Haikouichthys

Haikouichthys is an extinct genus of primitive jawless fish that lived during the Cambrian period, 518 million years ago, in what is now the Yunnan Province of China. Alongside Myllokunmingia and Zhongjianichthys, it is considered to be the earliest known vertebrate animal in the fossil record.

The type species, Haikouichthys ercaicunensis, was first described in 1999 in addition to its confamilial, Myllokunmingia, and more than 500 specimens were referred to this taxon as of 2003. The holotype was found in the Yuanshan member of the Qiongzhusi Formation in the 'Eoredlichia' Zone near Ercai Village in the Haikou Subdistrict (not to be confused with the city of Haikou in Hainan) of Xishan, Kunming, hence its specific name, which means "Haikou fish from Ercaicun". The fossil (along with Myllokunmingia's holotype) was recovered among the Chengjiang fauna, in one of a series of Lagerstätten sites where thousands of exquisitely preserved soft-bodied fossils have already been found.

Haikouichthys had a defined skull, vertebral elements and other characteristics that have led paleontologists to label it a vertebrate or at least a craniate, with the most recent phylogenetic analyses placing it, Myllokunmingia or the family Myllokunmingiidae on the vertebrate stem. Hou and colleagues have considered Haikouichthys to be synonymous with Myllokunmingia, but subsequent studies led by the British paleontologist Simon Conway Morris identified the genera to be distinct taxa on the basis of their gill arrangements, the absence of branchial rays in Myllokunmingia and the muscle segments (myomeres) having a more acute shape in Haikouichthys. Conversely, specimens of Haikouichthys, Myllokunmingia and Zhongjianichthys were suggested by Hou et al. in their book, The Cambrian Fossils of Chengjiang, China, to be taphonomic variants (i.e., of the same animal but in different states of decay), preferring to use the name Myllokunmingia to refer to myllokunmingiid specimens in their work in concurrence with Hou et al., 2002 due to what they argue to be the lack of certainty in the reliability of the distinguishing characters between the three taxa.

In a 2026 study, Haikouichthys and related animals were interpreted to have four camera-type eyes, which would make this the ancestral condition of the vertebrate total group.