Haikouichthys /?ha?ku?ks/ is an extinct genus of craniate (creatures with backbones and distinct heads) believed to have lived c. 530 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. Haikouichthys is about 2.5 cm (1 inch) 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 Lagerstatten 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 probably a notochord, although only a short segment is preserved in the single known specimen. 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. In the simplest sense craniates are chordates with heads, thus excluding members of chordate subphyla Urochordata (tunicates) and Cephalochordata (lancelets), but including Myxini, which have cartilaginous skulls and tooth-like structures composed of keratin. Craniata also includes all lampreys and armored jawless fishes, armoured fish, sharks, skates, and rays, and Teleostomians:spiny sharks, bony fish, lissamphibians, temnospondyls and protoreptiles, reptiles, birds and mammals. The craniate head consists of a brain, sense organs including eyes, and a skull. In addition to distinct crania (sing. cranium), craniates possess many derived characteristics which have allowed for more complexity to follow. Molecular-genetic analysis of craniates reveals that, compared to less complex animals, they developed duplicate sets of many gene families that are involved in cell signaling, transcription, and morphogenesis (see homeobox). In general, craniates are much more active than tunicates and lancelets and as a result have greater metabolic demands, as well as several anatomical adaptations. Aquatic craniates have gill slits whic
are connected to muscles and nerves which pump water through the slits (as opposed to lancelets, whose pharyngeal slits are used only for suspension feeding), engaging in both feeding and gas exchange. Muscles line the alimentary canal, moving food through the canal, allowing higher craniates like mammals to develop more complex digestive systems for optimal food processing. Craniates have cardiovascular systems which include a heart with two or more chambers, red blood cells, and O2 transporting hemoglobin, as well as kidneys. Paleontology or palaeontology (pron.: /?p?ln?t?l?d?i/) is the scientific study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments (their paleoecology). As a "historical science" it attempts to explain causes rather than conduct experiments to observe effects. Palaeontological observations have been documented as far back as the 5th century BCE. The science became established in the 18th century as a result of Georges Cuvier's work on comparative anatomy, and developed rapidly in the 19th century. The term itself originates from Greek: ? (palaios) meaning "old, ancient," , ?- (on, ont-), meaning "being, creature" and ? (logos), meaning "speech, thought, study". Palaeontology lies on the border between biology and geology, and shares with archaeology a border that is difficult to define. It now uses techniques drawn from a wide range of sciences, including biochemistry, mathematics and engineering. Use of all these techniques has enabled palaeontologists to discover much of the evolutionary history of life, almost all the way back to when Earth became capable of supporting life, about 3,800 million years ago. As knowledge has increased, paleontology has developed specialized sub-divisions, some of which focus on different types of fossil organisms while others study ecology and environmental history, such as ancient climates. Body fossils and trace fossils are the principal types of evidence about ancient life, and geochemical evidence has helped to decipher the evolution of life before there were organisms large enough to leave fossils. Estimating the dates of these remains is essential but difficult: sometimes adjacent rock layers allow radiometric dating, which provides absolute dates that are accurate to within 0.5%, but more often palaeontologists have to rely on relative dating by solving the "jigsaw puzzles" of biostratigraphy. Classifying ancient organisms is also difficult, as many do not fit well into the Linnean taxonomy that is commonly used for classifying living organisms, and palaeontologists more often use cladistics to draw up evolutionary "family trees". The final quarter of the 20th century saw the development of molecular phylogenetics, which investigates how closely organisms are related by measuring how similar the DNA is in their genomes. Molecular phylogenetics has also been used to estimate the dates when species diverged, but there is controversy about the reliability of the molecular clock on which such estimates depend.