Haikouichthys

Haikouichthys is an extinct genus of craniate (animals with notochords and distinct heads) that lived 518 million years ago, during the Cambrian explosion of multicellular life. The type species, Haikouichthys ercaicunensis, was first described in 1999. Haikouichthys had a defined skull and other characteristics that have led paleontologists to label it a true craniate, and even to be popularly characterized as one of the earliest fishes. More than 500 specimens were referred to this taxon and phylogenetic analyses indicates that the animal is probably a basal stem-craniate. Some researchers have considered Haikouichthys to be synonymous with the other primitive chordate Myllokunmingia, but subsequent studies led by the British paleontologist Simon Conway Morris identified both genera to be distinct, separate taxa on the basis of different gill arrangement, the absence of branchial rays in Myllokunmingia and the myomeres having a more acute shape in Haikouichthys.

Description
Haikouichthys is about 2.5 cm long and is narrower than Myllokunmingia, another putative chordate that comes from the same beds. The holotype of Haikouichthys ercaicunensis 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 name, which means "Haikou fish from Ercaicun". The fossil 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. Following the discovery of the holotype, additional Lower Cambrian fossils of Haikouichthys ercaicunensis have been discovered.

Researchers have identified eyes, cranial cartilages, at least six to nine gill arches, possible nasal sacs and otic capsules from its head. It is likely that the brain of Haikouichthys had the same major brain divisions found in extant vertebrates. The describers of this taxon initially reported its potential notochord, though some researchers consider this claim to be uncertain. Still, numerous segments (myomeres) with rear directed chevrons in the tail indicate that Haikouichthys was indeed a chordate, and complete dorsal, ventral and caudal fins were also found in its specimens. The fin radials of Haikouichthys show similarity to those of hagfish and lampreys, and they seem to angle "forward" toward the end thought on the basis of internal structures to be the head. There are 13 circular structures along the bottom that may be gonads, slime organs, or something else entirely.