Haikouichthys ˌhaɪkuˈɪkθɪs 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. 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. Cladistic analysis indicates that the animal is probably a basal chordate or a basal craniate; but it does not possess sufficient features to be included uncontroversially even in either stem group. It was formally described in 1999.
Haikouichthys is about long and is narrower than Myllokunmingia, another putative chordate that comes from the same beds. The holotype of Haikouichthys ercaicunensis was found in the Yuansshan member of the Qiongzhusi Formation in the 'Eoredlichia' Zone near Haikou at Ercaicun, Kunming City, Yunnan, China, hence its name "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.
The animal has a distinct head and tail. The head has at least six and perhaps nine probable gills. There are a number of segments (myomeres) with rear directed chevrons in the tail. There is a notochord. There is a prominent dorsal fin with fin radials similar, but not comparable, to those of hagfish and lampreys. The fin radials seem to angle "forward" toward the end thought on the basis of internal structures to be the head. This happens with a few modern fish but is an uncommon arrangement. There are 13 circular structures along the bottom that may be gonads, slime organs, or something else entirely.
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The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period of early Paleozoic when there was a sudden radiation of complex life and practically all major animal phyla started appearing in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.
In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consists of inner, vacuolated cells covered by fibrous and elastic sheaths, lies along the anteroposterior axis (front to back), is usually closer to the dorsal than the ventral surface of the embryo, and is composed of cells derived from the mesoderm.
A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the much more numerous Gnathostomata (jawed vertebrates). Formerly distinct from vertebrates by excluding hagfish, molecular and anatomical research in the 21st century has led to the reinclusion of hagfish as vertebrates, making living craniates synonymous with living vertebrates.