In lieu of an abstract, here is a brief excerpt of the content:

253 9 Evolution of Growth Rates and Their Implications Kevin Padian and Koen Stein To study the evolution of growth rates, it is necessary to develop two things: a sense of the ontogenetic patterns of individual species, and a phylogeny of the species in question. Studying ontogenies in phylogenetic context is the best approach to understanding how growth rates evolve. COMPARING GROWTH RATES: DISCRETE STAGES VERSUS OVERALL CURVES Growth rates can be compared among taxa in several ways. The simplest is to choose a single point or stage in ontogeny that can be assessed for each taxon and measure the growth rate at that stage in all taxa. Padian et al. (2001) used this approach for a variety of archosaurs. They chose the sub-adult stage to measure because it appeared to distinguish best the growth trajectories among different taxa. Very young taxa grow quickly, although even at the perinatal stage there are stark differences among amniotes (Horner et al. 2001); however, it is relatively rare to find perinatal specimens of fossil taxa, and even when they are found it can be difficult to identify them in isolation from other remains. Individuals that have attained asymptotic size (“adults” by most definitions) tend to grow relatively slowly and certainly so at the slowest rates of their trajectories, so this stage does not discriminate effectively among taxa. Padian et al. (2001) reasoned that animals that are still actively growing at the sub-adult stage contrast sharply with those that are not, and this may be the best discrimination. Figure 9.1 explains this observation. It shows the growth trajectories of the hadrosaur Maiasaura, the giant Cretaceous crocodile Deinosuchus, and a more typical 254    Evolution of Growth Rates (more typical croc or more typical growth rate?) living crocodile of more typical size (crocodile data from Erickson & Brochu 1999). The “sub-adult” stage can be defined in several ways, but regardless of definition it is easy to see how the slopes of these graphs differ as asymptotic growth is approached. (The measurements of body length were obtained for Maiasaura by reconstructing complete and partially complete skeletons of this well-represented dinosaur [Horner et al. 2001] and for Deinosuchus by estimating body length from the growth of cervical scutes [Erickson & Brochu 1999]). Discrete Stages Padian et al. (2001) took histological sections of long bones (femur and tibia) of a variety of archosaurian taxa and used information from the literature for other taxa. For each taxon, they located the section of the cortex that reflected growth at the sub-adult stage (i.e., before the onset of deposition of an external fundamental system but approaching that size). The amount and degree of vascularity were compared to those of living amniotes with similar tissues and for which rates of deposition are known (e.g., Castanet et al. 2000). (Rates of deposition can also be estimated by taking the thickness of the cortex between annual lines of arrested growth (LAGs) and dividing it by the length of the year to approximate µm deposited per day.) A similar approach was applied by Klein and Sander (2008) and subsequent authors (Klein et al. 2009; Woodward & Lehman 2009; Stein et al. 2010), but instead of comparing taxa of different clades, Klein and Sander (2008) compared different sauropod taxa. Based on the histologic changes through ontogeny of sauropod long bones (femur and humerus), Klein and Sander (2008) were able to formulate a series of histologic ontogenetic stages (HOS), which correlate well Figure 9.1. Comparative growth trajectories of the dinosaur Maiasaura and two crocodiles. From Padian et al. 2001. ] Project MUSE (2024-04-19 08:59 GMT) Evolution of Growth Rates    255 Figure 9.2. Plot of histologic ontogenetic stage (HOS, 35) versus body size as expressed by femur length in Magyarosaurus dacus compared to Europasaurus, Apatosaurus, Alamosaurus, and Phuwiangosaurus. From Stein et al. 2010. with femur length. These HOS diagrams (Fig. 9.2) illustrate how different taxa follow different growth trajectories and thus make it possible to distinguish taxa that look morphologically similar (Stein et al. 2010). Comparison of Ontogenetic Curves Another approach is to derive growth curves by matching the chronology of annual (as long as you know they are annual) growth lines against size increase over ontogeny and compare these among different animals. This is essentially the approach in the previous figures. These curves are based on linear measurements, which can be made on single elements or entire body lengths. Another approach used by Erickson et al. (2004) is...

Share