当看到鸟儿翘起扇形的短尾巴在空中调整飞行方向时，很难想象，它们的恐龙祖先曾拖着由30多节尾椎组成、长达一两米的大尾巴，在侏罗纪的原始森林里快速狂奔。
从“长尾龙”到“短尾鸟”，中间经历了什么？
不虞政和鸟（前）和奇异福建龙（后）生态复原图（张宗达绘制）
过去，化石记录留下了一道巨大的“演化鸿沟”。长尾鸟和短尾鸟几乎同时出现，也就是说现代鸟类的短尾巴（由4—9块尾椎和一块尾综骨构成）似乎是凭空冒出来的，瞬间取代了恐龙标志性的长尾巴，缺乏中间的过渡环节。
这让许多生物学家认为，那种“尾巴变短但缺少尾综骨”的过渡物种，在演化上从未出现过，尾巴变短不过是少数基因突变的结果，而非渐进演化。
Because long-tailed and short-tailed birds appear almost simultaneously in the fossil record, without intermediate forms, many evolutionary biologists argued that a transitional species with an abbreviated but no unfused pygostyle — the bony structure that characterizes the tails of modern birds — was unlikely present in bird evolutionary history.
pygostyle /ˈpaɪ.ɡə.staɪl/ 尾综骨
They suggested that tail shortening could result from just a few genetic mutations — a hypothesis seemingly supported by the abrupt appearance of the pygostyle in the Late Jurassic.
mutation /mjuːˈteɪ.ʃən/ 基因突变
不虞政和鸟正型标本、骨骼复原图和“小虞”卡通复原图（王敏供图）
如今，一件从福建出土的距今约1.5亿年的鸟类化石推翻了这一猜想。
However, the discovery of a 150-million-year-old bird fossil unearthed in Fujian province challenges that long-held assumption by providing a long-sought transitional form.
中科院古脊椎所和福建地科院联合考察队发现不虞政和鸟正型标本
近日，中国科学院古脊椎动物与古人类研究所和福建省地质科学研究院联合团队经过累计400余天的野外发掘，在福建政和动物群发现了一件保存精美的鸟类化石。
The fossil was discovered by a joint research team from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences and the Fujian Geological Science Research Institute. The discovery followed more than 400 days of fieldwork at the Zhenghe Fauna, a Late Jurassic fossil locality in Zhenghe county, Nanping, Fujian province, renowned for yielding exquisitely preserved vertebrate fossils.
他们将其命名为“不虞政和鸟”。其种名“不虞”出自《国语·周语》，意为“意料之外”，因为它独特的尾部结构完全颠覆学界过往认知。
Researchers named the new species Zhengheornis buyu. The genus name refers to the discovery site, while the species name is derived from Discourses of the States, or Guoyu, an ancient Chinese book, meaning "unexpected" — a tribute to the specimen’s surprising tail anatomy.
不虞政和鸟3D复原视频（王敏提供）
不虞政和鸟的尾部仅由15块变短的尾椎组成，且其末端并没有愈合成为尾综骨。相比之下，其他长尾鸟类（如始祖鸟）尾椎数超过23块；而与鸟类亲缘关系更近的多数兽脚类恐龙，尾椎数通常超过30块，尾巴长度接近甚至超过体长。
Dating to the Tithonian stage of the Late Jurassic, about 148 million to 150 million years ago, the well-preserved articulated skeleton has a tail composed of only 15 shortened caudal vertebrae and lacks a pygostyle. That is significantly fewer than in other long-tailed birds, such as Archaeopteryx, which possessed more than 23 caudal vertebrae, and other early bird-like dinosaurs, which typically exceeded 30.
Tithonian /taɪˈθəʊ.ni.ən/ 提通期
skeleton /ˈskel.ə.tən/ 骨骼
vertebrae /ˈvɜː.tɪ.briː/ 椎骨、脊椎骨
caudal /ˈkɔː.dəl/ 尾部的
中生代兽脚类恐龙（包括鸟类）后肢形态多样性比较（王敏供图）
论文通讯作者、第一作者、古脊椎所研究员王敏表示：这一尾骨形态结构有力地证明了在鸟类尾部的演化历史中，尾椎数量的减少和变短发生在尾椎愈合并形成尾综骨之前。这种“分步走”的演化模式，填补了鸟类尾部演化缺失的关键拼图。
"This tail morphology provides critical evidence for the stepwise evolution of the bird tail: vertebral reduction and tail shortening preceded pygostyle fusion in early bird evolution," said Wang Min, the study's lead and corresponding author and a professor at the Institute of Vertebrate Paleontology and Paleoanthropology.
"This landmark discovery not only reconciles long-standing academic debates regarding the timing of early avian diversification but also adds a crucial piece to the puzzle across the dinosaur-bird transition," Wang said.
morphology/mɔːˈfɒl.ə.dʒi/ 形态学
此外，这种缩短且没有过度僵化的短尾巴，能够有效减轻体重，使身体重心和升力中心向前移动。同时，较少的尾椎关节提升了尾部的刚性，更利于飞行控制；较短的力臂也让政和鸟能更轻松地操控尾羽，从而获得了比始祖鸟等更好的飞行稳定性和机动性。
The researchers said the shortened but flexible tail likely offered important functional advantages. By reducing body weight, shifting the center of gravity forward, and making the tail less rigid, Zhengheornis buyu could move its tail feathers more effectively. This would have improved its flight stability and control compared with its longer-tailed contemporaries like Archaeopteryx.
rigid /rɪˈdʒɪd/ 刚硬的
所以，下次看到鸟儿收拢尾羽俯冲而下，不妨想想1.5亿年前——恐龙祖先的古老印记，早已在那只小“意外鸟”未完成的尾巴里，悄然蜕变成了如今的轻盈。
记者：李梦涵
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