Haikouichthys ˌhaɪkuːˈɪkθɪs is an extinct genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of craniate
Craniata is a proposed clade of chordate animals that contains the Myxini , Petromyzontida , and Gnathostomata as living representatives...

 (creatures with backbones and distinct heads) believed to have lived c. 530 million years ago, during the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...

 of multicellular life. Haikouichthys had a defined skull and other characteristics that have led paleontologists
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...

 to label it a true craniate, and even to be popularly characterized as one of the earliest fishes
Prehistoric fish
Prehistoric fish refers to early fish that are known only from fossil records. They are the earliest known vertebrates, and include the first and extinct fish that lived through the Cambrian to the Tertiary. The study of prehistoric fish is called paleoichthyology...

. Cladistic analysis
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

 indicates that the animal is probably a basal chordate
Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...

 or a basal craniate.
but it does not possess sufficient features to be included uncontroversially even in either stem group.
It is about 2.5 cm (1 inch) long and is narrower than Myllokunmingia
Myllokunmingia is a chordate from the Lower Cambrian Maotianshan shales of China, thought to be a vertebrate, although this is not conclusively proven. It is 28 mm long and 6 mm high....

, another putative chordate that comes from the same beds.