Haikouichthys

Haikouichthys is an extinct genus of primitive, jawless fish. It lived approximately 518 million years ago during the Cambrian period. This animal lived in what is now the Yunnan Province of China. Along with other fossils like Myllokunmingia and Zhongjianichthys, it is one of the earliest known vertebrate animals. A vertebrate is an animal that possesses a skull and a backbone or vertebral elements. Studying Haikouichthys is vital for scientists. It helps them understand the very beginning of the vertebrate lineage in the fossil record.

The physical structure of Haikouichthys shows how early vertebrates functioned. It possessed a defined skull and cranial cartilages. Researchers also identified a notochord, which is a flexible rod, along with vertebral elements. The body featured zigzag myomeres, which are segments of muscle used for movement. It had a sail-like dorsal fin that was continuous with a caudal fin at the tail. A slender ventral fin was also present on its underside. These parts allowed the animal to be an active swimmer in its environment.

Haikouichthys had several distinct biological features. Its head contained at least six, and possibly up to nine, gills with fine filaments. It also possessed otic capsules, which are involved in hearing or balance. A unique feature was an "upper lip." Some researchers believe this organ functioned like a multipurpose hand. It might have draped over food particles on the ocean floor. It could also stir food into the water column for suspension feeding. This process allowed the fish to filter nutrients from the water.

One of the most surprising discoveries involves the eyes of Haikouichthys. A 2026 study reinterpreted the anatomy of these animals. Researchers found they had four camera-type eyes. These eyes were capable of image formation. There was a pair of lateral eyes on the sides of the head. There was also a pair of smaller, medial eyes. These medial eyes are thought to be homologous to the pineal or parapineal system in modern vertebrates. This complex visual system likely helped with navigation and avoiding predators.

The history of this discovery began in 1999. Scientists first described the type species, Haikouichthys ercaicunensis, in that year. The fossils were recovered from the Chengjiang fauna. This is a famous Lagerstätte, or a site with extraordinary fossil preservation. The holotype was found in the Yuanshan member of the Qiongzhusi Formation. It was located near Ercai Village in the Haikou Subdistrict. By 2003, more than 500 specimens had been identified as part of this taxon. The name specifically means "Haikou fish from Ercaicun."

There has been scientific debate regarding how Haikouichthys relates to other fossils. Some researchers, such as Hou and colleagues, suggested it was synonymous with Myllokunmingia. They argued the differences might just be taphonomic variants. Taphonomic variants are specimens of the same animal that look different due to decay. However, paleontologist Simon Conway Morris identified them as distinct taxa. He noted differences in gill arrangements and muscle shapes. For example, the myomeres in Haikouichthys had a more acute shape than those in Myllokunmingia.

Internal biology provides further links to other species. Haikouichthys had a reasonably well-developed brain. It likely shared the same major brain divisions found in modern vertebrates. The animal also had an intestine and a pericardic cavity. It possessed around 13 serially arranged gonads. This specific trait is considered a basal trait shared with lancelets. Additionally, the fin radials show similarities to modern hagfish and lampreys. These connections help scientists map the evolution of complex life on Earth.