Box 1 in Assessing dinosaur growth patterns: a microscopic revolution

Box 1. Assessing dinosaur longevity from growth-line counts Making growth curves for dinosaurs requires estimations of longevity for various individuals throughout development. To do this, osteohistologists sample bones from specimens spanning juvenile through adult developmental stages. The bones that are utilized show minimal remodeling (i.e. replacement or loss of the original bone during life because of metabolic, reproductive, biomechanical, or skeletal repair considerations [9]) and therefore provide a nearly complete record of development. Traditionally, dinosaur researchers have used the femur, but other bones have been shown to be equally or more efficacious in some taxa. For example, multi-element sampling in tyrannosaurs has shown (Figure Ia) that the bones shown in blue [pubis, fibula, ribs, gastralia (i.e. belly ribs) and some post-orbital skull bones] work better than the femur in these animals [12,24]. The bones are sectioned transversely at mid-shaft using a slow-speed saw fitted with a diamond-tipped blade (Figure Ib). The sections (Figure Ic) are then affixed to glass petrographic slides and sanded/polished for viewing with polarized and/or reflected microscopy. On very large specimens for which making entire cross-sectional slides is difficult, researchers polish the cut faces of the bones to reveal 'polished lines' that reflect hardness differences between individual growth lines [20,23,31]. Alternatively, Sanders has developed a method whereby a diamond-tipped drill coring-bit is used to extract a cylinder of bone from which polished-line preparations or petrographic slides can be made [23]. Aging is conducted by making total growth line counts within elements [28]. Here (Figure Ic), a thin-sectioned, dorsal rib from TYrannosaurus rex (Field Museum of Natural History, Chicago, FMNH PR 2081) shows growth lines (arrows) that were counted to age this specimen. Each line represents a period of slowed and/or potential stoppage in tissue deposition [9]. The highly vascularized regions between the rings are known as zones and represent periods of active growth [8,9]. The inset box denotes a region late in development composed of tightly stacked growth rings known as an external fundamental system that indicates when growth slowed precipitously 22,27]. At this point in development, somatic maturity and full-adult size was reached. (Figure I redrawn and reproduced with permission from [12]. Scale bar=10 mm.)Published as part of Erickson, Gregory M., 2005, Assessing dinosaur growth patterns: a microscopic revolution, pp. 677-684 in TRENDS in Ecology and Evolution 20 (12) on page 678, DOI: 10.1016/j.tree.2005.08.012, http://zenodo.org/record/373305