Haikouichthys

Haikouichthys ercaicunensis is a genus of extinct jawless fish that represents one of the earliest known vertebrates. Discovered in the celebrated Chengjiang fossil beds of Yunnan, China, this small, fish-like creature lived approximately 518 million years ago during the Cambrian Period. Its discovery provided crucial evidence for the early evolution of craniates, animals with a distinct skull. Physically, Haikouichthys was diminutive, measuring only about 2.5 centimeters in length. It possessed a distinct head and trunk, with clear evidence of a skull, paired eyes, a notochord (a primitive backbone), and gill arches. A dorsal fin, supported by fin rays, ran along its back, suggesting it was an active swimmer, likely propelling itself through the water with eel-like movements. Unlike later fish, it lacked jaws and paired fins. Its ecological role was likely that of a detritivore or a scavenger, feeding on small particles in the marine environment. The evolutionary significance of Haikouichthys cannot be overstated. As one of the first animals to possess key vertebrate features, it sits very close to the base of the vertebrate family tree, offering paleontologists a rare glimpse into the anatomical structures that would eventually give rise to all subsequent fish, amphibians, reptiles, birds, and mammals, including humans. The exceptional preservation of its soft tissues in the fine-grained mudstone of the Qiongzhusi Formation has allowed for detailed study of its internal anatomy, confirming its status as a foundational species in the story of vertebrate life.