科学家通过化石证据发现，鳄鱼和鸟类的共同祖先最初都是温血动物，具有较高的代谢率[1]。这一共同祖先在约2.5亿年前的三叠纪早期出现[1]。约6600万年前，当大多数恐龙灭绝时，两个谱系分道扬镳：鸟类保持了温血特征并最终进化出飞行能力，而鳄鱼则演化为冷血动物以适应水中伏击捕食的生活方式[1]。这种代谢方式的改变解释了为什么现代鳄鱼无法飞行[1]。
这项研究基于对81个物种化石腿骨血管孔洞的测量，该工作自2018年开始进行[1]。研究人员发现，温血动物无法长时间屏息，这使其作为水中伏击捕食者效率低下，而低代谢的冷血体质使鳄鱼能够屏息更久，从而更好地适应水生环境[1]。值得注意的是，现代鳄鱼虽然已是冷血动物，但仍保留着四腔心脏这一通常与温血动物相关的特征[1]。
Scientists have determined that the evolutionary divergence between crocodiles and birds stems from a fundamental metabolic shift that occurred approximately 66 million years ago [1]. Both groups descended from a common ancestor that originated in the Early Triassic period roughly 250 million years ago and possessed warm-blooded metabolisms with elevated metabolic rates [1]. However, when most dinosaurs went extinct around 66 million years ago, crocodiles underwent a dramatic physiological transformation, evolving into cold-blooded animals adapted for ambush predation in aquatic environments, while birds retained their warm-blooded characteristics [1].
This metabolic difference explains the stark contrast in modern capabilities between the two species. Warm-blooded animals cannot hold their breath for extended periods, a limitation that renders them ineffective as aquatic ambush predators; by contrast, the lower metabolism of cold-blooded crocodiles enables them to remain submerged far longer [1]. The research, which examined fossilized leg bone vascular pores from 81 species, revealed that both lineages maintained high metabolic rates approximately 66 million years ago before crocodiles transitioned to a low-metabolism physiology [1]. Additionally, modern crocodiles possess four-chambered hearts, a feature typically associated with warm-blooded animals, providing further evidence of their evolutionary heritage [1].
