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The Science of Sue
How To Build A Giant (cont.)
In most dinosaurs, as in living reptiles and other lower vertebrates, growth undergoes an annual cycle with a peak in growth that is followed by a slow-down and cessation of growth, often correlating with seasons of the year. A record of this growth cycle is preserved in the microscopic structure of an animal’s bones. Rapid growth causes wider bands of porous bone that are rapidly deposited and bear many blood channels, whereas denser bone marks a decrease in growth. When growth ceases, a line forms at the boundary of the bone tissue deposited during one growth cycle and that of the following year. By counting such “lines of arrested growth”, or LAGs, in a thin section of a bone under a microscope, it is possible to age a dinosaur specimen in much the same way as aging a tree. This is not as straightforward as it sounds, however, because bones remodel during growth, leading to loss of LAGs through bone erosion or replacement. This complicating factor is particularly prevalent in the hollow, weight-bearing limb bones of theropods. One of the challenges in our study was to find bones that are less prone to remodeling and retain a better age record, thus requiring less back-calculation of missing LAGs. Through sampling skeletons of alligators and monitor lizards of known ages, we found that some sections of the ribs, the gastralia, the fibula, and certain skull bones perform better than the limb bones traditionally used in studies of dinosaur histology. As luck would have it, most skeletons of theropods are found with numerous rib parts and gastralia that are too incomplete to mount and accessible for destructive sampling.
Armed with this new set of target bones, we sampled twenty specimens of North American tyrannosaurids: 7 Tyrannosaurus, 3 Daspletosaurus, and 5 each of the genera Albertosaurus and Gorgosaurus. Each specimen was sampled from multiple bones in order to double-check the obtained age counts. Other criteria were important to our sampling protocol. The specimens had to have parts of their hind limbs preserved so that their body mass, the other parameter necessary for growth curve determination, could be calculated using equations developed from the relationship between body mass and femoral circumference in living animals. Also, the specimens had to cover a range of sizes to insure that we covered juvenile through adult growth stages. Obviously, it was harder to get data for some species than for others. For instance, Daspletosaurus is a rare taxon, and we were limited to three specimens that fulfilled our criteria.
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