Key Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Palaeontology and Palaeoanthropology, Chinese Academy of Sciences, Beijing, People's Republic of China.
Proc Biol Sci. 2010 Jul 7;277(1690):2027-33. doi: 10.1098/rspb.2009.2281. Epub 2010 Mar 3.
In extant birds, the hand is permanently abducted towards the ulna, and the wrist joint can bend extensively in this direction to fold the wing when not in use. Anatomically, this asymmetric mobility of the wrist results from the wedge-like shape of one carpal bone, the radiale, and from the well-developed convexity of the trochlea at the proximal end of the carpometacarpus. Among the theropod precursors of birds, a strongly convex trochlea is characteristic of Coelurosauria, a clade including the highly derived Maniraptora in addition to tyrannosaurs and compsognathids. The shape of the radiale can be quantified using a 'radiale angle' between the proximal and distal articular surfaces. Measurement of the radiale angle and reconstruction of ancestral states using squared-change parsimony shows that the angle was small (15 degrees) in primitive coelurosaurs but considerably larger (25 degrees) in primitive maniraptorans, indicating that the radiale was more wedge-shaped and the carpal joint more asymmetric. The radiale angle progressively increased still further within Maniraptora, with concurrent elongation of the forelimb feathers and the forelimb itself. Carpal asymmetry would have permitted avian-like folding of the forelimb in order to protect the plumage, an early advantage of the flexible, asymmetric wrist inherited by birds.
在现存的鸟类中，手部永久性地向尺骨外展，腕关节可以在这个方向上广泛弯曲，以便在不使用时折叠翅膀。从解剖学上看，这种不对称的腕关节灵活性源于一块腕骨（桡骨）楔形的形状，以及近端腕掌骨上发达的滑车凸面。在鸟类的兽脚亚目祖先中，强烈的凸面滑车是虚骨龙类的特征，除了暴龙和伤齿龙外，还包括高度衍生的手盗龙类。桡骨的形状可以通过近端和远端关节面之间的“桡骨角”来量化。桡骨角的测量和使用平方变化简约法重建祖先状态表明，在原始虚骨龙中，角度较小（15 度），而在原始手盗龙中则相当大（25 度），这表明桡骨更楔形，腕关节更不对称。在手盗龙类中，桡骨角进一步增加，同时前肢羽毛和前肢本身也变长了。腕关节的不对称性使得前肢能够像鸟类一样折叠，以保护羽毛，这是鸟类继承的灵活、不对称的腕关节的早期优势。
