Haikouichthys is an extinct group of early jawless fish that lived during the Cambrian period, about 518 million years ago, in what is now Yunnan Province, China. Along with Myllokunmingia and Zhongjianichthys, it is considered the oldest known vertebrate animal found in the fossil record.
The main species, Haikouichthys ercaicunensis, was first described in 1999 along with Myllokunmingia. By 2003, more than 500 fossils were linked to this group. The holotype was discovered in the Yuanshan member of the Qiongzhusi Formation in the "Eoredlichia" Zone near Ercai Village in Haikou Subdistrict, Xishan, Kunming. This location is not the same as the city of Haikou in Hainan Province, which is why the species is named "Haikou fish from Ercaicun." The fossils were found among the Chengjiang fauna, a site known for preserving many soft-bodied fossils.
Haikouichthys had a clear skull and structures that suggest it was a vertebrate or at least a craniate. Recent studies place it, along with Myllokunmingia and the family Myllokunmingiidae, on the evolutionary path leading to vertebrates. Some scientists, like Hou and others, once thought Haikouichthys and Myllokunmingia were the same, but later research by Simon Conway Morris showed they were different based on differences in gill structures and muscle shapes. In their book The Cambrian Fossils of Chengjiang, China, Hou and colleagues suggested that Haikouichthys, Myllokunmingia, and Zhongjianichthys might be different stages of decay from the same animal, and they used the name Myllokunmingia for their work in 2002 due to uncertainty about the differences between the three groups.
A 2026 study found that Haikouichthys and related animals had four camera-like eyes, which may represent the earliest known eye structure in vertebrates.
Description
Haikouichthys is about 2.5 cm (1 in) long and has a thinner body than Myllokunmingia.
In the head area, scientists have found four eyes that can form images, a structure similar to an upper lip, cartilage in the head, at least six (possibly up to nine) gills with thin filaments, and structures around the ears. A part of the head resembling an upper lip was compared to a similar feature in young lampreys. Researchers believe this structure might have helped Haikouichthys and related species move food particles on the ocean floor or stir them into the water for feeding. Scientists think Haikouichthys had a brain with major sections similar to those found in modern vertebrates.
In addition to two side-facing eyes, structures once thought to be nasal sacs were later identified as two smaller, central eyes in 2026 by Lei and colleagues, based on studies of six Haikouichthys specimens and four myllokunmingid specimens. This new understanding suggests Haikouichthys and related species had four eyes capable of forming images. The two smaller central eyes are believed to be similar to the pineal/parapineal system in modern vertebrates. These four eyes likely helped with seeing in different directions, staying oriented, and avoiding predators.
A notochord with early signs of vertebrae has been found, along with zigzag-shaped muscle segments. The body has a sail-like dorsal fin connected to a tail fin, and a thin ventral fin separated from the tail fin by an opening. The fin structures in Haikouichthys are similar to those in hagfish and lampreys, and they appear to angle toward the head, center, and tail. Scientists have also identified an intestine, a space around the heart, and about 13 reproductive organs arranged in a line—a trait also seen in lancelets.
Paleoecology
Haikouichthys probably swam actively in bright, shallow seas. Its ability to swim actively might explain why not many fossils of Haikouichthys have been found. It could escape from sediment that would have buried it. It probably ate tiny particles from the water. Because it had a soft body, strong muscles, and provided a lot of energy, it was likely prey for other animals at that time. This may have led to the development of better eyesight, such as its four eye-like structures.