Growth series calculated for some ornithopod species, based on bone proportions, are partially to almost complete. Moreover, these data are supplemented by bone histology relating to growth lines and LAGs (Chapter 8). The Late Cretaceous hadrosaurid Maiasaura of North America is again well represented in this respect, with specimens ranging from possible newborns to full-sized adults. Partial growth series also are available for the Late Jurassic Dryosaurus and Late Cretaceous Hypacrosaurus of North America. For example, Dryosaurus skulls show that a typical juvenile trait is small skulls correlated with large orbits. The latter trait meant that the eyes were large relative to the rest of the face. With increased growth of the skull, the orbits became proportionally smaller. Similar ontogenetic trends are also observed for Hypacrosaurus, where both the orbits and frontals shrunk in proportion to the rest of the skull and the face became longer. Size-frequency analyses of bones in general also can be used to estimate growth rates. For example, an analysis of Maiasaura juveniles indicates that they may have achieved 3-m lengths within one year. This evidence implies that these ornithopods had high metabolic rates in the early stages of life, similar to some theropods (Chapter 9).
Of course, longer limb bones are assumed to belong to older individuals of any given species, which then are used as a relative scalar for other measurements, such as LAGs and amounts of fibrolamellar bone between LAGs. LAGs are possibly a result of annual periods of slow (arrested) growth, and if this is assumed they can be used to estimate how old a given dinosaur was when it died. However, environmental factors independent of seasonal changes can also cause LAGs, so they are not absolutely reliable for calculating ages. Regardless, LAGs in the Late Cretaceous hypsilophodontid Orodromeus suggest that this dinosaur had intermediate growth rates. Furthermore, thick deposits of fibrolamellar bone in between LAGs comprise a characteristic of fast growth, which has been recorded for Maiasaura juveniles.
When used in conjunction with size-frequency data, these deposits corroborate hypotheses about early altricial stages followed by rapid growth on the way to a precocial state. Again, these data point toward higher metabolic rates for these dinosaurs, consistent with an endothermic physiology (Chapter 8). Nonetheless, growth-rate estimates are based on limited data sets and require more samples and critical analysis before they can be considered reliable.
An independent source of estimates about growth series in ornithopods is provided by tracks made by juvenile and adult animals. As with other parts of the body, feet grow in some relative proportion with age. Therefore, a bedding plane containing tracks from a population of the same species of ornithopod should constitute a good sample of its growth series (Fig. 11.10). Caveats of using such trackway horizons for growth series estimates are:
1 they may have been made by more than one species of ornithopod in the same area with similarly shaped and sized feet; and
2 they may not have been contemporaneous, and the undertracks of adults possibly could have been transmitted to underlying (older) layers containing juvenile tracks; and
3 foot growth rates may have been different from growth rates for other body parts.
Nonetheless, if these problems are resolvable, then the sheer abundance of tracks in some instances provide censuses that typically far exceed any given ornithopod bone bed. Indeed, one study of iguanodontian tracks from a single bedding plane in an Early Cretaceous stratum of Colorado yielded hundreds of measurements showing a variety of sizes (20-48 cm long) for otherwise morphologically identical tracks. Localities with Early and Late Cretaceous strata that contain abundant ornithopod footprints, such as those in western Canada, South Korea, and other areas of the world, are also amenable to such an analysis. These data can thus provide insights into population structures of ornithopods that can supplement measurements of bones.
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