Computed tomography and three-dimensional reconstruction of the skull of the stem tetrapod <i>Crassigyrinus scoticus</i> Watson, 1929
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The early tetrapod <i>Crassigyrinus scoticus</i> was a large aquatic predator known from the lower- to mid-Carboniferous (upper Tournasian to upper Visean/lower Serpukovian, approximately 350–330 Ma) of Scotland and Canada. <i>Crassigyrinus</i> is enigmatic in terms of its phylogenetic position due to its unusual morphology, which features a mixture of primitive and derived characters. Previous reconstructions, based on five incomplete and deformed specimens, have suggested a dorsoventrally tall skull with a short and broad snout, large orbits and external nares, and an extended postorbital region. In this study, we scanned four specimens using computed tomography and segmented imaging data to separate bone from matrix and individual bones from each other. Based on these data, we present a revised description of the upper and lower jaws, including sutural morphology and abundant new anatomical information. Damage was repaired and the skull retrodeformed to create a hypothetical three-dimensional reconstruction of the skull of <i>Crassigyrinus</i> that is dorsoventrally flatter than earlier reconstructions, yet still morphologically unique amongst early tetrapods. Overall skull shape, the size and distribution of the teeth, sutural morphology, and the specialized anatomy of the jaw joint and mandibular symphysis all suggest that <i>Crassigyrinus</i> was a powerful aquatic predator capable of hunting and subduing large prey.
早期四足类（tetrapod）苏格兰厚迷螈（*Crassigyrinus scoticus*）是一类大型水生掠食动物，其化石产自苏格兰与加拿大的石炭纪早-中期地层（时代跨度为图尔奈阶上部至维宪阶上部/谢尔普霍夫阶下部，距今约3.5亿至3.3亿年）。厚迷螈属（*Crassigyrinus*）的系统发育位置始终存在争议，因其反常的形态特征兼具原始与衍生性状。此前基于5件残缺且受压变形标本的复原研究认为，该类群具有背腹向高耸的头骨，吻部短宽，眼眶与外鼻孔硕大，且眶后区域延长。本研究对4件标本开展计算机断层扫描（computed tomography）与分层成像，通过成像数据分离骨骼与围岩，并将单块骨骼逐一区分开。基于上述数据，我们对上、下颌骨进行了重新描述，涵盖骨缝形态与大量全新解剖学信息。研究团队修复了受损部位，并对头骨进行了变形复原，以此构建出苏格兰厚迷螈头骨的假想三维复原模型。该模型的头骨背腹向较此前的复原结果更为扁平，但仍属于早期四足类中形态独特的类群。整体头骨形态、牙齿的尺寸与分布模式、骨缝形态，以及颌关节与下颌联合的特化解剖结构，均证实厚迷螈属为强悍的水生掠食者，具备捕猎并制服大型猎物的能力。
