School of Biological Sciences, University of Utah, Salt Lake City, UT, USA.
J Anat. 2020 Nov;237(5):870-889. doi: 10.1111/joa.13256. Epub 2020 Aug 13.
Birds have lost and modified the musculature joining the pectoral girdle to the skull and hyoid, called the pectoral extrinsic appendicular and infrahyoid musculature. These muscles include the levator scapulae, sternomandibularis, sternohyoideus, episternocleidomastoideus, trapezius, and omohyoideus. As non-avian theropod dinosaurs are the closest relatives to birds, it is worth investigating what conditions they may have exhibited to learn when and how these muscles were lost or modified. Using extant phylogenetic bracketing, osteological correlates and non-osteological influences of these muscles are identified and discussed. Compsognathids and basal Maniraptoriformes were found to have been the likeliest transition points of a derived avian condition of losing or modifying these muscles. Increasing needs to control the feather tracts of the neck and shoulder, for insulation, display, or tightening/readjustment of the skin after dynamic neck movements may have been the selective force that drove some of these muscles to be modified into dermo-osseous muscles. The loss and modification of shoulder protractors created a more immobile girdle that would later be advantageous for flight in birds. The loss of the infrahyoid muscles freed the hyolarynx, trachea, and esophagus which may have aided in vocal tract filtering.
鸟类失去并改造了连接胸带与头骨和舌骨的外在附肢和舌下肌肉，称为肩胛外附肢和舌下肌肉。这些肌肉包括斜方肌、胸骨颌肌、胸骨舌骨肌、胸骨斜方肌、斜方肌和胸骨舌骨肌。由于非鸟兽脚亚目恐龙是鸟类最接近的亲戚，因此值得研究它们可能表现出的条件，以了解这些肌肉是何时以及如何失去或改变的。利用现存的系统发育框架、骨骼学相关因素和这些肌肉的非骨骼学影响，对其进行了识别和讨论。发现近颌龙类和基础手盗龙类是这些肌肉失去或改变的衍生鸟类状态最有可能的过渡点。为了控制颈部和肩部的羽毛区域，以进行绝缘、展示或在动态颈部运动后收紧/重新调整皮肤，可能需要增加对这些肌肉的控制，这可能是一些肌肉被改造为皮骨肌肉的选择力。肩伸肌的丧失和改变使肩部更不灵活，这在后来鸟类的飞行中具有优势。舌下肌肉的丧失使喉咽、气管和食管得到解放，这可能有助于声道过滤。
