Allometric growth of limb and body proportions in crocodylians

Crocodylia is the sole extant remnant of quadrupedal archosaurs playing a pivotal role in understanding the evolution of growth allometry in the archosaur locomotor apparatus. However, among crocodylians, the postnatal growth of the postcranial skeleton has almost exclusively been examined in Alligator mississippiensis, and whether other species share the same growth pattern is unknown. Here, we tested whether the following allometric trends are conserved across Crocodylia: (1) forelimb length grows isometrically relative to hindlimb length; (2) fore- and hindlimb lengths become relatively shorter with increasing body size; and (3) long bone cross-sectional geometry becomes more robust relative to body size. We examined the relationships of limb lengths, stylopodial circumferences and presacral length (body size proxy) in extant crocodylians using reduced major axis regressions and compared the slopes among species. The result revealed non-uniform growth patterns of limb architecture among living crocodylians. Generally, the hindlimb grows with negative allometry against the forelimb in non-gavialid crocodylians, whereas two gavialids (Gavialis gangeticus and Tomistoma schlegelii) showed isometry in hind- vs. forelimb length scaling, potentially reflecting their unique locomotor ecology. Femur circumference scales negatively against humerus circumference in most of the species examined, which may be related to the anterior shift of the center of mass during growth. Stylopodial circumferences scale variously against stylopodial lengths and presacral length in crocodylians, lending little support to hypotheses that these allometries correlate with adult body size or metabolism (i.e. ectothermic or endothermic) in tetrapods.