1006 Zoological Journal of the Linnean Society, 2019, 186, 1006–1025. With 8 figures. © 2019 The Linnean Society of London, Zoological Journal of the Linnean Society, 2019, 186, 1006–1025 Comparative morphology of presacral vertebrae in extant crocodylians: taxonomic, functional and ecological implications MASAYA IIJIMA 1,2, * and TAI KUBO 3 1 School of Resources and Environmental Engineering, Hefei University of Technology, Hefei, Anhui 230009, China 2 Department of Natural History Sciences, Hokkaido University, N10W8 Kita-ku, Sapporo, Hokkaido 0600810, Japan 3 The University Museum, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 1130033, Japan Received 27 June 2018; revised 22 November 2018; accepted for publication 25 December 2018 Despite its systematic and functional relevance, the axial skeleton of crocodylians has received considerably less attention than the cranial and appendicular skeleton due to the assumed evolutionary conservativeness. The current study provides comprehensive comparisons of presacral vertebrae in extant crocodylians to demonstrate: (1) taxonomic variation, (2) size-dependent shape changes and (3) potential integration between vertebrae and skull functional modules. Our comparisons highlighted the uniqueness of the Indian gharial, Gavialis gangeticus, among extant crocodylians. The presacral vertebrae of G. gangeticus are characterized by reduced level of regionalization and increased intervertebral mobility in the neck (more narrowly placed zygapophyses and short vertebral processes), which would help lateral head sweeping under water for foraging. The scaling relationships of vertebral dimensions against the body size proxy indicate that dorsal vertebrae become stiffer with increasing body size: positive allometries were observed in areas and heights of inter-central joints, and lengths of vertebral processes (neural spines and transverse processes). These structural changes presumably serve to resist increasing compression loads and dorsiflexion bending moment on dorsal vertebrae of larger individuals during terrestrial locomotion. The analyses of correlation between vertebral dimensions and snout shape revealed that slender-snouted species tend to have more laterally mobile necks (specifically narrow zygapophyses), implying the potential integration of cranial and neck modules to optimize the foraging strategy. KEYWORDS: aquatic adaptation – Crocodylia – postcranial skeleton – ontogeny. INTRODUCTION Crocodylians are the largest known living reptiles and at least 27 species are distributed in the tropics and subtropics around the world (Grigg & Kirshner, 2015). As derived members of Pseudosuchia, a diverse and successful group of reptiles that rivalled early dinosaurs in the Triassic (Brusatte et al., 2008), crocodylians have survived several large environmental crises (Brochu, 2003). Recent years have seen a rise in the number of comparative studies on postcrania of extant and fossil crocodylians. Such studies often focused on girdle and limb elements, revealing the morphofunctional differences between the two large groups, Alligatoridae and Crocodylidae. Compared to crocodylids, alligatorids are characterized by a stouter humerus and coracoid, shorter stylopodia (humerus and femur) in fore- and hindlimbs, shorter muscle fascicle lengths and larger cross-sectional areas for pectoral limb muscles, potentially reflecting the differences in their locomotor ecology (Chamero et al., 2013; Allen et al., 2015; Iijima et al., 2018). However, the axial skeleton of extant crocodylians has received considerably less attention, despite its potential taxonomic and ecological significance. This is partly because vertebral osteology and myology of extant crocodylian families were regarded as remarkably conservative and similar to each other (Hoffstetter & Gasc, 1969). Consequently, *Corresponding author. E-mail: miijima8@gmail.com Downloaded from https://academic.oup.com/zoolinnean/article-abstract/186/4/1006/5311613 by Nottingham Trent University user on 02 August 2019