Ontogeny of the postcranium in crocodylomorph archosaurs
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Quantitative analysis of crocodylomorph postcranial ontogeny involved simple bivariate analysis. In Alligator mississippiensis, limb bones become visually stouter during ontogeny, but this is because limb bone length scales in negative allometry with respect to body size. Scaling of limb bone width with respect to body size is isometric. The humerus of A. mississippiensis scales positively allometrically with respect to the femur, and distal limb elements scale negatively allometrically with respect to their proximal counterparts. No changes in girdle bone lateral shape are detectable, but girdle element thickness scales positively allometrically with respect to body size. Patterns in Crocodylus acutus are similar, but negatively allometric patterns in limb bones are not as distinct. In both A. mississippiensis and C. acutus, the length of the anterior sacral centrum scales in negative allometry with respect to body size, but the length of the axial centrum scales isometrically. Allometric differences between the two vertebrae are much greater in C. acutus. Morphological analysis involved the construction of matrices showing the distribution of ontogenetically variable characters among a set of specimens for a single taxon. These matrices were entered into PAUP and treated as phylogenetic matrices; nodes on 100% strict consensus trees resulting from searches through these matrices were converted into size-independant ontogenetic stages. All postcranial elements of A. mississippiensis for which matrices were constructed were staged except the astragalus and interclavicle. Based on discrete morphological transformations, the ontogeny of the crocodylian skeleton can be divided into four broad stages. Neonates lack the distal hook on the fibula and trochleae on the humerus and femur. A variety of late-stage changes may diagnose adults, but some characters are more consistent than others. Immature specimens are those that ontogenetically fall between neonates and adults. Gerontic individuals are diagnosed by the closure of certain sutures, most notably those in the scapulocoracoid, pelvis, and axis. Construction of size-independant ontogenetic stages based on morphological criteria permitted the following observations: 1. Vertebral neurocentral sutures close in a distinct caudal to cranial sequence. The axial neurocentral suture is the last to close. 2. Most ontogenetic stages ara highly variable with respect to size, demonstrating that although maturity is correlated with size, maturity and size are not equivalent. 3. Although the sequence of transformations seen in Alligator mississippiensis seems to be consistent throughout Crocodylia, heterochronic differences can be recognized. Most prominent among these are the faster rate of neurocentral suture closure in Crocodylus acutus relative to other taxa, the ontogenetically early closure of the scapulocoracoid suture in Caiman crocodilus, and the occurrence of gerontic characters at relatively small size in Alligator sinensis. 4. Some late stage changes, such as scapulocoracoid suture closure, had not been described in crocodylians before. The morphological ontogenetic transformations seen in extant crocodylians are observable in fossils, opening the possibility that size-independant ontogenetic stages may offer an objective means of discerning maturity in fossil crocodylomorph specimens. Such stages may also help refine current understanding of archosaur phylogeny and fossil population structure.