/ embryo/juvenile/adult established for Maiasaura is lacking for Hypacrosaurus, simply because not as many of the criteria are so clearly defined. Nevertheless, Hypacrosaurus eggs are well defined. They have a spherical geometry and about three times the volume of those reported for Maiasaura (1200 cm3 versus 3900 cm3), although the clutch sizes have not been established. A still unidentified lambeo-saurine egg clutch, also discovered in Late Cretaceous rocks of Montana, has an egg size and shape similar to that of Hypacrosaurus and a clutch size of 22. If this clutch size does indeed reflect a single egg-laying episode by a lambeosaurine mother, then it had an internal capacity in its reproductive tract of about 0.085 m3, which corresponds to about 85 liters of liquid volume. Such calculations indicate the probable large size of whatever species of lambeosaurine laid the eggs. Clutch size is i positively correlated with body size in modern crocodilians and the same condition is reasonably assumed for dinosaurs.

Other ornithopod embryos have been reported for the Late Jurassic iguanodon-tians Dryosaurus and Camptosaurus of North America, although nest structures and associated egg material for these species are still unknown. The disappointing lack of definite attribution of eggs, embryos, and nest structures for heterodontosaurids and other non-iguantonian ornithopods means that hypotheses concerning the evolution of nurturing behavior in these ornithopods are still untested. For example, the small ornithopod Orodromeus was originally considered the nestmaker and egglayer responsible for dinosaur egg assemblages in Late Cretaceous strata of Montana. This conclusion was made on the basis of the closely associated remains of near-embryonic juveniles. However, this hypothesis was falsified when the interior of one of the eggs revealed an embryonic Troodon, a small theropod (Chapter 9). The presumed parenthood of Orodromeus was dropped as a result of this information and a lesson was learned about avoiding "guilt by association" - that is, assuming that certain eggs and nests belong to dinosaurs whose remains happen to be nearby. The actual nests and eggs of Orodromeus are still unknown, despite the better-than-average establishment of the reproductive habits of its contemporaneous iguanodontian relatives.

Our knowledge of parental care among ornithopods specifically, and dinosaurs in general, is mostly limited as a hypothesis to iguanodontian fossil data from the Late Cretaceous of North America. Nevertheless, these fossils and their interpretations will give future researchers a better idea of what search patterns to assume when looking for similar fossil evidence from other ornithopods.

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